GSA Connects 2022 meeting in Denver, Colorado

Paper No. 102-15
Presentation Time: 9:00 AM-1:00 PM


BYKERK-KAUFFMAN, Ann, Earth and Environmental Sciences, California State Univ, Chico, 400 W. 1st St, Chico, CA 95929-0205

Detailed geologic mapping of the Coyote Mts of southern California constrains the geometry and net-slip of the active dextral Elsinore fault, which bounds the Coyote Mts on the SW and is responsible for the uplift that created the range. The Coyote Mts segment of the fault is a constraining bend, striking on average 292°, which is a 20° CCW deviation from the general 312° strike of the Elsinore fault. Structure contour maps of the fault surface reveal that the Coyote Mts segment of the Elsinore fault consists of 11 smaller segments that vary in strike from 265° to 301°. The segments that strike 265°–285° consistently dip 37°–47° N, underneath the Coyote Mts. The segments that strike 292°–303° have steeper dips.

Stratigraphic throw on the Coyote Mts segment of the Elsinore fault reaches a maximum of ~760 m, with the NE side (hanging wall) lifted up. The magnitude of strike-slip offset on the Coyote Mts segment of the fault is constrained to between 0.6 km and 5.2 km by the following evidence:

  • Near the NW tip of the range, the contact between the Pliocene Olla Formation and the Pliocene-Pleistocene Hueso Formation displays 0.6 km of dextral separation across the fault. This is a minimum estimate of the magnitude of right-lateral strike slip because uplift of the NE fault block tends to cause left-lateral separation of these WNW-dipping beds.
  • Across the central part of the range, the facies change between the Pliocene Arroyo Diablo Formation and the Pliocene Olla Formation displays a dextral separation between 2.5 km and 6.2 km. At least 1 km of this separation is likely accommodated by subsidiary faults within the Coyote Mts.
  • Just west of the Coyote Mts, the 95 Ma La Posta granite pluton crops out over 1400 km2 in northern Mexico and southern California. On the SW side of the Elsinore fault, the sharp N-trending eastern margin of the main mass of the La Posta pluton is truncated by the Elsinore fault. On the NE side of this dextral fault, the offset northern extension of the pluton margin should be located SE of that truncation. Yet, even though the northwesternmost exposure of crystalline basement in the Coyote Mts is just 5.2 km SE of the truncation, the La Posta pluton does not crop out in the Coyote Mts. Thus, the northern extension of the pluton margin must be buried under Neogene sedimentary rocks less than 5.2 km SE of where it is truncated by the Elsinore fault.