Working Kinematic Model for Geologic Slip Rates in Southern California

This is a work in progress (sort of), last updated 11/17/01. Feedback is welcome!


Working model for geologic slip rates on active faults in southern California. Numbers in white boxes are approximate long term rates for different faults (in mm/yr). Numbered yellow stations are locations of data control (see below). This compilation includes information available from geodetic studies, and attempts to match geologic slip rates with modern rates. There are still some problems with it. This model assumes that N-S shortening in the southern part of the eastern California shear zone (ECSZ) absorbs some of the total right lateral shear, thereby decreasing the amount and rate of slip that gets transferred down to the southern San Andreas fault, south of point #3. SBM = San Bernadino Mts; SJM = San Jacinto Mts.

Data control points on above map are from the following sources:

1. Cajon Pass (Weldon and Sieh, 1985): Offset river terraces dated with C-14 give 24.5 +/- 3.5 mm/yr slip rate on the San Andreas fault over the past 14,400 yrs.

2. San Gorgonio Pass (Yule et al., 2001): Trenching and mapping of offset fan surfaces yield oblique contraction on faults and related folds at ~10-15 mm/yr over the past 2,000-5,000 yrs. Strain vector is N45W, parallel to driving strike-slip faults. Because some slip may have been missed (complex fault zone), Yule considers this to be a minimum. Due to possible transfer of slip between two faults that were added in the calculation, there could be some overestimation included in the net shortening rate. In fact, the total slip rate is still quite poorly known through this area, and the actual measured rates are slow.

3. Biskra Palms (Keller et al., 1982; van der Woerd et al., 2001): Offset fan surface records ~ 14-16 mm/yr slip on San Andreas fault over the past 30.1±2.4 kyrs. This estimate differs from van der Woerd et al.'s, and is based on my re-measurement of fault offset using figures in Keller et al. (82) and air photos. The fault offset determined by Keller et al. was 700 meters. My re-measurement yields 485 ± 40 meters.

4. Anza (Rockwell et al., 1990): Slip rate on offset fans is bracketed widely between about 6 and 22 mm/yr over the past 50 ka. Ages estimated are from soil chronology. The authors favored a rate of 12-13 mm/yr. The minimum slip rate of 9 mm/yr determined by Sharp (1981) is discarded because a major unconformity invalidates his age estimate for the offset fan deposits.

5. Coyote Creek (Dorsey, in press): Offset of Pleistocene fan deposits (Qg) yields ~10±3 mm/yr on Coyote Creek fault since ~600±100 ka.

6. Clark Valley (Sharp, 1967; this study): Total offset on Clark fault is ~15 km. Slip was initiated at ~1.5 ± 0.2 Ma, giving a long term average rate of ~10 mm/yr. Age of initiation is inferred from well known age of the Ocotillo Formation in the Borrego Badlands (base = 1.2 Ma: Remeika and Beske-Diehl, 1996).

7. Laguna Salada (Mueller and Rockwell, 1995): Offset fan surfaces dated with soil chronology give slip rate (strike-slip component) of 2-3 mm/yr over the past 50 ka.

Combining 5 and 6 gives the following result: Recent study of Pleistocene stratigraphy and an offset alluvial surface shows that about 6±1 km of offset has accumulated on the Coyote Creek fault in lower Coyote Creek since 600 ± 100 ka, recording a long-term slip rate of about 10 ±3 mm/yr [Dorsey, 2002]. The nearby Clark fault reveals fresh fault scarps and offset modern drainages and alluvial fans, providing evidence for active faulting that must be added to the slip budget in this area. Morphometric analysis and estimated soil age for an offset Pleistocene fan yields a preliminary slip rate of about 4-7 mm/yr on the southeast Clark fault [Ryter, 2002]. These data suggest that the total late Pleistocene slip rate on the Clark and Coyote Creek faults can be bracketed between about 11 and 20 mm/yr at Clark Lake, similar to the likely range of slip rates at Anza. Slip rates on the Clark and Coyote Creek faults may have changed through time, but the details of these variatinos are presently not known.

Remarks:

The slip rates proposed above are considered here to be the consequence of early Pleistocene oblique collision along the San Andreas fault zone. I hypothesize that the San Jacinto fault zone was initiated at about 1.5 Ma when high topography in the northern San Jacinto Mountains began to impinge directly on (collide with) south-verging thrusts in the southern San Bernadino Mts at San Gorgonio Pass. Prior to this time, the southern San Andreas carried 35 mm/yr of slip down to the Salton Sea. When the San Jacinto Mountains block drove into the thrust belt it created a strong mechanical resistance to convergence between the San Jacinto Mts and the San Bernadino Mts (Morton and Matti's "structural knot"). This idea has been explored by others (Morton and Matti, 1993; Matti and Morton, 1993; Du and Aydin, 1996; Spotila and Sieh, 2000) but without including the role of topography and crustal buoyancy in the San Jacinto Mts, which may be important. Rapid thrusting, uplift and gravel deposition in the northern SBM occurred at about the same time (Meisling and Weldon, 1989) in response to this event. Oblique collision apparently caused slowing of SAF slip through San Gorgonio Pass and partial transfer of slip to the SJFZ on the west and the eastern California shear zone on the east.

An interesting aspect of this problem is that slip on the eastern California shear zone (ECSZ) probably feeds into the southernmost part of the San Andreas fault, but it is not clear where ECSZ slip merges with that of the southern SAF. N-S shortening in the southern Mojave block may be absorbing some of the regional right lateral shear (e.g. Bartley et al., 1990), which could cause a decrease in the amount and rate of strike slip offset that is transferred from the ECSZ to the southernmost San Andreas fault. This is characteristic of the uncertainties that currently prevent a full understading of Pleistocene and Holocene slip rates and related crustal processes active in this part of the transpressional plate boundary zone.

The main point of this exercise is to propose an alternative model for fault kinematics in southern California that (in my opinion) is at least as viable as the existing conventional wisdom. Although previous models have minimized the role of the SJFZ in the regional slip budget, new data seem to support the model proposed here. Existing discrepancies indicate that more data are needed to resolve competing hypotheses. There are many unknowns that currently prevent a full understanding of this complex problem.

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