We examine the structural complexities of a 28-km long part of the Clark fault of the San Jacinto fault zone in southern California in order to better document its lateral extent and the style of deformation of its southeast end. Changes in structural style are observed as the Clark fault’s damage zone widens from ~ 1-2 km in crystalline rocks of the Peninsular Ranges southeastward to ~ 18 km in the sedimentary rocks of the San Felipe-Borrego subbasin of the Salton Trough. The Clark fault extends into the San Felipe-Borrego subbasin as the Arroyo Salada segment for ~ 11-12 km to a newly defined northeast-trending structural boundary. This structural boundary, referred to herein as the Pumpkin Crossing block, is a ~ 3-km wide and ~ 8-km long fault zone dominated by northeast-striking sinistral-normal strike-slip faults. Southeast of the Pumpkin Crossing block the newly defined San Felipe Hills segment extends the Clark fault another ~12-13 km southeast to its intersection with the Extra fault zone. The Clark fault may have nearly 14.5-18 km of right separation represented in the surface deformation of the Arroyo Salada and San Felipe Hills segments, but the total amount of strain is masked by the wide diffuse fault zone with its complex deformation patterns and geometries. The lateral change observed in microseismicity patterns across the Arroyo Salada and San Felipe Hills segment boundary supports our structural interpretations about the existence, location, and structure of this boundary. Vertical patterns in the microseismicity suggest that the Clark fault zone narrows at depth, dips steeply northeast in the subsurface, and must interact with at least one weak decollement layer(s) beneath and/or with the sedimentary basin. Structural deformation within the late Miocene to Holocene silty- and clay-rich sedimentary basin of the Salton Trough includes features characteristic of strike-slip faults, such as stepovers, conjugate faults, folds, flower structures, and fault bends, as well as many unique structures that include pooch structures, ramp-flat geometries of strike-slip faults, complex en echelon fault zones with localized shear distributed across a high frequency network of faults, and heterogeneous kinematic indicators within individual fault zones.

We examine the structural complexities of a 28-km long part of the Clark fault of the San Jacinto fault zone in southern California in order to better document its lateral extent and the style of deformation of its southeast end. Changes in structural style are observed as the Clark fault’s damage zone widens from ~ 1-2 km in crystalline rocks of the Peninsular Ranges southeastward to ~ 18 km in the sedimentary rocks of the San Felipe-Borrego subbasin of the Salton Trough. The Clark fault extends into the San Felipe-Borrego subbasin as the Arroyo Salada segment for ~ 11-12 km to a newly defined northeast-trending structural boundary. This structural boundary, referred to herein as the Pumpkin Crossing block, is a ~ 3-km wide and ~ 8-km long fault zone dominated by northeast-striking sinistral-normal strike-slip faults. Southeast of the Pumpkin Crossing block the newly defined San Felipe Hills segment extends the Clark fault another ~12-13 km southeast to its intersection with the Extra fault zone. The Clark fault may have nearly 14.5-18 km of right separation represented in the surface deformation of the Arroyo Salada and San Felipe Hills segments, but the total amount of strain is masked by the wide diffuse fault zone with its complex deformation patterns and geometries. The lateral change observed in microseismicity patterns across the Arroyo Salada and San Felipe Hills segment boundary supports our structural interpretations about the existence, location, and structure of this boundary. Vertical patterns in the microseismicity suggest that the Clark fault zone narrows at depth, dips steeply northeast in the subsurface, and must interact with at least one weak decollement layer(s) beneath and/or with the sedimentary basin. Structural deformation within the late Miocene to Holocene silty- and clay-rich sedimentary basin of the Salton Trough includes features characteristic of strike-slip faults, such as stepovers, conjugate faults, folds, flower structures, and fault bends, as well as many unique structures that include pooch structures, ramp-flat geometries of strike-slip faults, complex en echelon fault zones with localized shear distributed across a high frequency network of faults, and heterogeneous kinematic indicators within individual fault zones.

The Clark fault is one of the primary dextral faults in the San Jacinto fault zone system, southern California. Previous mapping of the Clark fault at its southern termination in the San Felipe Hills reveals it as a broad right lateral shear zone that ends north of the crossing, northeast-striking, left-lateral Extra fault. We investigate the relationship between the dextral Clark fault and the sinistral Extra fault to determine whether the Clark fault continues to the southeast. We present new structural, geophysical and geomorphic data that show that the Extra fault is a ~7 km wide, coordinated fault array comprised of four to six left-lateral fault zones. Active strands of the Clark fault zone persists through the Extra fault array to the Superstition Hills fault in the subsurface and rotate overlying sinistral faults in a clockwise sense. New detailed structural mapping between the San Felipe and Superstition Hills confirms that there is no continuous trace of the Clark fault zone at the surface but the fault zone has uplifted an elongate region ~950 km. sq. of latest Miocene to Pleistocene basin-fill in the field area and far outside of it. Detailed maps and cross sections of relocated microearthquakes show two earthquake swarms, one in 2007 and another in 2008 that project toward the San Felipe Hills, Tarantula Wash and Powerline strands of the dextral Clark fault zone in the San Felipe Hills, or possibly toward the parts of the Coyote Creek fault zone. We interpret two earthquake swarms as activating the San Jacinto fault zone beneath the Extra fault array. These data coupled with deformation patterns in published InSAR data sets suggest the presence of possible dextral faults at seismogenic depths that are not evident on the surface. We present field, geophysical and structural data that demonstrate dominantly left-lateral motion across the Extra fault array with complex motion on secondary strands in damage zones. Slickenlines measured within three fault zones in the Extra fault array reveal primarily strike-slip motion on the principal fault strands. Doubly-plunging anticlines between right-stepping en echelon strands of the Extra fault zone are consistent with contraction between steps of left-lateral faults and are inconsistent with steps in dominantly normal faults. Of the 21 published focal mechanisms for earthquakes in and near the field area, all record strike-slip and only two have a significant component of extension. Although the San Sebastian Marsh area is dominated by northeast-striking leftlateral faults at the surface, the Clark fault is evident at depth beneath the field area, in rotated faults, in microseismic alignments, and deformation in the Sebastian uplift. Based on these data the Clark fault zone appears to be continuous at depth to the Superstition Hills fault, as Fialko (2006) hypothesized with more limited data sets.

Damage zones adjacent to or associated with faults are important to the geologic community because of their implications to hazards and their ability to preserve evidence for, and show history of, slip, fluid flow, and deformation associated with large strike-slip faults. We examine two fault zones in southern California where fault zone damage is expressed. We revisit the drilled crystalline core from the Cajon Pass California drill hole, 4 km northeast of the San Andreas fault (SAF), and 1 km north of the Cleghorn fault, to perform a systematic structural analysis of deformation and alteration associated with strike-slip faulting at the site. The core preserved 19 fault zones, 11 of which were not previously identified. The most significant fault is a fully intact steep-dipping fault zone at 3,402 m depth with potassium feldspar and epidote alteration. This fault correlates well with the nearby left-lateral Cleghorn fault. The extent of deformation varies within the core, and is controlled by the size of the fault zones intersected by the core. The extent of deformation varies and is controlled by the size of the faults the core intersected. We also examined the nature of right separation across the Clark fault damage zone along the Santa Rosa segment using a marker assemblage of biotite, hornblende-bearing tonalite - marble - bearing metasedimentary rocks - migmatite located in Coyote Mountain and the southeast Santa Rosa Mountains. Separation measured from this study is 16.8 km + 3.67 km / -6.03 km. Our measurement uses the updated location of the Clark fault in Clark Lake Valley and matches a distinctive lithologic contact across the fault instead of matching the diffuse western boundary of the Eastern Peninsular mylonite zone as previously used. We calculate the errors associated with projecting the contacts across Quaternary cover to the trace of the Clark fault, and consider a range of projections. Additional strain may have been accommodated in folds and small faults within the damage zone of the San Jacinto fault zone. Two large map-scale folds deform the marker assemblage near the San Jacinto fault zone and we tested whether Cretaceous ductile deformation or brittle late Quaternary right slip produced the folds.