Graduate Thesis Or Dissertation

 

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3r0750462

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  • Detailed subsurface structure contour maps and cross sections have shown the northern Los Angeles basin to be underlain by a south facing monocline that is complicated by secondary faults and folds. The monocline forms a structural shelf that marks the northern boundary of the Los Angeles central trough. The monocline and associated structures are called the Northern Los Angeles shelf. Isopach maps show that during the Miocene, the predominant structural style was extension. Thick accumulations of volcanic and volcaniclastic rocks, controlled by normal faults, had a very different depositional pattern than during the Pliocene. At approximately the beginning of the Pliocene extension changed to compression resulting in the reactivation of the Miocene normal faults in a reverse sense and the beginning of the formation of the monocline and secondary structures. Thick growth sequences were deposited to the south of the growing monocline toward the present day Los Angeles central trough. Fault-bend and fault-propagation fold models are inadmissible solutions to explain the growth of the monocline. A basement-involved shear model may explain some of the details of the secondary structures. Analysis of the Pliocene growth strata shows that the monocline and secondary structures, the South Salt Lake, the East Beverly Hills, and the Las Cienegas anticlines, all began to form near the beginning of the Pliocene. All of the secondary structures became inactive prior to the Upper Pico during the Late Pliocene. Thick accumulations of Upper Pico growth strata attest to continued monoclinal folding after the secondary structures became inactive. The growth strata record both the structural growth and the shortening associated with growth and therefore allow the dip of the monocline causing fault or shear zone (the Monocline fault) to be calculated. In the East Beverly Hills area, the growth strata yield a dip of 61°. At Las Cienegas the dip of the Monocline fault is 62°. These dips are maximum values based on the assumption the growth strata record all shortening. The fault slip rates for the Monocline fault are similar in both areas, 1.1-1.2 mm/yr in the East Beverly Hills and 1.3-1.5 mm/yr. in Las Cienegas. The resulting horizontal convergence rates are also similar, .5-.6 mm/yr and .6-.7 mm/yr respectively. The Quaternary marine gravels have been deformed into a broad east-west trending fold, the Wilshire arch. Elastic and non-elastic methods of modeling the blind fault (Wilshire fault), over which the deformation occurred, yield much greater shortening rates than for the Pliocene. The non-elastic method involves modeling the arch as a fault-bend fold. This model predicts a 15° north-dipping thrust with a slip rate of 1.5-1.9 mm/yr and a horizontal shortening rate of 1.4-1.8 mm/yr. The elastic method involves matching the observed deformation to that produced on the free surface by slip on a fault in an elastic half-space. The elastic dislocation model predicts a right-lateral reverse slip solution with an oblique-slip rate of 2.6-3.3 mm/yr. This solution yields a horizontal shortening rate of 1.4-1.8 mm/yr. These higher shortening rates suggest that there was a marked change in tectonic style at the end of the Pliocene from high-angle faulting and tectonic subsidence to shallow faulting and uplift.
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