Geologic framework of the El Casco 7.5’ quadrangle: southwestern gateway to the San Gorgonio Pass knot in the San Andreas Fault zone

Jonathan C. Matti

Published August 15, 2016, SCEC Contribution #6588, 2016 SCEC Annual Meeting Poster #123

The El Casco 7.5' quadrangle is located at the SW portal of San Gorgonio Pass (SGP), where Quaternary contractional deformation in SGP gives way to transtensional deformation associated with the right-lateral San Jacinto Fault. Important stratigraphic and structural elements include (1) sedimentary units that formed during the last 7 million years or so and record tectonic events within the San Andreas Fault (SAF) system, (2) the westernmost extent of the contractional SGP Fault zone, (3) multiple strands of the San Jacinto Fault zone, and (4) the San Timoteo Anticline, a NW-plunging, SW-vergent fold that deforms the Neogene succession.
The upper Miocene to middle Pleistocene sedimentary sequence is divided into three major units: (1) The Mt. Eden formation, (2) the overlying San Timoteo formation, and (3) sedimentary deposits of Live Oak Canyon (formerly interpreted as the upper member of the San Timoteo formation). The sedimentary sequence has the following significant attributes:

(1) Upper Miocene (~7 to 5 Ma) sediment in the Mt. Eden formation was sourced from local Peninsular Ranges-type rocks, and was dispersed radially into adjacent alluvial environments.

(2) Plio-Pleistocene (~5 to ~1.3 Ma) sediment in the San Timoteo formation was sourced from San Gabriel Mountains-type rocks NW of the El Casco quad and dispersed SE on a braidplain that lapped onto Peninsular Ranges landscapes.

(3) At ~1.3 Ma the evolving San Timoteo Anticline disrupted the San Timoteo depositional regime and sedimentary deposits of Live Oak Canyon began to accumulate in a synformal basin NE of the anticline and its ridge-like landscape; sediments of the Live Oak Canyon sequence buttressed unconformably against the anticlinal landform. Live Oak Canyon beds contain vertebrate fossils of the Shutt Ranch local fauna (~900 ka) and span the Brunhes-Matuyama magnetic reversal (780 ka).

These three Neogene depositional regimes correspond to sequential tectonic phases of the SAF system: (1) the Mt. Eden cycle (pre-5 Ma) corresponds with the Banning/San Gabriel Fault; (2) the San Timoteo cycle (5 Ma to ~1.3 Ma) corresponds with the San Andreas Fault; (3) the Live Oak Canyon cycle corresponds with a tectonic event that disrupted the San Timoteo braidplain, initiated the San Timoteo Anticline, and led to the synformal depositional basin NE of the anticline. This outcome most likely can be attributed to initiation of the San Jacinto Fault as dextral slip stepped left from the SAF. Morton and Matti (1993) suggest a 1.5 Ma age for this event; Matti and Morton (1993) favor a 1.2 Ma age. Either scenario suggests that the San Jacinto Fault initiated no earlier than 1.5 to 1.2 Ma, which yields a long-term average slip rate between 16.7 mm/yr and 20.8 mm/yr (based on total fault displacement of ~25 km). Kendrick and others (2002) proposed a similar long-term average slip rate (>20 mm/yr) based on dated surfaces in the San Timoteo Badlands.

Key Words
San Jacinto Fault, San Timoteo Badlands, San Gorgonio Pass

Matti, J. C. (2016, 08). Geologic framework of the El Casco 7.5’ quadrangle: southwestern gateway to the San Gorgonio Pass knot in the San Andreas Fault zone. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)