HR: 0800h
AN: GP21A-0106    [Abstracts]
TI: Paleomagnetic Data from Tertiary Mafic Dikes and Pseudotachylites from the Sierra Ladrones, a Spatially Focused, Large-Magnitude Extension Rift-Flank Uplift, Rio Grande Rift, New Mexico: Implications for Footwall Exhumation
AU: * Muggleton, S R
EM: smuggle@unm.edu
AF: Earth and Planetary Sciences, University of New Mexico, MSCO3-2040, Albuquerque, NM 87131, United States
AU: Geissman, J W
EM: jgeiss@unm.edu
AF: Earth and Planetary Sciences, University of New Mexico, MSCO3-2040, Albuquerque, NM 87131, United States
AU: Kelley, S A
EM: sakelley@ix.netcom.com
AF: Earth and Environmental Science, New Mexico Institute of Mining and Technology, MSEC 208 801 Leroy Place, Socorro, NM 87801, United States
AU: Read, A S
EM: adamread@gis.nmt.edu
AF: New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, United States
AU: Chamberlin, R M
EM: richard@gis.nmt.edu
AF: New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, United States
AB: The Sierra Ladrones (SL) are a fault-bounded, isolated, uplift of Proterozoic rocks reaching over 2700 m in elevation located at a transition in the central Rio Grande rift (RGR), where it widens around the southeast Colorado Plateau (CP). Rapid, Neogene uplift of the SL may reflect a structurally complex, shear couple allowing conversion of a salient of the essentially unextended southeast CP from the highly extended RGR at this latitude. The Ladron footwall block, which comprises most of the SL, is bound to the north and east by the east-dipping low-angle (20 to 30° dip) Jeter normal fault, to the south by the sinistral oblique-slip Cerro Colorado fault and to the west by the steep, east dipping, (presently) reverse sense Ladron fault. The SL is interpreted as an isostatic uplift with west-side down tilting due to unloading of ~6 km of hanging wall rock along the Jeter fault and shallowing of this fault from an original moderate dip of ~48° to a low angle of ~20°. Apatite fission track (AFT) study of Proterozoic granitoids from the eastern and central SL yield dates from 9±2 to 14±2 Ma, indicating rapid, denudation controlled exhumation from depths of about 3 to 4.5 km based on the ~110°C annealing temperature for apatite. Two samples from the northwest side of the SL have less well-defined AFT dates of 32±12 and 19±10 Ma but may support a hypothesis involving appreciable west-side down tilting. To further test this hypothesis five mafic dikes and 12 pseudotachylite veins that cut Proterozoic rocks were sampled for paleomagnetic data in the SE corner of the SL footwall near the trace of the Cerro Colorado fault. These dikes trend NW to ENE and their dips range from steep to shallow. Most Tertiary (dominantly Oligocene) mafic dikes in central New Mexico strike north-south and are near vertical, thus sub-perpendicular to the RGR least principal stress axis. The off-vertical orientations of dikes in the SL and other nearby footwalls have been interpreted to be the result of post-emplacement west-side down tilting. The five dikes sampled yield a mean (D=358°, I=59°, α₉₅=7.1°, κ=146; all normal polarity). Ten of the 12 pseudotachylite veins yield dual polarity paleomagnetic directions parallel to dike directions. Data from both dikes and pseudotachylites are statistically indistinguishable from mid-Cenozoic and younger NA reference directions and do not support significant structural reorientation following pseudotachylite and dike emplacement and remanence acquisition in this part of the SL. An alternative explanation for this relatively limited data set from the mafic dikes is that they are relatively old (i.e. Oligocene) and have been tilted after remanence acquisition by some 30° (west side down) and then the entire southwest part (if not all of) the Sierra Ladrones has been rotated in a counterclockwise fashion by about 20 to 30°. Further paleomagnetic work, as well as ⁴⁰Ar/³⁹Ar age dating of the dikes, together with apatite (U- Th)/He thermochronology of the Proterozoic rocks are underway to define the youngest phase of uplift.
DE: 1115 Radioisotope geochronology
DE: 1140 Thermochronology
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 8010 Fractures and faults
DE: 8109 Continental tectonics: extensional (0905)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting