I am a geologist in the Jackson School of Geosciences at the University of Texas at Austin. I am interested in the way the Earth works. I study how mountains form and how the Earth changed over geological time.

Specific research interests: metamorphic/tectonic evolution of mountain ranges, applying/developing accessory mineral geochronology to broad questions about Earth’s history, models for heat, mass, and fluid flow along fault systems


Himalayan evolution in the sands of time: Geochemical analysis of Siwalik garnets (Nepal)

Priimak, L., Catlos, E.J., Sorkhabi, R. (2023). Himalayan evolution in the sands of time: Geochemical analysis of Siwalik garnets (Nepal). 2023 AGU Fall Meeting, San Francisco, CA, 11-15 Dec


As the collision between India and Asia progressed, sediments in the foreland basin were incorporated into the Himalayas between two of its major faults, the Main Boundary Thrust and the Main Frontal Thrust. These sediments are termed the Siwalik Formation and provide a detailed record of the Himalayas’ erosional history, paleoclimate, transitions in paleobotany and biology, and exhumation. One critical question regarding Himalayan evolution that can be addressed by detailed analyses of Siwalik Formation sediments is how its record of metamorphic minerals tracks with our ideas of metamorphic processes recorded by bedrock geology. To address this question, we chemically analyzed garnets from the middle and upper sections of the Siwaliks exposed in the Surai Khola of Nepal. The Surai Khola samples cover the Middle Miocene through Pleistocene time interval (13-1 Ma). Garnets from the sections are abundant at some levels and appear chemically distinct. Their chemistry and compositional zoning allow the affiliation to be made to particular units within the Himalayas. For example, Greater Himalayan garnets have compositions and zoning consistent with higher-grade metamorphism (low Mn, flat zoning). In contrast, Lesser Himalayan Formation garnets preserve more prograde compositions (high Mn cores, bell-shaped profiles). We aim to use the appearance, relative abundance, and composition of metamorphic index minerals in the Siwalik sandstones, including garnets, to constrain the validity of models for Himalayan metamorphism and processes.

How old is the Ordovician–Silurian boundary at Dob’s Linn, Scotland? Integrating LA-ICP-MS and CA-ID-TIMS U-Pb zircon dates

Garza, H.K., Catlos, E.J., Chamberlain, K.R., Suarez, S.E., Brookfield, M.E., Stockli, D.F., Batchelor, R.A. (2023) How old is the Ordovician– Silurian boundary at Dob’s Linn, Scotland? Integrating LA-ICP-MS and CA-ID-TIMS U-Pb zircon dates. Geological Magazine, 160, 1775–1789.


Sedimentary rocks exposed at Dob’s Linn, Scotland, have significantly influenced our understanding of how life evolved over the Ordovician to Early Silurian. The current interpreted chronostratigraphic boundary between the Ordovician and Silurian periods is a Global Boundary Stratotype Section and Point (GSSP), calibrated to 443.8 ± 1.5 Ma (Hirnatian–Rhuddanian age), based on biostratigraphic markers, radioisotopic dates and statistical modelling. However, challenges arise due to tectonic disturbances, complex correlation issues and the lack of systematic dating in Ordovician–Silurian stratigraphic sections. Here, hundreds of zircon grains from three metabentonite ash horizons were dated using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). A subset of the grains were re-analyzed using Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS). We present a high-precision CA-ID-TIMS 238U-206Pb weighted mean date of 440.44 ± 0.55/0.56/0.72 Ma (±analytical/with tracer/with U-decay constant) for the Coronagraptus cyphus biozone. However, the study reports younger, and in certain cases, older LA-ICP-MS zircon dates within the Coronagraptus cyphusAkidograptus ascensus and Dicellograptus anceps zones, suspected as being influenced by Pb loss and LA-ICP-MS matrix mismatch. The study reports concerns about the suitability of Dob’s Linn as a GSSP section and examines various LA-ICP-MS maximum depositional age (MDA) approaches, suggesting the use of the TuffZirc date and the youngest mode weighted mean (YMWM) as suitable MDA calculations consistent with CA-ID-TIMS results.

Quantifying pore-fluid pressure ratios and analyzing deformation mechanisms in the Whipple Mountains brittle-ductile shear zone

Jaramillo, V., Yin, A., Catlos, E.J., Bell, E., Chin, E.J., Schmitt, A.K. (2023). Quantifying pore-fluid pressure ratios and analyzing deformation mechanisms in the Whipple Mountains brittle-ductile shear zone. GSA Abstracts with Programs, 55(6), 2023


Pore-fluid pressure is a key factor controlling the stress state and rock failure in Earth’s crust. Although its role in brittle deformation in the shallow crust (< 1-3 km) has been extensively examined, and in some cases quantified by direct bore-hole measurements, how pore-fluid pressure affects crustal deformation at brittle-ductile-transition depths (~15-25 km) remains poorly constrained. Deformation that occurred at brittle-ductile transition depths is commonly expressed by the development of semi-brittle shear zones characterized by coeval cataclastically (frictional sliding and fracturing) and crystal-plastically (dislocation and diffusion creep) of deformed rocks. The distinct deformation styles within the same shear zone require stress continuity across the contact between brittle and ductile structures. This stress-continuity condition in turn allows us to use paleobarometry and paleopiezometry to determine the stress state (i.e., the differential stress and mean stress) during semi-brittle deformation. Because the frictional coefficient (~0.6) and cohesive strength of crystalline rocks (<50 MPa) are well-known from laboratory experiments (i.e. Byerlee’s Law), we are able to use the estimated differential and mean stresses to determine the ratio between pore-fluid pressure and lithostatic pressure during the development of the Whipple detachment shear zone. Our results suggest pore-fluid pressure ratios between 0.92 and 1.11 which is consistent with observed tensile-fracture veins (pore-fluid pressure ratio >1.0) developed during the crystal-plastic deformation of quartzite in the shear zone. New P-T and monazite-in-garnet U/Th-Pb age constraints combined with the pore-fluid pressure ratios provide insights into the evolution of the Whipple detachment shear zone. A new P-T-t path may be established for the rocks in the Whipple shear zone and may suggest a deeper initiation or multiple metamorphic events not previously analyzed.