Paper No. 9
Presentation Time: 4:05 PM
TECTONIC CONTROLS ON LATE OLIGOCENE VOLCANISM IN THE GUAZAPARES MINING DISTRICT, NORTHWESTERN SIERRA MADRE OCCIDENTAL, MEXICO
MURRAY, Bryan P.1, BUSBY, Cathy J.
2, FERRARI, Luca
3 and SOLARI, Luigi
3, (1)Department of Earth Science, University of California, Santa Barbara, Webb Hall, BLDG 526, Santa Barbara, CA 93106-9630, (2)Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106-9630, (3)Centro de Geociencias, Universidad Nacional Autonoma de Mexico, Blvd Juriquilla 3001, Juriquilla, Queretaro, 76230, Mexico, bmurray@umail.ucsb.edu
The Guazapares Mining District of western Chihuahua, Mexico, is located in the northern Sierra Madre Occidental (SMO) along the western transition between the unextended core of the SMO silicic large igneous province and the highly extended Miocene basins of the Gulf Extensional Province. New geologic mapping of a ~425 km
2 area in the Temoris-Cerocahui region of the Guazapares Mining District and new zircon U-Pb Laser Ablation ICP-MS age results has found several previously unmapped NW-trending late Oligocene synvolcanic extensional basins. These half-graben basins are filled with a tilted mafic volcanic unit consisting of lava flows, flow breccias, and volcaniclastic sedimentary rocks that are deposited conformably on ~29 Ma welded silicic ignimbrites. Overlying these lower tilted and moderately extended mafic volcanic and sedimentary rocks are non-welded to partially-welded silicic ignimbrites, lava flows, and hypabyssal intrusions deposited at ~25 Ma. These silicic deposits are generally flat-lying and less extended than the underlying mafic volcanic rocks.
An angular unconformity forms the contact between the lower tilted mafic volcanic unit and the upper flat-lying silicic volcanic unit; however in some localities the tilted lower unit is transitional into the flat-lying upper unit. Evidence of synvolcanic half-graben extension includes lava flows and sedimentary deposits of the lower mafic volcanic unit that thicken and coarsen toward many of the basin-bounding normal faults and some deposits onlapping half-graben shoulders. Up to 250 m of vertical fault offset of the lower mafic volcanic unit occurred prior to eruption of the upper silicic volcanic unit, which is only offset up to ~60 m. Additionally, some normal faults appear to have controlled the location of subsequent silicic volcanism in the Guazapares Mining District. Vent-proximal volcanic assemblages including co-ignimbrite breccias, surge-like ignimbrites, and thick silicic lava flows are localized near normal fault trends, and hypabyssal silicic plugs intrude many of these faults. At least one silicic plug appears to be the source of a silicic lava flow in the region.
