Abstract

The Stockton fault zone (SFZ) and “arch”, a major tranverse feature oriented normal to the regional structural grain, segments the Great Valley into the Sacramento basin to the north and San Joaquin basin to the south. Several fields produce from fault-related traps, including the prolific Union Island and Lathrop Gas Fields.

The SFZ appears to have had a complex slip history, characterized by multiple periods of deformation beginning in the late Mesozoic. A detailed structural evaluation of the SFZ, integrating well-log, micropaleo, seismic-reflection, and outcrop data has been undertaken to evaluate the nature of Great Valley forearc basin deformation and the role of inherited structures on later modification of the remnant forearc basin.

Preliminary results suggest a two-phase slip history: initial normal (down-to-the-south) slip followed by reverse slip. The first phase began in the Late Cretaceous, prior to ~85 mya (Santonian or “Gzone” time) and ended in the early Maastrichtean, at about the time of Sawtooth Shale deposition (~70 mya). Outcrop relationships indicate that the second reverse phase occurred prior to deposition of the Valley Springs Formation in the early Miocene (20–22 mya). Erosionally thinned Cretaceous strata on the south side of the fault are as much as 5000 ft. thicker than to the north. Present normal separation of the Sierran crystalline basement and reverse separation of Cretaceous horizons suggests that initial normal slip exceeded 5000 ft. Thus, it appears that throughout most of the Late Cretaceous, the Stockton “arch” area represented a subsiding fault-bounded subbasin.

Research is presently in-progress to evaluate: 1) the amount of strike-slip on the SFZ; 2) Cretaceous and Paleogene depositional systems for evidence of syndepositional deformation; 3) the distribution of other transverse structures along the eastern Great Valley margin with similar slip histories; and 4) analogous modern convergent plate margins to develop a tectonic model for the SFZ.