High pore fluid pressure due to rapid sedimentation in the Salton Trough informed by the Fish Creek Vallecito Basin, California

Elaine K. Young, & Michael E. Oskin

Submitted September 10, 2023, SCEC Contribution #13283, 2023 SCEC Annual Meeting Poster #096

The Salton Trough is a seismically active region of southern California and northern Baja California characterized by several creeping strike-slip faults. High heat flow and overpressure conditions (pore fluid pressure in excess of hydrostatic or excess hydraulic head) due to rapid sedimentation and occasional low-permeability units likely affect fault behavior within the Salton Trough (e.g. triggering of seismic swarms or shallow fault creep), but direct regional-scale observations of the geologic conditions influencing overpressure development at seismogenic depth are lacking. Rapid and recent uplift of the Fish Creek-Vallecito Basin (FCVB) and Vallecito fault, a subbasin and basin-bounding fault within the western Salton Trough, provides an exposure of the upper 4-2 km of crust, analogous to present crustal conditions in the Salton Trough at depth. The FCVB thus provides an opportunity to understand regional overpressure conditions at depth and to explore the effects of rapid sedimentation and low permeability units on the mechanical conditions of faulting, particularly fault creep, in the southern San Andreas fault system.

The FCVB contains a ~6 km sedimentary section deposited across two half grabens in the hanging walls of the West Salton Detachment fault and the Proto-Vallecito fault between 8 -1 Ma that was uplifted since ~1 Ma. The compaction of strata in the FCVB is inconsistent with ~6 km burial, and recent work shows that basin subsidence was <4km. Maximum burial temperatures in the basin were ~<80-90 °C suggesting a geothermal gradient of ~20 °C/km. Though the sediments preserved within the FCVB are analogous to the Salton Trough, the geothermal gradient is much higher in the latter. We present results from a one-dimensional model using a finite difference scheme to predict the evolution of excess hydraulic head and over pressure conditions during deposition of the FCVB and the Salton Trough and compare these largely deltaic stratigraphic sequences to a reference column of only sand. We find that overpressure develops most significantly and there is less compaction with the deposition of impermeable units and that rapid and continued sedimentation is necessary for maintaining the excess pore fluid pressure. Once sedimentation is slowed or stops, the overpressure decays or disappears.

Key Words
Southern California, Fault Creep,

Young, E. K., & Oskin, M. E. (2023, 09). High pore fluid pressure due to rapid sedimentation in the Salton Trough informed by the Fish Creek Vallecito Basin, California . Poster Presentation at 2023 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)