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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


Wyoming Sedimentation and Tectonics; 41st Annual Field Conference Guidebook, 1990
Pages 183-185

The Case for a Continuous Heart Mountain Allochthon: Abstract

Thomas A. Hauge1

Abstract

Two fundamentally different models describing the geometry, kinematics, and mechanism of emplacement of the allochthon of the Heart Mountain detachment have been advocated. The traditional "tectonic denudation model" comprises catastrophic gravity sliding of numerous detached blocks, with widespread subaerial exposure of the detachment by tectonic denudation. In this view, catastrophic volcanism covered the allochthonous blocks and the denuded detachment immediately after implacement of the slide blocks. The alternative "continuous allochthon model" comprises non-catastrophic gravity spreading of a continuous allochthon, without tectonic denudation of the detachment. In this view, volcanism was noncatastrophic and coeval with, rather than subsequent to faulting; and volcanic rocks constitute a large volume of the allochthon. Recent field studies indicate that the continuous allochthon model is geometrically and kinematically in accord with field relationships, analog models, geometric models, and relationships characterizing other detachment complexes; whereas the tectonic denudation model is not similarly substantiated. In particular, the tectonic denudation model is disproved by field relationships which demonstrate that volcanic rocks once thought to postdate faulting are instead tectonically emplaced. This is demonstrated at specific localities where volcanic rocks have previously been interpreted as in depositional contact with the detachment.

Theoretical studies indicate that emplacement of the allochthon envisioned in the continuous allochthon model is mechanically plausible, whereas the catastrophic emplacement of detached blocks envisioned in the tectonic denudation model is mechanically implausible. Despite a broad range of proposed mechanisms [volcanic explosions, earthquake vibrations, acoustic fluidization, abnormal fluid pressure ("beer can," "hovercraft," "air-cushion," and "phreatomagmatic-hydraulic" versions), and "pneumatic-hydraulic plastic-wedge" models], no plausible mechanism has been found to allow detachment of numerous large slide blocks (areas of up to 10s of km2) and long-distance displacement of such blocks (up to 30 miles (50 km)), as is required by the tectonic denudation model. In contrast, the mechanics of gravitational spreading of continuous allochthon, both compressional and extensional, is well understood both theoretically and empirically. In the context of the continuous allochthon model, gravitational spreading explains the emplacement of the Heart Mountain allochthon.


 

wga0490183-fg1.jpg (3,608 bytes)Figure 1. The tectonic denudation model of Heart Mountain faulting, modified from Pierce (1960). Before faulting (1), volcanic rocks locally overlay Paleozoic strata. When the detachment formed (2), Paleozoic strata and overlying volcanic rocks detached along a basal Ordovician bedding plane and were emplaced as numerous slide blocks at catastrophic rates along the bedding-plane detachment and across the Eocene land surface. Emplacement was gravity-driven, aided by earthquake oscillations (Pierce, 1973). Immediately after faulting (3), catastrophic volcanism blanketed the disrupted terrane. Dotted line is present erosion surface. Patterns: random dash = Precambrian basement; dash = Cambrian shale; Previous HitbrickNext Hit = Ordovician to Mississippian sedimentary rocks, largely carbonate; dot-dash = late Paleozoic and younger sedimentary rocks; dash-v = Eocene volcanic rocks.

wga0490183-fg2.jpg (3,770 bytes)Figure 2. The continuous allochthon model of Heart Mountain faulting, modified from Hauge (1985). Before faulting (1), volcanic rocks 1 or more km thick overlay deeply eroded Paleozoic strata and younger strata to the southeast. When the detachment formed (2), Paleozoic strata and overlying volcanic rocks above the detachment underwent lateral translation and extension while structurally high (largely volcanic) rocks were downfaulted, tilted, and translated. Displacement was coeval with local volcanism (feeders out of plane of the section). After faulting had ceased (3), volcanism continued. Dotted line is present erosion surface. Patterns: random dash = Precambrian basement; dash = Cambrian shale; Previous HitbrickTop = Ordovician to Mississippian sedimentary rocks, largely carbonate; dot-dash = late Paleozoic and younger sedimentary rocks; dash-v = Eocene volcanic rocks.

Acknowledgments and Associated Footnotes

1 Exxon Production Research Company, P. O. Box 2189, Houston, TX 77252-2189

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