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Constraining basin geometry and fault kinematics on the santo tomas segment of the agua blanca fault through a combined ...

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Title:
Constraining basin geometry and fault kinematics on the santo tomas segment of the agua blanca fault through a combined geophysical and structural study
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Springer, Adam
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Gravity
Magnetics
Baja California
San Andreas
Fluvial Terraces
Dissertations, Academic -- Geology -- Masters -- USF   ( lcsh )
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non-fiction   ( marcgt )

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Abstract:
ABSTRACT: The Agua Blanca fault is a major transverse structure of northern Baja California, extending more than 120km east from the Punta Banda ridge near the city of Ensenada to the San Matais Pass in central Baja. Through much of its eastern extent slip on this fault appears to be pure strike slip, however, at the Valle de Santo Tomás the fault makes a ~25˚ change in orientation, which coincides with the formation of extensional basins on the fault. Recent evidence of the independent movement of the Baja Micro-plate relative to a stable Southern California Black leads to several possible hypotheses to explain this including: 1)That basins are localized structures, the result of a series of right steps or bends along the dextral Agua Blanca fault. 2)Basins are transtensional, possibly as a result of complexities associated with the northern boundary of the Baja Micro-plate . To test between these hypotheses it was necessary to constrain the fault kinematics on both basin bounding and in-basin faults, well as the basin geometry. This was accomplished through combined structural and geophysical surveys. Data collected suggest that the majority of dip-slip is confined to the Santo Tomás fault bounding the basin to the south, while the Agua Blanca fault bounding the basin to the north is primarily strike slip. This orientation typical in transtensional basins, suggesting that although Valle de Santo Tomás formed at a step over it is not a pull apart basin. Possible explanations for transtension in this area come from the orientation of the Agua Blanca fault in relation to the Baja Micro-plate. Where the fault is close to aligned with the relative motion of the plate there is little transtension, such as in Valle de Agua Blanca, however, where the fault makes a 25˚ change in orientation and becomes more oblique the motion of the Baja Micro-plate transtension is present.
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Thesis (M.S.)--University of South Florida, 2010.
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Constraining Basin Geometry and Fault Ki nematics on the Santo Toms Segment of the Agua Blanca Fault Through a Combined Geophysical and Structural Study by Adam Springer A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Geology College of Arts and Sciences University of South Florida Major Professor: Paul Wetmore, Ph.D. Chuck Connor, Ph.D. Sarah Kruse, Ph.D. John Fletcher, Ph.D. Date of Approval: January, 2010 Keywords: Gravity, Magnetics, Baja Calif ornia, San Andreas, Fluvial Terraces Copyright 2010, Adam Springer

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Table of Contents List of Figures iii List of Profiles iv Abstract v Chapter 1: Introduction 1 Chapter 2: Background 5 2.1 Pre Batholithic Strata 5 2.2 Peninsular Ranges Batholith 7 2.3 Post Batholithic Strata 10 2.4 Faulting 11 2.5 Previous Work 14 2.6 Geophysics 16 Chapter 3: Methods 17 3.1 Magnetics 17 3.2 Gravity 17 Chapter 4: Results 22 4.1 Structural Mapping 22 4.2 Geophysics 28 Chapter 5: Discussion 36 Chapter 6: Conclusions 41 References 42 i

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Appendices 50 Appendix A Table 1 Gravity Data 51 Appendix B Table 2 Magnetics Data 60 ii

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List of Figures Figure 1 Active Fault Map 2 Figure 2 Block Models 4 Figure 3 Regional Geology Map 6 Figure 4 Ances tral Agua Blanca Map 10 Figure 5 Study Area Fault Map 14 Figure 6 Gravity and Magnetic Stations 21 Figure 7 Photo of In-Basin Fault Scarp 23 Figure 8 Photo of Agua Blanca Fault 24 Figure 9 Photo of Fluvial and Strath Terraces 26 Figure 10 Geologic Map 27 Figure 11 Gravity Map 29 Figure 12 Projection fo r Basin Depth Calculation 37 iii

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List of Profiles Profile A-A 30 Profile B-B 31 Profile C-C 33 Profile D-D 34 Profile E-E 35 iv

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Constraining Basin Geometry and Fault Ki nematics on the Santo Toms Segment of the Agua Blanca Fault Through a Combined Geophysical and Structural Study Adam Springer ABSTRACT The Agua Blanca fault is a major transver se structure of northern Baja California, extending more than 120km east from the Punta Banda ridge near the ci ty of Ensenada to the San Matais Pass in central Baja. Through much of its eastern extent slip on this fault appears to be pure strike slip, however, at the Valle de Santo Toms the fault makes a ~25 change in orientation, which coincides wi th the formation of extensional basins on the fault. Recent evidence of the independent movement of the Baja Micro-plate relative to a stable Southern California Black leads to several possible hypothe ses to explain this including: 1) That basins are localized structures, the result of a series of right steps or bends along the dextral Agua Blanca fault. 2) Basins are transtensional, possibly as a re sult of complexities associated with the northern boundary of the Baja Micro-plate To test between these hypothe ses it was necessary to constr ain the fault kinematics on both basin bounding and in-basin faults, we ll as the basin geometry. This was accomplished through combined structural and geophysical surveys. Data collected suggest that the majority of dip-slip is confined to the Santo Toms fault bounding the basin to the sout h, while the Agua Blanca fault bounding the v

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vi basin to the north is primarily strike slip. This orientation typical in transtensional basins, suggesting that although Valle de Santo Toms formed at a step over it is not a pull apart basin. Possible explanations for transtension in this area come from the orientation of the Agua Blanca fault in relation to the Baja Micr o-plate. Where the fault is close to aligned with the relative motion of the plate there is li ttle transtension, such as in Valle de Agua Blanca, however, where the fault makes a 25 change in orientat ion and becomes more oblique the motion of the Baja Micro-plate transtension is present.

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Chapter 1: Introduction Plate boundary strain associated with th e Pacific and North American tectonic plates in southern California and northwes tern Baja California is dominated by regional scale, active faults comprising the San Andreas -Gulf of California fault system (SuerezVidal et al., 1991). Active fau lts in northern Baja Californi a can be divided into three groups: the first is the Imperi el, Cerro Prieto, Cucapa, and Laguna Salada, located within the Mexicali-Imperial Valley (Fig. 1). The seco nd group consists of faults related to the Main Gulf Escarpment, the San Pedro Mrtir fault, San Felipe lineament, and faults associated with the Sierra Juarez escarpment and the third group is faults that cross the Peninsular ranges, the Agua Blanca and San Miguel-Vallecitos faults (Fig. 1, SuerezVidal et al., 1991). The Agua Blanca fault syst em is an east-west to northwest trending dextral shear zone comprised of three or more sub-parallel wrench fa ult zones, all ending or merging with the Agua Blanca fault (Le gg et al., 1991). These fa ults, along with other fault zones within the submar ine Inner Continental Borderla nds, are thought to be the result of north-south convergence in the Tran sverse Ranges related to the Big Bend in the San Andreas Fault (Legg et al., 1991). The Agua Blanca fault is the southern-most in this system of faults that has been identif ied as transferring plate boundary shear out of the Gulf of California and into the Contin ental Borderlands circ umventing the Big Bend in the San Andreas(Allen et al., 1960; Legg et al ., 1991). Slip on Agua Blanca fault appears to be primarily strike slip through much of the eas tern extent, however, approximately 60 km southeast of Ensenada th e fault changes strike to a more northerly orientation. This coincides with the form ation of extensional basins along the 1

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Figure 1 Generalized map of active fa ults in northwestern Baja. These faul ts can be divided into three major groups: the Imperiel, Cerro Prieto, Cucapa, and Laguna Salada, located within the Mexicali-Imperial Valley, the Main Gulf Escarpment faults, including the San Pedro Martir, and the Agua Blanca and San MiguelVallecitos faults. Offshore faults comprise the Inner Co ntinental Borderlands and include the offshore extension of the Agua Blanca fault, the Coranado Bank fault as we ll as the San Clemente fault zone. Boxed on the map are major valleys along the fault. fault (Allen et al., 1960; Gatil, 1975) includi ng the Valle de Santo Toms and the Valle de Maneadero (Fig. 1). This raises an importa nt question because the strike of the fault is sub-parallel to several contractional structures (e.g. Puente Hills Fault; Dolan et al., 2007) associated with the shortening created by the Big Bend on the San Andreas Fault. 2

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Several recent geodetic studies have identifie d the Baja Peninsula as a distinct tectonic unit (Baja Micro-plate) from th e Pacific Plate (e.g. DeMets and Dixon, 1999; Plattner et al., 2007). However, all such studies tend to treat all of Baja southwest of the San Andreas as a single tectonic entity and have not considered the deta ils of slip along the myriad of faults that riddle the block, particularly at it s northern extreme. Thus, in considering the possible expl anations of the observed extension and pure strike-slip motion in a fault with an orientation sim ilar to several others that are primarily contractional there seems to be two plausible explanatio ns or hypotheses: 3) That basins are localized structures, the result of a series of right steps or bends along the dextral Agua Blanca fault. 4) Basins are transtensional, possibly as a re sult of complexities associated with the northern boundary of the Baja Micro-plate To test these hypotheses, it is important to understand the structures associated with each proposed hypothesis. From anal og modeling, a typical step over basin accommodates extension through master normal faults perpendicular to master strike slip faults (Rahe et al., 1998; McClay and Doole y, 1995), whereas extension in transtensional basins is typically accommodated on the master strike-slip faults, and sub-parallel dipslip faults (Fig. 2, Sanderson and Marchini, 1984; Wesnousky, 2005; Murphey and Burgess, 2006). Each of these would crea te distinctive basin geometry, and fault kinematics This study focuses on testing th ese hypotheses in the Valle de Santo Toms using a combined geophysical and structural investigation. To test these hypothesis it was necessary identify the three dime nsional structure of the Valle de Santo Toms as well as the position of mapped and concealed faults including determination of the relative 3

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amounts of dip-slip and strike-slip motion on basin-bounding faults. The remainder of this thesis will include a brief description of the regional geology, the methods employed, data collected and a discussion of the result s including interpretati ons and conclusions. B) A) Figure 2 Block models demonstrating basin structures ty pically associated with pu ll-apart and transtensional basins. In typical pull-apart basin formation, demonstrated in A) extension created in a step-over of two dextral strike-slip faults is accommodated by sub perpendicular normal faults, whereas in typical transtensional basins such as B) extension is accommodat ed on the basin bounding faults. 4

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Chapter 2: Regional Geology The Baja Peninsula extends 1300km sout h from the Transverse Ranges to Cabo San Lucas in Baja California Sur, varyi ng from between 45 and 240 km wide, and is separated from mainland Mexico by the Gulf of California. Geographi cally, it is defined by the north-south trending Peninsular Range batholith (PRb), a mountainous, spine-like terrain, with maximum elevati ons of 1800 to 3078 meters in th e north to 300 meters in the south where it merges into the low pl ains near La Paz (Fig. 3, Sedlock, 2003). The PRb is the largest segment of a Cretaceous age magmatic arc that was once continuous from eastern Alaska to through southern Mexi co, and is composed of hundreds of plutons with diameters varying between 1-50 km, as well as many dikes and irregular bodies (Silver and Chappell, 1988). 2.1 Pre-Batholithic Stratigraphy Prebatholithic stratigraphy along the axis of the peninsula consists of Triassic and Jurassic terrigenous turbidite s (Wetmore et al., 2003). Ou tcrops include the Bedford Canyon Formation, the French Valley Formation, Julian Schist, Vallecitos Formation, and the southernmost exposures north of the Agua Blanca Fault, in the Tres HermanosSanta Clara area. Also included with this are the central San Diego County Volcaniclastics, a group of submarine depos ited, epiclastically reworked breccias composed of andesites, dacites, and latite s. These deposits are exposed in sections containing turbidites, sandstones and shales inte rpreted to be deposited in submarine fans, which along with an age of 152 Ma leads to their correlation with th ese sequences (Balch et al., 1984; Anderson, 1991; Wetmor e et al., 2003). These strata have been interpreted to 5

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Figure 3 Regional geology map of Baja with the shad ed areas comprising the Peninsular Ranges batholith (PRb). The western segment of the PRb is divided into the Alisitos Arc, sout h of the Agua Blanca fault, and the Santiago Peak Arc north of the fault 6

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be a correlative group of moderately deep submarine fans that were combined and deformed within an accretionary prism complex associated with the Triassic-Jurassic North American continental margin arc, and are known collectiv ely as the Bedford Canyon Complex (Fig. 3, Gastil, 19 93; Wetmore et al., 2003). 2.2 Peninsular Ranges Batholith The PRb is composed of plutonic rocks primarily of Cretaceous age that outcrop continuously from 34 N to 28 N, with gravity and magnetic anomalies indicating the batholith continues in the subsurf ace at least as far south as 26 N (Sedlock, 2003). Based on differences in age, petrology, geochemistr y, and isotropic signatu res the batholith is subdivided into eastern and western zones. Th is nomenclature predates the distinction between the PRb and the independent Santia go Peak and Alisitos Arcs leading to the western batholith being further subdivided in to northwestern and southwestern zones, with a mid-Cretaceous ductile shear zone name d the ancestral Agua Blanca fault (aABF) as the juxtaposing structure (Fig. 3, We tmore et al., 2003). The boundary between the eastern and western zones is a point of some debate, since it s location varies on how it is defined, whether by differences in the type of prebatholithic rocks, ages or composition of plutons, isotope geochemistry, metamor phic assemblage, geophysical parameters, or cooling history (Sedlock, 2003). This generall y reflects that fact that northern and southern portions of the bat holith experienced different t ectonic evolutions during the mid-Cretaceous (e.g. Wetmore et al., 2003). The eastern batholith was emplaced be tween 105 and 80Ma and shows a general younging from its westward extent to the Gulf of California (Silver and Chappell, 1988). 7

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In comparison to the western PRb, the eastern zone can be characterized as a migrating or transgressive arc, whereas th e western PRb was static (Silver and Chappell, 1988). The rocks of the eastern PRb are primarily La Posta type (named for the largest and most studied pluton in the zone) which range from horneblende-biotite t onalite to two mica monzogranite (Gastil et al., 1990). The western PRb is divided into the Sa ntiago Peak arc segm ent (north) and the Alisitos arc segment (south). The two segments were initially juxtaposed by the aABf, a northeast-dipping, southwest-vergen t ductile shear zone interp reted to be a non-terminal suture between the Alisitos arc (south) a nd the North American continental margin (Wetmore et al., 2002, 2003). North of the fa ult is the Santiago Peak Arc formed an extension of the continental margin as it is an arc built atop a Late Triassic-Jurassic accretionary prism (i.e. Bedford Canyon Comp lex). The Santiago Peak Arc extends north from the aABF with a north-northwe st trend and ranges from 130km to 20 km wide. Lithologic units comprising this arc are characteristically resistant to erosion forming topographic highs (>1,000 a.m.s.l.; Ta naka et al., 1984). They consist of subaerially deposited sequences of basal tic andesite, andesite, dacite, rhyolite, volcaniclastic breccias, welded tuff and epic lastic rocks (Herzig, 1991). Contacts between the Santiago Peak Volcanics and the unde rlying Bedford Canyon Co mplex are primarily non-conformities or angular unconformities (Wetmore et al., 2003). The Alisitos Arc is interp reted to have formed as an island arc (Johnson et al., 1999; Wetmore et al., 2002, 2003). The dominant unit of this terrane is the Alisitos Formation, which is composed of coarsely pyroclastic and epicla stic basaltic and andesitic rocks with interbe dded biohermal limstones in the upper formation, with more 8

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finely clastic rocks composed primarily of tu ffs instead of volcanic breccias in the lower formation (Allison, 1974). In the study area for th is investigation pillow basalts were also observed in exposures of the Alisitos Form ation. Limestones within the formation contain Albian fauna (Allison, 1974), and alt hough limited, U-Pb ages of volcanic rocks indicated ages of 116 Ma (Carrasco et al., 1995) and 114.8.5 Ma (Johnson et al., 2003). In the latest Early Cretaceous the A lisitos Arc was sutured to North America, juxtaposed with the Santiago Peak Arc across a northeast dipping thrust fault, the eastern extent of which is the Main Mrtir Thrust and the northern extent is the ancestral Agua Blanca Fault (aABF) (Fig. 4). Motion along bot h the Main Mrtir Thrust and the aABF ceased by ~100 Ma (Wetmore et al., 2003; Al sleben et al., 2008). The active Agua Blanca Fault is spatially associated with aA BF (traces of both fau lts subparallel each other and are separated by ~2-3 km) throughout much of its western extent, however the aABF diverges toward the south near the cent er of the PRb where it is known as the Main Mrtir Thrust which, juxtaposes the Alisitos arc with the Triassic a nd Jurassic strata of the Bedford Canyon Complex (Wetmore et al., 2003). 9

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Figure 4 Fault map showing the spatial association between the ancestral Agua Blanca fault and the active Agua Blanca fault. 2.3 Post Batholithic Rocks Post batholithic rocks in northern Ba ja are comprised by the Upper Cretaceous Rosario Formation. This unit consists of marine distal to proximal submarine fan deposits (turbidites), sourced from the uplifting Late Cretaceous Arc to the east (Kimbrough et al., 2001), that outcrop as a narrow band parallel to the pres ent day shoreline, Cenozoic volcanic and associated non marine sediment ary rocks that form erosional remnants capping the Sierra Juarez and coastal hills north of Ensenada, as well as Quaternary terrace deposits, fan gravels, and alluvium (Schroeder, 1967). Marine terraces, many containing Quaterna ry invertebrates, are common on the west coast of the peninsula. These terraces have been used along the Pacific coast of North America to calculate uplift rates (Muhs et al., 1992; Ardoin and Miller, 2002; Metcalf, 1994). Just west of the study area, along the Punta Banda ridge, Rockwell et al. 10

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(1989) identified 14 of these terraces and was able to determine an uplift rate of 0.16-0.29 m/ka for the region. 2.4 Faulting The offshore Inner Continental Borderla nds fault system comprises a long (>300 km) and thin (<5 to 10 km) continuous zone of shear sub parallel to the onshore San Andreas transform system, and is comprise d of two major fault systems, the San Clemente and the Agua Blanca (Legg, 1985). The San Clemente Fault zone to the west resembles the San Andreas system onshore and is divided into two segments, the San Clemente and San Isidro fault zones that ar e linked by a major transp ressive, left bending fault segment similar to the Big Bend in the San Andreas (Legg, 1985). The near shore Agua Blanca fault system is much more comp licated and consists of three or more subparallel, northwest trending, dextral wrench fault zones (Legg, 1985). All major faults within this system are interpreted to be right slip although larg e structural relief indicates they may have an oblique component (Legg, 1985). Onshore, northwestern Baja California can be divided into two major fault zones, the Agua Blanca and San Miguel-Vallecitos fault systems (Rockwell et al., 1993). The San Miguel-Vallecitos fault zone extends sout heastward from Tijuana for a distance of 160km, and is comprised of the Vallecito s, Calabasas, and San Miguel Faults (Hirabayashi et al., 1996). It is currently th e most seismically activ e structure in Baja California, having produced six earthquakes of magnitude 6 or greater in a sequence between 1954 and 1956 on the San Miguel Fa ult segment (Reyes et al., 1975; Hirabayashi et al., 1996). Base d on Mesozoic features, recons tructed total offset on this 11

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fault zone is determined to be no greater than 0.5 to 0.6 km of right lateral displacement (Hirabayashi et al., 1996). Hirabayashi et al. (1996) used soil stra tigraphic techniques to date an offset alluvial ridge on the San Miguel-Vallecitos Fault, and found a maximum slip rate of 0.55 mm/yr, which agrees well with Dixon et al. (2002) coupling model estimate of 1.2.6 mm/yr. These slip rates are low when compared to similar estimates from the Agua Blanca Fault (Rockwell et al., 1987, 1993) of 6 mm/yr and suggests that the San Miguel-Vallecitos Fault transfers le ss than 1% of plate motion and contributes only minor slip to the Inner Continental Bord erlands fault system (Hirabayashi et al., 1996). The Agua Blanca Fault is a major transverse structure of northern Baja California, extending more than 120km east from Punta Ba nda. Allen et al. (1960) broke out several sections of the fault that include, from east to west: the Paso San Matis, Valle de La Trinidad, Caon de Dolores, Valle de Agua Bl anca, and Valle de Santo Toms, where the fault splits into northern (Agua Blanca fault) and southern (Santo Toms fault) branches, which intersect the Pacific coastline at th e Punta Banda and Bahia Soledad, respectively (Fig. 5). The Agua Blanca fault is well-d efined geomorphologically, exhibiting recent scarps, offset streams, shutterridges, fault sags and saddles, side-hill ridges, and fault controlled valleys (Allen et al., 1960). Alt hough the fault has not experienced a major historical earthquake, these e xpressions indicate that it ma y still be active. Mapping by Allen et al. (1960) suggest a minimum 11 and 22 kilometers of dextral displacement on the fault based on offsets of distinctive rock types. This displacement yields an average slip rate of 2 to 6 mm/yr based on a 3.5 to 5.5 Ma slip history (Atwater, 1970, 1989). Recent geodetic studies reveal a current s lip rate of 2.2 to 3.1 mm/yr from half space 12

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model results, and 6.2 1.0 mm/yr from viscoe lastic models for the Agua Blanca Fault (Dixon et al., 2002). A lack of well defined ge ologic rates means that both models agree within uncertainties; however Di xon et al. (2002) determines the viscoelastic model to be more reliable. Another fault, the Maximinos (Fig 5), exte nds just south of the Punta Banda ridge. It trends parallel to a 2 to 3 km long canyon just north of Canada Maximinos (Shug, 1987). Here the fault appears to displace a 125 ka terrace 50 to 220 meters in a dextral sense. Based on this offset the slip rate for the Maximinos fault may be as high as 1.6mm/yr (Shug, 1987). The trace of the fault b ecomes less apparent about 5km inland as evidence of Quaternary activity diminishes. Lack of exposure make it difficult to trace this fault to the Agua Blanca, but based on slip estimates the majority of Quaternary slip on the Agua Blanca has been concentrated on the well established, northernmost branch (Shug, 1987). The submarine extent of the Agua Blanca and Maximinos faults make a sharp turn from a 295 to 300 strike to a 325 to 335 strike just offshore of the Punta Banda ridge (Legg et al., 1991). Th ese faults merge into the Coranado Bank fault zone, a complex zone with numerous discontinuous, su bparallel, right and left stepping en echelon and anastomosing fault segments co mmonly associated with substantial structural relief (100 to 1000 meters) (Le gg et al., 1991). The Santo Toms fault in its offshore extent marks the boundary between the northern and southern continental borderlands and is associated with a regional seafloor drop of as much as 450 meter to the south (Krause, 1965). Displacement of the c ontinental shelf by the Santo Toms fault 13

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implied ~15 km of left lateral offset to Krause (1965), however, Legg (1985) concludes that the Santo Toms fault is primarily dextral. Figure 5 Aerial map with Valle de Santo Toms boxed. The Agua Blanca Fault (ABF) is shown exiting offshore at Valle de Meneadero, just north of the Punta Banda ridge. The southern branch, Santo Toms fault (STF) exits offshore at Bahia Soledad, with the Maximinos Fault (MXF) between them. 2.5 Previous Work The first references to a possible fault in the Agua Blanca-Santo Toms sector were Bos and Wittich (1913) and Beal (1948) who alluded to a fault of considerable importance on the north side of Punta Banda, beginning northeast of Isla Todos Santo (off the coast of Punta Banda) extending 115 in to the mountains of the peninsula. Work 14

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on the neotectonics of the Agua Blanca fau lt came from several theses from San Diego State University. Hillinski (1988) studied th e eastern terminus of the Agua Blanca in Valle de Trinidad and Valle San Matis. He c oncluded that within these valleys the Agua Blanca fault loses its dextral slip through rotation and extensio n, and stresses are absorbed or transferred to ar eas north of the fault and west of the Valle de Trinidad. Hatch (1987) constrained the late Quaternary activity of the Agua Blanca Fault in Valle de Agua Blanca, which Allen et al. (1960) identified as the type lo cality of this fault. Hatch (1987) estimated slip distances based on offset alluvial fans and age constraints from soil stratigraphy. Slip rate estimates from this study vary from 4 to 10mm/yr, with the best estimates being from 4 to 6mm/yr. This is comparable to Shug (1987) who found slip rates of 4 mm/yr on the northern branch of the Agua Blanca near the Punta Banda ridge segment. The difference in slip rates is attributed to the additional complexities of the fault west of Valle de Santo Toms (Hatch, 1987). Shugs (1987) study was performed in a similar manner to Hatch (1987) at several locations on the Punta Banda ridge. Although this study di d not extend to Valle de Santo Toms a description of Valle de Santo Toms is included, identifying it as west-northwest trending graben created by a right step in the Agua Blanca. Geomorphic expression of the fault both on the NE and SW margins is described as weak and locally discontinuous. The Santo Toms fault is described as principally dip-slip in which the expression appears to decrease westerly along strike with the fa ult dying out approximately 12km inland from the coast. 15

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2.6 Geophysics The most complete geophysical data sets for the region are from Gastil et al. (1975), who collected gravity data thr oughout northern Baja, and Langenheim and Jachens (2003) who compiled aeromagnetic data from the Baja peninsula. Gastil et al. (1975) noted a gravity anomaly, as much as 35 mG als, associated with the transition from Santiago Peak Arc to Alisitos Arc. A regional gravity high extends from the Agua Blanca fault south to lat 28 where it ends. Langenheim and Jachens (2003) were able to constrain total offset on the Agua Blanca fault to no more than 25 to 30 km of dextral strike slip based on offset of individual ma gnetic anomalies. The map generated in the Langenheim and Jachens study indicates that the Alisitos Arc is associated with a continuous magnetic high while the Santiago Peak Arc is associated with a less continuous magnetic signature. Small scal e gravity surveys near the study area are limited to the northern branch of the fau lt on the Todos Santos plain in Valle de Maneadero. These studies have constrained th e depth to basement and fault location in this basin (Pohle, 1977; Perez-Flores et al., 2004; Callihan, 2010). 16

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Chapter 3: Materials and Methods 3.1 Magnetics Magnetics data were collected in th e summer of 2008 using a Geometric G858 Cesium Vapor magnetometer. Streaming data were collected at one second intervals along the same transects as the gravity data (Fig. 6). Magnetics data was tied to a differential GPS unit set to collect position data at one second intervals. 3.2 Gravity Gravity data were collected during th e summers of 2008 and 2009. A total of 344 gravity stations (Fig 6) were acquired in and around the basin up to 10 km away to constrain anomalies associated with basin fill. Data collect ed during the summer of 2008 were acquired primarily using a Lacoste and Romberg gravity meter on loan from Florida International University. Towards the end of the summer USF acquired its own ZLS Calibrated screw gravity meter, however due to time constraint s this was only able to be used to collect two lines of data in the Santo Toms basin. However, the ZLS gravimeter was used exclusively du ring the summer of 2009.Data collection and processing procedures differ between the two gravity meters. Data collection with the Lacoste and Ro mberg gravity meter involves setting the meter on a relatively flat surface, leveling it, and then acquiring three readings by turning a dial to center a gauge and obt ain a counter reading. These r eadings were then averaged to obtain a final counter reading at each st ation. This counter reading is an arbitrary number and must be converte d to a mGal reading by using a conversion chart individual to the gravity meter. The ZLS involves a sim ilar process of levelin g; however, readings 17

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are obtained electronically by using a PDA that averages many readings to obtain a value. For this study we took three readi ngs and the third was recorded. Because gravity varies with elevation a nd latitude it is important to have the position of the data points located to within a few centimeters. To obtain this precision a Leica GS20 Differential GPS system was used. Two units were requir ed, one rover that collected the position of the data point, and a second base station that remained stationary to account for error due to m ovement of the satellites. When these are tied together accuracy is sub-centimeter. Once the data were converted to mGal re adings corrections could be made. These include drift and tidal, free air, Bouguer, latit ude and terrain correcti ons. Drift corrections use readings taken periodically through the day to account fo r changes in the position of the sun and moon, as well as small changes in the gravity meter. Due to the distance between some transects and the base station, re adings were obtained typically three times per day with some days limited to two read ings. The differences between the readings were then assumed to be a linear progression between each successive base station readings. Based on the time difference between the last base station reading taken and the time a point was taken the reading can be corrected. Tidal corre ctions similarly adjust for the gravitational pull of wate r during high and low tides, and are done based on charts and time of day the reading wa s taken. For this a FORTRAN code was used that gives a mGal correction for each data point. This correction was not necessary when the ZLS was used because the correction was made internally within the PDA. Free air corrections are one of three for gravity differences associated with differences in elevation. In a homogeneous ea rth, as elevation rises the pull of gravity 18

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becomes less, because the distance from the cen ter of the earth beco mes greater. Free air corrections adjust readings to the same elev ation, in this case the elevation of the base station. The equation used for this is: gFA=-0.3086*z Where gFA is the gravity difference in mGals as sociated with elevation differences relative to a datum and z is difference in elevation betw een the gravity station and the base station in meters. The second elevation corre ction is the Bouguer correc tion. The free air correction brings all readings to the same elevation, but it assumes th ere is only air between the two points of elevation, and ignores the additio nal mass associated with an increase in elevation due to the greater amount of rock underneath. This is compensated using the simple Bouger correction: gB=0.04193( z) Where is the densityassumed for material between the elevations of the point and the base station in g/cm3, z is the difference in elevation between the point and the base station in meters, and gB is the correction in mGals. The third elevation correction, terrain corrections, accounts for the additional mass and voids created adjacent valleys a nd mountains. A simple equation is not sufficient to correct for this and due to th e complexity involved is often accomplished using freely available computer codes. In our case a code written in Perl programming language was used to make corrections base d off a Digital Elevation Map (DEM) of the area. Equations used for this code are from Plouff (1976) and based off a flat top prism modeling method. 19

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Gravity values also vary based on latitude at a rate of one mGal for every 1248.5 meters. This is the result of the elliptical sh ape of the earth: as latitude increases the distance from the center of the earth decreases and there is an increase in the expected gravity readings. The formula for this is: gLAT=0.00081213sin2 This formula corrects all points to the latitude of the base station. is the difference in latitude between the point and the base station, and gLAT is the correction in mGals. 20

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N N Figure 6 Location map of gravity poin ts (top) and magnetic lines (bottom). Gaps in the magnet ic lines were due to noisy values having to be discarded. 21

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Chapter 4: Results 4.1 Mapping The Santo Toms fault bounds the basin to the south and is characterized by a steep range front with many large tria ngular faucets. Dip on this fault is 76 where the fault exits the basin to the west (Fig. 10). There are several scarps along the fault. The most evident scarps appear near the south eastern portion of the va lley and are visible from Mexico 1. A scarp height of ~2-3 meters was recorded here. Other scarps appear in the northwestern portion of the fault a lthough these are much more degraded and modified by human activities making scarp height s difficult to resolve. A smaller dip-slip fault merges with the Santo Toms fault near the town of Santo Toms and extends 310 for approximately 5-10 km (Fig. 10). It creates a distinctive aquatard effect that is easily discerned in air and satellite images as we ll as in the field. Th e springs rising up along this structure likely played a fundamental ro le in the foundation of the Mission at Santo Toms in the 1800s, and hence it has been named the Manantial (spring) fault. Reworking of the land due to agriculture may have accelerated the degradation of the fault trace, however, a scarp height of >3 m is observed near the town but decays to less than one meter by ~2.5 km to the southeast of the intersection of this fault with the Santo Toms fault. However, this feature may extend to the northeast side of a small hill, inferred to be a pop-up (800 meters long by 160 meters wide) located ~3.3 km southeast of the intersection of the faults (Q Fluvial 2 in Fig. 10). Another in-basin fault, also associated with the pop-up structure, is obser ved along the southwest side of the hill (Fig. 7). Cross sectional views from road cuts on th e structure, as well as striations, indicate 22

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that it is fault bound; however insufficient qua lity in the geophysical methods did not allow these faults to be resolved. Figure 7 Photo looking southeast (top) showing the south side of the pop-up structure with the north side being bounded by the Manantial fault. Bottom photo shows the cru sh zone associated with the fault on the south side of the structure. 23

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The Agua Blanca fault boundi ng the valley to the northeas t exhibits a steep range front as well, rising more than 500 meters from the basin floor. Dip on this fault was measured at two locations, bot h fairly steeply dipping, 85 on the eastern side and 60 just outside the basin to the northw est (Fig. 8, Fig. 10). Cretaceous strata on the northeast side of the fault are steeply southwest dipping. The fault trace is well exposed along most of the section parallel to Valle Santo Toms, but is covered by Quaternary deposits where it cuts alluvial fans near the southeastern pa rt of the valley, altho ugh aquatard effects and several small scarps delineate the trace of the fault in that area. The most evident scarps on the fault are located where th e fault overlaps and steps over behind a large butte in the middle of the basin. These scarps suggest ~2 meters of dip-slip motion. Figure 8 Photo looking northw est showing the Agua Blanca fault as it exits the basin, dipping approximately 60. 24

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Exposures of the Santiago Peak Arc are present on both the northeast and southwest side of the Agua Blanca fault. On the southwest side of the fault these exposures include a large bu tte rising 200 m from the basi n floor. This butte ends abruptly where Caon Verde, a large washout, enters the basin, although there are several much smaller, partially buried exposures of Santiago Peak bedrock northwest of the mouth of Caon Verde. Other geomorphic observations include fluvial and strath terraces that are present on both sides of the basin (Fig. 9). In the nor thwest corner of the basin, where Mexico Highway 1 enters from the north, strath terraces are located high on the hills at approximately 300 meters elevation (Q strath terrace in Fig. 10). These terraces dip towards Caon Las Animas, leading out of the basin towards Valle de Maneadero. Other fluvial terraces are located on the range front associat ed with the Santo Toms fault and are at approximately 250 meters elevation and dip towards Caon Santo Toms, the current path of drainage out of the basin to La Bocana, at the Pacific Ocean. The northwest side of the basin exhibits a steep range front as well; however alluvial fans make it difficult to establish th e presence of a fault. This was investigated further using geophysics, shown in Profile A-A. 25

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Caon Las Animas Caon Santo Toms Figure 9 Top photo is looking south from the northwes t corner of the basin showing a fluvial terrace dipping southeast. Middle photo looking northwest showing the mi rror image of the fluvial terrace shown in the top photo on the left, with the right line marking a strath terrace. Bottom pict ure is looking west showing fluvial terraces dipping west, toward Caon Santo Toms the current path o drainage toward the Pacific Ocean. 26

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Figure 10 Geologic map of Valle de Santo Toms. Major drainages are labeled and mapped faults are indicated by solid li nes. STF(Santo Toms fault), ABF (Agua Blan c a fa u lt ), Mf ( Manantial fa u lt ). 27

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4.2 Geophysics Magnetic data, both in collection and m odeling, presented several difficulties. Sensitivity in the magnetic field to locally rough terrain, locally variable litholgy and cultural noise (buried farming equipment, fen ces and power lines) lead to some data being thrown out. When the data was filtered to an acceptable range of values, data that were left proved to be insufficient to build accurate models of the basin. For this reason no models were constructed from the magnetic data, however, data collected has been included in this thesis. From the gravity map (Fig. 11), an incr ease of 32 mGals is observed, from a low in the northeast to a high in the southwest. This gradient is substantial, and similar to the anomaly associated with the transition from Santiago Peak Arc to Alisitos Arc observed by Gastil (1975). Location of the ancestral A gua Blanca fault on the gravity map is a general trace inferred from the locations of Santiago Peak Arc and Alisitos Arc outcrops; however all of these are outside of the basin. From the gravity map the trace of the aABf likely passes under the basin and is also likely cut by the Santo Toms fault and/or Agua Blanca fault, however since it is such a long wavelength feature it is difficult to determine hard constraints on its trace through the basin. Five basin perpendicular profiles were m odeled. Densities used in modeling were 2.76 g/cm3 for the Alisitos Arc (basaltic to andesitic volcanics) and 2.69 g/cm3 for Santiago Peak Volcanics (andesitic to rhyolitic volcanics), and a basin fill density of 2.0 g/cm3. In Profile B-B and C-C a quartz diorite pluton, mapped as part of the structural survey (Fig. 10), was modeled with a density of 2.81 g/cm3. In all models the locations of 28

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Figure 11 Gravity map overlain o a DEM of the study a rea. Faults mapped as pat of the structural survey are indicated by solid lines, while the dashed line is the inferred trace of the an cestral Agua Blanca fault through the valley. Transects used for modeling are marked as well. 29

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basin bounding faults was placed as close as possible to mapped locations, however there is some error associated with this. For Profiles B-B, C-C, and D-D, the data was detrended by fitting a least squares fit of a straight line to the data and removing the resulting function. This was to remove comp lexities associated with the transition for Alisitos to Santiago Peak Arc and focus on basin geometry. For these models a density of 2.72 g/cm3 was used outside of the basin. This is the average of the Alisitos Arc and Santiago Peak Arc densities and was used for these models since the effect of the transition between these two formations was removed by the detrending. For profile A-A detrending was not necessary since the model is entirely within Santiago Peak Arc. The southeastern most pr ofile, E-E was modeled by placing th e suture roughly in the middle of the basin, corresponding to th e interpreted location in Fig. 11. Profile A-A Profile A-A This profile test the possibility the northwest side of Valle de Santo Toms is fault bound. Best fit indicate gradually thickening sediment with a possible buried terrace. This model is located entirely with Santigo Peak Arc (SPA) with a density of 2.69 g/cm3. A density of 2.0 g/cm3 is used for basin fill. 30

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This profile extends across the range front on the northwest side of the basin to test the possibility of a fault being located there. The profile extends less than 1 km from the top of the range front to 300 to 400 meters into the basi n. The best fit model created from the data indicates gradually thickening sediment depth with a possible buried terrace between 200 and 400 meters north of the beginning of the profile. Due to the nonuniqueness of gravity modeling it is also possible to produce a model fitting the data by placing a small normal fault (< 200 m displacement) near th e base of the range front; however the model was shown becau se it produces lower error. Profile B-B Profile B-B This profile crosses the basin close to due no rth and south and extends ~4 km outside of the basin. Data along this line were de trended to remove regional effects caused by the transition from Santiago Peak to Alisitos Arc, and thus a density of 2.72 g/cm3 for basement rocks. Plutons north and south of the transect were modeled using a density of 2.81 g/cm3. ABF (Agua Blanca fault), STF (Santo Toms fault). This transect crosses the basin along highway Mexico 1 at the north end of the valley, almost due north and south, and extends ~4 km outside of the basin. This model 31

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was detrended, and a density of 2.72 g/cm3 is used for basement rock. North of the basin a quartz diorite pluton (D=2.81 g/cm3) was included in the model to represent feature KQuartz Diorite on the geologic map (Fig. 10). Another pluton of the same density was placed at a depth of ~3 km on the south side of th is model. This pluton was not mapped although it was necessary to fit the observed gravity and its existence and placement is reasonable. Additionally, there are a few small intrusive bodies located only a few hundred meters beyond the southern limit of the model line as mapped by Gastil et al., 1971. Hence, it is conceivable that either this body or a simila r intrusion is responsible for the observed increase in gravity at the south limit to th e model line. The model basin is observed to thicken to a maximum depth of ~600 m agai nst the Santo Toms fault. In the field, sediments are observed pinching out south of the Agua Blanca fault on the north side of the transect leading its placement outside of the basin. Sediment thickness on north side is attributed to large fans at the mouth of Caon Las Animas, which this transect follows out of the basin. 32

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Profile C-C Profile C-C This profile is on the sa me transect as B-B, but only extends ~500 m outside of the basin to focus on basin geometry. Densities used were 2.81 g/cm3 for plutonic rock, 2.72 g/cm3 for basement, and 2.0 g/cm3 for basin fill. ABF (Agua Blanca fault), STF (Santo Toms fault). This profile is along the same transect as Profile B-B, but is shortened to extend <1 km outside of the basin to focus on basin geometry. Data from this line were detrended. Basin sediment is observed to thicken against the Santo Toms to ~600 m, similar to that in Profile B-B. The goal of this line was to locate any in-basin faults that may be present; however none we re able to be discerned. 33

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Profile D-D Profile D-D This profile crosses the basin ~1 km west of Profile B-B and C-C. Data on this transect were detrended. Solid lines mark possible in basin fa ults. Densities used were 2.72 g/cm3 for basement, and 2.0 g/cm3 for basin fill. ABF (Agua Blanca fault), STF (Santo Toms fault). Profile D-D extends northeast-southwest, perpendicular to basin bounding faults. Data in this line were detrended. Two in-basin faults were observed on this profile. The southernmost of these is interpreted to be basement offset at depth by the Manantail fault observed in the structural survey. This fau lt is modeled to dip to the north at ~50 Another in-basin fault is modeled ~400 m nor th of the Manantial fault, dipping to the south at ~40 Sediment thickness against the Sant o Toms fault is ~400 m. Sediment thickness on the north side of th is transect is attributed to fluvial fans at the mouth of Caon Verde abutted against basement rock. In the structural survey sediments were observed pinching out against the fault resulti ng in the Agua Blanca being placed north of the basin. 34

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Profile E-E Profile E-E This profile crosses the basin on the southe ast side. Data were not detrended in this model. The suture between Alisitos Arc (AA) (2.76 g/cm3) and Santiago Peak Arc (SPA) (2.69 g/cm3) is placed in the middle of the basin. ABF (Agua Blanca fault), STF (Santo Toms fault), aABF (ancestral Agua Blanca fault). Profile E-E crosses the basin perpen dicular to basin bounding faults on the southeastern end of the basin. No detrending was done to the data on this transect. The suture between Alisitos Arc and Santiago P eak Arc was placed n ear the middle of the basin, close to its interprete d location in Fig. 11. Basin sedi ments were thickest on this profile, reaching a maximum depth of ~800 m. Here the Agua Blanca fault is placed on the north end of the basin sin ce it is observed cutting alluvial fans in the area. Sediment thickness on this end of the basi n is attributed to the structural juxtaposition of alluvial fans from the southeast against the northern range front, and not a dip slip component on the Agua Blanca fault. 35

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Chapter 5: Discussion Data collected as part the structural and geophysical study allo ws testing between two possible explanations for extensional basins forming along the western half of the Agua Blanca fault; 1) That basins are localized structures, the result of a series of right steps along the dextral Agua Blanca fault. 2) Basins are transtensional, possibly as a re sult of complexities associated with the northern boundary of the Baja Micro-plate The Agua Blanca fault in Valle de Sant o Toms appears to be primarily strike slip. The only evidence of a dip slip component in this portion of the fault, is in form a small scarp exhibiting approximately ~2 m of dip-slip observed in one locality ~1 km southeast of the point the Agua Blanca Fault crosses the mouth of Caon Verde. Basin sediments are only rarely cut by the Agua Blanca fault but more often pinch out against basement rocks to the southwest. Localized sediment thicknesses of 150 to 200 meters are attributed to large alluvial fans, part icularly from Caon Verde, which are derived from fluvial systems to the northeast of th e Agua Blanca Fault and spill out across the fault into the basin. The majority of dip-slip motion appears to be confined to the Santo Toms fault on the south side of the basi n. Large triangular faucets demo nstrate a substantial normal component with basin sediments thickening ag ainst this fault up to 1 km. Several ~2-3 meters dip-slip scarps indi cate recent dip-slip motion on the fault. Geophysical data indicate sediments thicken adjacent to the Sant o Toms fault to as much as 1 km. Without additional knowledge of subsurface geology it is difficult to substantiate these models, 36

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however when topographic profiles from the we stern side of the buttes southwest of the Agua Blanca Fault in the southeas tern part of the valley are projected to the fault plane a depth of ~400 m is calculated, similar to the modeled basin depth in that section of the Santo Toms fault (Fig. 12). Figure 12 INEGI topography map show ing line along which a projection was made to estimate basin depth against the Santo Toms fault. An elevation drop of 160 m is observed across a distan ce of ~1 km on the butte on the northeast side of the line. When this is project ed onto the Santo Toms fault ~1.5 km southwest of the edge of the butte a basin depth of ~240 m is calculated, similar to the basin depth agai nst the Santo Toms fault on Profile D-D ~2 km northwest of the projection. The Manantial fault was the only in-basin fault that could be discerned. The Manantial fault extends from the Santo Toms fault and appears to be primarily dip slip as well, with scarp heights of 4 m. Modeling of geophysical data over the fault suggest that there may be another in-basin fault di pping opposite the Manati al fault ~400 m north 37

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of its location. A possible expl anation for these faults is that they are synthetic or antithetic to the Santo Toms fault. Best fit models of the no rthwest range front of Valle de Santo Toms appear to show an alluvial fan spilling into the basin and possibly covering a terrace. If a fault does exist in this location large alluvial fans have covered much of the trace, and no scarps or aquatard effects were observed suggesting that this fault, if it exists, has been inactive longer than either the Agua Blanca or Santo Toms faults. Fluvial and strath terraces on both sides of the basin sugge st that erosion played a role in basin formation. Large fluvial te rraces are located high on the hills on the northwest side of the basin dipping out of the basin to the northwest parallel to the Agua Blanca Fault and towards Caon Las Animas and Valle de Maneadero. High on the hills on the south and west side of Valle de Santo Toms there are several alluvial fan surfaces that dip towards Valle de Santo Toms, some are cut and uplifted by the Santo Toms fault. The current path of drainage out of the basin is to the northwest out Caon Santo Toms towards the Pacific Ocean at Punta China. This seems to suggest that at one point drainage shifted from flowing towards the Pa cific out of Caon Las Animas to its current position and deeply down cut in the process. Th e valley floor is essentially flat, however, suggesting that this down cutt ing was either very limited in magnitude or followed by lacustrine sedimentation or both. Comparison of the results of this stud y with observations made of analogue models of pull-apart basins (e.g. Rahe et al., 1998; McClay and Dooley, 1995), some significant differences are apparent. In cases of symmetrical basin models where opposing sides of the basin move at appr oximately the same absolute rate, and 38

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asymmetric basin formation where one side of the basin remains stationary with respect to the basement, master normal faults formed on both sides of the basin (symmetrical) or one side of the basin (asymmetrical), perp endicular to the basi n bounding strike slip faults. In the case of Valle de Santo Toms, the majority of normal slip appears on one of the basin bounding strike slip faults, the Sa nto Toms fault. While a small amount of normal faulting may be interpreted to have o ccurred on an inferred fault at the northwest end of Valle Santo Toms, it is clear that th e overwhelming majority of dip slip motion is confined to the basin-bounding Santo Toms fau lt. The fact that there is little if any evidence for normal faulting perpendicular to the basin-bounding fau lts strongly suggests that a step-over model is an unlikely explanation for the formation of Valle Santo Toms and hence, hypothesis 1 is excluded. The observa tion that the majority of dip slip motion is accommodated by the Santo Toms fault is consistent with the interpretation that Valle Santo Toms has formed due to transt ension (Sanderson and Marchini, 1984; Wesnousky, 2005; Murphey and Burgess, 2006) on the Santo Toms fault as a result of the orientation of this structure relative to the motion of the crustal block(s) north of the Agua Blanca Fault (e.g. Wetmore et al., in review) This oblique slip component is only pres ent after the fault takes a northwesterly bend, displacement in the Valle de Agua Blanca, just east of Valle de Santo Toms, is primarily strike slip. Possible explanati ons for this observation come from recent evidence that the Baja peninsula south of th e Agua Blanca fault behaves as a separate micro-plate (Plattner et al ., 2007). Geodetic studies have found its movement to be separate from the Pacific plate to a 99 percent confidence, with the boundaries of the plate being the Bonfil, Carrizal, and San Jose Cabo faults to the south, an area west of 39

PAGE 47

Guadalupe Island to the east, and the Agua Bl anca-San Pedro Mrtir Faults to the north (Plattner et al., 2007). 24 geodeti c stations north of the Agua Blanca fault were used to calculate vectors of th is block with respect to a stable ridged Baja California block and yielded residual vectors suggesting an averag e movement of 122 7 at a rate of 2.4 mm/yr. This vector is similar to the strike of the Agua Blanca fault in the eastern sections, explaining primarily strike slip motion thr ough Valle de Agua Blanca, and transtension where the fault makes a 25 change in orientation and becomes more oblique to the motion vector of the micro-plate. 40

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Chapter 6: Conclusions Evidence from the structural and geophysical survey indicates that Valle de Santo Toms is a transtensional basin. Extension within the basin is primarily accommodated on the Santo Toms fault which bounds the basin to the southwest and exhibits a substantia l normal component. The Agua Blanca fault on the northeast is primarily strike-slip with basin sediments thinning and often pinching out against the fault. From geophysical data basin sediments a ppear to thicken against the Santo Toms fault and only thicken to the northeast where alluvial and fluvial fans spill out into the basin. These features are typical of those found in transtensional basins. Possible explanations for transtension in this area come from the or ientation of the Agua Blanca fault in relation to the Baja Micro-plate Wh ere the fault is close to aligned with the relative motion of the plate there is little tr anstension, such as in Valle de Agua Blanca, however, where the fault makes a 25 change in orientation and becomes more oblique the motion of the Baja Micro-plate transtension is present. 41

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References Allen, Clarence Roderic, Leon T. Silver, and Francis Greenough Stehli. "Agua Blanca Fault--A Major Transverse Structure of Northern Ba ja California, Mexico." Geological Society of America Bulletin 71.4 (1960): 457-82. Allison, E. C. "The Type Alisitos Forma tion (Cretaceous, Aptian-Albian) of Baja California and its Bivalve Fauna." A Guidebook to the Geology of Peninsular California Ed. G. Gastil and J. Lillegraven. United States (USA): AAPG--SEPM-SEG, Pac. Sect., United States (USA), 1974. Alsleben, H., et al. "Complex Deformation during Arc-Continent Co llision; Quantifying Finite Strain in the Accreted Alisitos Arc, Peninsular Ra nges Batholith, Baja California." Journal of Structural Geology 30.2 (2008): 220-36. Anderson, C. L. Zircon Uranium-Lead Isotopic Ages of the Santiago Peak Volcanics and Spatially Related Plutons of the Peninsular Ranges Batholith, Southern California. M.S. Thesis San Diego Stat e University, 1991 San Diego. Ardoin, Charles R., and M. Meghan Miller. "Vertical Deformation Near Willapa Bay, Washington; Geological Society of America, 2002 Annual Meeting." Abstracts with Programs Geological Society of America 34.6 (2002): 376. Balch, Duane C., Sarah H. Bartling, and Patrick L. Abbott. "Volcaniclastic Strata of the Upper Jurassic Santiago Peak Volcanics, San Diego, California; Tectonics and 42

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Sedimentation Along the California Margin." Field Trip Guidebook Pacific Section, Society of Economic Pa leontologists and Mineralogists 38 (1984): 157-70. Beal, Carl Hugh. "Reconnaissance of the Geology and Oil Possibilities of Baja California, Mexico." Memoir Geological Society of America 56.12 (1948): 138. Bose, E., Wittich, E., 1913, Informe relativo a la exploracion de la region norte de la costa occidental de la Baja California Inst. Geologico de Mexico, Parergones IV, p. 307-533. Calllihan, S. M. Constraining the Geometry and Evolu tion of the Meneadero Basin Baja California, Mexico M.S. Thesis University of South Florida, 2010 Tampa. Carrasco, A. P., David L. Kimbrough, and Ch arles Herzig. "Cretaceous Arc-Volcanic Strata of the Western Peni nsular Ranges: Comparison of the Santiago Peak and Alisitos Group." Abstracts of Peninsular Geologic al Society Intern ational Meeting on Geology of the Baja California Peninsula v. III (1995): p. 19. DeMets, Charles, and Timothy H. Dixon. "N ew Kinematic Models for Pacific-North America Motion from 3 Ma to Present; I, Evidence for Steady Motion and Biases in the NUVEL-1A Model." Geophysical Research Letters 26.13 (1999): 1921-4. Dixon, Timothy, et al. "Seismic Cycle and Rh eological Effects on Estimation of PresentDay Slip Rates for the Agua Blanca and Sa n Miguel-Valecitos Fau lts, Northern Baja California, Mexico." Journal of Geophysical Research 107.B10 (2002): 23. 43

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Dolan, James F., David D. Bowman, and Charles G. Sammis. "Long-Range and LongTerm Fault Interactions in Southern California." Geology 35.9 (2007): 855-8. Gastil, R. G., Richard P. Phillips, and Edwin C. Allison. Reconnaissance Geology of the State of Baja California. United States (USA): Geological Society of America (GSA), Boulder, CO, Unite d States (USA), 1975. Gastil, Gordon, et al. "The Problem of the magnetite/ilmenite Boundary in Southern and Baja California, California; the Nature & Origin of Cordilleran Magmatism." Memoir Geological Society of America 174 (1990): 19-32. Hatch, Michael E. Neotectonics of the Agua Blanca Fault, Valle Agua Blanca, Baja California, Mexico Map San Diego State Universit y, San Diego, CA, United States (USA), 1987 United States (USA). Herzig, Charles Thomas. Petrogenetic and Tectonic Development of the Santiago Peak Volcanics, Northern Santa Ana Mountains, California Doctoral University of California at Riverside, Riverside, CA United States (USA), 1991 United States (USA): University Microfilms, Ann Arbor, MI, United States (USA). Hilinski, Thomas E., and Thomas K. Rockwell. "Structure and Quat ernary Faulting about the Eastern Terminus of the Agua Blan ca Fault, Baja California, Mexico; the Geological Society of America, Cord illeran Section, 82nd Annual Meeting." Abstracts with Programs Ge ological Society of America 18.2 (1986): 117. 44

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Hirabayashi, C. Kenji, et al. "A Neotect onic Study of the San Mi guel-Vallecitos Fault, Baja California, Mexico." Bulletin of the Seismological Society of America 86.6 (1996): 1770-83. Johnson, S. E., et al. "Structure, Emplacemen t and Lateral Expansion of the San Jose Tonalite Pluton, Peninsular Ranges Bat holith, Baja California, Mexico." Journal of Structural Geology 25.11 (2003): 1933-57. Johnson, Scott E., Marcus C. Tate, and C. Mark Fanning. "New Geologic Mapping and SHRIMP U-Pb Zircon Data in the Peninsul ar Ranges Batholith, Baja California, Mexico; Evidence for a Suture?" Geology (Boulder) 27.8 (1999): 743-6. Krause, Dale C. "Tectonics, Bathymetry, and Geomagnetism of the Southern Continental Borderland West of Baja California, Mexico." Geological Society of America Bulletin 76.6 (1965): 617-50. Kimbrough, DL, et al. "Forearc-Basin Sediment ary Response to Rapid Late Cretaceous Batholith Emplacement in the Peninsular Ra nges of Southern and Baja California." Geology 29.6 (2001): 491-4. Langenheim, V. E., and R. C. Jachens. "Crustal Structure of the Peninsular Ranges Batholith from Magnetic Data; Implicati ons for Gulf of Ca lifornia Rifting." Geophysical Research Letters 30.11 (2003): 4. 45

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Legg, Mark R., Victor Wong O, and Francisco Suarez-Vidal. "Geologic Structure and Tectonics of the Inner Continental Borderla nd of Northern Baja California; the Gulf and Peninsular Province of the Californias." AAPG Memoir 47 (1991): 145-77. Legg, Mark Randall. Geologic Structure and Tectonic of the Inner Continental Borderland, Offshore Northern Baja California, Mexico Doctoral University of California at Santa Barbara, Santa Barb ara, CA, United States (USA), 1985 United States (USA): University Microfilms, Ann Arbor, MI United States (USA). McClay, Ken, and Tim Dooley. "Analogue Models of Pull-Apart Basins." Geology (Boulder) 23.8 (1995): 711-4. Metcalf, Joel Gilbert. Morphology, Chronology, and Deforma tion of Pleistocene Marine Terraces, Southwestern Santa Barbara County, California Master's University of California at Santa Barbara, Santa Barb ara, CA, United States (USA), 1994 United States (USA). Muhs, Daniel R., Thomas K. Rockwell, and George L. Kennedy. "Late Quaternary Uplift Rates of Marine Terraces on the Pacific Co ast of North America, Southern Oregon to Baja California Sur; Impacts of T ectonics on Quaternary Coastal Evolution." Quaternary International 15-16 (1992): 121-33. Murphy, Michael A., and W. Paul Burgess. "Geometry, Kinematics, and Landscape Characteristics of an Active Transtension Zone, Karakoram Fault System, Southwest Tibet." Journal of Structural Geology 28.2 (2006): 268-83. 46

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Perez-Flores, Marco A., et al. "Structural Pattern of the Todos Santos Coastal Plain, Based on Geophysical Data--Patron Estructura l De La Planicie Costera De Todos Santos, Con Base En Datos Geofisicos." Ciencias Marinas 30.2 (2004): 349-64. Plattner, C., et al. "New C onstraints on Relative Motion be tween the Pacific Plate and Baja California Microplate (Mexic o) from GPS Measurements." Geophysical Journal International 170.3 (2007): 1373-80. Plouff, D. "Gravity and Magnetic Fields of Polygonal Prisms and Application to Magnetic Terrain Corrections." Geophysics 41.4 (1976): 727-41. Pohle, Chester R. A Gravity Survey Along the North Side of the Aua Blanca Fault, Baja California Undergraduate Thesis San Diego State University, 1977 San Diego. Ed. R. Gordon Gastil and R. H. Miller. The Prebatholithic Stratigraphy of Peninsular California Vol. 279. United States (USA): Geol ogical Society of America (GSA), Boulder, CO, United States (USA), 1993. Rahe, Bret, David A. Ferrill, and Alan P. Morris. "Physical Analog Modeling of PullApart Basin Evolution." Tectonophysics 285.1-2 (1998): 21-40. Rockwell, Thomas K., et al. "Uranium-Series Ages, Faunal Correlations and Tectonic Deformation of Marine Terraces within th e Agua Blanca Fault Zone at Punta Banda, Northern Baja California, Mexico; Ge ologic Studies in Baja California." Field Trip Guidebook Pacific Section, Society of Economic Paleontologists and Mineralogists 63 (1989): 1-16. 47

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Rockwell T. K. Hatch M. E. Schug D. L. (1987). Late Quaternary rate s: Agua Blanca and borderland faults, U.S. Geological Survey Final Technical Report for Contract No. 14-08-0001-22012, 122 pp. Rockwell T. K. Schug D. L. Hatch M. E. ( 1993). Late Quaternary slip rates along the Agua Blanca fault, Baja California, Mexico, in Geological Investigations of Baja California Abott P. L., (Editor) South Co ast Geological Society,, 53-92. Sanderson, David J., and W. R. D. Marchini. "Transpression." Journal of Structural Geology 6.5 (1984): 449-58. Schug, David L. Neotectonics of the Western Branch of the Agua Blanca Fault Baja California, Mexico. Master of Science San Diego State University, 1987. Sedlock, Richard L. "Geology and Tectonics of the Baja California Peninsula and Adjacent Areas; Tectonic Evolution of Nort hwestern Mexico and the Southwestern USA." Special Paper Geological Society of America 374 (2003): 1-42. Silver, L. T., B. W. Chappell, and P. E. (chairperson) Brown. "The Peninsular Ranges Batholith; an Insight into the Evolut ion of the Cordilleran Batholiths of Southwestern North America; the Origin of Granites." Transactions of the Royal Society of Edinburgh: Earth Sciences 79.2-3 (1988): 105-21. Suarez-Vidal, Francisco, et al. "Framework of Recent and Active Faulting in Northern Baja California; the Gulf and Penins ular Province of the Californias." AAPG Memoir 47 (1991): 285-300. 48

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Tanaka, H., T. E. Smith, and C. H. Huang. "The Santiago Peak Volcanic Rocks of the Peninsular Ranges Batholith, Southern California; Volcan ic Rocks Associated with Coeval Gabbros." Bulletin Volcanologique 47.1 (1984): 153-71. Wesnousky, S. G. "The San Andreas and Walk er Lane Fault Systems, Western North America; Transpression, Transtension, Cumu lative Slip and the Structural Evolution of a Major Transform Plate Boundary." Journal of Structural Geology 27.8 (2005): 1505-12. Wetmore, Paul H., et al. "M esozoic Tectonic Evolution of the Peninsular Ranges of Southern and Baja California; Tectonic E volution of Northwestern Mexico and the Southwestern USA." Special Paper Geological Society of America 374 (2003): 93116. Wetmore, Paul H., et al. "Tectonic Implicat ions for the Along-Stri ke Variation of the Peninsular Ranges Batholith, Southern and Baja California." Geology (Boulder) 30.3 (2002): 247-50. Worzel, John Lamar, and G. Lynn Shurbet. "G ravity Anomalies at Continental Margins." Proceedings of the National Academy of Sc iences of the United States of America 41.7 (1955): 458-69. 49

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Appendices 50

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Appendix A Table 1 Gravity Data This table includes the UTM coordina tes for each point (Zone 11) and the corrected gravity value after being tied to th e absolute gravity station located at the Hildalgo monument in the city of Ensenada. Easting Northing Absolute Gravity 553722.6 3491975 979372.6733 553787.8 3492458 979372.3501 553955.3 3492949 979371.3733 554182.1 3493429 979370.128 554296.3 3493797 979369.6395 554360.9 3494204 979368.4154 554361.9 3494203 979362.5865 554295.6 3493798 979365.8239 554182 3493432 979365.8371 553955 3492951 979365.5375 553788.1 3492468 979365.4787 553721.6 3491976 979365.3164 555644.7 3491051 979370.8194 555684.3 3490251 979374.8513 555594.1 3489865 979376.4265 555955.2 3489420 979376.808 556043.7 3488872 979377.6145 556240.9 3488624 979377.8739 556216.9 3488037 979378.9792 556295.8 3487259 979380.6984 556909 3486995 979380.123 557371.6 3486634 979379.8245 557406.5 3486268 979380.9955 557593.7 3485608 979381.9624 557656.2 3485188 979382.1468 557903.3 3484776 979382.0447 557992 3484224 979383.3826 558270.2 3483479 979385.0356 558478.5 3482795 979386.984 558623.1 3482174 979388.7636 558750.6 3481384 979390.741 51

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557975.1 3480527 979395.2896 563016.8 3487865 979362.1506 562990.6 3488373 979363.1145 563026.1 3488832 979361.0195 563066.5 3489332 979359.1434 563064.2 3489944 979358.1959 558926.1 3489823 979366.9344 559354.2 3490080 979365.067 559648 3490350 979363.3455 559825.2 3490710 979362.6886 560148.3 3491070 979363.0845 558506.2 3489560 979354.1424 556585.9 3490695 979367.4799 556791.8 3491109 979366.1203 556996.3 3491542 979365.0252 557213 3492009 979363.6511 557472 3492432 979362.2094 558032.4 3492731 979360.7911 556813.9 3487099 979380.4956 557204.4 3487366 979378.6535 557536.9 3487710 979376.1193 557835.6 3488057 979374.4632 557994.8 3488510 979372.4789 558050.2 3489047 979370.4983 562526.9 3487710 979364.588 562462.6 3487290 979366.0986 562443.3 3486774 979368.983 562429.4 3486395 979372.2763 561992.9 3485968 979373.9326 561944.2 3485721 979372.5124 561752.4 3485479 979373.3958 559832.6 3492092 979360.0217 560087.5 3492354 979359.9614 560369.1 3492607 979357.9039 560455.4 3492791 979359.3697 564399 3488282 979363.5967 564855.4 3488197 979364.383 565254.7 3487896 979365.7517 565650.2 3487698 979365.4736 563551.3 3489688 979359.4859 563961.8 3489747 979363.0204 52

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564350.2 3489917 979362.6613 564652.9 3490199 979360.2656 565062.8 3490435 979364.2058 565283.1 3490577 979364.5994 539872.6 3491769 979397.9064 540383.5 3492055 979397.0389 540832.8 3492424 979395.8103 541217.5 3492773 979394.3501 541696.6 3492884 979393.3585 542122 3493212 979391.7424 542637.2 3493437 979389.9998 543148.8 3493517 979388.4711 543669.8 3493412 979387.8245 544012.3 3493077 979388.9637 544471.5 3493053 979387.4367 545058.1 3493216 979384.7353 545487.1 3493496 979384.181 545955 3493580 979382.6383 546593.1 3493455 979381.3607 547205.5 3493535 979380.749 547684.8 3493600 979379.9798 548164.2 3493506 979380.2551 548471.1 3493068 979379.1416 549029.3 3492982 979380.0121 549655 3492961 979379.9044 550125 3492862 979380.0412 550567.5 3493043 979379.0255 551047.9 3493203 979375.2271 551599.6 3493403 979373.0293 551989.7 3493781 979372.2541 552523.2 3493921 979370.8581 553471.7 3493985 979368.9717 553909.4 3494148 979368.1592 554361.5 3494210 979367.9565 558989.2 3490417 979363.6965 558526.9 3490560 979364.3305 558021 3491359 979363.621 557513.1 3491666 979364.2838 556792.8 3492240 979365.239 556465.2 3492507 979365.6238 555986.2 3492551 979366.811 53

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559341.9 3490066 979364.221 559695.3 3489778 979364.0692 560011.5 3489469 979363.988 560362.9 3489190 979364.2666 560824.1 3489098 979363.6015 561286.5 3489048 979363.371 561750.1 3488900 979363.2573 562207.6 3488751 979363.24 562631.6 3488697 979362.4159 562998.7 3488452 979363.4773 563405 3488320 979363.4843 563871.8 3488275 979363.5202 574379.2 3491193 979370.4394 573743.8 3491013 979370.3128 573395.5 3490742 979366.5674 572895.5 3490657 979366.4803 572444.6 3490516 979366.3704 571999.7 3490068 979365.883 571470.3 3489974 979364.7627 570871.6 3490196 979364.2409 570422.3 3489983 979364.9626 569947.1 3489966 979364.7012 569485.8 3489682 979366.9135 569118.8 3489263 979369.7781 568569.6 3489308 979371.8511 568101.3 3489289 979372.8663 567881.2 3488813 979370.6361 567447.2 3488639 979369.5909 567458.6 3488191 979369.2189 567124.9 3487885 979368.4097 566781.2 3487588 979367.0244 566694.3 3487270 979365.6911 566229.4 3487011 979367.3676 566111.5 3486965 979366.7567 558720.5 3492964 979361.611 559080.9 3493284 979360.6869 558931.2 3493679 979362.1339 559016.4 3494033 979360.7049 559275 3494379 979360.7145 559336.6 3494785 979361.5387 556117 3489557 979375.3149 54

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556251.8 3489775 979374.1336 556354 3489998 979372.7458 556392.7 3490153 979370.912 556439.7 3490413 979370.887 556648 3490821 979370.2646 556725.9 3490976 979369.8368 556866.3 3491260 979368.7276 556937.5 3491424 979368.0505 557059.1 3491681 979367.5117 557160 3491882 979367.1048 557330.8 3492246 979365.7132 557384.9 3492335 979365.5764 557580.2 3492582 979364.9816 557729.5 3492624 979364.4687 558420.3 3492917 979363.8608 552225.2 3495603 979371.4821 552229.6 3495413 979370.8048 552171.8 3495253 979372.9853 552185.6 3495068 979372.5206 552282.2 3494853 979372.3609 552377.4 3494692 979370.0532 552468.7 3494508 979371.7892 552650.4 3494506 979371.3071 552779.5 3494428 979370.519 552787.6 3494258 979371.093 552858.8 3494101 979372.3845 552976.2 3493984 979371.9985 552930.8 3493789 979372.5594 552872.1 3493582 979373.2545 556169.9 3488808 979377.7397 555967.7 3489141 979377.1875 555946.1 3489570 979376.1752 555440.1 3490056 979375.7802 555720.5 3490629 979371.9165 555591 3491539 979369.5342 555471.2 3492160 979368.529 555421.8 3492582 979368.0167 555336.4 3493141 979367.5715 555274.5 3493636 979368.3732 555120.2 3494171 979367.9665 554843.8 3494921 979366.5792 55

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554641.3 3495133 979366.771 554444.5 3495432 979367.6584 554309 3495550 979368.7873 554511.9 3495919 979369.8263 554331.2 3496176 979371.0299 554018.4 3496484 979372.0247 553486.4 3496947 979373.3318 553334.7 3497332 979374.562 553093.2 3497622 979374.2591 552734.8 3497786 979374.5586 552157.5 3498233 979374.7488 551965.6 3498190 979374.4383 555249.3 3491690 979369.4638 554899 3491796 979370.0857 554555.4 3491947 979370.8902 554195.5 3491989 979371.7259 553899.2 3491996 979372.359 553394.1 3492049 979373.1601 553053 3492128 979373.5235 552733.1 3492150 979373.7179 552412.1 3492229 979374.4483 551940.2 3492273 979376.0701 551526.6 3492430 979376.1113 551174 3492460 979376.8446 550831.1 3492475 979377.2895 550526.9 3492473 979378.2178 549919.3 3492619 979378.7854 554473.9 3488179 979383.7318 554015.1 3488787 979382.9428 553569.1 3489381 979381.6927 553229.3 3490044 979381.5542 552554.3 3490258 979382.4426 551588.3 3490586 979382.1714 551036.9 3491033 979382.062 550676.5 3491497 979381.6444 550182.8 3491928 979380.4286 549313.3 3492290 979377.5599 552255 3492512 979374.6354 552284.9 3492715 979374.4058 552328.6 3492917 979374.2855 552367.3 3493088 979373.846 56

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552385.7 3493276 979373.1715 552417.6 3493508 979372.4861 552432.7 3493743 979371.563 554367.5 3493977 979368.884 554231 3493587 979369.4817 554058.4 3493132 979370.657 553777.9 3492717 979371.9622 553758.7 3492234 979372.6642 556046.6 3491594 979368.049 555957.7 3491874 979367.8794 555942.6 3492117 979367.6937 556013.8 3492331 979367.4436 556097.5 3492549 979366.3693 556214.8 3492869 979365.4808 556278.3 3493115 979365.5212 556373 3493306 979364.8122 556480.3 3493563 979364.5854 556516.1 3493663 979364.949 556544.5 3493751 979365.263 556570.9 3493797 979365.459 556641.8 3493908 979365.6338 556078.5 3491182 979369.4707 556943.6 3490482 979368.6827 557397.2 3490208 979368.5701 557811.5 3489970 979368.239 558340.5 3489616 979368.0696 559004.5 3489227 979366.6888 560136.3 3488836 979365.4726 560723.9 3488534 979365.8442 561484.9 3488210 979365.3291 562195.5 3487940 979364.0041 563337.6 3487715 979362.5444 563854.4 3487540 979363.5673 564463.3 3487327 979363.5652 565036.8 3486938 979365.523 565654.9 3486261 979370.501 565602.5 3485547 979373.6041 566047.4 3485148 979374.8576 566512.6 3484468 979376.9274 566694.2 3483643 979378.3089 559678.9 3488914 979365.2086 57

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555649.1 3494139 979366.6358 555979.7 3493724 979366.0566 556863.8 3493336 979364.2722 557293.4 3493031 979363.4472 557760.6 3491971 979363.4445 558152.6 3491556 979363.1397 558403.3 3491266 979363.0126 558378.1 3491016 979363.8934 558318.5 3490760 979364.5413 558273.8 3490469 979365.7014 558278.4 3490225 979366.9895 558142.2 3490011 979367.8724 558105.4 3489895 979368.2555 558403.6 3491266 979361.3846 561766.6 3488615 979363.8714 561645 3488407 979364.1828 561254.1 3488759 979363.7073 560757.9 3488882 979364.1991 560299.5 3489045 979364.7956 560065.5 3489051 979364.962 560057.2 3489239 979364.7292 560129 3489408 979363.7836 560323.8 3489404 979363.7137 559810.3 3489595 979364.0568 559413.1 3489857 979364.3453 558708.5 3489714 979366.5759 559167.3 3489937 979364.9222 559538.1 3490238 979363.3013 559744.1 3490626 979361.321 560085.8 3490998 979359.5433 559926.5 3490343 979361.971 560229.7 3490685 979360.2593 560572 3491087 979358.4255 561041.2 3491023 979358.8352 561483.1 3490773 979359.6192 561766.9 3490358 979360.4507 562135.7 3489920 979360.1523 562582.6 3489549 979359.8825 563039 3489550 979359.4822 563060.6 3489301 979359.8848 563037.6 3489014 979361.1033 58

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563002.6 3488445 979363.6557 562980.1 3488060 979363.5975 558643.3 3490122 979365.7867 557774.4 3490622 979366.1324 557331.8 3490827 979366.5799 557003.4 3491067 979366.7991 556529 3491542 979367.0695 558207.3 3491871 979362.0359 558270 3492077 979362.0219 558510.4 3492449 979361.2878 558972.1 3492170 979360.2432 559403.8 3491963 979359.6307 559721.3 3491531 979357.7261 560495 3489689 979363.5057 560757.1 3489979 979361.154 561031.8 3490102 979361.8906 561039.3 3490397 979362.0865 565028.4 3489253 979362.9096 564879.7 3489070 979362.3291 564736.1 3488895 979362.1401 564641.2 3488830 979362.0485 564469.6 3488639 979361.8516 564108.5 3488522 979362.461 564092.1 3488253 979363.5425 563979.4 3488012 979363.4514 563891 3487821 979363.511 59

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60 Appendix B Table 2 Magnetics Data Data collected as part of the magnetic portion of the survey is too numerous to include in the thesis; however a CD with this data can be obtained by contacting the author through the University of South Florida Department of Geology. Department of Geology University of South Florida 4202 E. Fowler Avenue, SCA 528 Tampa, FL 33620-8100 USA Phone: 813.974.2236 Fax: 813.974.2654


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Springer, Adam.
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Constraining basin geometry and fault kinematics on the santo tomas segment of the agua blanca fault through a combined geophysical and structural study
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ABSTRACT: The Agua Blanca fault is a major transverse structure of northern Baja California, extending more than 120km east from the Punta Banda ridge near the city of Ensenada to the San Matais Pass in central Baja. Through much of its eastern extent slip on this fault appears to be pure strike slip, however, at the Valle de Santo Toms the fault makes a ~25˚ change in orientation, which coincides with the formation of extensional basins on the fault. Recent evidence of the independent movement of the Baja Micro-plate relative to a stable Southern California Black leads to several possible hypotheses to explain this including: 1)That basins are localized structures, the result of a series of right steps or bends along the dextral Agua Blanca fault. 2)Basins are transtensional, possibly as a result of complexities associated with the northern boundary of the Baja Micro-plate To test between these hypotheses it was necessary to constrain the fault kinematics on both basin bounding and in-basin faults, well as the basin geometry. This was accomplished through combined structural and geophysical surveys. Data collected suggest that the majority of dip-slip is confined to the Santo Toms fault bounding the basin to the south, while the Agua Blanca fault bounding the basin to the north is primarily strike slip. This orientation typical in transtensional basins, suggesting that although Valle de Santo Toms formed at a step over it is not a pull apart basin. Possible explanations for transtension in this area come from the orientation of the Agua Blanca fault in relation to the Baja Micro-plate. Where the fault is close to aligned with the relative motion of the plate there is little transtension, such as in Valle de Agua Blanca, however, where the fault makes a 25˚ change in orientation and becomes more oblique the motion of the Baja Micro-plate transtension is present.
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