Master of Science / Department of Geology / Mary Hubbard / 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.
| Identifer | oai:union.ndltd.org:KSU/oai:krex.k-state.edu:2097/88 |
| Date | January 1900 |
| Creators | Robbins, Rebecca |
| Publisher | Kansas State University |
| Source Sets | K-State Research Exchange |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Format | 22672234 bytes, application/pdf |
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