HR: 14:25h
AN: V53F-04    [Abstracts]
TI: Effects of A Weak Crustal Layer in a Transtensional Pull-Apart Basin: Results from a Scaled Physical Modeling Study
AU: Dooley, T P
EM: tim.dooley@beg.utexas.edu
AF: Bureau of Economic Geology, Jackson School of Geosciences, The , University of Texas at Austin, University Station, Box X, Austin, TX 78713, United States
AU: * Monastero, F C
EM: francis.monastero@navy.mil
AF: U.S. Navy Geothermal Program Office, China Lake, Ridgecrest, CA 93555, United States
AU: McClay, K R
EM: ken@gl.rhul.ac.uk
AF: Fault Dynamics Research Group, Royal Holloway University of London, Egham Hill, Egham, TW20 0EX, United Kingdom
AB: Results of scaled physical models of a releasing bend in the transtensional, dextral strike-slip Coso geothermal system located in the southwest Basin and Range, U.S.A., are instructive for understanding crustal thinning and heat flow in such settings. The basic geometry of the Coso system has been approximated to a 30? dextral releasing stepover. Twenty-four model runs were made representing successive structural iterations that attempted to replicate geologic structures found in the field. The presence of a shallow brittle-ductile transition in the field known from a well-documented seismic-aseismic boundary, was accommodated by inclusion of layers of silicone polymer in the models. A single polymer layer models a conservative brittle-ductile transition in the Coso area at a depth of 6 km. Dual polymer layers impose a local elevation of the brittle-ductile transition to a depth of 4 km. The best match to known geologic structures was achieved with a double layer of silicone polymers with an overlying layer of 100 µm silica sand, a 5° oblique divergent motion across the master strike-slip faults, and a thin-sheet basal rubber décollement. Variation in the relative displacement of the two base plates resulted in some switching in basin symmetry, but the primary structural features remained essentially the same. Although classic, basin-bounding sidewall fault structures found in all pull-apart basin analog models formed in our models, there were also atypical complex intra-basin horst structures that formed where the cross-basin fault zone is situated. These horsts are flanked by deep sedimentary basins that were the locus of maximum crustal thinning accomplished via high-angle extensional and oblique-extensional faults that become progressively more listric with depth as the brittle-ductile transition was approached. Crustal thinning was as much as 50% of the original model depth in dual polymer models. The weak layer at the base of the upper crust appears to focus brittle deformation and facilitate formation of listric normal faults. The implications of these modeling efforts are that: 1) Releasing stepovers that have associated weak upper crust will undergo a more rapid rate of crustal thinning due to the strain focusing effect of this ductile layer; 2) The origin of listric normal faults in these analog models is related to the presence of the weak, ductile layer; and, 3) Due to high dilatency related to major intra-basin extension these stepover structures can be the loci for high heat flow.
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8015 Local crustal structure
DE: 8094 Instruments and techniques
DE: 8111 Continental tectonics: strike-slip and transform
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting