AN ASSESSMENT OF SYNEXTENSIONAL DIKES EMPLACED DURING EARLY SLIP ON THE MOHAVE WASH LOW-ANGLE NORMAL FAULT, CHEMEHUEVI MOUNTAINS, SE CALIFORNIA

Dikes adjacent to and in the damage zone to the Mohave Wash low-angle normal fault (LANF), exposed in the Colorado River extensional corridor (California, USA), were documented to understand the relationship between dike emplacement/magmatism and early slip on the Miocene Chemehuevi LANF system. Dike composition, geometry, orientation, distribution, and structural relations with the Mohave Wash fault (MWF) were characterized. Field work was concentrated on two areas with contrasting paleodepths of ~5 km (SW Chemehuevi Mts) and >10 km (NE Chemehuevi Mts). Collectively, dikes comprising the swarm define a subalkaline trend of dominantly magnesian, metaluminous to peraluminous sheets. Dikes make up ~2% of the footwall in the SW (updip) area, and are dominantly mafic to intermediate compositions (52–63% wt. % SiO₂) with lesser dacite–rhyolite dikes (68–73 wt. % SiO₂). Cross-cutting relations indicate mafic–intermediate dikes postdate more felsic compositions. These dikes exhibit sharp contacts, commonly with chilled margins, vary in thickness from 0.5 to >12 m, and intrude all structural levels of the MWF. Individual dikes cannot be traced across the MWF and appear to be truncated or rarely to have intruded into a network of brittle fractures defining the damage zone. Dike emplacement appears to have been episodic. In contrast, dikes in the NE (downdip) area comprise up to ~10% of the footwall, are dominantly felsic (typically >67 wt. % SiO₂), notably thinner (typically <2 m), and have well developed deformation fabrics (L>S). In the SW, dikes occur with two general orientations striking nominally E-W and N-S with moderate to steep dips. Orientations are not consistent with emplacement via mode 1 cracks formed during regional extension oriented at ~050°, and instead may have been controlled by some preexisting fracture system. Based on structural analysis, hanging wall dikes have been rotated up to 24° counter clockwise (axis at 285°) relative to footwall dikes in this area. In contrast, dikes exposed downdip in the NE exhibit contacts striking dominantly E-W to SE-NW-SE, gentle dips <20° on average, and uniform lineations trending ~240°. These observations indicate that extension at greater paleodepths was partitioned, at least in part, into the synextensional dikes and resulted in rotation and thinning.