Download Kristin McCurdy, Bates College

Kristin McCurdy, Bates College
For her senior thesis, Kristin will investigate the thermal evolution of Maine following the Acadian
Orogeny by combining age data from throughout Maine with three-dimensional thermal modeling.
She has compiled the published metamorphic and cooling ages from Maine and will determine 3
new muscovite cooling ages in a transect that runs NW from Sydney, Maine to Bald Mountain,
Weld, Maine. Using tectonic models, including that of Tucker et al. (2001) and a model of flat
subduction, she will develop three-dimensional models which investigate the thermal evolution.
These models will be used to contrast the thermal evolution expected from a scenario with a
west-dipping subduction zone with that expected to result from an east-dipping subduction zone.
Predictions from her 3D thermal models will also be compared with her compiled age data to
provide new insight into the thermal evolution of Maine.
Phaedra Upton, University of Maine, Bates College (Sept 05 – May 06)
My research is focused on the mechanical evolution of the Maine crust during the Acadian
orogeny using three-dimensional mechanical modeling in conjunction with constraints provided by
other members of the project. My models will advance upon preliminary modeling efforts and will
bring together several aspects of the Acadian orogen into an overall geodynamic framework. The
work is divided into three experiments
1) Thermal and rheological evolution of deep crust during the Acadian orogeny.
Models based on previous 3-D thermal models, including those currently being run by Kristin
McCurdy, will investigate the effect of the thermal evolution of the Arcadian on the mechanical
evolution of the deforming crust.
2) Orogen parallel strain variation during Acadian orogeny.
Heather Short has shown that deformation within the Arcadian-age orogen in central-eastern
Maine was partitioned spatially and temporally with the dominant direction of accommodation of
convergence within the mid-lower crust switching from orogen-perpendicular to orogen-parallel
near the end of the Devonian. Kinematic observations combined with 3-D thermo-mechanical
models of the Acadian will be used to evaluate the role of lateral and vertical variation on strain
partitioning. The models will have a mechanical resolution on the order of one to five kilometers
and thus will be comparable to field observations. They will be run for dextral transpression with a
varying degree of obliquity. The models will also have an upper surface conditioned by an
orographic erosion regime and we will test the sensitivity of our results to this condition.
3) Enhanced exhumation within the transpressive Norumbega shear zone
Preliminary field mapping, microstructural and petrologic analysis in south-central Maine suggests
that pre-Silurian rocks of the Liberty/Orrington belt and Silurian rocks of the Fredericton belt
record higher Devonian-age shear strains and peak metamorphic temperatures than adjacent
rocks to the southeast, and that these strains and temperatures taper off into the Central Maine
Basin Sequence to the northwest.
The higher-strain rocks consist of more-competent
metavolcanics and gneisses interlayered with less-competent metapelites that locally contain
porphyroblasts of garnet, sillimanite, cordierite, and andalusite. Preliminary microprobe monazite
ages from the Liberty-Orrington belt suggest a component of transpression. These rocks were
then exhumed during Silurian-Devonian time. The age and kinematic data suggest a geodynamic
model incorporating localized exhumation during transpression is appropriate for Acadian-age
orogenesis in central-eastern Maine.
Combining these three experiments with field and laboratory will allow us to place separate field
areas into an overall geodynamic context within the Acadian orogeny. Although there remain
more degrees of freedom than observations, we now have good constraints upon the length
scales of deformation, the extent of the rock types and the thermal history during the Acadian
orogeny, providing bounds for our models.