Introduction:
Structural analysis of active folds in East Los Angeles was completed in collaboration with other Group C researchers in 1997 with the aim of defining: 1) kinematic mechanisms which act to build the Boyle Heights and City Terrace anticlines, 2) the geometry, sense, and recency of slip on blind thrusts which drive uplift of these folds, 3) segment boundaries which may limit the extent of blind thrust earthquakes, in particular near the Downtown area along the current Los Angeles River channel. Work completed in 1997 has resolved parts of these questions through construction of three cross sections which extend north-south through East LA.
Results:
Well data for the sections was kindly provided by Bob Yeats, of Oregon State University. Nearly all the useful well data in East Los Angeles is derived from deviated boreholes. We used directional surveys to project each wellbore onto a horizontal plane, which are shown on Figure 1. Deviated well traces were then projected horizontally onto three cross sections using two methods. The first method projected all well data in an easterly or westerly direction onto the nearest cross section. Results of this suggest that both the Boyle Heights and City Terrace anticlines are complex, noncylindrical structures. This was indicated where dipmeter data from adjacent wellbores (in an E-W direction) were projected onto the same section with little agreement or consistent structural geometry. The second projection method transferred borehole dipmeter data and stratigraphic information horizontally onto the sections in the same direction as the local strike of the Coyote Pass Escarpment and Boyle Heights anticline (i.e. adjacent to the wellbore; Figures 2a, 2b, 2c). This method yielded better results than our earlier efforts and resulted in the sections presented in this report (Figures 2a, 2b, 2c).
Stratigraphy:
We correlated about 25 stratigraphic contacts across the region defined by sand and shale beds in Miocene and Pliocene strata deposited as turbidites. By first identifying key strata that extended across the entire region (usually thick shale beds), we were able to pick other lithostratigraphic points based on sand/shale percentage. Comparison of our sand/shale picks with biostratigraphic data in well reports suggests the lithostratigraphic boundaries are broadly correlative with time lines (e.g. "Base Repetto", etc...) across the area we studied. In general Miocene and Pliocene strata thicken basinward to the south, with some noteable exceptions, including the base of the section between the biostratigraphic markers "Top Mohnian" and "Top Delmontian" which displayed significant (100-150%) thickening towards the north over short lateral distances (e.g. < 1-2 km, based on available well spacing).
Although these pre-growth strata are useful in determining fold geometry and repeated section across blind thrusts at depth they do not constrain the rates and magnitude of recent shortening. To address these questions, we also correlated the base of a sequence of late Quaternary gravels (the Los Angeles River Gravels) across the region they are preserved (i.e. along the current channel of the Los Angeles River). Relief across folds developed in the gravels exceeded 300 meters over lateral distances of less than a kilometer.
Structual Geology:
A number of oil wells penetrated repeated sections that are readily correlative (i.e. correlations have high confidence) along the section lines. We interpreted these to be indicators of blind thrusts which extend upward as steep, mostly north-dipping faults into the cores of the active folds mapped by Oskin and Sieh at the surface (i.e. the Boyle Heights and City Terrace anticlines). Closely-spaced wells in the Union Station and Boyle Heights oilfields suggest these faults are steep, dip about 60 degrees and display 200-600 m of vertical stratigraphic separation (here measured as structural relief). At least one backthrust (top to the north) was noted under the Boyle Heights anticline that has mapped surface expression (work in progress by Oskin). Many of the blind thrusts mapped in the subsurface can be shown to extend to within 1 km of the surface.
The geometry of pre-growth strata indicates that the Boyle Heights and City Terrace anticlines form as fault propagation folds with steep, south-facing front limbs, an interpretation consistent with geotechnical data analysed by Sieh and Oskin for the Eastside Metrorail Project that suggest these limbs achieve steep dips with very little overall shortening. Kinematic implications of this relationship suggest that active folding is likely to be focused along the south-facing fold limbs exposed in the region, in particular the Coyote Pass Escarpment. Progressive limb rotation, rather than kink-band migration is thus the primary folding mechanism that acts to build these structures.
Mapping of folded late Quaternary geomorphic surfaces by Oskin indicate the active folds in this region do not have a history of lateral propagation, in other words the area uplifted in the initial stages of shortening is the same as that expressed today. As discussed earlier, the pre-growth stratigraphic correlations suggest abrupt thickening in the Miocene section (from North to South across north-dipping faults). We interpret this as plausible evidence for an earlier history of Miocene extension on normal slip faults that later became reactivated in the Late Quaternary as blind thrusts. This is supported by the lack of evidence for lateral propagation of the folds, which we interpret to reflect the idea that reactivation of older faults, rather than creation of new ones occurred.
Conclusions:
Based on the structural analysis completed for this project, north-dipping blind thrusts exist beneath the City Terrace and Boyle Heights anticlines. Individual faults have areas up to ~140 km2, suggesting they are capable of generating ~M6.2 earthquakes, if current segment boundaries act to halt ruptures. Projection of the thrusts from the subsurface to the ground surface indicates they mimic the current shape of active folds. The lateral extent of the Boyle Heights and City Terrace anticlines is thus a reasonable proxy for definition of segment boundaries, the most important of which lies along the current Los Angeles River Channel (e.g. ruptures that were initiated in East Los Angeles would not extend westward under downtown LA.