Research Interests

My research interests are in reconstructing the thermal histories for rocks. Argon thermochronometry is my favorite, however I also use U-Th/He, U/Pb, and fission track chronometry as well. Basically, any geologic investigation that warrants argon thermochronometry (field-based or experimental) is of interest to me. Here are some projects I am working on.

Extension

In the late Cretaceous (~75-65 Ma) a widespread extensional event took place in the southern hinterland of the Sevier Orogen. This event is characterized by rapid cooling of mid-crustal rocks and motion along shear zones, commonly with a top-to-the-southwest transport direction. The Granite Mountains are a Mesozoic mid-crustal plutonic complex consisting of Jurassic and Cretaceous plutons capped with several small metamorphosed Paleozoic roof pendants. U/Pb zircon geochronology and ⁴⁰Ar/³⁹Ar thermochronometry indicate the Cretaceous plutons cooled very rapidly following their intrusion into the middle crust from ~73-65 Ma. A cooling rate gradient across the range leads us to believe the cause for rapid cooling is indeed tectonic and not an effect of regional cooling due to a change to shallow slab subduction. Additionally, thermal signatures (⁴⁰Ar/³⁹Ar K-feldspar, apatite fission-track, and U-Th/He apatite) and field relations with adjacent mountain ranges indicate the Granites represent a "mid-crustal horst" in the extensional domain of the hinterland. Studies are currently ongoing to further document this extensional event in the southwest and constrain the timing and rates of extension and possible mechanisms driving extension. Collaborators are Terry Spell (UNLV), Michael Wells (UNLV), and Keith Howard (USGS).

Shortening?

Recently, we have identified a previously unrecognized mylonite zone in the northeast Granite Mountains. In this zone, granitic mylonite sits above quartzite mylonite (Zabriskie?) with foliations striking 019^o and dipping ~20^o NW. Sense of shear is consistently top towards 019^o in both the granitic and quartzite mylonite. U/Pb zircon ion probe data from a sample of the granitic mylonite indicates a protolith crystallization age of 150-155 Ma. These data seem to be pointing to an earlier record of crustal thickening as documented in the Clark Mountains thrust complex, the Clipper Mountains, East Sierran thrust system, etc. More work will be done to attempt to better constrain the timing of deformation and understand the significance of these mylonites.

Just across the Nevada-California border extending south from the Spring Mountains is the Clark Mountains area of Walker et al. (1995). This region was mapped decades ago by Burchfiel and Davis (1971, 1977, 1981, 1988). This area is significant to the geologic history of western North America because it represents the triple junction between the eastern margins of the magmatic arc, the Sevier fold-thrust belt, and the east Sierran fold-thrust belt. Contained within the Clark Mountains thrust complex are exposures of crystalline Precambrian basement, Precambrian to Mesozoic miogeoclinal and cratonal strata, and Mesozoic plutonic and volcanic rocks. The thrust faults from structurally highest to lowest (and east to west) vary from deep-seated ductile thrusts involving arc rocks and Precambrian basement (Winters Pass/Pachalka/Sterling thrusts) to a three-sheet thrust system progressively shallowing from ductile to brittle thrusts (Mesquite Pass thrust) to a brittle decollement style thrust (Keaney/Mollusk Mine thrust) typical of thin-skinned Sevier thrusting to the north. My interest in this area is in the Ivanpah Pluton. This pluton appears to intrude into the core of an anticline, however it has been dated as Late Jurassic (Walker et al., 1995) and the anticline has been dated as mid-Cretaceous (Fleck et al., 1994). Hmmmm. Evidently, the pluton was present during folding. The pluton is cut by the Morning Star and Sunnyside thrust faults, that may represent propagating 'blind thrusts' to the eastward overturned anticline. By initially attempting to track mica ages across the aerial exposure of the pluton to identify thermal boundaries across these two faults, we have discovered a ~20 Ma discrepancy in biotite ages from the edges of the pluton to the interior. This is way too long to be the result of protracted cooling, and is likely reheating, or (as I'm more inclined to think right now) representative of episodic construction of the Ivanpah pluton - more appropriately, the Ivanpah plutons. Now, thermal history analyses is being combined with U/Pb geochronology of any distinguishable pluton bodies discovered. Collaborators are Michael Wells (UNLV).

New Zealand, Australia, and Antarctica were connected making up the Pacific margin of Gondwana. Following Paleozoic and Mesozoic episodes of subduction, arc magmatism, continental collision, and accretion, a reorganization of plates at ~110 Ma possibly related to interaction between a spreading center and the Gondwanan margin subduction zone resulted in continental extension and eventual fragmentation during the Cretaceous. This extensional event resulted in the opening of the Tasman Sea ca. 85 Ma. In western South Island, this extensional event is recorded in the Paparoa Metamorphic Core Complex, which was exhumed from middle crustal levels episodically along two detachment faults. Recent mapping by Allibone and Tulloch (1997) revealed the presence of extensional structures on Stewart Island. These findings include coastal mylonites with top-to-the-southeast kinematics along with coarse grained conglomerates exposed offshore as islets. Because the bedding orientation of the conglomerates is coaxial with the solid-state mylonitic fabric, we interpret these rocks as the upper plate to a detachment fault that extends southeast beneath the Great South Basin and Campbell Plateau. Also of interest is that this structure represents an extension direction nearly perpendicular to that preserved in the Paparoa, and therefore likely records separation of New Zealand from Marie Byrd Land, Antarctica and may be related to mylonites of the Ross Sea (Siddoway et al., 2003; Fitzgerald and Baldwin, 1997). Collaborators are Terry Spell (UNLV) and Andy Tulloch (IGNS)