Earth and Planetary Sciences ETDs

Publication Date

7-1-2002

Abstract

The northeast-trending Virgin Mountain anticline (VMA) of the North Virgin Mountains straddles the boundary between the unextended Colorado Plateau and the highly extended crust of the central Basin and Range province. The anticline is 50 km long and 8 km wide, is doubly-plunging, and has overturned Paleozoic beds on both limbs. The crystalline core of the VMA is composed of Paleoproterozoic supracrustal and intrusive rocks that record four tectonic events from circa 1740 Ma to 1550 Ma (d1-D4) characterized by distinct structural fabrics and associated metamorphic assemblages and microstructures. The most prominent Proterozoic structure is the Virgin Mountains shear zone (VMSZ), which was initially defined as a crustal weak zone during D1 northeast-southwest contraction and may represent an important crustal suture, as it contains "exotic lithologies" commonly associated with ophiolites. A clockwise-rotating strain field during progressive east-west (D2) and northwest-southeast (D3) contractional events resulted in the prominent northeast-strike of this zone. Late Paleoproterozoic, east-west contraction across this zone (D4) resulted in a complex array of linked, dextral, transpressive shear zones which partitioned strain into strike-slip, pure-shear, reverse, and normal sense deformation zones. Deformation within the VMSZ continued to 1600 and 1550 Ma based on syn-tectonic monazite rim ages that are associated with sub 500 degree Celcius (greenschist facies) metamorphic assemblages and textures, and sub 500 degrees Celcius deformational microstructures. Development of these highly fissile shear zones later controlled the geometry of Laramide and Miocene brittle deformation. The Proterozoic basement in the core of the VMA now resides at elevations of more than 2 km above sea level, roughly 2.5 km higher than the elevation of basement in the adjacent Colorado Plateau. The northeast trend and vertical uplift of the anticline is a result of Miocene east-west extensional deformation superimposed on the pre-existing northeast- and north-trending structural grains created during Paleoproterozoic (D2, D3, D4) and Laramide contractional tectonic events. Outward-verging monoclinal reverse faults similar in style to Laramide-aged faults in the Colorado Plateau are present on the east and west limbs of the VMA, and we propose that much of the vertical uplift of the anticline occurred during this time. The geometry of Miocene deformation was both strongly partitioned and directed by these pre-existing structures, and also manifest as steeply dipping conjugate normal faults in the Mesozoic and Paleozoic section that soled in to basal detachments in the Cambrian Bright Angel Shale and at the Great Unconformity. Apatite-fission track (AFT) dates range from 21.7 ± 2.3 Ma directly below the Cambrian-Precambrian unconformity to 14.0 ± 2.5 Ma in the core of the VMA, and indicate that the anticline was unroofed in Miocene time. Short AFT lengths (<13m) with large standard deviations (>2.5) from the 22-20 Ma AFT ages suggest that Proterozoic rocks of the VMA cooled slowly through the AFT partial annealing zone at 22-20 Ma, prior to extension, while longer AFTs suggest rapid cooling and synextensional exhumation from 16-14 Ma. We interpret the former ages to represent pre-extensional erosional cooling of a regionally elevated terrain, contrary to most models which assume a peneplain pre-extensional surface. Our model is consistent with geophysical, sedimentological, and tectonic studies of the region. MOOTS/ASTER Airborne Simulator (MASTER) remote sensing data were acquired over the YMA in order to evaluate the utility of the data for geological mapping in a structurally complex area. These data proved invaluable in producing more than 60 km2 of new geologic mapping by the first author, and recognizing many of the key structures which led to our Proterozoic, Laramide, and Miocene tectonic interpretations.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Selverstone, Jane

Second Committee Member

Brearley, Adrian

Language

English

Keywords

Tectonics, Tectonic Development, Proterozoic, Laramide, Miocene, North Virgin Mountains

Document Type

Thesis

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