HR: 0830h
AN: T31E-0880    [PDF]
TI: Timing, Style, and Magnitude of Upper Crustal Extension, Sierra San Felipe, NE Baja California, Mexico: Constraints on Rift Processes in the NW Gulf of California Extensional Province
AU: * Givler, R
EM: givler@lettis.com
AF: Dept. Earth \& Space Sci., UCLA, Los Angeles, CA 90095
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
EM:
AF: Earth \& Atmos. Sci., Georgia Inst. Tech., Atlanta, GA 30332
AU: Kent, G M
EM:
AF: IGPP, Scripps Inst. Oceanography, San Diego, CA 92093
AU: Harding, A J
EM:
AF: IGPP, Scripps Inst. Oceanography, San Diego, CA 92093
AU: Holbrook, W S
EM:
AF: Dept. Geol. \& Geophys., U. of Wyoming, Laramie, WY 82071
AU: Umhoeffer, P J
EM:
AF: Dept. Geol., No. Arizona U., Flagstaff, AZ 86011
AU: Fletcher, J M
EM:
AF: Ci\'{e}ncias de la Tierra, CICESE, PO Box 4843, San Diego, CA 92073
AU: Gonzalez-Fernandez, A
EM:
AF: Ci\'{e}ncias de la Tierra, CICESE, PO Box 4843, San Diego, CA 92073
AB: The Gulf Extensional province (GEP), NSF-MARGINS Rupturing Continental Lithosphere focus site, has extended primarily since Late Miocene time. Rift processes in NE Baja California include ENE-directed extension, distributed dextral shear, and detachment faulting, which were partly synchronous with formation of proto-oceanic pull-apart basins in the adjacent Gulf of California. New geologic mapping in the Sierra San Felipe constrains the age and magnitude of extension in the onshore part of the rift, allowing better understanding of strain partitioning in this strongly oblique rift. Strata in the Ca\~{n}on El Parral (CEP) area, SSW of San Felipe, record early gulf-related extension. Early Miocene sedimentary strata intercalated with olivine basalt (Tb1) are overlain by the 12.6 Ma tuff of San Felipe (Tmr1). Unconformably above Tmr1 are tuffaceous sandstone (Tsst) and laterally equivalent fault-scarp facies, and higher $\sim$6.3 Ma ignimbrite deposits (Tmr3-4). These are overlain unconformably by Plio-Pleistocene conglomerate (TQcg1). Thickness changes, paleoflow indicators, and grain-size changes show that an ENE-striking, W-down fault controlled deposition of Tsst. Extension magnitude increased after Tmr3-4 time and began to wane during TQcg1deposition. Few faults cut TQcg1, implying very little Pleistocene strain; area-balanced cross sections indicate $<$40% extension there. The top-to-the-E Las Cuevitas detachment (LCD), W of San Felipe, was active in Miocene-Pleistocene time, controlling marine incursion. Pre-extensional hanging-wall conglomerate and eolian sandstone are overlain by basalt and ignimbrite, then by syn-LCD fluvial and marine beds, which were deposited in N and S sub-basins. Marine strata ($\sim$6.0-1.8 Ma) in the N sub-basin grade W and upsection from silts to fault-scarp facies breccia at the LCD. A dextral E-striking fault separates the sub-basins, probably controlling the S edge of the deeper northern basin and offsetting a distinctive marine-fluvial contact $\sim$1 km. Associated fault-related topography suggests that it is still active. Initial extension in the Sierra San Felipe appears to be recorded in the CEP area and on the top-to-E Santa Rosa detachment (SRD, N of CEP and S of LCD) between $\sim$12.5-8.9$\pm$1.2 Ma. The age of earliest LCD slip is unknown, but by $\sim$6.0 Ma the LCD bounded a deep marine basin. Sedimentation changed to fluvial deposition at $\sim$1.8 Ma, possibly due to slowed slip rate on the LCD. The LCD and SRD are en echelon structures; slip on them overlapped in time and may have been entirely synchronous. Major extension in the Sierra San Felipe apparently ended in Pleistocene time; active extension now occurs on the Sierra San Pedro M\'{a}rtir fault to the W and in the Delfin Basin to the E, indicating a change of strain partitioning in the NW GEP in Pleistocene time. It appears that $\sim$E-directed extension in the onshore NW part of the GEP was minor (10-25 km) compared with the $\sim$270 km of NW-directed gulf extension since $\sim$6 Ma.
DE: 5475 Tectonics (8149)
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting