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. / Graduation date: 1994
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/35965 |
| Date | 08 March 1994 |
| Creators | Schneider, Craig L. |
| Contributors | Yeats, Robert S. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
Page generated in 0.0019 seconds