Joint Meeting Pacific Section, AAPG & Cordilleran Section GSA April 29–May 1, 2005, San José, California
Neotectonics of the Rinconada and Reliz Fault
Zones, Salinas Valley, California
Lewis I. Rosenberg1 and Joseph C. Clark2
1 Environmental Division, San Luis Obispo County Planning Department, County Government Center, San Luis Obispo, CA 93408,
[email protected]
2 P.O. Box 159, Glen Campbell, PA 15742
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.
Posted with permission of The Geological Society of America; abstract also online (http://gsa.confex.com/gsa/2005CD/finalprogram/abstract_85242.htm). © Copyright 2005 The Geological Society of America (GSA).