Basin thickness variations at the Junction of the Eastern California Shear Zone and the San Bernardino Mountains, California: How thick could the Pliocene sections be?

We estimate the thickness of Neogene basin fill along the junction of the Eastern
California Shear Zone and the North Frontal thrust system of the San Bernardino Mountains
using gravity data with geologic and well log constraints. The geometry of the basin fill is of
interest for groundwater assessment and location of potential faults, as well as providing an upper
bound on the thickness of any potential, buried Pliocene sediments. Nearly one thousand new
gravity measurements were collected along the North Frontal thrust system from Hesperia to
Johnson Valley. Three-dimensional inverse modeling of the new and existing gravity data shows
that Neogene deposits are segmented into basins along the range front of the San Bernardino
Mountains. The Helendale fault, a dextral fault in the Eastern California Shear Zone, separates
shallower basement (approximately 300 m depth) beneath Lucerne Valley east of the fault from
deeper basement (approximately 550 m or more) west of the fault. The thickest part of the basin
fill is generally located near the San Bernardino Mountains and the basin shallows northward.
The amount of throw on the North Frontal thrust appears to decrease eastward, as the gravity
gradient associated with the fault diminishes in amplitude. The thickness of basin fill away from
the North Frontal system and east of the Helendale fault is less than 100 to 200 m, except for local
pockets generally developed along strike-slip faults of the Eastern California Shear Zone and local
east-west oriented depressions associated with folding of the basin fill.