Abstract

Miocene synextensional strata flanking the Chemehuevi and southern Sacramento Mountains in southeastern California and western Arizona indicate deposition during seismically active low-angle normal faulting. These data complement existing structural, isotopic, and fission-track studies, which show that the regionally developed Chemehuevi-Sacramento detachment-fault system was initiated and allowed movement of the hanging wall over more than 18 km within the seismogenic regime at moderate to low angles of dip (≤ 30°). Individual faults within the detachment-fault system are corrugated parallel to the east-northeast transport direction, resulting in broad mullion structures (50–550 m amplitude and 1.5–10 km wavelength). Slip occurred along the rooted fault system between ca. 23 and 12 Ma; the average slip rate was ∼7–8 mm/yr during peak tectonic extension from ca. 19 to 15 Ma.

Tertiary strata preserved in tilted hanging-wall blocks are between 2 and 3 km thick. Mafic and intermediate volcanic rocks (ca. 23 to 18.5 Ma) at the base of the section constitute 40% to 50% of most basin fills. Alluvial-fan conglomerate, sedimentary breccia, and megabreccia (somewhat >15.5 to somewhat <13.9 Ma) dominate the unconformably overlying sedimentary succession. Decreasing dip angles in progressively younger strata through this volcanic-sedimentary succession, and angular unconformities between units, form corroborative evidence with geochronologic data and show that these strata accumulated during extension. Sedimentary facies and clast types indicate proximal deposition in small basins distributed along east-northeast–trending regions that parallel and overlie synforms in the underlying corrugated Chemehuevi-Sacramento detachment fault. The composition, thickness, and distribution of these volcanic and sedimentary strata support their accumulation near a gently dipping normal fault. Clast types show an inverted stratigraphy recording erosion to progressively deeper structural levels in the source region. Emplacements of thick (<750 m) megabreccias (rock-avalanche deposits), derived from both the hanging wall and the footwall, were likely triggered by earthquakes. Tilted and displaced conglomerate and megabreccia (younger than ca. 15 Ma) contain footwall clasts and indicate breaching of the detachment fault, erosion of the footwall, and late movement on the gently dipping Chemehuevi-Sacramento detachment fault at the Earth's surface.

These data show that the gentle dip and corrugated shape of the Chemehuevi-Sacramento detachment-fault system, when it was seismically active and allowed movement at and near the Earth's surface, controlled the location and fill of basins during progressive extension. Stratigraphic, structural, and thermal data therefore corroborate and challenge the assumption that low-angle normal faults are aseismic.

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