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Active folding within LA Basin

Xiangyan Tian. Ge 277. 6 December 2008. Active folding within LA Basin. Readings: Shaw, J., and P. Shearer, An elusive blind-thrust fault beneath Metropolitan Los Angeles, Science , 283 , 1516-1518, 1999

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Active folding within LA Basin

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  1. Xiangyan Tian Ge 277 6 December 2008 Active folding within LA Basin Readings: Shaw, J., and P. Shearer, An elusive blind-thrust fault beneath Metropolitan Los Angeles, Science, 283, 1516-1518, 1999 Davis, T. L., and J. S. Namson, A balanced cross-section of the 1994 Northridge earthquake, southern California

  2. Argus et al. (2005) From Aron’s presentation

  3. From Aron’s presentation Main Problem: Faults creeping to within 6 km of the surface seems inconsistent with large earthquakes breaking a brittle lithosphere down to 15 km depth. In metropolitan L.A. the seismogenic depth is 15–20 km, the approx. maximum depth of seismicity and the depth to which 3 large modern EQs ruptured. This disagreement suggests that the model, in which an edge dislocation occurs along a planar reverse fault in an elastic continuum, may be unsatisfactory. Additional Inconsistency: The total horizontal rate estimated by the elastic dislocation model across the Puente Hills and Elysian Park thrusts is more than double the Holocene rates estimated from geology. The difference between the two estimates could be accounted for by slip along faults outside the zones studied by Dolan et al. (2003) and Oskin et al. (2000), or the convergence rate over the past several years could have been faster than the mean Holocene convergence rate. Argus et al. (2005)

  4. Maps showing the locations of the following profiles Contour map of the Puente Hills thrust system overlain on a LandsatTM image Shaded relief map of Los Angeles region Shaw and Shearer (1999) Fuis et al. (2003)

  5. Velocity models and reflectivity of LARSE lines 1 and 2, aligned along surface trace of San Andreas fault Fuis et al. (2003)

  6. Fault slip rates 0.5 – 2 mm/yr The geometry of folded growth strata in the Santa Fe Spring structures indicates that at least 800 m of slip occurred on the underlying blind thrust in the Quaternary. Use of the maximumage of Quaternary strata (1.6. million years ago) yields a minimum slip rate of 0.5 mm/year. Maximum slip rate (2.0 mm/year) is taken as the portion of the shortening (7.5 to 9.5 mm/year) measured by geodesy across the Los Angeles basin that remains unaccounted for on previously recognized fault systems. Shaw and Shearer (1999) The best-fitting trishear models produce a good match to the modeled part of the structure. The initial tip line position of the Puente Hills thrust was located in the same part of the crust as the Whittier Narrows event (Allmendinger and Shaw, 2000) .

  7. Balanced cross-section across the Northridge portion of the Transverse Range fold-and-thrust belt Main event of the Northridge 1994 earthquake and aftershocks below 5 km occurred along the Pico thrust, a backthrust off the north-dipping Elysian Park thrust ramp. Shallower aftershocks are the result of fold growth (flexural-slip faults) and propagation of the fold hinge into the Santa Clara synclinorium. Davis and Namson (1994)

  8. b. Balanced cross-section showing the Santa Susana Mountains and Santa Monica Mountains anticlinorium. c. Restoration of the cross-section shows that it balances and is therefore a viable solution. Fault slip rates: The Pico thrust 1.4-1.7 mm/yr The Elysian Park thrust 3.9 – 5.9 mm/yr Davis and Namson (1994)

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