AGU Fall 2009 | Poster T31A-1783 | 16 December | 8:00-12:20 Poster Hall, Moscone South

Constraints on early Gulf of California rifting from seismic images across the eastern margin of Guaymas Basin

Abstract

Multi-channel seismic (MCS) images and seismic velocity models from three transects crossing the eastern margin of Guaymas Basin provide new insights into early rifting and the transition from rifting to spreading in the Gulf of California. Two distinct kinematic models have been proposed for early rifting (~13-6 Ma) in the Gulf, one in which Pacific/North America relative motion is partitioned between right-lateral dextral shear west of the Baja Peninsula and east-west extension in the proto-Gulf prior to lithospheric rupture and the onset of spreading, and a second model in which "oblique" extension oriented in the direction of Pacific/North America relative motion (NW-SE) has persisted since the onset of rifting at ~13 Ma. The three MCS transects all reveal a smooth, bright, undulating reflector that we interpret as the top of a ~2-km-thick, diapiric salt body overlying igneous basement. This body underlies 0.5 to 2 km of sediment with deformation indicative of diapiric motion of the salt body. Seismic tomography images the interpreted salt body as a shallow, high-velocity (~4.0-4.5 km/s) region above igneous basement and just inboard of the continent ocean transition, consistent with an interpretation of a thick salt unit formed within an early rift basin on extended continental crust. The western (seaward) edge of the interpreted salt body is oriented north-south, suggesting that this early rift basin also trended north-south. This observation favors a kinematic model in which some portion of Pacific/North America relative motion is partitioned onto east/west dipping normal faults within the proto-Gulf prior to ~6 Ma. Comparison of crustal-scale tomograms along these same transects provides additional insight into structural orientations and magmatic emplacement at the time of lithospheric rupture.

Poster