STRUCTURAL EVOLUTION OF THE NORTHERN LOS-ANGELES BASIN, CALIFORNIA, BASED ON GROWTH STRATA

Subsurface structure contour maps and cross sections show that the nor thern Los Angeles basin is underlain by a south facing monocline compl icated by secondary faults and folds. The monocline and associated str uctures 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, sh ows that during the Miocene, the predominant structural style was exte nsion. Growth strata show that at approximately the beginning of the P liocene (5 Ma), contraction reactivated the Miocene normal faults as r everse faults and formed the monocline, its secondary structures, and the Los Angeles central trough. Lensing-out of earliest Pliocene (Delm ontian Stage) growth sandstones onto the flanks of the monocline and s econdary structures (the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines) indicates that these contractional struct ures all began to form at this time. All of the secondary structures b ecame largely inactive prior to the deposition of upper Pico member of early Pleistocene age (1.2 Ma). However, thick accumulations of growt h strata of the upper Pico member of the Fernando Formation within the Los Angeles central trough attest to continued monoclinal folding aft er the secondary structures became largely inactive. The growth strata record both vertical and horizontal components of structural growth o f the monocline so that the dip of the blind fault zone causing the mo nocline (the Los Angeles fault) can be calculated. In the East Beverly Hills area, the dip of the Los Angeles fault is 61 degrees and at Las Cienegas, the dip is 62 degrees. These are maximum values based on th e 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 conver gence 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, indica ting that the monocline grew by progressive limb rotation rather than by kink-band migration. Therefore, fault-bend and fault-propagation fo ld models based on kink-band migation are inadmissible solutions to ex plain the growth of the monocline. We suggest a basement-involved shea r zone to explain the geometry of the northern Los Angeles shelf.