HR: 1330h
AN: T22A-0497    [PDF]
TI: Episodic Accelerations in Extensional Fault Displacement Rate Determined From Reconstructed Delta Architectures: Loreto Basin, Baja California Sur, Mexico.
AU: * Mortimer, E
EM: estellem@glg.ed.ac.uk
AF: University of Edinburgh, School of Geosciences West Mains Road, Edinburgh, EH9 3JW United Kingdom
AU: Gupta, S
EM: s.gupta@ic.ac.uk
AF: Imperial College London, Department of Earth Science and Engineering, Imperial College, London, SW7 2BP United Kingdom
AU: Cowie, P
EM: patience.cowie@glg.ed.ac.uk
AF: University of Edinburgh, School of Geosciences West Mains Road, Edinburgh, EH9 3JW United Kingdom
AB: The Pliocene Loreto basin is a fault-bounded half graben situated on the western margin of the obliquely extending Gulf of California. The 35 km long, basin-bounding Loreto fault has ~1500m of throw and has accommodated largely east-west bulk extension. Episodic displacement accumulation on the Loreto fault has been proposed by previous authors to be the controlling factor on the development of 16 cycles of coarse-grained delta progradation. They showed that the frequency of progradation cycles was too rapid to be explained by Pliocene eustatic fluctuations. Here we present a high-resolution study of the facies architecture to show that previously unrecorded systematic variations in depositional architecture of individual cycles of progradation can characterise the temporal variations in fault-displacement rate. Our analysis provides reconstructions of the depositional architecture of 11 cycles of delta progradation, of which 7 exhibit a transition from a shoal-water to Gilbert-type delta with increasing foreset height and thickness when traced palaeo-seaward. We show that the geometry, and specifically the nucleation of Gilbert-type deltas and basinward thickening of foresets cannot be generated by eustatic fluctuations, but that they are tectonically controlled. Furthermore, the geometry of a single cycle of delta progradation requires a continued basinward increase in bathymetry that can only be generated by an acceleration in the rate of tectonically-controlled basin subsidence along the basin-bounding Loreto fault. This transition in delta morphology plus the subsequent drowning of the delta tops requires an acceleration in fault displacement rate over a time period of 5-20 kyr. A physical explanation for this acceleration is that fault rupture is controlled by time-dependent damage accumulation, where the damage accumulation rate depends on applied stress. Finally, we hypothesise that the recurring episodes of movement on the Loreto fault are characteristic of strain partitioning between extensional and strike-slip faults in the Gulf of California, i.e., the degree of partitioning is time-varying.
DE: 7221 Paleoseismology
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8107 Continental neotectonics
DE: 8110 Continental tectonics--general (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting