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Wilder, Douglas T.
Relative motion history of the Pacific-Nazca (Farallon) plates since 30 million years ago
h [electronic resource] /
by Douglas T. Wilder.
[Tampa, Fla.] :
University of South Florida,
Thesis (M.S.)--University of South Florida, 2003.
Includes bibliographical references.
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ABSTRACT: Relative plate motion history since 30 Ma between the Pacific and the southern portion of the Nazca (Farallon) plates is examined. The history is constrained by available seafloor magnetic anomaly data and a two-minute grid of predicted bathymetry derived from satellite altimetry and shipboard sensors. These data are used to create a new plate motion reconstruction based on new magnetic anomaly identifications and finite poles of motion. The new identified magnetic isochrons and tectonic reconstruction provides greater resolution to the tectonic history between chrons 7y (24.73 Ma) and 3 (4.18 Ma) than previous interpretations. Shipboard magnetics and aeromagnetic data from over 250 expeditions were plotted and used to extrapolate magnetic anomalies picked from 2D magnetic modeling from selected cruises. Magnetic anomalies were further constrained by tectonic features evident in the predicted bathymetry. Previously published magnetic anomaly locations consistent with this work were used where interpretation could not be constrained by 2D modeling and map extrapolation. Point locations for anomalies were used as input for calculation of finite poles of motion for chrons 10y, 7y, 6c, 5d, 5b, 5aa, 5o, 4a and 3a. An iterative process of anomaly mapping, pole calculation and anomaly point rotations was used to refine the finite poles of motion. Eleven stage poles were calculated from the nine finite poles from this study and two published instantaneous Euler vectors. Tectonic reconstructions indicate a history dominated by two major southward ridge propagation events, the first starting by 28 Ma and completed by 18 Ma. The second event initiated in association with breakup of the Farallon plate around 24 Ma and ceased by about 11 Ma. Lithosphere was transferred from Nazca to Pacific during the first event and in the opposite sense during the second. Development of the Mendoza microplate east of the later propagator occurred at about 20 Ma and this dual spreading process appears to have lasted until about 15 Ma.
Co-adviser: Naar, David F.
Co-adviser: Tebbens, Sarah F.
marine geology and geophysics.
x Marine Science
t USF Electronic Theses and Dissertations.
RELATIVE MOTION HISTORY OF THE PA CIFIC-NAZCA (FARALLON) PLATES SINCE 30 MILLION YEARS AGO by DOUGLAS T. WILDER A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science College of Marine Science University of South Florida Co-Major Professor: David F. Naar, Ph.D. Co-Major Professor: Sarah F. Tebbens, Ph.D. Sarah E. Kruse, Ph.D. Date of Approval: July 18, 2003 Keywords: marine geology and geophysics, tectonics, seafloor spreading, magnetic anomaly Copyright 2003, Douglas T. Wilder
Dedication Humbly, to Yvonne and Maeve who endured for me.
Acknowledgments This work would not have been possi ble without the guidance, patience, dedication, support and enthusiasm provided th roughout every phase of this research by my advisor Dr. David F. Naar. I am extrem ely grateful to him for providing me the opportunity to conduct this research and comple te my degree. Special thanks also to Dr. Sarah F. Tebbens, my co-major professor, who provided invaluable assistance during the course of this study beginning with initial data acquisition from Lamont-Doherty and including revision of programs used for finite and stage pole calculati ons. Thanks also to Dr. Sarah E. Kruse, my third committee me mber, who was extremely helpful at several points during completion of this work and he lped immensely in tightening up the text for the final draft. There are many others who provided he lp or otherwise offered support that contributed to the successful completi on of this work. Ruoying He provided programming expertise in the early stages of the work. Doug Myhre committed a great deal of time rewriting the Ma gBath program in Java and was always there to troubleshoot a myriad of computing problems. Dr. Mark Hafen was instrumental in getting me acquainted with the labs and procedures of the department (now college). William Self offered sight-unseen mathematical assistan ce from somewhere in cyberspace. Special thanks also to the following for comments a nd scientific assistance: Dr. Steve Cande, Dr. Yasushi Harada, Dr. Paul Wessel, Dr. Tanya Atwater, Dr. David Sandwell, Dr. Ben Horner-Johnson, Dr. Walter H. F. Smith, Dr. R. Dietmar Mller, Dr. Robert Detenbeck, and Dr. Dawn Wright. The National Scie nce Foundation provided funds that allowed this work to occur.
Crucial administrative support that made graduation possible was provided by Barb Daugherty, Nadina Piehl and Flo Cole. Jane t R. Giles, of the gr aduate school office, provided cheerful help with finalizing and submitting this document for graduation. Dr. Ted VanVleet was especially helpful as th e graduate coordinator for the college and provided special attention for me on several occasions. As my employer for much of the time I worked on this thesis, Henry Norris e xhibited encouragement and flexibility that allowed me to pursue this work. My co-worker, Jim Burd, provided invaluable encouragement and prodding near the completi on of this work. Finally, this work was greatly facilitated by the mora l support of Michelle McInty re, my comrade in arms, who lent encouraging words and an open ear along the way.
i Table of Contents List of Tables iii List of Figures iv Abstract vi Chapter 1 Introduction 1 Chapter 2 Methods 7 Data 7 Anomaly Identification 7 Finite and Stage Poles 9 Reconstruction 14 Chapter 3 Results 16 Magnetic Anomaly Identifications 16 Spreading Rates 16 Asymmetrical Spreading 25 Anomaly Juxtapositions 26 Finite Poles 27 Stage Poles 29 Tectonic Reconstructions 30 Chron 10y 30 Chron 7y 37 Chron 6c 37 Chron 5d 37 Chron 5b 38 Chron 5aa 38 Chron 5o 39 Chron 4a 39 Chron 3a 39 Present 40 Chapter 4 Discussion 41 Chapter 5 Conclusions 47 References 49
ii Appendices 53 Appendix A: Data File Information 54 Appendix B: 2D Magnetic Modeling 58
iii List of Tables Table 1. Calculated fini te poles for PAC-NAZ. 13 Table 2. Covariance matrices for fin ite poles calculated in this study. 14 Table 3. Stage poles calcu lated from finite poles. 14
iv List of Figures Figure 1. General tectonic setti ng of southeast Pacific. 3 Figure 2. Cruises used in 2D magnetic modeling. 8 Figure 3. Synthetic magnetic anom aly model and magnetic timescale of Cande and Kent (1995) used in this study. 10 Figure 4. Magnetic profiles for crui ses CATO4c (A) and EEL29d (B). 11 Figure 5. Magnetic data used in this study. 12 Figure 6. Interpreted magnetic isochrons. 17 Figure 7a. Grey-shade predicte d bathymetry, magnetic data, interpreted tectonic lineations (blue lines) and volcanics (red hachures) with anomaly pick s used to calculate finite poles for panel A. 18 Figure 7b. Detail of data for panel B. 19 Figure 7c. Detail of data for panel C. 20 Figure 7d. Detail of data for panel D. 21 Figure 7e. Detail of data for panel E. 22 Figure 7f. Detail of data for panel F. 23 Figure 8. Full-spreadi ng rates calculated along estimated flowlines for Pacific-Nazca spreading. 24 Figure 9. Spreading asymmetry cal culated from spreading rates (Figures 8). 25 Figure 10. Finite pole comparisons. 28 Figure 11. Stage poles for Pacific-Nazca relative motion. 29
v Figure 12. Full spreading rate vectors compared to interpreted tectonic features and predicted bathymetry. 31 Figure 13. Tectonic reconstruction fo r chrons 10y (grey) (A) and 7y (olive) (B). 32 Figure 14. Tectonic reconstruction for chrons 6c (lt. red) (A) and 5d (red) (B). 33 Figure 15. Tectonic reconstruction fo r chrons 5b (orange) (A) and 5aa (lavender) (B). 34 Figure 16. Tectonic reconstruction fo r chrons 5o (purple) (A) and 4a (green) (B). 35 Figure 17. Tectonic reconstruction for chrons 3a (lt. blue) (A) and 5d (lt. purple) (B). 36
Relative Motion History Of The Pacific-Nazca (Farallon) Plates Since 30 Million Years Ago Douglas T. Wilder ABSTRACT Relative plate motion history since 30 Ma between the Pacific and the southern portion of the Nazca (Farallon) plates is examined. The history is constrained by available seafloor magnetic anomaly data and a two-minute grid of predicted bathymetry derived from satellite altimetry and shipboard sensors. These data are used to create a new plate motion reconstruction based on new magnetic anomaly identifications and finite poles of motion. The new identified magnetic isochrons and tectonic reconstruction provides greater resolution to the tectonic history between chrons 7y (24.73 Ma) and 3 (4.18 Ma) than previous interpretations. Shipboard magnetics and aeromagnetic data from over 250 expeditions were plotted and used to extrapolate magnetic anomalies picked from 2D magnetic modeling from selected cruises. Magnetic anomalies were further constrained by tectonic features evident in the predicted bathymetry. Previously published magnetic anomaly locations consistent with this work were used where interpretation could not be constrained by 2D modeling and map extrapolation. Point locations for anomalies were used as input for calculation of finite poles of motion for chrons 10y, 7y, 6c, 5d, 5b, 5aa, 5o, 4a and 3a. An iterative process of anomaly mapping, pole calculation and anomaly point rotations vi
was used to refine the finite poles of motion. Eleven stage poles were calculated from the nine finite poles from this study and two published instantaneous Euler vectors. Tectonic reconstructions indicate a history dominated by two major southward ridge propagation events, the first starting by 28 Ma and completed by 18 Ma. The second event initiated in association with breakup of the Farallon plate around 24 Ma and ceased by about 11 Ma. Lithosphere was transferred from Nazca to Pacific during the first event and in the opposite sense during the second. Development of the Mendoza microplate east of the later propagator occurred at about 20 Ma and this dual spreading process appears to have lasted until about 15 Ma. vii
1 Chapter 1 INTRODUCTION In the southeast Pacific Ocean basin, th e Pacific-Nazca (Farallon) seafloor spreading history since the Oligocene invol ves several plate boundary reorganizations (Handschumacher, 1976; Tebbens and Cande, 1997; Okal and Cazenave, 1985), microplate formation and extinction events (Herron, 1972a; Herron, 1972b; Anderson et al., 1974; Handschumacher et al., 1981; En geln and Stein, 1984; Hey et al., 1985), propagating rift events (Hey et al., 1985, Hey et al., 1995; Naar and Hey, 1986; Naar and Hey, 1991; Rusby, 1992; Searle et al., 1989) a nd the fastest active seafloor spreading documented on Earth (Rea, 1981; Naar and He y, 1989; DeMets et al., 1994; Martinez et al., 1997). While numerous studies have desc ribed relative plate motion history between these plates, complex tectonic structures an d a dearth of magnetic, bathymetric and age data, have prevented a clear understanding of the history from 30 Ma to present. The period 25 Ma to 5 Ma (chrons 7 to 3) re presents the least unde rstood portion of the history and appears to be marked by cyclic rift propagation and microplate formation (Liu, 1996; Cornaglia, 1995). The spreadi ng history for the Pacific-Nazca boundary during this period has benefite d only indirectly from resear ch on the Nazca-Antarctic and Antarctic-Pacific spreading centers (Te bbens and Cande, 1997; Tebbens et al., 1997; Cande et al., 1995). While a significant lack of knowledge concerning Pacific-Nazca relative spreading history between 25 Ma and 5 Ma remains, several studies provide some
2 constraint. Initial breakup of the Farallon plate occurred around 24 Ma (Searle et al., 1995; Okal and Cazenave, 1985; Lonsdale, 1989 ) along the Galapagos-Grijalva fracture zone (Hey, 1977a; Hey et al., 1977). Reorient ation of the Pacific-Farallon spreading direction from NE-SW to WNW-ESE occu rred soon after the breakup. WNW-ESE spreading persisted until present. Further reorganization of the Pacific-Nazca divergent boundary occurred at about 5 Ma (Bird and Naar, 1994; Klaus et al., 1991; Naar and Hey, 1991; Okal and Cazenave, 1985; Rusby, 1992; Sear le et al., 1995; and Searle et al., 1993 as referenced in Liu, 1996) when the Easter and Juan Fernandez microplates are thought to have formed by rift propagation from w ithin transform fault zones (Bird and Naar, 1994; Naar and Hey, 1991). Prominent NE-SW-trending fracture zone s on the Pacific and Nazca plates (Marquesas-Mendana, Austral-Nazca, Resoluti on-Challenger) and the East Pacific Rise are the major tectonic features in the study area (Figure 1). Other significant features include the more recent Easter and Juan Fernandez microplates and the defunct microplates, Bauer and Mendoza. Less promin ent oblique features on the Pacific and Nazca plates we interpret as pseudofaults fr om major southward propagators. There are distinctive volcanic seamount chains trending roughly east -west across the southeast Pacific basin that may highlight absolute motion for the Pacific and Nazca plates but obscure magnetic anomaly identifications. Ne w magnetic anomaly picks, finite poles, and the tectonic reconstructi on presented here are limited to the region bounded to the north by the Marquesas-Mendana fracture zo nes and to the south by the ResolutionChallenger fracture zones mentioned above and to the east and west by longitudes 80 W
3 Figure 1. General tectonic setting of southeast Pacific study region.
4 and 140 W. This work divides the study area into Â“northernÂ” and Â“southernÂ” sections separated by the Austral-FZ2-SO EST-Nazca fracture zone system. The Mendoza Rise, by virtue of its location east of the Ea st Pacific Rise and west of the traceable extensions of the Menda na and Nazca fracture zones (Figure 1), represents an important event following th e spreading direction change due to the breakup of the Farallon plate. Okal and Cazen ave (1985) used SEASAT altimetry data to suggest that a southward propagator marked the birth of the Mendoza Rise. Using the same line of evidence, they also suggest th at at around 18 Ma, the Pacific-Farallon ridge ceased propagating and instantaneously jumped 500 km to the west leaving behind what became the Mendoza microplate. The Okal and Cazenave (1985) reconstruction accommodates the 24 Ma spreading direction ch ange by this and similar westward ridge jumps. Herron (1972a) suggests that the Me ndoza Rise was actively spreading between 21 Ma and 9 Ma before spreading along the EPR became dominant within the last 10 m.y. Lonsdale (1989), by examining ridge segmentation along the EPR, suggests the Mendoza microplate formed via a northward propagation of the EPR around 15 Ma. Liu (1996) suggests that the Mendoza microplate wa s just starting to form at 19.6 Ma (chron 6) and had terminated between 13.4 Ma (chron 5ab) and 12.3 Ma (chron 5a). Liu (1996) also suggests that the Mendoza microplate deve loped as the result of north and southward propagators that formed the east and west boundaries of the micropl ate. Spreading was re-established in a WNW-ESE direction begi nning approximately at 12.3 Ma (chron 5a) and persisting until 4.71 Ma (chron 3) (Liu, 1996). The end of this period marks the approximate initiation of the present Easter and Juan Fernandez microplates (Bird and Naar, 1994).
5 Other work on the various ridges/chains and hotspots of the Nazca Plate also provide constraints on the Pacific-Nazca sp reading history. The reconstruction of Cornaglia (1995) used a suite of available magnetic and bathymetric data to delineate major tectonic boundaries including the oblique features on the Pacific and Nazca plates that, when rotated together, form distin ctive Â“VÂ” shapes indicative of southward propagation of the Pacific-Fa rallon spreading system. This work corroborates previous interpretations. Liu (1996) reconstructed the tectonic history for the Nazca-Pacific spreading center between 28.5 (c hron 10) and present using cr ustal ages inferred from bathymetry, isochron identification (using the newly calibrated geomagnetic time-scale of Cande and Kent, 1995), new age data for the Easter Seamoun t Chain, GLORIA side-scan and bathymetry data, and bathymetry derived from recently declassified satellite altimetry data (Smith and Sandwell, 1994). The Resolution-Chile fracture zone (FZ) bounded the reconstruction to the south while the Austral-FZ2-SOEST-Nazca FZ served to constrain the history within the central portion of the stu dy area. The northern extent of LiuÂ’s reconstruction is bounded by the Marquesas-Mendana FZ although the Bauer and Garret fracture zones were not used to c onstrain the history since these features are short lived and spatially discontinuous due to formation of the Mendoza and Bauer microplates (Goff and Cochran, 1996). The most comprehensive interpretation to date (Liu, 1996), attempts to describe the complicated spreading history from chron 7 to 5 with very little data. Historic details for this period remain sparse and a closer l ook at available bathymet ric and magnetic data is needed to resolve the complexities rela ted to the change from Pacific-Farallon to Pacific-Nazca seafloor spreading. In th is thesis the higher resolution two-minute
6 predicted bathymetry of Smith and Sandwe ll (1997) and magnetic data from numerous cruises since 1967 are used to describe seaf loor-spreading history (back to 30 Ma) along the Pacific-Nazca (Farallon) boundary by calcula ting new Pacific-Nazca (Farallon) finite rotation poles. By integrating our understa nding of propagating rifts and microplates with these new poles, we present a plate tect onic reconstruction for this area from 28 Ma to present.
7 Chapter 2 METHODS Data Shiptracks from 250 cruises obtain ed from the Lamont-Doherty Earth Observatory (Appendix A) were plotted in map form to allow selection of cruises for 2D magnetic modeling and aid in magnetic anomal y extrapolation across the seafloor. The 2-minute predicted bathymetry of Smith a nd Sandwell (1997) (at 2-minute resolution) served to constrain major tectoni c features and anomaly interpol ations and extrapolations. Anomaly Identification Magnetic anomalies were identified us ing a 2D modeling program, MAGBATH (Hey et al., 1986; Naar and Hey, 1989; Naar and Hey, 1991). Using estimated spreading rates and the magnetic timescale of Cande and Kent (1995) as input (in addition to parameters listed in Appe ndix B), MAGBATH generates a synthetic magnetic anomaly model and plots it against shipboard magne tic and bathymetric data. Varying the estimated spreading rates allows for identif ication of magnetic anomalies by comparing the synthetic model with shipboard data. Figu re 2 shows the locations of cruise tracks used in magnetic modeling. Ma jor tectonic features evident in the pr edicted bathymetry and documented in other studie s were used as a secondary control on magnetic anomaly delineation. Magnetic anomaly picks of Mayes et al. (1990), Cornaglia (1995), Searle et al. (1995), Liu (1996), Tebbens and Cande (1997) and Tebbens et al. (1997) were used where interpretation could not be constrai ned by 2D modeling and map extrapolation.
8 Figure 2. Cruises used in 2D magnetic m odeling. Boxes show locations of profiles provided in Figure 4. Longitude and latitude degrees are west and south.
9 Selection of positions of magnetic anomalies, such as young edge or old edge, followed the selection scheme of Atwater and Severingha us (1989) with revised ages of Cande and Kent (1995) magnetic timescale (Figure 3). This selection scheme, developed for the northern Pacific, provides standardized refe rencing of magnetic anomalies by prominent magnetic polarities within each anomaly. Figure 4 demonstrates the 2D magnetic modeling method for selected segments of cruise data lines CATO4 and ELT29 (entire sh ip tracks shown in Figure 2). Appendix B contains plots of the magnetic modeling comp leted in this study. A synthetic magnetic anomaly was generated for each magnetic data line using the 2D bathymetry by varying spreading rates along the profile For details see Naar a nd Hey (1991). Magnetic data used to extrapolate a nomalies in map view are shown in Figure 5. Full seafloor spreading rates and directio ns are derived by tr eating the calculated stage poles as instantaneous Euler vectors via a program called MakeVec used in Naar and Hey (1989). Points along an approximate d spreading flow path for both the northern and southern sections were selected for anomalies 1o, 2a, 3a, 4a, 5o, 5aa, 5b, 5d, 6c, 7y and 10y as input, along with modified stag e pole parameters, for MakeVec. Finite and Stage Poles Finite rotation poles for magnetic anom alies 3a, 4a, 5o, 5aa, 5b, 5d, 6c, 7y, and 10y were calculated using the method of He llinger (1981). Magnetic anomaly picks were rotated to the conjugate plate for the sake of comparison. Displaying the magnetic anomalies superimposed on predicted bathym etry allowed identification of magnetic
10 Figure 3. Magnetic timescale of Cande and Ke nt (1995) used in this st udy. Bracketed sets of bl ack (normal polarity) and white (reverse polarity) blocks represen t magnetic anomalies. Anomaly Ids are indicated by the brackets. Ages (shown at an angle above the timeline) are in millions of years. A constant-rate synthetic magnetic anomaly model is shown in green. Arrows and red circles indicate the ages for finite poles calcula ted in this study. Blue ci rcles indicate the ages for finite poles of Naar and Hey (1989) (0.78 Ma) and DeMets et al. (1990) (3.03 Ma). Red squares indicate ages for attempted calculations of fi nite poles in this study.
11 Figure 4. Magnetic profiles for cruises CATO 4c (A) and EEL29d (B). Vertical red lines represent seafloor spreading centers. Vertical blue lines represent pseudofaults (pf) and failed rifts (fr). The top prof ile in each graph is magnetic data. Immediately below is the synthetic anomaly model followed by seafloor ba thymetry. Dark blocks represent normal magnetic polarity and are la beled according to Cande a nd Kent (1995). Appendix A contains all magnetic prof iles used in this study. A B 1 2a 2 2a 3 3a 3b 4 4a 5 5a 5aÂ’s 5b 5c 5d 5e 6 pf pf fr pf pf fr 5a 5 4a 4 3b 3a 3 2a 2 2 2a 3 3a 3b 4 4a 5
12 Figure 5. Magnetic data used in this study. Appendix A lists da ta files used to gene rate magnetic profiles.
13 anomaly-pseudofault intersections that furthe r served to constrain pole locations since conjugate pairs of such intersections should theoretically rotate back together as one point. An iterative process of pole calculation, anomaly pick refinement, and comparison of pole positions and angles for the different anomalies -all constrained by predicted bathymetry -lead to the nine new finite poles (plus two publishe d poles) presented in Table 1. Table 2 contains cova riance matrices for the eleven finite poles. Eleven stage poles (Table 3) were also calculated from th e eleven finite poles to further constrain relative plate motions. Table 1. Calculated finite poles for PAC-NAZ. Chron Age (Ma) Latitude S Longitude E Angle (deg.) Degrees per Million Years 1o* 0.78 -48.12 89.50 1.056 1.35 2a** 3.03 -53.80 91.80 4.30 1.42 3a 6.57 -54.38 85.34 9.68 1.47 4a 8.86 -58.23 86.36 13.18 1.49 5o 10.95 -60.01 86.73 16.86 1.54 5aa 13.07 -62.00 87.21 19.63 1.50 5b 15.10 -64.52 83.61 23.10 1.53 5d 17.45 -72.22 89.27 26.49 1.52 6c 24.06 -59.68 88.98 38.45 1.60 7y 24.73 -61.15 88.86 38.88 1.57 10y 28.28 -65.60 82.61 43.83 1.55 Calculated from Naar and Hey (1989) best fitting pole for PAC-NAZ ** Calculated from DeMets et al. (1990) best fitting pole for PAC-NAZ
14 Table 2. Covariance matrices for fin ite poles calculated in this study. Chron DF N k a b c d e f 3a 39 48 0.170 6.063E-07 1. 322E-06 5.068E-06 6.805E07 2.477E-06 1.299E-06 4a 34 43 0.250 6.934E-07 1. 490E-06 6.870E-06 7.910E07 3.403E-06 1.796E-06 5o 33 42 0.340 6.685E-07 1. 223E-06 7.005E-06 6.481E07 3.296E-06 1.653E-06 5aa 35 44 0.294 5.201E-07 6. 460E-07 4.202E-06 3.717E07 1.897E-06 9.640E-07 5b 21 30 0.294 8.484E-07 1. 455E-06 9.443E-06 7.957E07 4.349E-06 2.188E-06 5d 19 28 0.074 1.669E-06 1. 717E-06 1.132E-05 1.325E06 6.068E-06 3.551E-06 6c 20 29 0.208 4.171E-07 -6.925E-07 1. 143E-05 -2.758E-07 6.238E-06 3.641E-06 7y 28 39 0.176 3.173E-07 -1.012E-06 7. 515E-06 -5.263E-07 3.803E-06 2.115E-06 10y 38 47 0.219 3.451E-07 -2.641E-07 4. 126E-06 -1.120E-08 1.772E-06 8.988E-07 DF = degrees of freedom. N = number of points. k = estimated correctness of the calculated uncertainties. a-f = calculated values for the 9-element error matrix The covariance matrix for Table 2 is defined as Cov(u)=(1/k) where u represents the finite pole vector a nd k is the squared quotient of an assigned uncertainty estimate to the true (or scaled) es timate for each data poi nt. The k parameter indicates the correctness of the calculate d uncertainties; the circumflex over the k indicates that this parameter is an estimate rather than a true value. Table 3. Stage poles calcul ated from finite poles. Pacific Fixed Nazca Fixed Chrons Age (Ma) Lat Lon Angle Lat Lon Angle Degs/my 1o, present 0.78 -48.12 89.50 1.06 -48.12 89.50 -1.06 1.35 2A,1o 3.03 -55.65 92.56 3.25 -55.61 92.80 -3.25 1.44 3A,2A 6.57 -54.34 80.10 5.40 -54.85 80.12 -5.40 1.53 4A,3A 8.86 -68.76 87.47 3.59 -68.23 93.87 -3.59 1.56 5o,4A 10.95 -66.39 86.12 3.70 -66.14 90.72 -3.70 1.77 5AA,5o 13.07 -73.89 85.02 2.84 -73.09 98.95 -2.84 1.34 5B,5AA 15.10 -71.89 43.40 3.65 -77.85 52.69 -3.65 1.80 5D,5B 17.45 -66.28 -98.25 4.81 -62.53 -138.43 -4.81 2.05 6C,5D 24.06 -36.00 98.49 13.79 -36.07 79.08 -13.79 2.09 7y,6C 24.73 -50.66 -58.85 1.07 -51.80 -116.54 -1.07 1.59 10y,7y 28.28 -61.46 -0.17 6.17 -78.13 -58.75 -6.17 1.74 ^ a b d b c e d e f
15 Reconstruction Relative Pacific-Nazca (Far allon) spreading history was reconstructed for the period 28.2 Ma to present based initially on the reconstructi on of Liu (1996) and constrained by anomaly selections and pred icted bathymetry. Major pseudofaults and fracture zones were delineated by examina tion of the interpreted magnetic data in conjunction with fault zones identified in the predicted bathymetry. Due to the proximity of the magnetic equator and l ack of identifiable magnetic anomalies in the northern areas of the Nazca plate, the reconstruction is bounded by the Marquesas-Mendana and Resolution-Challenger fracture zone systems. Individual reconstructions for each calculated finite pole were created by rotating together the magnetic anomaly picks on th e Pacific and Nazca plates. For each time frame, interpreted tectonic features younge r than the modeled time were removed and remaining (older) features were rotated together. The matched line of anomaly picks between each plate represents the seafloo r-spreading axis active at that time.
Chapter 3 RESULTS Magnetic Anomaly Identifications Interpreted magnetic anomaly isochrons and present-day major tectonic features are shown in Figure 6. Isochron widths correspond to the age ranges depicted in Figure 3 (Cande and Kent, 1995). Detail views of magnetic data and anomaly picks used in finite pole calculations are shown in Figures 7a through 7f. Significant gaps in coverage of the interpreted anomalies exist between chron 6c (24.095 Ma) and 5d (17.446 Ma) due to complexities in the tectonic framework due to plate boundary reorganizations and microplate formation thought to have occurred during this period (Liu, 1996; Lonsdale, 1989; Okal and Cazenave, 1985; Searle et al., 1995; Cornaglia, 1995; Cornaglia and Francheteau, 1993). Spreading Rates Interpreted isochrons indicate full spreading rates ranging from approximately 119 to 221 mm/yr between chrons 10y (28.283 Ma) and 3a (6.567 Ma) in the study area (Figure 8). The fastest rates occur between chrons 6c (24.059 Ma) and 5d (17.446 Ma) while the slowest occur between chrons 7y (24.730 Ma) and 6c (24.059 Ma). Full spreading rates generally decrease from chron 5d (17.446 Ma) to present with sharper decreases at chrons 5d (17.446 Ma) and 5aa (13.065 Ma). Rotation rates from stage pole calculations are also plotted in Figure 8. Rotation rates generally agree with the full-spreading rates although the low rate observed at chron 5d (17.446 Ma) in the spreading rates is not seen in the rotation rates. Similar to the full16
Figure 6. Interpreted magnetic isochrons. Labeled, colored bands represent magnetic anomalies according to the timescale of Cande and Kent (1995). Anomalies south of the Chile FZ (see Figure 1) are from Tebbens et al. (1997). Anomalies north of about 15S are from Liu (1996). Anomalies younger than 3a (5.894 Ma) and between the Marquesas-Mendoza FZ system are from Naar and Hey (1991) and Bird and Naar (1994). 17
Figure 7a. Grey-shade predicted bathymetry, magnetic data, interpreted tectonic lineations (blue lines) and volcanics (red hachures) with anomaly picks used to calculate finite poles for panel A. Magnet anomaly picks are also shown rotated (colored circles with out dot (see legend)). 18
Figure 7b. Detail of data for panel B. 19
Figure 7c. Detail of data for panel C. 20
Figure 7d. Detail of data for panel D. 21
Figure 7e. Detail of data for panel E. 22
Figure 7f. Detail of data for panel F. 23
Full Spreading Rates951151351551751952152350510152025Age (Ma)Rate (mm/yr ) 1.001.201.401.601.802.002.20Rotation Rate (degs/my ) North South Full Rotation 6c 5b 5o 10y 3a 2a 4a 5d 1o 5aa 7y Figure 8. Full-spreading rates calculated along estimated flowlines for Pacific-Nazca spreading. Anomaly IDs are included within the graph. Spreading rates are based on the time scale of Cande and Kent (1995) and calculated using MakeVec (see text for details). Estimated error is +/10 mm/yr for each spreading rate. spreading rate plot, a decreasing trend is evident in the rotation rate plot from chron 5d (17.446 Ma) to present. The observed rotation rate increase and decrease from chron 7y (24.730 Ma) to 5d (17.446 Ma) is probably a manifestation of the breakup of the Farallon plate and accompanying change in spreading direction. Difficulties in anomaly identification in this complex area prevent determination of any clear spreading rate pattern during this time interval (28 Ma to 17.5 Ma). The later decrease observed at chron 5aa (13.065 Ma) in both the spreading and rotation rates, may reflect development of the Mendoza microplate in association with cessation of the later southward propagator interpreted in the northern section of the study area. 24
Asymmetrical Spreading Significant asymmetrical spreading between chrons 10y (10.949 Ma) and 3a (6.567 Ma) is observed in both the northern and southern sections with rates generally greater to the east (Figure 9). Spreading asymmetry is more pronounced in the northern section with Spreading Asymmetry-70.0-60.0-50.0-40.0-30.0-20.0-10.00.010.020.00510152025Age (Ma)%Asymmetry Northern Section Southern Section 5aa 4a 7y 10y 2a 3a 5d 5o 1o 6c 5b Figure 9. Spreading asymmetry calculated from spreading rates (Figure 8). Anomaly IDs are included within the graph. Positive asymmetry indicates more lithosphere added to the Pacific plate. Dashed blue line represents asymmetry primarily related to the existence of the Mendoza paleomicroplate. spreading to the east exceeding that to the west by approximately 15 mm/yr as compared to an average difference of 5 mm/yr to the west in the south. Maximum asymmetry of approximately 57% to the east is observed in the northern section between chrons 6c (~24 Ma) and 5d (~17 Ma) while no asymmetry is observed in the southern section between chrons 7y and 6c (representing a span on only about 0.5 m.y.). Although rigorous 25
statistics have not been applied, there is a clear pattern of about 10% faster spreading (more seafloor) toward the east. Anomaly Juxtapositions Interpreted anomalies in the southern section of the project area show a normal progression of seafloor spreading back in time to about chron 5d (17.446 Ma). Chron 6c (24.095 Ma) through 12 (32.782 Ma) in the southern section (Figure 7, panels D and F) are displaced across oblique features interpreted to be conjugate pseudofaults (the Pamatai Trough and its conjugate oblique lineament on the Nazca plate in Figure 1 between 25S and 35S). Chrons 8 (25.823 Ma) and older are dextrally offset across the pseudofaults while no offset is clearly seen in the younger anomalies (although chron 6c (24.095 Ma) on the Pacific plate can be interpreted as sinistrally offset). The dextral orientations of chrons 8 and older, along with the observed trends in the pseudofaults, suggest a southward propagation event that transferred relatively little lithosphere to the Pacific plate from about the beginning of chron 12 (~32.8 Ma) to chron 8 (~26.0 Ma). By the start of chron 7a (25.648 Ma), lithosphere ceased being transferred to the Pacific plate but propagation continued southward until about chron 5d (17.446 Ma). Due to the sparseness of data and uncertainty in navigation, we cannot determine if lithospheric transfer occurred since chron 7a (~25 Ma) or not. The young edge of anomaly 9 (27.027 Ma) in the southwest quadrant, north of the interpreted pseudofault, intersects the pseudofault at approximately half the distance between the Austral and Resolution fracture zones. In contrast, the young edge of chron 11 (29.401 Ma) intersects the mid-distance point between the Nazca and Challenger fracture zones on the Nazca plate. The intersection of the young edge of chron 10 (used 26
to calculate a finite pole) and the pseudofault is similarly offset. Since the points of intersection are expected to rotate back together, a greater length of pseudofault is required (and observed) on the Nazca plate at time 10y (28.283 Ma) that would imply asymmetric spreading. Anomaly-pseudofault point intersections in the northern half of the study area also present difficulties in the reconstructions. In the case of the northern section, interaction of the Mendoza rise, as a microplate or northward propagator (Liu, 1996), may help explain the offsets. Finite Poles Calculated finite poles and associated error matrices are given in Tables 1 and 2. Figure 10 compares the poles to those of Tebbens and Cande (1997) and Mayes et al. (1990). The chron 2a (~3 Ma) best-fitting Euler vector of DeMets et al. (1990) and the best fitting Euler vector for 1o (0.78 Ma) from Naar and Hey (1989) are also shown. Confidence regions (95%) for each of the calculated poles are also shown in Figure 10. Major changes in the polar curve are observed at chron 6c (24.095 Ma) and 5d (17.446 Ma) with less significant changes at chrons 7y (24.730 Ma), 5b (15.095 Ma) and 5aa (13.065 Ma). From time 10y (28.283 Ma), the poles migrate northward until chron 6c (24.095 Ma), which marks a sharp cusp leading to chron 5d (17.446 Ma) at about 75 S latitude. The large 95% confidence ellipse for the 5d pole may indicate a less extreme cusp than that reflected in Figure 10. The curve appears stable after chron 5aa (13.065 Ma) although calculated rotation rates show a significant change between 5aa (13.065 Ma) and 4a (8.862 Ma) suggesting that while rates of plate motion and global position of the 27
entire system were changing, relative orientations were not. In general, finite pole positions tend to remain between 90 and 80 E longitude and display steady northward migration after chron 5b (15.095 Ma). Figure 10. Finite pole comparisons. Ninety five percent confidence regions are shown for the nine finite poles calculated in this study as well as for the 1o and 2apoles. Poles are the positions about which the Pacific and Nazca plates were moving relative to one another at the time of the indicated magnetic anomaly identification. Map projection is stereographic with a projection center at 70S, 180W. Rotations from this study do not correlate well with those of Tebbens and Cande (1997) or Mayes et al. (1990) although comparison is difficult since neither of these studies used the same set of magnetic anomaly time periods, except for chron 6c (~23.7 Ma), 6o (20.131 Ma) and 5a (~12.8 Ma) (Tebbens and Cande, 1997). Comparison of the error ellipses for these studies might reveal greater correlation than initially apparent. 28
Stage Poles Stage poles calculated from the finite poles are given in Table 3 and Figure 11. Finite poles are reconstructed across known ridge jumps, microplates and areas affected by asymmetrical spreading. Stage poles for intervals in which these events occurred wont include these details. Prior to chron 5b (15.095 Ma), calculated angles and pole positions vary widely, with a significant increase in rotation angle (and therefore spreading rate) for stage chron 5d (17.446 Ma) to 6c (24.095 Ma) corresponding to break up of the Farallon plate and reestablishment of a new plate boundary. The chron 5d (17.446 Ma)-6c (24.095 Ma) stage pole location is also radically displaced from the bounding stages. Figure 11. Stage poles for Pacific-Nazca relative motion. Interpreted tectonic lineations are shown in blue to the right of the figure. Map projection same as for Figure 10. 29
Ignoring the chron 5d (17.446 Ma), 6c (24.095 Ma) stage, the overall trend shows a gradual shift of poles across Antarctica from the western hemisphere to the eastern spanning approximately 70 of latitude. Stage poles between chron 5o (10.949 Ma) and 4a (8.862 Ma) suggest a comparatively small deviation from this overall trend. Stage pole locations appear more stable after chron 5b (15.095 Ma), which may indicate greater stability in plate motions or simply reflect greater control in anomaly identification for the younger time periods in this study. A lack of known significant ridge jumps and microplates in this region immediately after chron 5b supports the former case and suggests that plate motions after this time were governed by simple tectonics. Spreading velocity vectors calculated from the stage poles are shown in Figure 12. Full spreading rate vectors are generally parallel to interpreted tectonic lineations except for stages 5d-5b and 7y-6c in both the northern and southern sections and on both the Pacific and Nazca plates. Tectonic Reconstructions Time frames for each of the nine calculated poles are depicted in Figures 13 through 17. Chron 10y (28.283 Ma) Spreading between the Pacific and Farallon plates was occurring in an ENE-WSW direction in the northern and southern sections of the study area along a ridge dextrally offset by the Austral-Nazca fracture zone. The Tuamotu and Nazca volcanic chains were converging near the southern terminus of the northern section spreading center (Figure 13a). A southward propagator in the southern section was well-established by this time, probably having initiated around chron 12. 30
Figure 12. Full spreading rate vectors compared to interpreted tectonic features and predicted bathymetry. Vector lengths and azimuths represent spreading rates and directions for each stage with longer vectors representing faster spreading. Vectors originate at the young edge of each stage. Labels for the eleven stages depicted are shown for the northwest quadrant only. 31
A B Figure 13. Tectonic reconstruction for chrons 10y (grey) (A) and 7y (olive) (B). Red stippled areas represent volcanic chains. Interpreted tectonic lineations are shown as thick blue lines. Active spreading center is shown as double lines matching the color of the anomaly isochrons. Active fracture zones also match the color of the anomaly. 32
A 33 B Figure 14. Tectonic reconstruction for chrons 6c (lt. red) (A) and 5d (red) (B). Symbols as in Figure 13.
A B Figure 15. Tectonic reconstruction for chrons 5b (orange) (A) and 5aa (lavender) (B). Symbols as in Figure 13. 34
A B Figure 16. Tectonic reconstruction for chrons 5o (purple) (A) and 4a (green) (B). Symbols as in Figure 13. 35
A B Figure 17. Tectonic reconstruction for chrons 3a (lt. blue) (A) and present (B). Symbols as in Figure 13. 36
Chron 7y (24.730 Ma) While southward propagation continued in the southern section, a second southward propagation event developed in the northern section by chron 7y (24.730 Ma). The earlier southward propagator continued as evidenced by the pseudofault traces, however actual southward progress appears to be limited and may be associated with initiation of the 24 Ma plate boundary reorganization. The Tuamotu chain is no longer being formed although the Nazca and Crough volcanic chains continued to develop south of the Austral-Nazca fracture zone, suggesting independent melt sources. Chron 6c (24.059 Ma) A northward propagator has initiated by chron 6c at the spreading center along the Austral-Nazca fracture zone marking the beginning of the Mendoza Rise/microplate. Southward propagation continued in the northern and southern sections. A major plate boundary reorganization was imminent around this time (Hey, 1977b; Hey et al., 1977; Searle et al., 1995) involving the break up of the Farallon plate that has been estimated to occur at 24 Ma. Chron 5d (17.446 Ma) Many tectonic event occurred during this time period with very little data to outline details. A generic model is proposed, but until additional data and interpretation come forth, we can only propose this schematic model. By chron 5d (17.446 Ma), spreading direction was reestablished in a WNW-ESE direction longitudinally across the entire study area (Figure 14b). Southward propagation in both the northern and southern sections continued as the new spreading direction (and an increase in spreading asymmetry) was established resulting in skewed pseudofault traces. The Mendoza Rise 37
is interpreted to have propagated well into the northern section by chron 5d cutting across the pseudofaults of the later southward propagator in the northern section. The southward propagator in the southern section ceased sometime between chron 6c (24.059 Ma) and 5d after intersecting the newly formed Chile fracture zone. The Crough and Nazca volcanic chains continued to develop and it appears clear that they continued to have independent sources. The trends evident at chron 5d (17.446 Ma) suggest that the interpreted southward migration of a magmatic source (with respect to the two plates) for these chains had ceased by this time. Chron 5b (15.095 Ma) The Mendoza Rise is interpreted to have continued spreading at chron 5b (15.095 Ma) but may have ceased north-northeastward propagation (Figure 15a). The southbound propagator in the northern section is interpreted to have intersected the inner pseudofault of the Mendoza propagator in a manner similar to the Easter micropolate (Naar and Hey, 1991). Normal spreading continued in the southern section of the study area and new material continued to accumulate on the Crough and Nazca chains. By this time, the trends of these volcanic chains are clearly parallel to the dominant spreading direction affirming that the interpreted southward migration of source material had ceased. Chron 5aa (13.065 Ma) By chron 5aa (13.065 Ma), the Mendoza Rise is interpreted to have ceased seafloor spreading. A more southward, versus a southeastward, propagation in the northern section was reestablished by chron 5aa apparently after the cessation of seafloor spreading and propagation of the Mendoza propagator near the end of chron 5b (14.8 38
Ma). Southward propagation continued in the northern section and may have intersected the Austral-Nazca fracture zone system by this time. The Crough and Nazca chains continued to develop as dominant WNW-ESE spreading proceeded, still apparently being formed by independent melt sources. Chron 5o (10.949 Ma) The southward propagator that initiated in the northern section is interpreted to have severed the Austral-Nazca fracture zone and continued south to form a sinistrally offset relationship with the now failing rift of the southern section (Figure 16a). The Crough and Nazca chains appear to have grown closer to the propagating tip suggesting there may have been at a common source (or mixed source) for the volcanoes located near the tip of the southward propagator. Chron 4a (8.862 Ma) Southward propagation is interpreted to have ceased by this time forming a new transform fault with the ridge in the southern section. The propagation appears to have ceased soon after passing over a common or mixed source of magma for the Crough and Nazca chains located beneath the spreading axis (Figure 14b). Additional age and geochemical data from new sample sites, especially along the Crough and Tuamotu lineaments, would enable a test of this proposal. Chron 3a (6.567 Ma) Seafloor spreading continued with an apparent cessation of propagation events in the region. The Crough and Nazca chains are interpreted to intersect at the spreading center north of the newly-formed transform fault, which later forms the FZ2-SOEST fracture zones. 39
Present At about 5 Ma, new propagation events led to the formation of the Easter and Juan Fernandez microplates (Bird and Naar, 1994; Hey et al., 1995; Hey et al., 1985; Larson et al., 1992; Naar and Hey, 1991; Rusby, 1992; Searle et al., 1989; and Zukin and Francheteau, 1990). These microplates and their associated new propagation events suggest on-going restructuring of the Pacific-Nazca boundary that appears to involve multiple propagation events and microplates, much like an analog plate tectonic model using freezing liquid wax done by Bodenschatz and Tebbens (personal communication, Tebbens, 2002). 40
41Chapter 4 DISCUSSION Anomaly patterns south of the Austral-Nazc a fracture zone system (the southern section of the study area) suggest that lithos phere was transferred to the Pacific plate from the Farallon plate by a southward propaga tor that initiated around chron 12 (31 Ma). Interpreted offsets between the propagator and failing ridge suggest relatively little material was transferred. Th e reconstruction presented here further suggests that the southward propagator and failing rift may have had minimal overlap resulting in an overlapping spreading center and causing the two rift tips (one propagating and one failing) to curve towards each other. Although interpreted tectonic features suggest the younger southward propagator in the northern section of the study area initi ated about mid-way between the MarquesasMendana fracture zone (Figure 13b), anomal y patterns do not preclude this propagator starting at chron 10y (28.283 Ma) time and thus requiring relativel y rapid propagation and minimal lithospheric transfer to reach the interpreted configuration of chron 7y (24.730 Ma). Southward propagation in the southern section, howev er, does not appear to have progressed significantly by chron 7y. We suggest that the later propagation event (north of the Austral-Nazca fault zone sy stem) initiated mid-way between the bounding fracture zones rather than farther north si nce the latter case re quires much faster propagation to reach the mid-way point than ge nerally observed in this region. However, given the resolution of our analysis, we ca nnot preclude initiation of propagation at the
42 fracture zone farther north since propagation approa ching 1,000 mm/yr have been documented elsewhere. The later propagation event, like the ear lier southern event, appears to have developed essentially in-line with the failin g rift resulting in no observed offset in magnetic anomalies across the pse udofault in the vi cinity of 20 S, 88 W. This also implies no net transfer of lit hosphere for at least the period between chrons 7a (~25.5 Ma) and 6c (~23 Ma). Verification of this conclusion requires additional interpretable data in the conjugate area on the Pacific plate. The in -line orientation of these anomalies may be explained as resulting from a southward-migrating overlapping spreading center such as described by Macdonald et al. (1992). Calculated spreading asymmetries from interpreted anomalies, however, are as high as 57% between chrons 6c (~24 Ma) an d 5d (~17 Ma) and generally indicate that more material accreted to the Nazca plate between chrons 10y (28.283 Ma) and 3a (6.567 Ma). Overlapping spreading centers and/or microplate evolution, however, obscure the calculations but also add significant asymmetry (asy mmetry accretion by way of lithospheric transfer) between the Pacific and Nazca plates. The large percentages of observed asymmetry also may be due in pa rt to spreading center jumps (transferred lithosphere), although such details are not di scernable at the scale of this study. Calculated spreading rates and asymmetries (F igures 8 and 9) indi cate generally faster spreading in the southern section and w ith less asymmetry relative to the northern section. In the northern sect ion, overlapping spreading cente rs and microplate evolution (perhaps dominating the northern section more so than the southern) tend to add to these calculations and make spreading ch aracteristics difficult to assess.
43 The later propagator in the northern section was still active by chron 5d (17.446 Ma) and is interpreted to ha ve curved southeasterly as proposed by Liu (1996) during microplate activity. The failing ridge associat ed with this Â“inward-curving (southeast)Â” propagator is interpreted to have ceased spreading by chron 5d (17.446 Ma) having been replaced by a southward propagator when the Mendoza Rise to the east, stopped spreading and the microplate be came inactive. The discontin uous nature of the Mendoza Rise, along with the interpreted southeasterly curve of the ear lier southward propagator is characteristic of microplate formation (slower spreading rates due to dual spreading) and rotation. The northward propa gation of the Mendoza Rise is interpreted to have cut across the existing eastern ps eudofault of the southward propagator placing portions conjugate with the western pseudofault farthe r east and therefore le aving a complicated reconstruction (Figure 14b). Likewise, the southward propa gator in the northern section is interpreted to have cut across the inner pseudofault of the Mendoza propagator while the Mendoza Rise was active around chron 5b (~15 Ma). The interpretation of the Mendoza Rise as a northward propagator is in general agreement with the interpretation of Liu (1996) who suggested initiation of a northward propagator for the Mendoza Rise around chron 6 (19.6 Ma). The interpreted ti ming here, however, suggests the Mendoza Rise completed its northward propagation by chron 5b (15.095 Ma). The southward propagator and NNE Me ndoza propagation event may have had pseudofaults falling in roughly the same location between them (Figure 14b). Unfortunately, resolutions in both the magnetic and altimetry data do not allow definitive delineation of features that may serve as pseu dofaults for this system. This system of propagators in the north ern section may have resulted in an overlapping spreading center
44 that became a microplate--the Mendoza microplat e. This scenario differs, again, from that of Okal and Cazenave (1985) who suggested that spreading jumped from the Mendoza Rise westward which then became the East Pacific Rise. Volcanic island chain trends suggest a ge neral southward migration of the PacificFarallon plates and the spreading centers from before chron 10y (28.283 Ma) to sometime between chrons 6c (~23.7 Ma) and 5d (17.446 Ma ). The Tuamotu chain appears to have formed in the same manner as the Nazca Ridg e when one or more hotspots were on or near the active spreading center. The Tu amotu volcanic chain a ppears to have ceased development by chron 7y (24.730 Ma). Liu (1996) proposed redire ction of magmatic source material from the Crough hotspot was terminated after the fracture zone on the Pacific plate migrated north over the Crough hotspot. Southward migration appears to have ceased sometime between chrons 6c and 5d (the period marked by breakup of the Farallon plate). Young-end positions of the Tuamotu and Nazca ridges by chron 10y (28.283 Ma) suggest a common melt source beneath the spreading ridge just north of the AustralNazca fracture zone. Interpreted positions after chron 10y (28.283 Ma) and up to chron 5 (~10.5 Ma) suggest either two melt sources or bifurcated channeling from a common source for the Crough and Nazca ridges. By chron 5o (10.949 Ma), the young-end volcanics are interpreted to again converge suggesting th e reemergence of a common melt source. However, the primary trend of the Salas y Gomez chain for chron 5o (10.949 Ma) (Figure 16a) appears to be offs et to that of the Crough chain suggesting either an error in reconstruction or conti nued two-hotspot source for these chains. The latter appears more tenable since the Chile frac ture zone is consistent across the ridge.
45 The interpreted configurations for chrons 4a (8.862 Ma) (Figure 16b) and 3a (6.567 Ma) (Figure 17a), however, more clearly suppor t reemergence of a common melt source. Present-day configurations (F igure 17b) again suggest dualmelt sources although by this time, development of the Easter microplate may be affecting magmatic plumbing to the chains. Geochemical sampling might resolve whether these two chains shared a single melt source. Cessation of the apparent relative sout hward migration of a melt source noted above appears to correlate with establishm ent of the new spreading direction after Farallon breakup. The redevelopment of a single magma source by chron 5o (10.949 Ma) or 4a may reflect a lag in manifestati on of re-plumbing after the Farallon breakup. However, the reappearance of dual-sour ce magma by chron 4a (8.862 Ma) suggests comparatively rapid manifestation of magma tic re-plumbing. Without additional ages and geochemical data along the Pacific chain that may have formed from two hotspots, however, we can not be certain whether or not two or just on e hotspot is responsible for the chains and if they became mixed at time s. Geochemical and ag e data of volcanoes on the Nazca plate suggest a simple binary mixi ng of an enriched mantle source and midocean ridge basalt (Duncan et al., 2003; Ray et al., 2003). The interpretation of the major southward propagators both north and south of the Austral-Nazca fracture zone relies on the interpretatio n of bounding oblique features between 123W and 135W and 81W and 104W as pseudofaults. Delineation of these features is difficult, especially on the Nazca pl ate. Additional difficulties in delineation of anomalies and tectonic features in the vi cinity of the Mendoza Rise further reduce the detail and certainty of our in terpretation in this region.
46 There are numerous features evident in th e predicted bathymetry that are left out of this discussion and may exercise significan t influence on interpreted events. Notably, a north-south-trending feature south of the Easter Seamount Chain, at roughly the same longitude of the Mendoza Rise, may represen t a similar propagation/failed rift and/or microplate event. Delineation of pseudofau lts and anomalies in the areas around this feature as well as the other interpreted ps eudofaults is particularly problematic. Cornaglia (1995) recognized linear structures oblique to known spreading directions and attempted to delineate magnetic isochrons in accordance with these features. Cornaglia (1995) also recognized a mechanism for lithos pheric transfer involving propagation and microplate formation for the Pacific-FarallonNazca relative spreading history. Greater resolution in both the predicted bathymetry (from altimetry or ships) and more magnetic data are needed to improve the interpreta tion. New seafloor and volcano age data for these regions, especially in the Mendoza area, are greatly needed to resolve the details on the significant transfer of lithosphere from the Pacific to the Nazca plate that occurred from approximately 23 Ma to 17.5 Ma.
47Chapter 5 CONCLUSION Nine new and two published finite poles are presented that bracket PacificFarallon to Pacific-Nazca rela tive plate motion. Reconstr uctions are generated using these poles, magnetic anomalies and predicted bathymetry from altimetry. Additional bathymetric and magnetic data are needed to better constrain the history from chron 6c (24 Ma) to 5d (17 Ma) especi ally on the Nazca plate near and south of the Mendoza failed rift. The interpreted relative spreading hi story between the Pacific and Nazca (Farallon) plates is marked by two major south-going propagation events immediately preceding and following the 24 Ma change in spreading direction (Farallon breakup). The first propagation occurred south of the Austral-Nazca fracture zone and initiated sometime before chron 10y (probably around chron 12 (~31 Ma)) and terminated at the Chile fracture zone sometime between chron 6c (24.059 Ma) and 5d (17.446 Ma). This propagator transferred comparatively little li thosphere from the Nazca (Farallon) to the Pacific but may have begun transferring lithosph ere in the opposite direction by chron 6c (24.059 Ma). The second propagator initiat ed north of the Austral-N azca fracture zone near or at the existing spreading center around ch ron 7y (24.73 Ma) and terminated at the present-day FZ2-SOEST fract ure zones by chron 5o (10.949 Ma). This propagator overlapped with the existing rift forming the Mendoza microplate. The Mendoza Rise appears to have developed as a failed dueli ng northward propagator marking the eastern
48 boundary of the Mendoza microplate sometime between chron 6c (24.059 Ma) and 5d (17.446 Ma). This propagation event adds gr eat uncertainty to th e interpreted history during these approximately six million years in which a lack of interpretable data prevent calculation of poles of rotation for the micr oplate of Pacific-Nazc a plate pairs. The southward propagator associated with th e western edge of the Mendoza microplate appears to have been redire cted to a southeast direc tion by chron 5b (15.095 Ma) in response to normal microplate tect onics (e.g., Bird et al., 1998). Reconstruction of relative plate moti on history from chron 10y (28.283 Ma) to 3a (6.567 Ma) supports previous work suggesting stationary hotspots but with apparent magmatic channeling for development of the Tuamotu, Nazca and Crough chains. As noted, additional bathymetric, magnetic and geochemical data are needed, particularly in the Mendoza region, to further c onstrain the details of relative motion and constrain the global plate-motion circuits. Furthermore, to constrain the melt sources responsible for the Tuamotu and Crough volcanic linements a nd their association with the Nazca Ridge and Easter/Salas y Gomez volcanic linements, additional samples are needed from the Tuamotu and Crough chains for comparison of age and geochemical characteristics with samples from conjugate chains on the Nazca pl ate (Naar et al., 2002; Duncan et al., 2002; Harada et al., 2002; Ray et al., 2002).
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51 Hey, R. N., D. F. Naar, M. C. Kleinrock, J. P. Morgan, E. Morales, and J.G. Schilling, Microplate tectonics along a superfast seafloor spreading system near Easter Island, Nature 317 320-325, 1985. Klaus, A. W. Icay, D. F. Naar, and R. N. Hey, SeaMARC II survey of a propagating limb of a large non-transform offset near 29 S along the fastest spread ing East Pacific Rise segment, J. Geophys. Res ., 96 9985-9998, 1991. Larson, R. L., R. C. Searle, M. C. Kleinrock, H. Schouten, R. T. Bird, D. F. Naar, R. I. Rusby, E. E. Hooft, and H. Lasthiotakis, Ro ller-bearing tectonic e volution of the Juan Fernandez microplate, Nature 356 571-576, 1992. Liu, Z., The origin and evolution of the Easter seamount chain, Ph.D. Dissertation, University of South Florida, 1996. Lonsdale, P. Segmentation of the Pacific-Nazca spreading Center, 1 N Â– 20 S, J. Geophys. Res ., 94 12197-12225, 1989. Macdonald, K. C., P. J. Fox, S. Miller, S. Carbotte, M. H. Edwards, M. Eisen, D. J. Fornari, L. Perram, R. Pockalny, D. Scheirer, S. Tighe, C. Weiland, and D. Wilson, The East Pacific Rise and its flanks 8-18N: History of segmentation, propagating and spreading direction based on SeaMARC II and Sea Beam studies, Marine Geophys. Res ., 14 299-344, 1992. Martinez F., R.N. Hey and P.D. Johnson, The East ridge system 28.5 -32 S East Pacific Rise: Implications for overlappi ng spreading center development. Earth, Planet. Sci. Lett ., 151 13-31. 1997. Mayes, C. L., L. A. Lawver, and D. T. Sa ndwell, Tectonic history and new isochron chart of the South Pacific, J. Geophys. Res ., 95 8543-8567, 1990. Naar, D. F., and R. N. Hey, Fast rift propaga tion along the East Pacifi c Rise near Easter Island, J. Geophys. Res 91 3425-3438, 1986. Naar, D. F., and R. N. Hey, Recent Pacific-Easter-Nazca plate motions, in Evolution of Mid Ocean Ridges Geophys. Monogr. Ser., 57 J. M. Sinton, ed., 9-30, AGU, Washington, D.C., 1989. Naar, D. F., and R. N. Hey, Tectonic evolution of the Easter microplate, J. Geophys. Res ., 96 B5 7961-7993, 1991. Naar, D. F., K. T. M. Johnson, P. Wessel, and D. G. Pyle, Preliminary mapping and dredging results along the Nazca Ridge and Easter/Salas y Gomez Chain, Eos Trans AGU 83,4 OS273, 2002.
52 Okal, E. A., and A. Cazenave, A model for th e plate tectonic evolution of the east-central Pacific based on Seasat investigations, Earth Planet Sci. Lett ., 72 99-116, 1985. Ray, J. S., J. J. Mahoney, K. T. M. Johnson, D. G. Pyle, D. F. Naar, P. Wessel, and Y. Harada, Geochemistry of volcanism along the Nazca Ridge and Easter seamount chain, Proceedings of the EGS-AGU-EUG Joint Assembly Nice, France, P0074 48, April, 2003. Rea, D. K., Tectonics of the N azca-Pacific divergent plate boundary, Mem. Geol., Soc. Am. 154 27-62, 1981. Rusby, R. I., Tectonic pattern and evolution of the Easter microplate, based on GLORIA and other geophysical data, Ph.D. Disse rtation, University of Durhan, 1992. Searle, R. C., R. T. Bird, R. I. Rusby, and D. F. Naar, The development of two oceanic microplates: Easter and Juan Fernan dez microplates, East Pacific rise, J. Geol. Society London 150 965-976, 1993. Searle, R. C., J. Francheteau, and B. Cornaglia, New observations on mid-plate volcanism and the tectonic history of the Paci fic plate, Tahiti to Easter microplate, Earth and Planetary Sci. Lett ., 131 395-421, 1995. Searle, R. C., R. I. Rusby, J. Engeln, R. N. He y, J. Zukin, P. M. Hunter, T. P. LeBas, H. J. Hoffman, and R. Livermore, Comprehensive sonar imaging of the Easter microplate, Nature 341 701-705, 1989. Smith, W. H. F. and D. T. Sandwell, Ba thymetric prediction from dense satellite altimetry and sparse shipboard bathymetry, J. Geophys. Res ., 99 21803-21824, 1994. Smith, W. H. F. and D. T. Sandwell, Global seafloor topography from satellite altimetry and ship depth soundings. Science 277 195-196, 1997. Tebbens, S. F., and S. C. Cande, Southeas t Pacific tectonic evolution from early Oligocene to present, J. Geophys. Res ., 102 B6 12061-12084, 1997. Tebbens, S. F., S. C. Cande, L. Kovacs, J. C. Parra, J. L. Labrecque, and H. Vergara, The Chile ridge: A tectonic framework, J. Geophys. Res ., 102 B6 12035-12059, 1997. Zukin, J. and J. Francheteau, A tectonic test of instantaneous kinematics of the Easter microplate, Oceanologica Acta 10 183-198, 1990.
54 Appendix A: Data File Information In all, 250 separate shiptracks of data were downloaded from Lamont-Doherty Earth Observatory file servers. All of thes e were converted to mgd format and 34 were selected for processing. Th ese 34 were further culled to remove portions of the ship tracks that did not apply we ll to the study at hand (i.e ., cruise tracks oriented perpendicular to known direction of seafloor sp reading). Each of the culled ship tracks was divided into segments of not more than 2000 km in length. Experiments showed that mgdpro and magbath (Fortran programs used to prepare the data for magnetic modeling) produced strange results on data lines longer than 2000 km. To facilitate progress, seven ideal ship tracks from the 34 were selected for final processing. These seven were divided into 40 segments of less than 2000 km in length. Segments were chosen to bisect spreading ridges where possible. Segments from the same ship track were also selected such that some overlap occurred to facilitate comparison. The seven ship tracks of focus we re eventually reduced to four totaling 22 segments for modeling. The four final tracks were P6702, CATO4, EEL29, and C1306. The ship track C0905 was added later in an attempt to so rt out the magnetics in the Mendoza microplate area. These data were carefully modeled and anomaly picks were transferred to a map of all 250 ship tracks depicting the magnetic signals obtained for each cruise. This Â“wiggleÂ” map was ge nerated using the Generic Mapping Tools subroutine Â“pswiggleÂ”. A Geographic Information System was used to compare magnetic data and anomaly picks from 2D modeling with the pred icted bathymetry data. Anomaly picks on
55 Appendix A: (Continued) the map were extrapolated based on the magne tic data and constrained by the predicted bathymetry. File names: a2542.cdf c2309.cdf ew9105.cdf jch87.cdf mw87a.cdf sot05.cdf amph1.cdf c2606.cdf ew9106.cdf jch88.cdf mw87b.cdf sot06.cdf amph2.cdf c2608.cdf ew9203.cdf jch89.cdf mw87c.cdf sot07.cdf amph3.cdf c2901.cdf ew9204.cdf jr111.cdf nor03.cdf sot08.cdf ant17.cdf caphb.cdf ew9416.cdf jr112.cdf nor04.cdf sot09.cdf aria1.cdf caphc.cdf ew9602.cdf jr137.cdf ocpbn.cdf tha81.cdf aria2.cdf carr1.cdf ew9709.cdf jr138.cdf opfz1.cdf trip1.cdf ars04.cdf carr2.cdf fd771.cdf jr147.cdf p6702.cdf trip2.cdf ars1a.cdf cato3.cdf fd772.cdf kk005.cdf p7008.cdf tuga01wt.cdf ars1b.cdf cato4.cdf fd773.cdf kk006.cdf p7101.cdf v1705.cdf ars1c.cdf cato5.cdf fd77b.cdf kk007.cdf p7301.cdf v1706.cdf bb11b.cdf cctw2.cdf fd77d.cdf kk008.cdf p7302.cdf v1707.cdf bb31a.cdf cctw3.cdf fdrk1.cdf kk009.cdf p7303.cdf v1814.cdf c0806.cdf cctw4.cdf fdrk3.cdf kk010.cdf p7304.cdf v1815.cdf c0904.cdf cn010.cdf gecsj.cdf kk017.cdf papa2.cdf v1904.cdf c0905.cdf cn011.cdf gecsk.cdf kk018.cdf pasc2.cdf v1905.cdf
56 Appendix A: (Continued) c1004.cdf crcs1.cdf ggl08.cdf kk019.cdf pasc3.cdf v1906.cdf c1005.cdf d1102.cdf ggl09.cdf kk020.cdf pasc4.cdf v2104.cdf c1006.cdf dd01a.cdf ggl16.cdf kk021.cdf piq01.cdf v2105.cdf c1110.cdf dd41b.cdf ggl33.cdf kk022.cdf piq02.cdf v2113.cdf c1111.cdf dolp2.cdf ggl34.cdf kk023.cdf piq03.cdf v2403.cdf c1203.cdf eel03.cdf ggl35.cdf kk025.cdf piq04.cdf v2809.cdf c1212.cdf eel05.cdf ggl54.cdf kk026.cdf piq05.cdf v2810.cdf c1305.cdf eel06.cdf ggl68.cdf kk027.cdf piq06.cdf v3210.cdf c1306.cdf eel08.cdf ggl69.cdf kk028.cdf piq08.cdf vlcn4.cdf c1307.cdf eel09.cdf ggl70.cdf kk029.cdf plds1.cdf vlcn5.cdf c1308.cdf eel10.cdf ggl71.cdf kk041.cdf plds2.cdf vlcn8.cdf c1501.cdf eel17.cdf ggl83.cdf kk042.cdf plds3.cdf we772.cdf c1502.cdf eel18.cdf ggl85.cdf kk046.cdf plds4.cdf yq69f.cdf c1503.cdf eel19.cdf ggl92.cdf kk048.cdf prt01.cdf yq713.cdf c1713.cdf eel20.cdf gh8a1.cdf kk049.cdf prt02.cdf yq716.cdf c1714.cdf eel21.cdf gh8b1.cdf kk053.cdf ram01.cdf yq717.cdf c1802.cdf eel24.cdf hu122.cdf kk054.cdf risp1.cdf yq718.cdf c1803.cdf eel25.cdf hu127.cdf kk055.cdf risp2.cdf yq719.cdf c1804.cdf eel28.cdf hu132.cdf kk056.cdf sb015.cdf yq733.cdf
57 Appendix A: (Continued) c2011.cdf eel29.cdf hu140.cdf msn07.cdf scan8.cdf yq734.cdf c2107.cdf en112.cdf hud01.cdf msn08.cdf scan9.cdf yq735.cdf c2108.cdf enap2.cdf hud02.cdf mw852.cdf scn10.cdf yq736.cdf c2117.cdf erd11.cdf hud03.cdf mw853.cdf sot01.cdf yq737.cdf c2303.cdf ew9102.cdf hud04.cdf mw854.cdf sot02.cdf yq738.cdf c2304.cdf ew9103.cdf igu03.cdf mw8707.cdf sot03.cdf c2305.cdf ew9104.cdf inm15.cdf mw879.cdf sot04.cdf
58 Appendix B: 2D Magnetic Modeling The magnetic modeling information in Appendix B is given in the following order: 1. Parameter file used as input. 2. Rate file listing spreading rates in mm/yr for specified age. The following terms are used in the rate plots: a. Â“iÂ” initial spreading velocity b. Â“vÂ” normal spreading velocity (no asymmetry) c. Â“aÂ” percent of spreading asymmetry (positive values indicate more spreading to west) d. Â“jÂ” spreading center jump (distance, time) e. Â“zÂ” end of file 3. MagBath plot of magnetic data (fuscia), generated model (green), seafloor bathymetry (blue) as measured when ma gnetic data were collected and modeled rate (lower fuscia). Black and white blocks bounded by identical bathymetry curves represent the effective magnetic thickness of oceanic crust. Black blocks denote normal magnetism while white blocks indicates reversed magnetism. Vertical blue lines when present in a plot represent modeled spreading center jumps and resulting pseudofaults. Vertical red lines represent th e spreading axis. Parameters and plots are given for all ma gnetic ship track lines modeled in this study. Not all of the plots were used to in terpolate magnetic anoma lies on the seafloor. Some of the plots contain out put plotting errors manifested as straight lines either horizontally or diagonally acro ss the plot and also as overlapping, repeating rates in the rate (fuscia) graph. Modeled magnetic blocks may be compared to Figure 3.
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