NEOTECTONICS AT THE INTERSECTION BETWEEN THE MANIX AND DOLORES LAKE FAULTS, HARVARD HILL, CENTRAL MOJAVE DESERT, CA

Detailed geologic mapping and structural analysis at Harvard Hill, 30 km east of Barstow, CA, reveal complex interactions between the ENE-striking, sinistral Manix fault and NNW-striking, dextral Dolores Lake fault. A fault strand that cuts earliest Holocene alluvial fan deposits and Pleistocene Lake Manix deposits along the southern edge of the hill is presumably the most active local strand of the Manix fault. We have also identified at least two possibly active strands of the Dolores Lake fault traversing western Harvard Hill.

Our study confirms a previously recognized major structural and lithologic break, marked by a strand of the Manix fault, between northern and southern Harvard Hill. Northern Harvard Hill consists of Miocene Barstow Formation, with deformation presumably associated with both the Dolores Lake and Manix faults. Southern Harvard Hill consists of different Barstow Formation strata and several coeval and younger rock-avalanche breccia deposits. Structures here indicate deformation from one or more Manix fault strands and associated secondary structures. Along the east side of Harvard Hill, secondary N- to NNE-striking structures are present, such as bedrock faults and graben and scarp features in late Pleistocene alluvial fan deposits. Immediately adjacent to the main fault intersection to the southwest, Barstow Formation strata are thrust southwestward over Pleistocene Lake Manix deposits and NW-trending folds occur in the footwall lake deposits. In addition, deformation in this same area has uplifted the Lake Manix beds such that they now lie at least 50 m above their original depositional elevations.

The observed structures and lithologic relationships at Harvard Hill provide critical information on the timing and magnitude of strain that is accommodated by faulting and uplift near the intersection between the Manix and Dolores Lake faults. More generally, structural analyses at Harvard Hill substantiate the previously proposed late Cenozoic NNE shortening and WNW extension for the central Mojave Desert.