SLIP AND KINEMATIC HISTORY OF THE GARLOCK FAULT FROM ANALYSIS OF THE LAVA MOUNTAINS MIOCENE VOLCANIC-SEDIMENTARY SEQUENCE, SAN BERNARDINO COUNTY, CALIFORNIA

The potential of geothermal resources in areas of strong brittle shear in the presence of Neogene volcanism was investigated by detailed geologic mapping adjacent to the Garlock fault (GF) in the Lava Mountains (LM). Our results show the relationships between the Bedrock Spring Fm. (BSF) and two packages of volcanic rocks. The BSF was deposited unconformably on an older volcanic sequence and is interbedded with ~8 Ma volcanics. The basal BSF seals faults that cut the older volcanics. The older volcanics are undated but are similar to 11-15 Ma Summit Diggings volcanics (SDV), 10 km to the west. All three units are deformed into tight ENE-plunging folds.

The older volcanic rocks and SDV are separated by a fault. Reconstructing basalts of these two areas gives ~11 km of sinistral slip. This fault joins the GF to the east and continues westward as left step-over of the left-lateral GF. The older volcanic units in the LM correlate across the GF with the lower Dove Spring Fm. in the El Paso Mountains (EPM) for slip of ~50 km. The BSF correlates across the GF to the EPM with a similar amount of slip.

Above the BSF is a distinct package of cobble conglomerates containing abundant silicified felsic volcanic, hornblende diorite, altered basalt clasts and a ~7.9 Ma tuff. The clast composition of this overlying unit grades eastward into a set of clasts that resemble the Paleozoic Garlock Series of the EPM. This reflects a dramatic change from the south-sourced granitic and intermediate volcanic detritus of the BSF. The sediment sources for the clasts in these two facies above the BSF closely match the rock types exposed along the southern EPM. Reconstructing these offset sources to the LM yields ~45 km of slip. These correlations imply that the GF changed at ~8 Ma from a topographically low-relief strike-slip fault to a fault with significant topographic relief. Therefore this may record the time at which regional stress vectors changed to transtension loading the GF with more compressive stress.

This interpretation suggests that the GF has had two distinct periods of activity: ~15 km of strike slip between 15-13 Ma followed by 50 km of strike-slip with fault-parallel compression starting at ~8 Ma. This scenario is positive for a geothermal system as rapid faulting creates pathways for fluid movement and uplift leads to increased geothermal gradients.