Contributions to Geology 19.2

Plate-tectonic mechaism of Laramide deformation

Warren Hamilton

Pages
87-92

Keywords
Plate-tectonic mechaism of Laramide deformation

Abstract
The Laramie compressive deformation of the craton was caused by a clockwise rotation of about 2 or 4 degrees of the Colorado Plateau region relative to the continental interior, during late Late Cretaceous and early Tertiary time. The Euler pole of this rotation was in the near northern Texas. This rotation absorbed a few percent of the convergence between North American plate and the Farallon (eastern Pacific) plate that was being subducted beneath it.
Late Paleozoic and Neogene deformation of the craton also were produced by motion of a southwestern subplate relative to the continental interior.

Review of thrusting in the Wyoming Foreland

R. R. Berg

Pages
93-104

Keywords
Review of thrusting in the Wyoming Foreland

Abstract
The major foreland uplifts of Wyoming are asymmetric, fault-bounded folds of Precambrian crystalline rocks. Their structures are well documented tint he Wind River Mountains by geophysical and subsurface data. Seismic surveys in the 1950’s showed that the southwest flank of the Wind River is underlain by a thrust fault that dips and average of 30° beneath the mountains. Horizontal displacement is greater than 13 km (8 mi), and total vertical uplifts is on the order of 14 km (9 mi). Based on wells drilled through similar structures, the thrust zone is believed to consist of overturned Paleozoic and Mesozoic rocks. Interpretations of gravity data confirm the general configuration of the mountain flank but cannot establish the nature of deep-crustal structure.
Results of more recent drilling further support the structural interpretation of the Wind River flank as well as document the similarities in structural style of other foreland uplifts. In fact, all recent data continue to favor the hypothesis of “fold-thrust” uplift. However, major problems remain. These concern (1) the deformation of the brittle , Precambrian rocks into the antiform structure, (2) the nature of deep structure beneath the uplifts, and (3) the origins of compressive forces within the crust. Solutions to these problems will come primarily from examination and interpretation of subsurface and geophysical data, aided perhaps by studies of the exposed Precambrian rocks in the cores of uplifts.

Regional stresses and deformation associated with arc-trench complexes and the Wyoming foreland province; a different view

Vincent Matthews, III

Pages
123-126

Keywords
Regional stresses and deformation associated with arc-trench complexes and the Wyoming foreland province; a different view

Abstract
Deformation of the upper crust in the modern and ancient arc-trench system is coeval with subduction of oceanic lithosphere beneath the deformed belt. Stress indicators in both ancient and modern arc-trench systems suggest that a horizontal compressive force applied to the margin of the overriding plate is not the sole cause of the stress filed in the overriding slab.
Deformation in the backarc area is primarily a result of an extensional stress system in the crust. Deformation in the arc-massif occurs in response to a stress field generated primarily by differential vertical forces acting on the base of the brittle crust. Deformation in the subduction zone probably results from an inclined shear couple rather than from a horizontal compressive force.
The subduction of oceanic lithosphere is a process believed to be much more passive than previously thought.

grand Canyon monoclines: vertical uplift or horizontal compression?

Peter W. Huntoon

Pages
127-137

Keywords
grand Canyon monoclines: vertical uplift or horizontal compression?

Abstract
Most of the monoclines in the Grand Canyon region developed over high-angle reverse faults in the underlying crystalline basement rocks. It is tempting to infer from the steep dips of the basement faults that monoclinal development was associated with a vertical uplift mechanism. However, the observed high-angle faults were inherited from Precambrian time, so the crystalline basement was structurally anisotropic during Laramide folding. Because of this, no simple relationship exists between the dips of the reactivated faults and principal stress directions.
One segment of the Meriwhitica monocline in the western Grand Canyon developed over a Laramide fault in the Precambrian Crystalline rocks. This unique fault can be related to the Laramide stress and is a low-angle thrust. Its low dip implies horizontal compression that was unaffected by pre-existing structural anisotropy in the basement. Other evidence supporting horizontal principal stresses includes imbricate and conjugate sets of minor thrusts and fractures in the overlying Paleozoic section. All of these structures result in shortening of the crust in the area which is consistent with the horizontal compression.

Laramide crustal shortening in the northern Wyoming Province

Lisa R. Kanter, Russ Dyer and Ted E. Dohmen

Pages
135-142

Keywords
Laramide crustal shortening in the northern Wyoming Province

Abstract
Migrated COCORP seismic reflection profiles (Lynn, 1979) across the southern Wind River Mountains, Wyoming, show that the bounding fault of the Wind River uplift is a thrust; thus the Wind River uplift is a compressional feature. The thrust maintains a dip of about 30° to a depth of 24 kilometers where it begins to shallow. The fault zone extends more than half way through the crust, although the Moho shows no apparent offset in the seismic section or in gravity models (Smithson and others, 1978). The Wind River uplift and adjacent basins seem to be representative of the structural style of the entire Wyoming Province. We have applied the fault geometry observed in the Wind River area to other Laramide age structures in the northern Wyoming Province and calculated a first order, minimum estimate of regional crustal strain using several methods. The upper crust of the Wyoming Province was shortened and thickened by at least 5% in Laramide time with the shortening directed mostly northeast-southwest.

The Pacific Creek anticline: buckling above a basement thrust fault

Mark K MacLeod

Pages
143-160

Keywords
The Pacific Creek anticline: buckling above a basement thrust fault

Abstract
The Pacific Creek anticline is a broad, generally north-south trending, geologic structure in the Green River Basin north of the Rock Springs uplift and south of the Wind River Mountains in the southwestern Wyoming. The anticline was formed during the Late Cretaceous-Early Tertiary Laramide orogeny. The structural elements within the anticline suggest an origin by horizontal compression evidenced by basement thrust faulting. Beneath the anticline, on the COCORP seismic Line 1, there is evidence of a basement thrust fault that is similar in character, though not in displacement, with the larger Wind River thrust to the north, Antithetic reverse faults developed in the upthrown thrust block and produced a “pop-up” block pattern. Depositional thinning over these blocks is seen first in the Late Cretaceous Lance-Lewis time and continues into Paleocene Fort Union time.

Mechanisms of deformation within Laramide and Precambrian deformation zones in basement rocks of the Wind River Mountains

Gautam Mitra and B. Ronald Frost

Pages
161-173

Keywords
Mechanisms of deformation within Laramide and Precambrian deformation zones in basement rocks of the Wind River Mountains

Abstract
The Precambrian basement rocks of the Wind River uplift show at least three generations of deformation zones. The earliest zones, which may record several deformation events, are recrystallized and apparently formed during or before the last Archean regional metamorphism. Late Precambrian deformation zones, dating from 900 to 600 million years ago, cut all crystalline rocks. These zones are characterized by greenschist retrogression, particularly by extensive development of chlorite. These deformation zones show strong preferential weathering and form most of the distinct lineaments which are visible on air photos. Laramide deformation zones, characterized by closely spaced fractures and zones of granulation with only minor amounts of secondary mineral growth, are seen to cut underformed basement, Late Precambrian deformation zones, and the overlying Paleozoic sedimentary cover. It is believed that the 10% crustal shortening seen across the Wind River uplift was accommodated in the basement by movement along these Laramide deformation zones.