Earth and Planetary Sciences ETDs

Publication Date

Fall 12-17-2019

Abstract

Metasedimentary and igneous basement rocks in northern New Mexico record episodic pulses of tectonism during cratonic growth from 1.8 to 1.38 Ga. Continued challenges involve parsing the deformational features attributable to the Yavapai Orogeny (1.71-1.68 Ga), Mazatzal orogeny (1.66-1.60 Ga), and Picuris orogeny (1.5-1.38 Ga) in this region and understanding how older structures may have been overprinted and reactivated to explain the observed strain. This paper presents regional cross-sections of the 1.7 Ga Vadito, 1.68 to 1.50 Ga Hondo, and 1.5-1.45 Ga Trampas groups of the Picuris Mountains and Rio Mora areas of northern New Mexico combined with new geochronologic and thermochronologic constraints. These data suggest the following tectonic evolution. An early bedding-parallel S1 fabric is present only in pre-Trampas Group rocks and is interpreted to be related to early isoclinal folds and thrusts. All groups were folded by F2 and F3 fold generations during the Picuris orogeny. New U-Pb dates from monazite cores of about 1.5 Ga in aluminous gneisses from the Vadito- Hondo group aluminous contact zone within the Pecos thrust sheet are interpreted as onset of Picuris orogeny prograde tectonism. This onset of heating/deformation is thus close in v age, and apparently pre-dates deposition of the Trampas Group at ~1488 ± 6 Ma in the footwall of the Pecos and Plomo thrusts. Growth of metamorphic garnets at 1.45- 1.40 Ga from previous studies and new monazite rim dates of ~1.45 Ga may record metamorphism close in age to deposition of the thrust-bound 1.453 ± 10 Ga Marqueñas Formation. Regional cross sections show that the modern surface exposure represents an exhumed ~ 15 km middle crustal duplex system that juxtaposes, from north to south: 1) 1.45 Ga triple point (>550 C, 3.5 kbars) polyphase metamorphic tectonites in the Copper Hill anticline- Hondo syncline thrust sheet made between the Pilar and Plomo thrusts, 2) a similar grade thrust-bound Marquenas tectonite that makes up the Plomo shear zone, 3) the 1.45 Ga Penasco granite that intrudes Vadito Group rocks between the Plomo shear zone and the Pecos master thrust, and 4) overriding volcanic rocks of the ~ 1.72-1.65 Ga Pecos volcanic complex. Juxtaposition of amphibolite-grade upper plate rocks against 1.48-1.45 Ga metasedimentary lower plate rocks as the product of progressive thrusting from 1.50 to 1.40 Ga synchronous with development of syntectonic foreland basins. The 1.45 Ga granite and pegmatites show only minor penetrative strain but are considered broadly syntectonic with waning deformation at 1.45- 1.40 Ga. A modern analog is the Tien Shan – Tarim basin region of the interior Tibetan Plateau inferred for the Picuris tectonism. New 40Ar/39Ar dates on hornblende and muscovite constrain cooling through ~500 °C and ~350-400 °C respectively and provide insight into cooling rates following the Picuris orogeny, and hence the nature and timing of demise of Picuris orogenic uplifts. Hondo and Trampas group rocks cooled through 500 °C 1420-1381 Ma and vi through 350 °C from 1378-1359 Ma. This is similar to new data with 40Ar/39Ar cooling data from muscovite from the Petaca pegmatite district (mean cooling age of 1375 ± 10 Ma), and similar ages in basement exposures from neighboring mountain ranges. Slow cooling from >500 to <350 >°C from ~1420 to ~1360 Ma suggests cooling rates of ~ 3 °C/Ma over 60 Ma following peak triple point metamorphism. To explain this protracted cooling we propose that an orogenic plateau was built by thrust duplexing and penetrative folding in northern New Mexico during the Picuris orogeny followed by relatively slow erosional removal. Erosion of much of the region initiated by 1.1-1.3 Ga, resulting in deposition of the Debaca Group and correlative strata.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Karl E. Karlstrom

Second Committee Member

Matthew T. Heizler

Third Committee Member

Adrian Brearley

Language

English

Keywords

Picuris orogeny, tectonics, Precambrian geology

Document Type

Thesis

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