GEOLOGIC AND GEOMORPHIC CONSTRAINTS ON THE QUATERNARY SEPARATION RATE OF THE MIDLAND FAULT, SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA

The Midland fault is an approximately north to north-northwest-striking, west-dipping buried or “blind” fault zone underlying the central Sacramento-San Joaquin Delta. Structure contours of a late (?) Miocene unconformity beneath the Delta indicates uplift of the hanging wall of the Southern Midland fault between the towns of Brentwood and Rio Vista, terminating abruptly south of Lindsey Slough in Solano County. Structural relief on the Miocene unconformity is about 213 m +/- 61 m. These values suggest long-term average reverse slip rate on the Southern Midland fault ranging between about 0.07 mm/yr and 0.1 mm/yr, for an assumed range of fault dip from 45° to 75°. Comparison of historic and pre-historic drainage patterns in the Delta with structural relief on the Miocene unconformity provides evidence of possible Holocene uplift above the southern Midland fault. The complex network of streams and sloughs drained broad areas of swampy deposits and appears to have been extremely sensitive to topography and tidal levels. Small, sinuous sloughs in the Delta primarily developed east of the Midland fault converged abruptly westward across the fault into the major channels of the San Joaquin and Sacramento Rivers. Similarly, the thickest concentrations of Holocene peat are located directly east of the Midland fault. These relations suggest that west-side-up topographic relief was present in the hanging wall of the Midland fault during late Pleistocene to Holocene time and was sufficient to affect drainage patterns and peat thickness. Inferred structural relief on late Pleistocene deposits across the fault estimated from borehole data suggests a possible late Quaternary vertical separation rate between 0.2 to 0.9 mm/yr. Variations in the thickness of Holocene peat deposits from subsurface borings suggests a minimum of 2 to 4 m of structural relief on the base of peat, with an implied a middle to late Holocene vertical separation rate of 0.3 to 0.6 mm/yr across the Midland fault. Based on the very low modern relief of the central Delta, we favor the lower values in the ranges of late Quaternary separation rates.