At collisional mountain ranges the tectonic history of crustal shortening and
subsequent post-collisional erosion is preserved in the form of the presently observed
gravity anomalies. In this study, models of erosion and isostatic rebound at various stages
of collision illustrate the evolution of crustal structure, topography, and resulting gravity
anomalies.
The Ouachita Mountains of Arkansas, which show a low/high Bouguer gravity
couple characteristic of the initial stages of collision, have undergone just 8 km of erosion
during the process of completely rebounding the syn-orogenic crustal root. This minor
rebound means that the Ouachitas retain a crustal geometry similar to the continental margin
prior to collision, including thin transitional and oceanic crust.
At more advances stages of collision Bouguer gravity anomalies show a broad low
reflecting a thickened crustal root. The width of this low, which relates directly to the
amount of crustal shortening, is retained during subsequent erosion and elastic rebound,
but the amplitude decays gradually. Thus, the width and amplitude of the low can be used
to estimate the degree of convergence and amount of erosion, respectively, for a specific
mountain range. For the Scandinavian Caledonides results are consistent with 20 km of
erosion following 200 km of crustal shortening. Following a larger magnitude of
convergence, about 300 km, the southern Appalachians are estimated to have undergone
28 km of post-collisional erosion. Bouguer gravity profiles across the recently-active Alps
compare with a model of 200 km of crustal shortening and 8 to 12 km of erosion. While
the Alps have undergone a similar amount of shortening as that estimated for the
Caledonides, erosion and post-collisional rebound is at an initial stage, such that a thick
section of exotic crust still overlies the underthrusted European Platform.
The results of these model comparisons suggest that the crustal geometry ofa
collisional mountain range should be viewed as a consequence of the degree of crustal
shortening as well as the amount of erosion and isostatic rebound. In models at moderate
to advanced stages of shortening ( 200 km), and mature stages of erosion (e.g.,
Caledonides, Appalachians), the geometry of the crustal "suture" between overthrusting
and underthrusting crusts is present as a shallow, subhorizontal de collement beneath the
foreland. In the hinterland the suture abruptly steepens, a result of differential uplift during
isostatic rebound. This crustal geometry, characteristic of seismic-reflection profiles
across many ancient mountain belts, suggests: (1) that the "low angle detachment"
observed beneath collisional mountain ranges was originally much deeper and steeper than
it is at present; and (2) that steep-dipping seismic reflectors towards the hinterland represent
the thrusted contact between converging crustal blocks, but have been steepened as a result
of isostatic uplift following erosion. / Graduation date: 1992
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/36718 |
| Date | 17 March 1992 |
| Creators | Enos, Robert A. |
| Contributors | Lillie, Robert J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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