Currently seeking a graduate student to work on minor mid-Cenozoic igneous occurrences in eastern New Mexico, which form a patchy discontinuous belt representing the most distal periphery of Cordilleran magmatism emplaced approximately 50-200 km east of the closest major alkaline magmatic centers. They have received little attention and present excellent opportunities for exciting fieldwork, novel research, and impactful student mentorship. Initial reconnaissance of these igneous rocks is building towards holistic studies addressing basic aspects of these occurrences through mapping, petrography, geochemistry, and geochronology. This work will lead to bigger questions on the relationship between these peripheral intrusions and more major alkaline magmatic centers, exhumation and heat flow histories recorded in these rocks, and significance for tectonics of paleo-plate dynamics of the SW US Cordilleran margin!
Volcanic ash and associated aerosol layers in glacier ice offer a uniquely complete record of explosive volcanism. Investigation of these layers, both in bare ice areas of and in ice cores offers insight into eruptive processes, local and regional ice flow processes, and the impact of eruptions on global systems (climate and ozone depletion). The Antarctic ice sheet is an ideal place to preserve a record of volcanic eruptions. The combination of chemical fingerprinting of glass shards, and chemical analysis of volcanic aerosols associated with tephra layers in Antarctic blue ice allows establishment of a high-resolution chronology of local and distant volcanism that can help understand patterns of significant explosive volcanism, atmospheric loading, and climatic effects associated with volcanic eruptions.
Cosmogenic dating techniques have been successfully applied to dating of geomorphically-young surfaces, such as glacial moraines, beach terraces, and basaltic lava flows that have intact surface features, and hence have undergone little erosion (e.g. Phillips et al., 1997a and b; Phillips et al, in review, Dunbar and Phillips, 1996; Zreda et al., 1991, 1993; Zreda, 1994; Anthony and Poths, 1992, Laughlin et al., 1994). These techniques rely on measurement of cosmogenic nuclides that begin to build up as soon as a rock is exposed to cosmic rays. Therefore, cosmogenic techniques can be applied to dating of any surface that is composed of material that was not exposed to cosmic rays prior to formation of the surface, and has been exposed more-or-less continuously since. In the case of an extrusive volcanic rock, buildup of cosmogenic nuclides begins when the rock is erupted, so measurement of the ratio of a cosmogenic isotope to a non-cosmogenic isotope can provide an estimate of eruption age (Phillips et al., 1986).
Directly dating the timing of deformation remains a challenging task. An ongoing collaboration seeks to establish U-Pb dating of titanite grains involved in ductile deformation as a promising new deformation chronometer by applying this technique to Laramide-age shear zones in Joshua Tree National Park.
Two Bureau of Geology scientists, in collaboration with scientists at the United State Geological Survey, have discovered similarities between ground water systems that formed ore deposits 10 million years ago and modern ground water in the Rio Grande Rift. They reported their work in an invited presentation at the 2000 Annual Meeting of the Geological Society of America.
Dr. Virgil Lueth, mineralogist/ economic geologist, and Lisa Peters, senior lab associate at the New Mexico Geochronological Research Lab, have been studying the mineral jarosite in ore deposits from Chihuahua, Mexico, to Albuquerque.
Dr. Matthew Heizler (geochronologist) has just been awarded a three year grant from the NSF tectonics division to study the "Great Unconformity" exposed in western North America. An unconformity is a span of time for which no rock record is represented because it has been eroded away or because sediment was never deposited. The Great Unconformity was coined by John Wesley Powell during his epic run of the Colorado River through the Grand Canyon, AZ in 1876. Here he noticed that deformed ancient metamorphic rocks were covered by much younger undeformed sedimentary rocks. New Mexico has some of the best exposures of the contact between these very old Precambrian rocks (1.7 billion years) and younger sediments (300 million years) of anywhere in North America.
The depositional age of the Gatuña Formation in the Pecos Valley of southeastern New Mexico is poorly constrained, with estimates that vary from as old ca. 13 Ma at its base to as young as ca. 100 ka at its highest levels. As part of geologic mapping program efforts, we are applying detrital sanidine Ar-Ar geochronology and detrital zircon U-Pb geochronology to more tighlty bound the depositional age and duration of these alluvial deposits and their context within the late Cenozoic paleo-landscape of the ancestral Pecos River.
A new dating method, being developed at the NMBG&MR, uses our state-of-the-art geochronology laboratory, funded by NSF and NM Tech, to determine the age of detrital sanidine (tiny volcanic minerals) from sediments.
Pre-Cordilleran rocks of western North America are predominantly composed of inboard, more stratigraphically coherent assemblages and more outboard assemblages with tectonostratigraphic histories obscured by extensive deformation, magmatism, and metamorphism. Inboard assemblages generally represent autochthonous deposits of the western Laurentian continental margin that formed in response to the breakup of the Rodinian supercontinent whereas outboard packages define a tectonic collage representing westward continental growth since mid-Paleozoic time . Detrital zircon U-Pb geochronology of metasedimentary strata across western North America has revealed varied sedimentary sources from both within and without the Laurentian craton that shift through time and space.
