HR: 15:25h
AN: T32D-08    [PDF]
TI: Late Miocene (Proto-Gulf) Extension and Magmatism on the Sonoran Margin
AU: * Gans, P
EM: gans@geol.ucsb.edu
AF: UC-Santa Barbara, Department of Geological Sciences UC Santa Barbara-Building 526, Santa Barbara, CA 93106-9630
AU: MacMillan, I
EM: ian_macmillan@umail.ucsb.edu
AF: UC-Santa Barbara, Department of Geological Sciences UC Santa Barbara-Building 526, Santa Barbara, CA 93106-9630
AU: Roldan-Quintana, J
EM: jaimer@servidor.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Blvd. L. D. Colosio s/n Apdo. Postal 1039, Hermosillo, Son 83000 Mexico
AB: Constraints on the magnitude and character of late Miocene (Proto-Gulf) deformation on the Sonoran margin of the Gulf of California extensional province are key to understanding how and when Baja California was captured by the Pacific plate and how strain was partitioned during the early stages of this transtensional rift system. Our new geologic mapping in southwestern Sonora and $^{40}$Ar/$^{39}$Ar dating of pre-, syn-, and post-tectonic volcanic units indicate that late Miocene deformation and volcanic activity were largely restricted to a NW-trending, 100-120 km wide belt adjacent to the coast. Inboard of this belt, NW-SE extension is mainly older ($>$15 Ma) and occurred in an intra-arc or back-arc setting. Proto-Gulf deformation within the coastal belt was profoundly transtensional, with NW-striking, dextral strike slip faults operating in concert with N-S and NNE-striking normal and oblique slip faults to produce an inferred NW or NNW tectonic transport direction. The total amount of late Miocene NW directed dextral shear within the coastal belt is still poorly constrained, but may exceed 100 km. The locus of deformation and volcanic activity migrated westward or northwestward within the Sonoran coastal belt. in the eastern portion (Sierra Libre and Sierra El Bacatete) major volcanic activity commenced at $\sim$13.0 Ma and peaked at 12.0 Ma, and major faulting and tilting is bracketed between 12.0 and 10.6 Ma. Further west in the Sierra El Aguaje/San Carlos region, major volcanic activity commenced at 11.5 Ma and peaked at 10.5 Ma, and most faulting and tilting is bracketed between 10.7 and 9.3 Ma. On the coastal mountains northwest of San Carlos, rift related faulting and tilting continued after 8.5 Ma. Voluminous late Miocene (13-8 Ma) volcanic rocks within the Sonoran coastal belt were erupted from numerous centers (e.g. Sierra Libre, Guaymas, Sierra El Aguaje). These thick volcanic sections are compositionally diverse (basalt to rhyolite, with abundant dacite and andesite), and are intimately associated in space and time with tectonic activity. Overall, the Sonoran coastal belt provides a spectacular example of distributed transtension and associated magmatism that ultimately led to rupturing of the continental lithosphere.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
DE: 9350 North America
SC: Tectonophysics [T]
MN: 2003 Fall Meeting