PALEOGEOLOGIC PATTERNS, CRUSTAL BLOCKS, AND EVOLUTION OF THE SAN ANDREAS FAULT SYSTEM IN SOUTHERN CALIFORNIA

The Pacific and North America (NA) tectonic plates (large crustal blocks) made contact at ≈ 30 ± 2 Ma, resulting in dextral motion between the two that continues today. In California (Ca), the San Andreas Fault (SAF) is now the principal structure between the two plates. Understandably, much has been made of the implications of sea-floor tectonics for the evolution of the SAF system, but further clarity regarding where, when, how, and why the boundary between these two plates shifted on-land into NA is gained from geologic studies in Ca. Particularly relevant is the differential development of Ca’s major physiographic-structural provinces (medium-size crustal blocks) within the NA plate. These include: Salinian block (SB); Great Valley (GB); Sierra Nevada (SN); Basin and Range (BR); Mojave Desert (MD); Transverse Ranges (TR); Peninsular Ranges (PR); and Salton Trough (ST).

In a narrow sense, the SAF is a continuously mapped feature that extends from Pt Arena in N Ca to the S end of the Salton Sea in S Ca. Along its length, the SAF bounds several of Ca’s physiographic-structural provinces—including the GV, SN, and MD east of the fault and the SB, PR, and ST to the west—and transects the TR province. The SAF enters S Ca through Tejon Pass (TP) at a nexus where the GV, SN, MD, TR, and SB provinces converge.

In the broadest sense, the SAF system comprises not only the SAF itself, but many other strike-slip faults that intersect the SAF or otherwise share in plate-margin strain partitioning. Sets of dextral faults that transect the SB, PR, and TR west of the SAF and sinistral faults that transect the TR east of the SAF cut these provincial crustal blocks into smaller crustal blocks and their offsets contribute to that of the SAF.

The distribution in space and time of strain on faults of the SAF system in S Ca is controversial. Total cumulative displacement measured along the SAF itself in S Ca is well established as ≈ 300 km along the reach between the Gabilan Range to the N and its intersection with the San Gabriel Fault to the S, ≈ 160 km between TP and San Gorgonio Pass (SGP), and ≈ 180 km between SGP and the central part of the Chocolate Mts. These segmental differences are related to the strain accommodated by other faults of the SAF system and constrained by paleogeologic patterns that developed before, during, and after movement on various faults in the system.