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Petrology, structure and exhumation of the southern Sawatch mountains, south-central Colorado

by Robbins, Rebecca

Abstract (Summary)
The southern Sawatch Range of the Southern Rocky Mountains of south-central

Colorado is composed of Precambrian crystalline igneous and metamorphic rocks that

have undergone at least three major mountain building events during the Phanerozoic, the

Ancestral Rockies Orogeny, the Laramide Orogeny, and Rio Grande rifting. In order to

determine how the ancient basement structures might have influenced later episodes of

deformation, a small area of basement terrain was mapped along the western margin of

the Poncha Pass transfer zone between the San Luis and Upper Arkansas basins in the

northern Rio Grande rift.

The two dominant rock types in the map area, (hornblendic) amphibolite gneiss

and (felsic) quartzofeldspathic gneiss, may represent interlayered metabasalt/metadiorite

and metarhyolite/metagranite, with lenses of exotic lithologies throughout. Metamorphic

foliations were found to be oriented predominantly N35ºW 47ºNE and to have had an

influence on younger brittle structures related to the rifting episode. Lineations and

fractures in the gneissic fabric also are parallel to brittle deformation structures.

Apatite Fission-Track (AFT) analysis provided a means of determining when this

crust was exhumed and cooled by the removal of overburden in response to erosion

and/or tectonics. The resultant AFT age distribution revealed that exhumation occurred

at the higher elevations during the Laramide orogeny (~299-46 Ma), and at lower

elevations during Rio Grande rifting (~29-19 Ma). Although it is commonly thought that

these mountains were exhumed during the rifting episode, the results of this study

indicate that older events played a significant role in the exhumation.

Bibliographical Information:

Advisor:

School:Kansas State University

School Location:USA - Kansas

Source Type:Master's Thesis

Keywords:precambrian paleoproterozoic southern sawatch range felsic hornblendic gneiss exhumation apatite fission track thermochronology geology 0372

ISBN:

Date of Publication:01/01/2005

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