Structural evolution of the northern Los Angeles basin, California, based on growth strata

Abstract

Subsurface structure contour maps and cross sections show that the northern Los Angeles basin is underlain by a south facing monocline complicated by secondary faults and folds. The monocline and associated structures form a structural high, the northern Los Angeles shelf, which marks the northern margin of the Los Angeles central trough. Analysis of “growth strata,” sediments deposited during structural growth, shows that during the Miocene, the predominant structural style was extension. Growth strata show that at approximately the beginning of the Pliocene (5 Ma), contraction reactivated the Miocene normal faults as reverse faults and formed the monocline, its secondary structures, and the Los Angeles central trough. Lensing‐out of earliest Pliocene (Delmontian Stage) growth sandstones onto the flanks of the monocline and secondary structures (the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines) indicates that these contractional structures all began to form at this time. All of the secondary structures became largely inactive prior to the deposition of upper Pico member of early Pleistocene age (1.2 Ma). However, thick accumulations of growth strata of the upper Pico member of the Fernando Formation within the Los Angeles central trough attest to continued monoclinal folding after the secondary structures became largely inactive. The growth strata record both vertical and horizontal components of structural growth of the monocline so that the dip of the blind fault zone causing the monocline (the Los Angeles fault) can be calculated. In the East Beverly Hills area, the dip of the Los Angeles fault is 61° and at Las Cienegas, the dip is 62°. These are maximum values based on the assumption that the growth strata bed lengths record all shortening. The Pliocene‐Pleistocene average fault slip rates for the Los Angeles fault are 1.1 – 1.3 mm/yr in the East Beverly Hills and 1.3–1.5 mm/yr at Las Cienegas. The resulting Pliocene‐Pleistocene horizontal convergence rates are 0.5 – 0.6 mm/yr and 0.6 – 0.7 mm/yr respectively. The Pliocene‐Pleistocene growth strata show increased dip with age, indicating that the monocline grew by progressive limb rotation rather than by kink‐band migration. Therefore, fault‐bend and fault‐propagation fold models based on kink‐band migration are inadmissible solutions to explain the growth of the monocline. We suggest a basement‐involved shear zone to explain the geometry of the northern Los Angeles shelf.