Sedimentation patterns support seismogenic low-angle normal faulting, southeastern California and western Arizona

Miocene synextensional strata flanking the Chemehuevi and southern Sacramen to Mountains in southeastern California and western Arizona indicate deposi tion during seismically active low-angle normal faulting. These data comple ment existing structural, isotopic, and fission-track studies, which show t hat the regionally developed Chemehuevi-Sacramento detachment-fault system was initiated and allowed movement of the hanging wad over more than 18 km within the seismogenic regime at moderate to low angles of dip (less than o r equal to 30 degrees), Individual faults within the detachment-fault syste m are corrugated parallel to the east-northeast transport direction, result ing in broad mullion structures (50-550 m amplitude and 1.5-1.0 km waveleng th). Slip occurred along the rooted fault system between ca. 23 and 12 Ma; the average slip rate was similar to 7-8 mm/yr during peak tectonic extensi on 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-fa n conglomerate, sedimentary breccia, and megabreccia (somewhat >15.5 to som ewhat <13.9 Ma) dominate the unconformably overlying sedimentary succession . Decreasing dip angles in progressively younger strata through this volcan ic-sedimentary succession, and angular unconformities between units, farm c orroborative evidence with geochronologic data and show that these strata a ccumulated during extension. Sedimentary facies and clast types indicate pr oximal deposition in small basins distributed along east-northeast-trending regions that parallel and overlie synforms in the underlying corrugated Ch emehuevi-Sacramento detachment fault. The composition, thickness, and distr ibution of these volcanic and sedimentary strata support their an cumulatio n near a gently dipping normal fault. Clast types show an inverted stratigr aphy recording erosion to progressively deeper structural levels in the sou rce region. Emplacements of thick (<750 m) megabreccias (rock-avalanche dep osits), derived from both the hanging wad and the footwall, were likely tri ggered by earthquakes. Tilted and displaced conglomerate and megabreccia (y ounger than ca. 15 Ma) contain footwall clasts and indicate breaching of th e detachment fault, erosion of the footwall, and late movement on the gentl y 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 allo wed movement at and near the Earth's surface, controlled the location and f ill of basins during progressive extension. Stratigraphic, structural, and thermal data therefore corroborate and challenge the assumption that low-an gle normal faults are aseismic.