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Outline South American Plate Relationships Rebecca Williams Kelly McGuire, Sarah Craig z Introduction z South American Plate z Coupling Theory: Relationship to African Plate z Relationship with North American Plate z Caribbean Plate z North American Analogy? South American Plate z General westward movement from Mid Atlantic Ridge (MAR) since break up of Gondwana. z Has variations in lithosphere thicknesses. Coupled with African Plate z The South Atlantic Ridge has had constant spreading rate for 80 Myrs. z African Plate slowed abruptly 30 Myrs ago. z Caused westward acceleration of SA Plate. z Seen by increase in cordilleran activity along western edge-Andean Deformation. z Plate velocity ~2-2.8 cm/yr 1 Affect on Andean Deformation? Relationship of SA and NA Plates z Assuming SA Plates trenchward velocity, since 30Ma, is taken up entirely by E-W shortening, the predicted amount of cordilleran shortening from plate motion is 120 km. z Estimate comparable to previous estimates for the Central Andes. z Present day shortening rate of 1cm/yr. z Diverged from Late Triassic to E Campanian (84 Myrs ago). z Time when Gulf of Mexico and ProtoCaribbean Seaway formed. z Since then only minor relative motion. z Only meet (present day) east of Caribbean. Is the North American Plate coupled? Evidence from Mud Diapirism z Westward acceleration of NA Plate expected from SA-AF plate analysis. z If the NA Plate is not coupled, an E-W differential motion between N and S America should be evident. z Motion of NA Plate relative to Iceland Hot Spot is consistent with westward acceleration. z Mud Diapirs ahead of Barbados accretionary wedge. z Triggered by change in relative plate motion from Compression to Dextral Strikeslip. Evidence from Mud Diapirism Northern SA Border 2 Northern SA Border z NA Plate, discussed previously. z Caribbean Plate: Atlantic Seafloor part of Northern SA is subducting beneath Caribbean Plate. z In NE Venezuela, deformation dominated by right lateral, strike slip along E-W striking faults. z N-S Compression z Caribean Plate moving E/SE @ 1-1.5 cm/yr relative to SA. z Subduction occurring @ 1.5cm/yr Relation to Caribbean Plate Terranes z Northern SA border is most complex border of Caribbean Plate z Undergone diachronous, right-oblique collision with overriding Lesser Antilles arc system. z Began 35-40Ma. z Drove Caribbean Terranes from west to east to point North of Trinidad-present day collision. z 3 Caribbean Terranes between arc system and SA continent: 1. Tobago Terrane 2. Paria-Trinidad Terrane: exhumed schists. 3. Araya-Margarita Terrane. z P-T Terrane thrust above Mesozoic shelf strata and locally, Miocene Strata: ¾ Piggy-Backed Foreland Basin ¾ Obducted Terranes Piggy Backed Foreland Basin 3 Subduction? Spatial Variation in Subduction z Irregular Seismicity z Dipping zone below continent but attached to SA lithosphere. z Contradicts normal thinking-would expect seismicity north of exposed plate edge. z Continent has detached from slab and moved north above slab. z Wedged between overriding terranes and underriding slab. Tectonic Wedging Model North American Analogy? z Mechanism for thickening crust z Rationale for depressing mantle. z Supported by P Wave Velocities z Supported by gravity model. z Oblique convergence common in Cordillera e.g. Offshore British Colombia. z Thrust belts common along western margin. z No known analogy for slab detachment and continent wedging. References z Bosch & Rodriguez, 1992. North Venezuela collisional crustal block: The boundary between the Caribbean and South American Plates. J of South American Earth Sciences 6 (3) 133-143. z Mantovani et al, 2001. Tidal gravity anomalies as a tool to measure rheological properties of the continental lithosphere: application to the South American Plate. J of South American Earth Sciences 14 (2001) 1-14. z Meijer & Wortel, 1992. The dynamics of motion of the South American Plate. Journal of Geophysical research 97 (8) 1191511931. z Pindell et al, 1988. A plate-kinematic framework for models of Caribbean evolution. Tectonophysics 155 (1-4) 121-138. z Russo & Speed, 1992. Oblique collision and tectonic wedging of the South American continent and Carribean Terranes. Geology 20. 447-450. z Silver et al, 1998. Coupling of South American and African Plate Motion and Plate Deformation. Science 279 (5347) 60-63. z Sumner & Westbrook, 2001. Mud diapirism in front of the Barbados accretionary wedge: the influence of fracture zones and North America-South America plate motions. Marine and Petroleum Geology 18. 591-613. 4