THERMAL-MECHANICAL IMPLICATIONS OF A SHALLOW ASTHENOSPHERE IN A DEFORMING OROGEN: CONSTRAINTS FROM 3-D NUMERICAL MODELING

A high temperature region associated with Acadian deformation extends 200 km through north central New England. Knowledge about the scale of the heat source that produced this region could lead to a better understanding of 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 NE-SW thermal gradient. The isogradic sequence, geobarometry, and the mineral assemblages indicate Buchan metamorphism, moderately low pressures of 3 kbar, and temperatures ranging from 550 to 450 °C 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-D conductive thermal models test heat sources of various sizes, temperatures, and geometries emplaced into regions with initially normal or elevated geotherms. 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 the thermal models we find that, on the local scale, the quality of gravity data is such that it cannot confirm or contradict 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 expected from a region of shallow, cooled asthenoshpere. 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, we constructed 3-D mechanical models that deform the orogen obliquely with west dipping subduction. The patterns of strain localization that emerge from this modeling are compared to measurements of strain from southern Maine.