Project Goals:

• Determine what lithospheric conditions are favorable for "core-complex" sytle rifting
• Examine lithospheric conditions in Arizona during the onset of mid-Tertiary Cordilleran rifting using xenoliths from syn-rift volcanic deposits
• Contrain input to numerical models that simulate rift environments with low-angle normal faults

Overview:

The primary research objective of this project is to study the mechanical state of the lower crust and upper mantle during detachment faulting and metamorphic core complex formation in the mid-Tertiary Cordilleran rift. I seek to answer the questions: A) was lithospheric extension accommodated by 1) rolling-hinge style detachment faults requiring lithospheric flexure, 2) very weak planar, low-angle detachment faults in strong lithosphere, or 3) flow of weak lower-crustal rock beneath a low-angle detachment fault, and B) what lithospheric conditions favor core-complex-style rifting? I will address these questions by examining the thermal state and deformation textures in the lower crust and upper mantle using thermobarometry, textural, and geochemical analyses of xenoliths from Transition Zone alkaline volcanic deposits that erupted concurrent with detachment fault activity. The intellectual merits of this project are that is has the potential to help resolve uncertainties related to the development of metamorphic core complexes that have stood since the 1970’s. These xenoliths provide a unique opportunity to examine lower-crustal and mantle processes from within and below the zone of detachment fault activity. Results of a previous thermobarometry analysis performed on one such Transition Zone xenolith suite indicate that the lithosphere was far colder during rifting than many numerical models suggest. It is important to verify and expand upon this prior investigation by including other suites in order to understand Cordilleran core-complex rifting.

Current field work (summer, 2007) and laboratory work are supported by a CSU-AAUP research grant. I am applied for funding to continue this project through a NSF early career devellopment grant proposal; currently pending.

The region addressed by the project includes the Harcuvar-Harquahala core complex within the Basin and Range tectonic province and several volcanic fields across the Transition Zone tectonic province. The Transition Zone separates the Basin and Range, which was a high mountain range with thick crust prior to being extended and thinned by ~100% during Tertiary rifting, from the Colorado Plateau, which has been a relatively stable crustal block through several Mesozoic and Cenozoic tectnoic events. The Harcuvar-Harquahala core complex is an exposure of middle-crustal rock that was denuded from beneath the Transition Zone in the footwall of a low-angle normal fault (detachment fault). The xenolithos used in this study were likely sampled by ascending alkaline mantle melts during the onset of rifting. This project will include mineral chemistry and textural analysis of xenoliths as well as Ar-Ar geochronology to constrain the age of volcanic deposits relative to phases of rifting.