Large Holocene ruptures on the Cañada David detachment, Baja California, Mexico; implications for the seismogenesis of low-angle normal faults

Abstract

We present the results of detailed mapping and paleoseismic investigations along a section of the complex fault scarp array associated with the Cañada David detachment, a low-angle oblique normal fault that controls the southwestern flank of the Sierra El Mayor, Baja California, Mexico. We used high-resolution unmanned aerial vehicle imagery to map portions of a sequence of alluvial fans that comprise a bajada that flanks the southwestern range front. Their ages are distinguished based on a regional soil chronosequence. Three paleoseismic trenches were excavated across active surface faults that root into the Cañada David detachment at depth. Fine-grained stratigraphic units in the trenches were dated by optically stimulated luminescence techniques. The three trenches in this study show strong evidence for four large (Mw≥7) Holocene earthquakes at or just prior to about 1.7, 4.3–6.1, 7.7–7.6 and 8.4–10.7 ka, as well as weak to moderate evidence for two additional late Pleistocene earthquakes at or just prior to 12.7–14.1 ka and 15.2 ka, indicating an average recurrence of such events of ∼3 ka. This earthquake recurrence interval is slightly longer than that for the Laguna Salada fault to the north that flanks the Sierra Cucapah. The regular recurrence of major surface ruptures strongly suggests that severely-misoriented low-angle normal faults like the Cañada David detachment have a mechanical behavior that is very similar to other more optimally oriented faults suggesting that earthquakes on such faults should be common and may be difficult to recognize from seismologic methods due rolling-hinge ramps and/or multifault ruptures.