HR: 0800h
AN: S31A-10    [Abstracts]
TI: Transition From Proto-Gulf Extension to Transtension, Coastal Sonora, Mexico
AU: * Bennett, S E
EM: bennett@unc.edu
AF: Department of Geological Sciences University of North Carolina at Chapel Hill, Campus Box #3315 Mitchell Hall, Chapel Hill, NC 27599-3315, United States
AU: Oskin, M
EM: oskin@unc.edu
AF: Department of Geological Sciences University of North Carolina at Chapel Hill, Campus Box #3315 Mitchell Hall, Chapel Hill, NC 27599-3315, United States
AB: Transtensional deformation during the proto-Gulf time period (12-6Ma) may have operated to focus lithospheric rupture into the Gulf of California. Currently, it is unknown whether this oblique rupture was preceded by either: 1) partitioned deformation of dextral strike-slip on the Tosco-Abreojos fault zone (TAFZ) west of Baja California and E-W extension in the Gulf Extensional Province, or 2) a zone of distributed shear along en echelon, dextral, continental, strike-slip faults, similar to the modern plate boundary. Once the onset of transtension is recognized in the Gulf, we can infer whether the timing and the style of dextral deformation played an integral role in lithospheric necking. We have identified two well-exposed sedimentary basins in coastal Sonora that record both proto-Gulf and modern Gulf history within syn-tectonically emplaced volcanic and sedimentary rocks. These basins contain two regionally extensive pyroclastic flows that serve as marker units: the ~12.6Ma Tuff of San Felipe (Tsf) and the 6.3±0.2Ma Tuff of Mesa Cuadrada (Tmc). These markers bracket basin fill deposited pre-extension (pre-12Ma), during the proto-Gulf period (12-6Ma), and during the modern Gulf period (post-6Ma). Average dips of beds shallow from 74° in pre-extension rocks (n=20), to 56° in Tsf (n=48), to 34° in modern Gulf rocks (n=43). Modern Gulf conglomerates unconformably overlie Tmc, which in turn lies unconformably on Tsf, suggesting continuous exposure and erosion of older proto-Gulf rocks during modern Gulf time. Fault kinematics (n=22) in pre-extension rocks, which integrate all subsequent Gulf deformation, display ENE-directed (azimuth ~261°) extension with a near-vertical σ₁ principal stress, after 25°±5° correction for clockwise vertical axis rotation. Fault kinematics (n=8) in modern Gulf conglomerates are dominantly strike-slip and display NW-directed transtension (azimuth ~314°), with a σ₁ principal stress that plunges 40° to the NE. The azimuth of this transtension could be more westerly after correction for clockwise vertical axis rotation. Paleomagnetic analysis of Tmc and Tsf, in progress, will quantify the timing of vertical axis rotation coeval with basin development.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8157 Plate motions: past (3040)
SC: Seismology [S]
MN: 2007 Joint Assembly