Oceanic-ridge subduction vs. slab break off: Plate tectonic evolution along the Baja California Sur continental margin since 15 Ma F. Michaud Ge´osciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France J.Y. Royer CNRS-UBO Domaines Oce´aniques, Place Copernic, 29280 Plouzane´, France J. Bourgois Ge´osciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France J. Dyment Institut de Physique du Globe de Paris, 4 place Jussieu, 75252 Paris, France T. Calmus Instituto de Geologı´a, Universidad Nacional Auto´noma de Me´xico, AP 1039, 83000, Hermosillo, Sonora, Mexico W. Bandy Instituto de Geofisica, Universidad Nacional Auto´noma de Me´xico, Ciudad Universitaria, 04510 Mexico D.F. M. Sosson Ge´osciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France C. Mortera-Gutie´rrez Instituto de Geofisica, Universidad Nacional Auto´noma de Me´xico, Ciudad Universitaria, 04510 Mexico D.F. B. Sichler Ifremer, BP 70, 29280 Plouzane´, France M. Rebolledo-Viera Instituto de Geofisica, Universidad Nacional Auto´noma de Me´xico, Ciudad Universitaria, 04510 Mexico D.F. B. Pontoise Ge´osciences Azur (UPMC-IRD-CNRS), BP48, 06235, Villefranche/Mer, France ABSTRACT The interaction of the Pacific-Farallon spreading centers with the North American convergent margin off Baja California, Mexico, supposedly ceased at 12 Ma, when plate convergence and seafloor spreading stopped. We propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 238N to 278N (Famex cruise of the R/V L’Atalante, April 2002). The data unveil a major clockwise rotation of the Pacific-Farallon spreading direction, starting ca. 14 Ma, that formed a series of short spreading centers that became extinct ca. 8–7 Ma. We suggest that the transcurrent motion between the Pacific and North America along Baja California was accommodated by seafloor spreading and oblique convergence along the trench. This change in spreading direction was followed by a concomitant progressive demise of both Pacific-Farallon seafloor spreading and Farallon–North America subduction that are attributed to the breakoff of the Farallon slab. This also resulted in the opening of a trench-parallel slab window beneath Baja California. Keywords: slab-window, Pacific-Farallon oceanic ridge, trench-spreading center interaction, Mexico. INTRODUCTION For 90 m.y., oceanic spreading ridges have subducted beneath the North American plate. This process explains many of the characteristics of the geodynamical evolution of western North America (Dickinson and Snyder, 1979). Interaction between the PacificFarallon ridge and the North American margin occurred along the Baja California margin from 25 to 12 Ma, at which time seafloor spreading and subduction supposedly stopped (Stock and Hodges, 1989). It is widely accepted that the Farallon plate broke into the Guadalupe and Magdalena plates and that remnants of these plates are still present off Baja California due to the initiation of transform motion between the Pacific and North America plates ca. 12 Ma (inset, Fig. 1). In this general geodynamic framework, several points remain unconstrained. Off Baja California south of 288N, the seafloor fabric is poorly known, the age of the oceanic crust is not well constrained, and the occurrence of abandoned Pacific-Magdalena spreading centers has yet to be described in detail (e.g., Lonsdale, 1991). The isochrons *E-mails:
[email protected]; jroyer@ univ-brest.fr.
younger than the well-identified magnetic anomalies 5B (15 Ma) and 5AD (14.5 Ma) are discontinuous and difficult to identify (Lonsdale, 1991). Moreover, recent sampling of submarine volcanoes indicates Quaternary volcanic eruptions (Lonsdale and Castillo, 2004). The duration and amount of transform motion along the Baja California margin after subduction ceased are still being debated (Dixon et al., 2000; Marsaglia, 2004). Moreover, transform motion along the Baja California margin supposedly lasted until the opening of the Gulf of California at 5–3.6 Ma (Spencer and Normark, 1989), although there is evidence for continuous strike-slip faulting along the Baja California margin since at least 5 Ma (Fletcher and Munguia, 2000), and possibly since 8–7 Ma (Michaud et al., 2004). Volcanism along Baja California prior to 12 Ma is predominantly characterized by a typical calc-alkaline subduction signature (Martı´nBarajas et al., 2000). After 12 Ma the composition of magmatic products drastically changed to mainly magnesian andesite (Rogers et al., 1985), including 8 Ma–Holocene bajaite (Calmus et al., 2003), and 11–9 to 6–5 Ma adakites (Defant and Drumond, 1990).
The close association of adakite and niobiumenriched basalt, which erupted at the same time (11–9 Ma) in the Santa Clara volcanic field, is consistent with melting of altered Pacific mid-oceanic-ridge basalt (Aguillon et al., 2001). Such pressure-temperature conditions for slab melting have been attributed to spreading-ridge subduction (Bourgois and Michaud, 2002; Benoit et al., 2002), and should be incompatible with the presence of abandoned spreading centers off Baja California. Swath bathymetry and magnetic and seismic profiles were collected off Baja California during the Famex cruise (R/V L’Atalante, April 2002; Fig. 1). These data suggest that the identified spreading centers accommodated part of the transcurrent motion between the Pacific and North American plates prior to their extinction at 8–7 Ma. SEAFLOOR SPREADING FABRIC OFF BAJA CALIFORNIA On the western part of the survey area, the seafloor displays a regular abyssal hill fabric trending N1608–1658, parallel to magnetic lineation 5B (Fig. 1B, dashed blue lines). To the north, this fabric abuts a N808E-oriented trough (Shirley fracture zone). East of chron 5B, the seafloor fabric becomes complex and includes a series of ridge segments, conjugate fan-shaped abyssal hills, troughs, volcanic highs, and probable spreading discontinuities with various orientations. In the fan-shaped areas, the abyssal hills trend N208 to N258E, and in some places N-S (Fig. 1B, dashed red lines). Linear, N1208E-striking, 100-km-long, 4000-m-deep depressions divide the study area in several parts. From 278309N to 268N, three elongated volcanic ridges rise to 1500 m depth and trend N108N to N458E. Between 268309N and 258509N, a wide, N458E-oriented volcanic ridge is bounded on each side by N208–258E-trending oceanic fabric (Fig. 2). Near 258309N, some abyssal hills trend N658E (Fig. 1B, green lines) and are perpendicular to elongated, N1508E-oriented, wide troughs. Farther south, antisymmetrical fan-
q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; January 2006; v. 34; no. 1; p. 13–16; doi: 10.1130/G22050.1; 4 figures.
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Figure 1. Inset: Location of A and B. Guadalupe (Gua) and Magdalena (Mag) plates are two remnants of Farallon plate. Extinct Pacific (Pac)-Farallon spreading centers (double lines) after Lonsdale (1991) (NAM—North America Plate ). A: Bathymetric chart based on compilation of swath bathymetry data collected by R/V L’Atalante (Famex cruise; tracks in red), R/V Marion Dufresne, and additional profiles from the National Geophysical Data Center. Magnetic lineations for chrons 5B and 5AD (black lines) are modified from Lonsdale (1991). B: Structural interpretation; limit (thick black dotted line) between initial Pacific-Farallon oceanic fabric (in blue) and late oceanic fabric (in red and green); strike of abyssal hill fabric (dashed lines); and transform faults (continuous line). High elongated volcanoes are interpreted as extinct spreading centers. Abandoned spreading segments (double black dashed) were hypothesized by Lonsdale (1991).
Figure 2. Close-up of Figure 1A in oblique Mercator projection with magnetic anomaly profiles plotted along track (positive pointing to top of figure). Heavy lines are data from Famex cruise. We identify symmetric sequence of magnetic anomalies from chron 5B (black and gray), to 5AD (purple), 5AC (orange), 5AB (red), and 5AA (yellow). Open symbols are tentative correlations (along celestially navigated tracks). Double line marks locations of extinct fossil axes. Height of central elongated volcano and high amplitude of magnetic signal above it indicate that volcanism may have continued while spreading was stalled. Oceanic fabric and magnetic lineations show ~508 clockwise rotation of spreading direction prior to cessation of spreading. 14
shaped abyssal hills define several conjugate propagators and associated pseudo-faults, which accommodate the spreading direction change. It is, however, difficult to locate the abandoned spreading centers. Between 248309N and 248009N, an elongated ridge surrounded by curved abyssal hills strikes N608E, perpendicular to the trend of the margin; it is associated with a narrow N1508Estriking fault, parallel to the margin. We attribute these features to the very last stage of spreading activity, suggesting that seafloor spreading finally ceased when the spreading direction became parallel to the margin. We interpret the series of elongated ridges at 278209N, 268509N, 268109N, 258309N, 258009N, and 248159N, whose orientation progressively shifts from N108E to N508E from north to south, as extinct spreading centers. These spreading centers are discontinuously connected by transform faults whose orientation progressively becomes parallel to the Baja California margin. This system of clockwise-rotating spreading centers, propagators, and transform and pseudo-faults accommodates a 908 change of spreading direction. The oceanic fabric off Baja California is thus more complex than the staircase succession of N–S-trending fossil spreading centers GEOLOGY, January 2006
posed by Lonsdale (1991). This model requires the spreading rates to increase from 43 to 51 mm/yr prior to extinction. We favor an alternate model (Fig. 3B), where the spreading rates gradually decrease after chron 5AC (14 Ma) from 72 to 6 mm/yr (half rate) ca. 8–7 Ma (chron 4). The modeling of the magnetic anomalies younger than chron 5AA is not unequivocal. Nevertheless, the amount of crust (;60 km) between the symmetric chron 5AA lineations, and the size of the volcano, indicate that seafloor spreading continued for at least several millions years after 12 Ma, and possibly until 7–8 Ma. The demise of this spreading center followed an ;508 clockwise rotation of the magnetic lineations and the abyssal hill fabric, which started just after chron 5AD (14.5 Ma). We find evidence for a late seafloorspreading direction parallel to the margin and to the Tosco-Abreojos fault zone. Extinction of the southernmost spreading centers may thus be concomitant with the 8–7 Ma onset of strike-slip motion along the Tosco-Abreojos fault zone and the 8–7 Ma cessation of compressional deformation at the toe of the continental margin (Fig. 3 in Michaud et al., 2004). In any case, the drastic change in spreading direction that caused the PacificFarallon spreading center to break into several segments with a spreading direction progressively parallel to the margin (i.e., parallel to the Pacific–North America motion) suggests that the relative motion between these two plates has been accommodated both by these spreading centers and by convergence along the trench until 8–7 Ma. Consequently, from 15 Ma until 8–7 Ma, subduction along the trench became more and more oblique (Fig. 4). If compression along the trench continued after spreading ceased, as observed in similar geodynamic situations (Barker and Austin, 1998), it is possible that some of the PacificMagdalena ridge segments ceased spreading slightly earlier than 8–7 Ma.
Figure 3. Magnetic models for extinction of spreading center of Figure 2 (dotted profile in Fig. 2). Both models are identical from chron 5B to 5AA (12.9 Ma). Model A assumes extinction of spreading at 11.9 Ma (end of chron 5A), as postulated by Lonsdale (1991), whereas our favored model B assumes gradual slowing of spreading rates and cessation of spreading at 7.5 Ma (chron 4) (models based on the Cande and Kent [1995] magnetic reversal time scale and computed with routines from Mendel et al., 2005).
hypothesized by Lonsdale (1991; inset, Fig. 1). TIMING OF THE DEMISE OF THE PACIFIC-MAGDALENA SPREADING CENTERS As shown by the swath bathymetry data, the demise of the Pacific-Magdalena spreading centers is more progressive and gradual than the cessation of spreading between the Pacific and Guadalupe plates, north of the Shirley fracture zone, where seafloor spreading gradually slowed and stopped after chron 5AB (Lonsdale, 1991). The new magnetic profiles provide new constraints on the timing of the seafloor-spreading extinction. In the area of Figure 2, symmetric sequences of magnetic anomalies 5AD (14.5 Ma) to 5AA (13 Ma) are clearly identifiable on either side of a high volcano (Figs. 2 and 3), which, from its strike, height, and elongated shape, we identify as the extinct spreading center. In Figure 3A, we assume that spreading ceased at 12 Ma, as proGEOLOGY, January 2006
Figure 4. Schematic evolutionary model of plate boundaries off Baja California from 15 to 7 Ma, assuming a fixed North American plate (NAM; full circle). Propagation of slab window is shown in light gray. 1—At 15 Ma, subduction between Farallon plate (FAR) and NAM is subperpendicular to trench. Pacific plate (PAC) moves at 52 mm/yr (Atwater and Stock, 1998); Pacific-Farallon ridge spreads at 156 mm/yr (Fig. 3). 2—As breakoff of the young Farallon slab propagates southward, Farallon plate breaks into Guadalupe (GUA) and Magdalena (MAG) plates. GUA slows rapidly relative to PAC. Farther south, diminishing slab pull induces increasing coupling between MAG and NAM, causing clockwise change in spreading direction between MAG and PAC. Subduction
of MAG relative to NAM slows and becomes oblique. 3—At 10 Ma, north of Shirley fracture zone (FZ) subduction has stopped and dextral shear occurs. Extension is predicted at junction between Shirley FZ and trench. South of Shirley FZ, spreading ridge segments reorganize perpendicular to PACNAM direction of motion; very oblique subduction still produces compression along trench. 4—At 8 Ma, MAG-NAM subduction and MAG-PAC seafloor spreading progressively stop from north to south while ToscoAbreojos propagates to south, all boundaries accommodating PAC-NAM motion. 5— After 8–7 Ma, MAG-PAC seafloor spreading and MAG-NAM subduction cease concomitantly while strike-slip motion along ToscoAbreojos fully accommodates PAC-NAM motion. 15
DISCUSSION Since ridge subduction apparently did not occur south of 308N (Yeats et al., 1981; Lonsdale, 1991), the development of a slab window beneath Baja California, accounting for the Miocene to recent magmatism, has yet to be explained. If subduction of the PacificFarallon spreading ridge occurred beneath Baja California, the observed abandoned spreading segments would be remnants of westward ridge jumps. Such spreading reorganization is invoked at the Chile Triple Junction (Bourgois et al., 1999). In this hypothesis the ridge would have first subducted, after which a new ridge segment formed west of the trench, breaking through the preexisting oceanic crust, thus creating a discontinuity of oceanic crust age west of the present abandoned ridges. While this hypothesis is possible, the bathymetric data do not show evidence for such discontinuity, typically evidenced by deep bathymetric troughs (Mammerickx and Sandwell, 1986). To explain both the geometry of the abandoned ridge segments and development of a slab window beneath Baja California, we propose that the young Farallon slab broke off before the Pacific-Farallon spreading center reached the trench (Fig. 4), possibly due to the southward migration (relative to a fixed North America) of the slab window beneath California. As the oceanic ridge approached the trench from north to south, increasing coupling between the subducting and overriding plates caused the young upper part of the subducted plate (the Magdalena plate) to detach from the old, cold, sinking Farallon slab. Buoyancy of young hot material began to compensate for slab pull, and PacificMagdalena spreading slowed. Since the slab pull was essentially removed, the coupling between the Magdalena and North American plates increased. Consequently, subduction of the Madgdalena plate became increasingly oblique and progressively ceased; the long Pacific-Farallon spreading center slowed and broke off into a series of short spreading segments as they started to accommodate Pacific– North America motion instead of PacificFarallon motion, hence the drastic change in spreading direction observed after chron 5B. On land, increasing heating and melting of the young subducted Magdalena plate produced the peculiar magmatism observed along Baja California. North of the Shirley fracture zone, although the spreading ridge is further away from the margin, the slab reorganization to the south also reduced the slab pull and led to the extinction of the Guadalupe spreading center. South of the Shirley fracture zone, PacificMagdalena seafloor spreading stopped when the ridge segments became perpendicular to the trench (no more slab pull), the remnants of the Magdalena plate transferred onto the Pacific plate, and strike-slip motion initiated between the North American (i.e., Baja Cali16
fornia) and Pacific plates. This scenario better accounts for our observations, although the cause of the slab detachment is not yet understood. A similar slab-detachment control was proposed to explain a mafic pulse and mantle heterogeneity in central Mexico (Ferrari, 2004). CONCLUSION Off Baja California, from 238N to 278N, the oceanic fabric displays a continuous clockwise change in spreading direction. This rotation started after chrons 5B–5AD (15–14.5 Ma) and caused the reorganization of a long Pacific-Farallon spreading center into a series of short spreading segments that became extinct. We suggest that prior to the cessation of subduction at 8–7 Ma the motion between the Pacific and North America was accommodated along these spreading centers and along the trench. We propose that interaction between the spreading ridge and the margin induced a slab detachment opening a trench-parallel slab gap or slab window. Once this occurred, the slab pull was essentially removed, both subduction beneath North America and seafloor spreading relative to the Pacific plate ceased, and slab melting developed. ACKNOWLEDGMENTS We thank the crew of R/V L’Atalante. This work was supported by Institut National des Sciences de l’Univers du Centre National de la Recherche Scientifique. We thank J. Stock, C. DeMets, J. Fletcher, and W. Normark for their constructive reviews.
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