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1 the Volume of the Complete Palos Verdes Anticlinorium: Stratigraphic

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1 the Volume of the Complete Palos Verdes Anticlinorium: Stratigraphic The Volume of the Complete Palos Verdes Anticlinorium: Stratigraphic Horizons for the Community Fault Model Christopher Sorlien, Leonardo Seeber, Kris Broderick, Doug Wilson Figure 1: Faults, earthquakes, and locations. Lower hemisphere earthquake focal mechanisms, labeled by year, are from USGS and SCEC (1994), with location of 1930 earthquake (open circle) from Hauksson and Saldivar (1986). The surface or seafloor traces, or upper edges of blind faults are from the Southern California Earthquake Center Community Fault Model (Plesch and Shaw, 2002); other faults are from Sorlien et al. (2006). Figure 2 is located by red dashed polygon. L.A.=Los Angeles (downtown); LB=Long Beach (city and harbor); PVA=Palos Verdes anticlinorium, Peninsula and Hills, PVF=Palos Verdes fault; SPBF=San Pedro Basin fault; SMM=Santa Monica Mountains; SPS=San Pedro Shelf. Structural Setting If the water and post-Miocene sedimentary rocks were removed, the Los Angeles (L.A.) area would have huge relief, most of which was generated in the Plio-Pleistocene (Fig. 1; Wright, 1991). The five km-deep L.A. basin would be isolated from other basins to the SW by a major anticline-ridge, the Palos Verdes Anticlinorium (PVA), parallel to the current NW-SE coastline (Davis et al., 1989; Figures 2, 3). The Palos Verdes Hills have been long recognized as a contractional structure, but of dimensions similar to the peninsula (e.g., Woodring et al., 1946). This prominent topographic feature is instead only the exposed portion of the much larger PVA (Fig 2). Submerged parts of the PVA had previously been interpreted as separate anticlines (Nardin and Henyey, 1978; Legg et al., 2004) related in part to bends in strike-slip faults (Ward and Valensise, 1994), rather than a single complex structure above low-angle (oblique) thrust faults. Davis et al (1989), Shaw and Suppe (1996), and Broderick (2006) recognized the scale of the regional onshore-offshore structure. Palos Verdes Anticlinorium: Scale and Activity The PVA is a regional NW-trending 80 km-long post-Miocene growth structure (Figs. 3, 4). It implies an underlying thrust-fault system of similar dimensions. The Plio-Pleistocene growth rate of the PVA is not as evident as its size. The Santa Monica and Los Angeles Basins have subsided about 4 km in the last 5 m.y., and this subsidence has probably continued through Quaternary time (Fig. 3; Wright, 1991; Sorlien et al., 2006). If the basins are simply marking regional lithospheric subsidence, structural growth of the PVA must be correspondingly rapid to keep the top of this structure near sea level. The PVA is undeniably a large structure, one of the largest active structures in the Los Angeles metro area, yet particulars about the thrust fault system associated with it and rates of deformation are still controversial. Oblique slip on the onshore restraining segment of the right- lateral Palos Verdes fault can only explain uplift of the Palos Verdes Hills relative to a sea level datum and cannot fully explain growth of structural relief of the much larger regional anticlinorium (Fig. 2, Ward and Valensise, 1994; Broderick, 2006). Possibly because of the large 1 Fig. 2: Oblique view to northeast of bathymetry and topography.The Shelf Projection, Palos Verdes Hills, and San Pedro Shelf are part the Palos Verdes antclinorium, a regional topographic and structural high extending 80 km along trend. The 700 m-high San Pedro Escarpment is continuous for 40 km, before stepping right to the southwest side of San Pedro shelf (Fig. 3). The smaller dashed oval is the 0.3 mm/yr uplift contour modeled by Ward and Valensise (1994). The south edge of the outlined area is the limit of the anticlinorium, but a fold-and-thrust belt continues farther southeast. uncertainty on its seismogenic potential, this fault system has not been considered in hazard calculations (e.g. www.consrv.ca.gov/CGS/rghm/psha/fault_parameters/pdf/B_flt.pdf). High- resolution seismic reflection data and coreholes recently showed folding of inferred Holocene strata at the base of the Compton backlimb in south-central Los Angeles (Leon et al., 2006). Figure 3: Map showing faults related to the Palos Verdes anticlinorium. Blue lines locate Figure 4 (D- D’; E-E’, and 5 (A-A’). Gray fill is the southwest limb of Palos Verdes anticlinorium, depth contours are base Repetto (~top Miocene) from Wright (1991). The Red curves with triangles on NE hanging-wall side represent parts of the same thrust system, respectively the tip of the San Pedro Escarpment fault (SPEF), and the top of the Compton ramp. The Compton is responsible for the regional NE-dipping fold limb that forms the southwest margin of LA basin (Fig. 5). Near top Miocene 3D representations have been made of most of the offshore part of the anticlinorium (Broderick, 2006; Fig. 6) and we are now digitizing the Wright (1991) map to produce views of the full structure. Folding of the offshore northwest plunge of the PVA started during early Pliocene time (Broderick, 2006). Strata below a ~1.8 Ma horizon do not thin onto the PVA’s limb that underlies the southwest escarpment of San Pedro Shelf (Fig. 4). Therefore, the southwest part of this structure did not start folding until after ~1.8 Ma. The southwest limb has over 1 km of relief at D-D’ (Fig. 4). A simple model of motion of the hanging-wall up a fault ramp can be used to 2 calculate 1.6 mm/yr slip for a fault dipping 20º, or 1.1 mm/yr for a fault dipping 30º. Present-day slip rates can be inferred if thrust faulting and folding are required in order to keep the Shelf Projection, Palos Verdes Hills, and San Pedro Shelf from sinking concurrently with the adjacent basins. In a simple fault-bend fold model that assumes uniform post-Miocene subsidence rates, slip=0.8 mm/yr/sin(dip), or ~2 mm/yr for a Compton ramp dipping 22 degrees. Additional evidence for activity includes GPS-measured contraction occurring near downtown Los Angeles (Argus et al., 2005), that may stem from creep on the down-dip part of the fault system. Despite different names for its different parts, a single major thrust fault may account for the west flank of the LA Basin (Fig. 5). Figure 4: Located Figures 3 and 6. Left: USGS profile along E-E’ illustrates southwest propagation of the thrust front, and continued activity of the system. Late Pliocene Lower Pico strata were deposited, onlapping sea floor relief “a”; unconformity U2 was cut; the thrust front propagated southwest after 1.8 Ma forming this part of San Pedro Escarpment; turbidites “b” onlapped U2; with continued tilting; unit “c” was deposited, onlapping U1; the thrust front propagated to the fault beneath “d”, folding “c” and U1, and finally unit “e” was deposited in a “piggyback” basin. See Fisher et al. (2004). Right: USGS profile along D-D’, showing that the strata deposited between ~2.5 (top Repetto) and 1.8 Ma (top lower Pico) do not thin on the southwest limb of Palos Verdes anticlinorium, and thus it did not start folding here until after ~1.8 Ma. Figure 5: Regional cross section illustrating the structural link between Palos Verdes anticlinorium and the Los Angeles basin. This section is across the northwest plunge of the anticlinorium; relief of top Miocene is more than 4 km. It would be ~ 3 km greater on a section across Palos Verdes Hills and the deepest part of Los Angeles basin. Located on Figure 3. 3 The onshore restraining segment of the Palos Verdes fault contributes locally some contraction that may account for enhanced uplift of the Palos Verdes Hills (Ward and Valensise, 1994; also Woodring et al., 1946), but cannot account for the major part of the PVA. Broad contractional folding is inconsistent with the young extension near a releasing double bend on the sub-vertical Palos Verdes fault (Fig. 6). The offshore anticlinorium can instead be explained by the NE- dipping San Pedro Escarpment fault (SPEF), which aligns in 3D with the Compton thrust ramp of Shaw and Suppe (1996) and may be the same fault (Figs 3, 5, 6). The combined Compton- SPEF has twice the area of the Compton ramp alone, and could generate a M7.3 earthquake. The fault area and maximum magnitude are even larger if the Compton-SPEF is continuous down-dip with the lower Elysian Park fault (Fig. 6; Shaw et al., 2002). A Puente Hills thrust earthquake is expected to be catastrophic; rupture on the underlying Lower Elysian Park-Compton fault may have a comparable effect. Figure 6: View to the northwest of San Pedro shelf and escarpment along the right-lateral Palos Verdes fault. Red box locates in inset. The folded horizon, base “Repetto”, is just above top Miocene (~5 Ma). The labeled releasing double bend is associated with two dozen high-angle normal-separation faults that cut to the seafloor (Fisher et al., 2004). Absorption of blind thrust slip on the NE-dipping San Pedro Escarpment fault can explain uplift of the anticlinorium relative to subsiding basins, as well as growth of the San Pedro Escarpment. Color scheme base Repetto from 0.07 to 2.2 s two-way travel time (TWTT), while volume shown is 0 to 4 s TWTT. Other SCEC-related Efforts The majority of my time went to co-organizing a SCEC and NSF-supported workshop held in Istanbul, including creating the web page for it: http://projects.crustal.ucsb.edu/NAF-SAF- 2006/. Several additional revisions of our manuscript on the Santa Monica-Dume fault absorbed a lot of effort during 2006.
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