HR: 17:45h
AN: S22E-08    [PDF]
TI: Did Flow of Weak Middle or Lower Crust Accommodate Extension in the Gulf of California and Salton Trough?
AU: Black, N
EM: nblack@ess.ucla.edu
AF: Dept. Earth \& Space Sci., UCLA, Los Angeles, CA 90095
AU: * Axen, G J
EM: gaxen@ess.ucla.edu
AF: Dept. Earth \& Space Sci., UCLA, Los Angeles, CA 90095
AU: Lizarralde, D
AF: Earth \& Atmos. Sci., Georgia Inst. Tech, Atlanta, GA 30332
AU: Kent, G M
AF: IGPP, Scripps Inst. Oceanography, San Diego, CA 92093
AU: Harding, A J
AF: IGPP, Scripps Inst. Oceanography, San Diego, CA 92093
AU: Holbrook, W S
AF: Dept. Geol. \& Geophys., U. of Wyoming, Laramie, WY 82071
AU: Umhoeffer, P J
AF: Dept. Geol., No. Arizona Univ., Flagstaff, AZ 86011
AU: Fletcher, J M
AF: Ci\'{e}ncias de la Tierra,CICESE, PO Box 4843, San Diego, CA 92073
AU: Gon\'{a}lez-Fern\'{a}ndez, A
AF: Ci\'{e}ncias de la Tierra,CICESE, PO Box 4843, San Diego, CA 92073
AB: The strength of middle and/or lower crust (MLC) is key to understanding continental deformation and controls whether or not brittle upper crust and mantle lithosphere are coupled: weak MLC may flow and decouple the adjacent layers. We analyze the extent of MLC flow during rifting in the northern Gulf of California and Salton Trough (NGC/ST; in the NSF MARGINS Rupturing of Continental Lithosphere focus site) using area-balance techniques to compare upper crustal extension and MLC extension. Mid-Tertiary extension in SE CA, W AZ and W mainland Mexico thinned the crust before $\sim$E-W extension localized in the GC/ST at $\sim$12 Ma. By $\sim$6 Ma, NW-SE Pacific-N. American plate separation was concentrated in the NGC/ST, which began to subside below sea level. By $\sim$3.6 Ma, transform faults connected pull apart basins forming ocean crust (S gulf) and proto-oceanic crust in the NGC/ST. The Peninsular Ranges west of the NGC/ST were not extended at the surface. MLC flow into the NGC/ST is suggested by (1) the fact that the Moho dips W under the unextended Peninsular Ranges (W of the rift) to a maximum depth well W of the range crest (Lewis et. al., 2000, 2001), (2) by an apparent surplus of MLC relative to upper crust within the rift (e.g., Couch et al., 1991), and (3) by the magnitude of extension ($\sim$255 km) between initially adjacent areas on either side of the NGC/ST (Oskin et al., 2001). We constrain the extent of MLC flow two ways: by comparing known upper crustal extension (e.g., Lewis and Stock, 1998; Axen and Fletcher, 1998) to (1) whole-crustal and (2) lower-crustal extension. Whole and lower crustal extension are obtained from cross-sectional areas constrained by depth to Moho and/or to the base of the upper crust (references above), by the extent of new proto-oceanic crust (e.g., Fuis et.al., 1984), and by assuming end-member initial crustal thicknesses (e.g., $\sim$37-40 km maximum thickness of the Peninsular Ranges or $\sim$25 km minimum from previously extended crust east of the NGC/ST). In general, the data are much better W of the NGC/ST than to the E, and constrain Moho depth much better than depth to the base of the upper crust (which may not equate to the base of the brittle crust in any case). Therefore we focus on comparisons of whole crust to upper crust in the Peninsular Ranges and western NGC/ST. Preliminary calculations of whole-crustal extension typically exceed known upper crustal extension by up to $\sim$4 times, strongly favoring MLC flow into the rift. However, for the end-member case of minimum initial crustal thickness, the higher estimates of known upper crustal extension are subequal to the calculated whole crustal extension, requiring no MLC flow. In most cases, area balance requires a present-day deficit of MLC material beneath the unextended Peninsular Ranges. This is key, because MLC deficits cannot be explained by magmatic processes, unlike the surpluses inferred below the rift (e.g addition of basaltic lower crust). These conclusions support much previous work that favors flow of weak MLC in extended terrains. Better constraints should be provided once reflection and wide-angle refraction data from MARGINS seismic profiles across conjugate margins of the gulf are fully processed.
DE: 5475 Tectonics (8149)
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
DE: 8159 Rheology--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting