Author

Lucy E. Brown

Date of Award

2006

Level of Access

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Peter O. Koons

Second Committee Member

Scott E. Johnson

Third Committee Member

Martin G. Yates

Abstract

A high-temperature region associated with Acadian deformation extends 200 km through north-central New England. The scale of this heat source is investigated in order to better understand the rheology and mechanics of deforming orogens. In central Maine, on the boundary of the high-temperature region, isograds in pelitic rocks are normal to the accretionary structures and record a steep northeast-southwest thermal gradient. The isogradic sequence, geobarometry, and the mineral assemblages indicate low-pressure, high-temperature metamorphism, moderately low pressures of 3 kbar, and temperatures ranging from 600º to 450ºC over a 10 km region. The spacing between these isograds and the temperature at each isograd are used in combination with thermal modeling to extract information about the scale and nature of the heat source. Three-dimensional conductive thermal models fall into two broad categories based on scale and type of heat source: models with a local plutonic heat source and models with a regional asthenospheric heat source. The results show that the likely source of the isograds in central Maine is local granitic plutons that dip shallowly to the northeast, but the regional thermal structure is best described by models with an asthenospheric heat source at crustal depths. Integrating existing Bouguer gravity data with thermal models, I found that, on the local scale, the quality of gravity data is such that it cannot confirm or refute the presence of a shallowly dipping pluton. On a regional scale, there is a large mass deficiency over southern Maine that cannot be explained entirely by low-density plutons and implies an overthickened crust. The presence of a thick crust contradicts the high-density signature explected from a region of shallow, cooled asthenosphere. These opposing pieces of evidence may be a sign of flat subduction in which the asthenospheric heat source was replaced by crustal material. Using information from the thermal and gravity modeling, I constructed three-dimensional mechanical models that deform the orogen obliquely with west-dipping subduction. The patterns of strain that emerge show curvature at the transition between the strong and weak rheological zones, and high uplift in the weakened zone relative to the strong region. This curvature in the orogen is not observed in Maine. Further numerical modeling could explore possible ways of accomodating both a shallow asthenosphere and a lack of curvature, but this discrepancy suggests that there may not be an abrupt change in rheology in central Maine. In turn, the possibility of along-strick rheological uniformity supports the idea that central Maine represents a change in the erosional level associated with underplating.

Included in

Geomorphology Commons

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