Petrologic and thermobarometric studies, coupled with geologic mapping and structural analysis, provide critical evaluation of several different models for Acadian (Late Silurian to Middle Devonian) dome evolution in southern Vermont. Previous models considered diapiric uprise and composite nappe-stage crustal thickening and subsequent diapirism as likely causes of dome formation. Both of these previous models result in symmetrical distribution of P-T values about the dome structures with corresponding coreward increases in temperature, and typically, coreward decrease in associated pressures. Thermobarometric calculations made during this study demonstrate that both P and T increase eastward across the entire region and are not symmetrically distributed about dome axes. The P-T data coupled with petrographically derived relative age relationships and available geochronology also suggest that attainment of peak metamorphic conditions and concurrent dome-stage deformation are diachronous and young from west to east. These relationships are consistent with new geologic mapping and structural analysis which show that all of the domes in southern Vermont are low-amplitude fold interference structures. A current tectonic model indicates that Acadian Barrovian metamorphism in this region was a consequence of west-directed crustal thickening of an eastward dipping tectonic wedge, presumably from the Bronson Hill Terrane; an Ordovician arc sequence. The basal surface of this allochthonous mass projects above the present land surface within this area. Accretion of lower-plate rocks (of this study) into the thrust complex and continued west-directed thrusting of the accreted package over a seismically recognizable east dipping ramp structure provided the necessary geometry and mechanism for dome-stage fabric development, calculated uplift rates (1.2 to 1.7 km/m.y. and west to east younging of Acadian structural and metamorphic evolution.
Thermobarometric and geochronologic estimates of metamorphic pressure - temperature (P-T) conditions and metamorphic cooling ages were used to constrain the required thermal and tectonic input parameters for use in one-dimensional thermal modeling of an Acadian (Silurian-Devonian} tectonotherma! regime within the pre-Silurian Taconide zone of southern Vermont. This regime includes: 1) garnet-grade rocks from the eastern flank of an Acadian composite dome structure (Sadawga Dome; the western domain); 2) staurolite/kyanite-grade rocks from the western flank of a second composite structure, the Athens dome (eastern domain). Results from thermal modeling include development of P-T paths, temperature-time (T-t) and pressure-time (P-t) curves, related values of maximum temperature and pressure, pressure conditions at maximum temperature, predicted closure ages for radiogenic phases, and integrated uplift and cooling rates.
Thermal modeling results are remarkably similar to independently obtained data for Acadian regional metamorphism in western New England, and provide some important constraints on regional thermal evolution: 1) pressure values contemporaneous with peak temperature on P-T paths may be substantially lower than actual maximum pressure (> 2.5 kbars); 2) differences in peak temperature for rocks initially loaded to similar crustal depths (garnetgrade vs. staurolite-grade), differences in calculated uplift rates, and differences in Ar closure ages, are consequences of variations in durations of isobaric heating events (or "residence periods"), and differences in actual tectonic uplift rates. These modeling results are internally consistent with structural model that suggests west to east younging of specific Acadian deformations and resultant diachroneity of peak metamorphic and Ar closure ages. Regional variations in timing and conditions of metamorphism may be controlled by diachronous deformational events coupled with variations in crustal levels to which rocks were initially loaded during the ca. 400 Ma onset of Acadian orogenesis in western New England. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/37857 |
| Date | 21 May 2008 |
| Creators | Armstrong, Thomas Robert |
| Contributors | Geosciences, Tracy, Robert J., Hewitt, David A., Ratcliffe, N. M., Law, Richard D., Glover, Lynn III |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | xiii, 189 leaves, BTD, application/pdf, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 31211034, LD5655.V856_1993.A767_drw01.pdf, LD5655.V856_1993.A767_drw02.pdf, LD5655.V856_1993.A767.pdf |
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