2009 Portland GSA Annual Meeting (18-21 October 2009)
Paper No. 15-6
Presentation Time: 9:25 AM-9:40 AM


BUSBY, Cathy J.1, MURRAY, Bryan P.1, and FERRARI, Luca2, (1) Department of Earth Science, University of California, Santa Barbara, Webb Hall, BLDG 526, Santa Barbara, CA 93106-9630, cathy@crustal.ucsb.edu, (2) Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, Queretaro, 76230, Mexico

The Sierra Madre Occidental (SMO) of western Mexico is the least deformed silicic large igneous province on Earth, as well as the largest Cenozoic one. Three-fourths of the volume of the Late Eocene – early Miocene ignimbrite flareup of North America was erupted from the SMO, and two-thirds of this occurred in a first pulse at about 38 – 28 Ma, triggering global cooling. We present new map and U-Pb LA ICPMS zircon age data on rocks of the first pulse in the northern SMO, which have apparently not been overprinted by the second (Miocene) pulse. Our goal is to understand the strain regime extant at the birth of the silicic LIP, in a place where its underpinnings are well exposed, and how this may have influenced volcanic styles. Deep dissection in the Copper Canyon region affords spectacular cross-sectional views of these supervolcanoes.

In a recent summary of all LIPs on Earth, Bryan and Ernst (2008) listed the SMO as the only one that did not form atop an extensional terrane. However, our new work shows that, in the Temoris-Cerocahui region of Copper Canyon, the underlying andesite arc rocks (referred to in the region as “Lower Volcanic Complex”, LVC) were offset hundreds of meters on syndepositional normal faults immediately prior to the ignimbrite flareup (Upper Volcanic Complex, UVC). These mainly NNW-SSE trending normal faults and ENE-WSW-trending transfer zones were then partially reactivated during eruption of the UVC, and served to localize lines of plugs and fissure-like intrusions several kilometers long and 100’s of meters wide. Some of these linear intrusions pass upward into lava flow belts, and are fringed by linear belts of proximal ignimbrite. The fissure-proximal ignimbrite belts have abundant andesite blocks in cross-stratified units with set heights of 5 – 20 m. We infer that the blocks were derived from intensely fractured LVC andesites along the fissures; the distinctive mega-surge-like character of the lithic ignimbrite may reflect a higher flux for explosive eruption along fissures, relative to central vents.

Bryan, S., and Ernst, R.E., 2008, Revised definition of Large Igneous Province (LIP): Earth and Planetary Science Letters, v. 86, p. 175-202.

2009 Portland GSA Annual Meeting (18-21 October 2009)
General Information for this Meeting
Session No. 15
Supervolcanoes, Ignimbrite Flare-ups, and Their Impacts: Definition, Debate, and New Developments
Oregon Convention Center: B117/118/119
8:00 AM-12:00 PM, Sunday, 18 October 2009

Geological Society of America Abstracts with Programs, Vol. 41, No. 7, p. 56

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