Introduction:
Structural and geomorphic analysis of the northern San Joaquin Hills was completed in collaboration with other Group C researchers in 1997 with the aim of defining: 1) kinematic mechanisms which act to build the San Joaquin Hills anticline, 2) the recency of uplift of the fold (i.e.whether Holocene sediments were deformed in a manner consistent with the long- term history of uplift), 3) long-term rates of uplift and its relation to fault slip rates on the blind thrust proposed to underlie the fold, and 4) the geometry of the proposed blind thrust.
Work completed in 1997 has resolved a number of these questions through structural modeling of 3D fold geometry, mapping of deformed drainage networks, generation of high resolution digital elevation models and triangulated irregular networks in Arc/Info, and compilation of water well data made available from the Orange County Water District. These studies were coordinated with ongoing U-series age dating, compilation of geotechnical data and mapping of deformed Late Quaternary deposits by Grant, Edwards, Kennedy, Gath and Munro (see other report by Grant).
Results:
Modeling of the geometry of the San Joaquin Hills suggests it forms by fault-bend folding mechanisms, based on the geometry of axial surfaces deforming marine terrace deposits (Figure 1) and stream drainage networks (Figure 2) which are sharply divided between the limbs of the fold and its NW-dipping plunge panel. The history of lateral propagation of the fold also supports this model, as defined by drainage networks which indicate that the plunge panel is progressively consumed by the NE-dipping fold limb (e.g. Shaw and others 1994). Based on the subaerial morphology of the anticline and the geomorphically consistent structural model, consumption of the plunge panel occurs at the top of the NE-facing fold limb (Figures 2, 3). This implies that the active axial surface at the top of the NE-facing fold limb is pinned to the top of a blind thrust ramp at depth (Figure 3). Note that the base of the SW-facing fold limb is located offshore where it is not available for geomorphic analysis.
The recency of uplift of the fold has been assessed by mapping and U-series dating of folded marine terrace deposits (see companion report by Grant). These results suggest Marine Stage 5a sediments (~80 Ka) are folded in a manner and location consistent with the long-term uplift history of the fold (Figure 1). Well data in the water gap between Huntington and Newport Mesa also indicate the Holocene Talbert Aquifer is deformed in a style consistent with the structural models completed for this study (Figure 4; Sprotte and others, 1980). Besides evidence for uplift of Holocene marshes in the Newport Back Bay area currently being investigated by Grant, this aquifer data may represent the strongest evidence yet for classifying the San Joaquin Hills anticline as an active, potentially seismogenic structure.
The geometry of the blind thrust which drives folding of the San Joaquin Hills anticline is not directly imaged by either earthquake microseismicity, or available subsurface well and seismic data in onshore regions of Orange County. Construction of a cross section across the Newport oilfield and wildcat wells across Newport Mesa did not yield information useful for determining the geometry of the blind thrust (i.e. wells usually do not penetrate deeper than ~ 1 km). Construction of triangulated irregular networks (TIN's) from digital elevation models in Arc/Info, however, allows some constraints on subsurface blind thrust geometry to be defined. These digital models confirm initial modeling results from earlier work (Figures 5a, 5b, 5c) and indicate that the NE-facing limb of the fold clearly folds marine terrace deposits in the region of Bonita Creek (Figures 5b, 5c). In this area Stage 7 and 9 terrace platforms are folded downward to the NE from their otherwise constant elevation around the SW side of the fold. These efforts support the mapping efforts by the co investigators (Grant, Gath and Munro) which indicate the difficulty (i.e. uncertainty) in correlating isolated terrace surfaces which do not extend along a constant elevation in this area. On the landward side of the fold (i.e. the NE-facing fold limb), the folded terraces nearly merge into an single surface, but still exhibit minor vertical separation before being buried by alluvial sediments derived from the El Toro embayment.
These geomorphic observations imply several key geometrical characteristics for the otherwise unimaged, and unaccessible blind thrust. These include: 1) the active axial surface on the NE-facing fold limb is located at the crest of the fold (Figure 3), or along the boundary of the adjacent plunge panel (i.e. this is an active axial surface that is pinned to a bend in the blind thrust at depth), 2) minor uplift occurs on the NE-facing back limb, suggesting the dip of the blind thrust beneath this area is shallow, but not flat, and 3) a steeper ramp segment is located beneath the area under the plunge panel and SW-facing fold limb.
Conclusions:
Based on the structural and geomorphic analysis completed for SCEC- funded work in 1997, it is likely that the blind thrust proposed to exist beneath the San Joaquin Hills is active. For a range of given geometries, fault slip rates are likely to be in the range of ~1.5+1.0 mm/yr. Because subsurface fault geometry is unconstrained, the area likely to rupture in a single seismic event is uncertain, however, a maximum credible earthquake for this part of coastal Orange County would extend along strike ~ 25 km, with a minimum fault width of ca. 8 km. This raises the possibility that a near Northridge-sized event is possible on a SW-dipping thrust in this region, and that directed effects of strong ground motion may be severe in the densely populated urban corridor along Interstate 5 between San Juan Capistrano and Irvine.
References
Shaw, J. H., Hook, S. C., and Suppe, J., 1994, Structural Trend Analysis by Axial Surface Mapping, American Association of Petroleum Geologists Bulletin, Vol. 78, p 700-721.
Sprotte, E.C., Fuller, D.R., Greenwood, R.B. and Mumm, H.A., 1980, Classification and Mapping of Quaternary Sedimentary deposits for purposes of seismic zonation, south coastal Los Angeles Basin, Orange County, California: California Division of Mines and Geology Open File Report 80-19, 250p.
SCEC Supported Publication List
Walls, C., Rockwell, T.R., Mueller, K, Bock, Y., Williams, S., Pfanner, J., Dolan, J., P. Fang, Escape From L.A.: Extrusion Tectonics in Southern California and Implications for Seismic Risk: Nature (In Review).
Mueller, K., and Suppe, J., 1997, Growth of Wheeler Ridge Anticline, California: Implications for fault-bend folding behaviour during earthquakes,Journal of Structural Geology Vol. 19, p. 383-396.
Mueller, K., and Talling, P., 1997, Geomorphic evidence for tear faults accommodating lateral propagation of an active fault-bend fold, Wheeler Ridge, California,Journal of Structural Geology Vol. 19, p. 397-411.