NEOTECTONICS OF THE RINCONADA AND RELIZ FAULT ZONES, SALINAS VALLEY, CALIFORNIA

The Rinconada fault is a major structural element of the Salinas Valley, extending 230 km from the Big Pine fault on the south to King City on the north. The Reliz fault, which appears as a right stepover from the Rinconada fault, trends northwestward along the northeastern base of the Sierra de Salinas of the Santa Lucia Range and beyond for 60 km to near Spreckels, where it is largely concealed. Recent aeromagnetic data suggest that the Reliz fault continues northwestward another 25 km into Monterey Bay, where it aligns with a high-definition magnetic boundary.

Evidence of Pleistocene movement along the Rinconada fault zone includes right-lateral offset of stream channels, apparent truncation of probable upper Pleistocene alluvial fan deposits, and 5 m of vertical offset of a 300ka to 400ka alluvial fan. In exploratory trenches excavated along topographic breaks in slope, the fault offsets stream terrace deposits and colluvium, suggesting possible Holocene displacement.

Most of the offset on the Reliz fault appears to have been reverse dip-slip, with the Sierra de Salinas uplifted to the southwest by as much as 3,000 m. Late Pleistocene age alluvial fans cross the trend of the Reliz fault at the base of the Sierra de Salinas without apparent disruption. Near Spreckels, a trench exposed a fault that cuts a late Pleistocene age duripan, but does not offset late Pleistocene colluvium.

Because displacement on the Rinconada fault zone is largely strike-slip and that on the Reliz fault is primarily reverse dip-slip, the continuity of these two faults is uncertain, at least during Pleistocene time. The total documented right-slip on the Rinconada fault zone is 38 km, whereas the maximum possible right-slip on the Reliz fault zone probably does not exceed 23 km and may be less. Some, if not all, of the strike-slip motion along the Rinconada trend may have been partitioned northwestward to the Tularcitos fault of upper Carmel Valley. The Tularcitos fault is Holocene active and may have at least 16 km and possibly as much as 40 km of right-lateral displacement.

Thus, we believe that the Reliz fault is a separate zone because of the difference in type of displacement and the change in overall strike, with the likelihood that slip was partitioned at different rates between the Rinconada fault zone and the Reliz fault zone.