Gary Axen, Associate Professor of Geology
Research Areas (see publications)
- Fault mechanics and evolution of fault rocks: A raging controversy exists in geology and geophysics over the strength of natural faults relative to laboratory friction measurements—are they strong or weak?? Various lines of evidence suggest that faults are weak relative to lab friction, but the cause for this is unclear. This controversy centers on the San Andreas fault, but only very limited samples will be returned from the SAFOD drill site. Mechanically enigmatic low-angle normal faults (aka detachment faults) provide numerous opportunities to study fault mechanics and fault-rock evolution, because their large-magnitude slip delivers directly to the surface fault rocks formed at depth (i.e., in the seismogenic zone). Detachment faults are the “weak end-member” of all natural faults: they form at high angles to the maximum principal stress as well as under low differential stress relative to thrusts or strike-slip faults, so share important similarities with more hazardous faults such as the San Andreas. Recent work characterized the paleo-stress field around the Whipple and West Salton detachment faults tested several mechanical models of weak-fault slip and fault evolution. These studies also showed that stress-drop during West Salton detachment earthquake events was nearly complete. More recent and ongoing work is focused on the textural evolution of fault rocks formed at paleosismogenic slip rates versus slow slip rates, or at siesmogenic depths (at confining pressure that cannot be replicated in fast-slip laboratory experiments) versus low-confining pressure slip (using fault rocks formed along landslide bases). Support from NSF Tectonics Program, Southern California Earthquake Center, Rock Mountain Association of Geologists, and the New Mexico Geological Society.
- Age and dynamics of emplacement of the Socorro Magma Body (SMB) and related surface uplift: The SMB is the second largest known magma body on Earth: it is a sill about 130 m thick at a depth of 19 km and with a map-view area of ~3400 km 2 (so ~340 cubic km of magma). Surface uplift above the SMB occurs in a bulls eye-shaped region with rates up to ~2.5 mm/yr at the center. We use arched river terraces to show that the SMB had at least one earlier inflation event, between ~27,000 and 3,000 years ago, so is not a “one-shot deal”;. Terraces are deformed both by the SMB and by active normal faults, so this study also yields information about normal fault activity. Terrace remnants and flights were mapped and correlated regionally using soils, surface geomorphology, and terrace-fill stratigraphy, and dated using 36 Cl depth profiles and 14 C. These data will also provide information on climatic forcing (glacial cycling) of river terrace formation. Numerical geodynamic modeling separates out uplift effects due to pressurization of the SMB (inflation) versus thermal expansion of rocks above the SMB, showing that the two processes result in different spatial-temporal patterns of surface uplift. These results show that the SMB actively inflates, and strongly suggest a mechanism by which that inflation causes earthquake swarms. Co-PIs are Jolante van Wijk, Fred Phillips and Bruce Harrison. This project forms the core of PhD research of Brad Sion (river terrace work) and M.S. work of Rediet Abera and Shuoyu Yao (geodynamic modeling). Support from NSF Tectonics Program.
- 3D palinspastic reconstructions of the highly extended Death Valley terrain, from the southern Sierra Nevada to Las Vegas, Nevada: 3D reconstructions are being made at various time steps between ~14 Ma and the present, and will serve as input to a forward geodynamic-hydrologic model that predicts surface evolution and sedimentation and groundwater flow during the tectonic evolution of the area. These predictions will be tested against spring water chemistry and subsurface residence times, as well as biological diversity and endemism of spring fish, snails and microbes. NMT Co-PIs include Fred Phillips (lead PI), Jolante van Wijk, John Wilson, plus professors and students at UNLV, Purdue, Desert Research Institute and University of the Pacific. This project supports three NMT PhD students and one Postdoctoral Researcher. Support from NSF Integrated Earth Systems Program.
- Extensional tectonics of the Great Kavir (Desert), central Iran: Thermochronology ( 40 Ar/ 39 Ar on biotite, muscovite amphibole and K-feldspar multi-domain diffusion modeling) applied to the footwall of a normal-sense detachment fault system shows that large-magnitude extension occurred in this part of Iran in Late Cretaceous time, earlier than exhumation of other known metamorphic core complexes in Iran. Stratigraphic relationships suggest that extension continued to Eocene time as part of a regionally developed extensional system. PhD research of Ahmad Malekpour-Alamdarie. Collaboration with Matt Heizler of the New Mexico Bureau of Geology.
- Large-magnitude gravity-slide tectonics, Sawtooth Mountains, New Mexico: Remnant klippen of one or more gravity-driven sheets record motion on an(?) allochthon, originally at least several tens of square kilometers in area, underlain by a zone of soft-sediment deformation ~50-100 m thick. Sediments involved are coarse, volanogenic sandstones and conglomerates of the alluvial apron from a mid-Cenozoic andesitic stratovolcano. We use detailed geologic mapping and cliff-mapping of the klippen (most exposures are on cliffs) in conjunction with structural measurements and outcrop- to thin-section-scale structural observations to determine if sliding occurred in a single or multiple events, catastrophically or slowly, and to determine the direction(s) of sliding, which will help to understand the cause(s) of the gravity-driven tectonics. M.S. thesis of Jeff Dobbins, co-advised by Steve Cather of the New Mexico Bureau of Geology, and M.S. project of Michael Chirigos.
- Decollement tectonics of Laramide and Rio Grande Rift age: Paleozoic rocks east of Socorro, in an area of thousands of square kilometers in size, are detached along Permian evaporite-rich strata and display structures mainly consistent with internal extension and top-east motion of the upper plate. Folds in the decollement zone are cut by dikes of ~32 Ma age and both folds and dikes are cut by ramp-flat low-angle faults with younger-on-older stratigraphic separation. These faults controlled local deposition in a volcanic-sedimentary half graben. It appears that the decollements formed in front of (in the footwall of) a Laramide uplift, and were reactivated during Rio Grande rift time. Undergraduate research of Santiago Flores and M.S. project of Mark Green, co-advised by Steve Cather of the New Mexico Bureau of Geology.