ML Crawford and WA Crawford Research
Our petrology research at Bryn Mawr centered on the evolution of convergent continental margin orogenic belts, particularly the processes that occurred in the mid- to lower crust. The research goal was understanding the processes by which continental crust forms as plate action adds pieces of oceanic and/or continental crust to a pre-existing continental nucleus and as molten igneous rock generated by plate subduction intrudes the overlying rocks of the growing continental margin.
The research combined petrological, mineralogical and geochemical studies to answer questions that include: crustal history during orogen development, based on a study of the metamorphic history of the rocks in the orogen; the role of igneous processes in contributing to the thermal history of the crust as well as to changes in crustal composition; the sources of the magmas that form the igneous rocks and the information they provide on tectonic processes; and the nature and timing of deformation.
Southeastern Pennsylvania and northern Delaware
In Pennsylvania the tectonic processes formed mountains along the present eastern margin of North America during the early Paleozoic (550-350 million years ago). Our studies involve the thermal and structural history of the metamorphic rocks in the region. A secondary aspect, pursued mainly by colleagues at West Chester University and the Delaware geological Survey involves determining the timing of emplacement of the associated igneous rocks.
Two Paleozoic events can be clearly distinguished: the older one is a low pressure regional aureole surrounding igneous rocks in northern Delaware. The younger is a regional event that is more extensive than the first and, where the two occur together, overprints the low pressure event.
Continued investigations center on the tectonic setting for the igneous rocks throughout the area, deduced from their chemistry. In addition, additional age data will help to constrain the timing of the events and their connection to the rocks along strike both to the north and south.
Southeastern Alaska and adjacent parts of British Columbia
The Pacific Northwest focuses on the igneous and metamorphic rocks as well as the structures of a much younger (100-50 million years old) eroded mountain belt. The research goal is understanding the processes by which continental crust formed as plate action added pieces of oceanic crust, an oceanic plateau, and slivers of continental crust, to a continental nucleus and as molten igneous rock generated by plate subduction intrudes the overlying accreting rocks of the continental margin. This orogen has the advantage of recording a single major convergent orogenic event coupled with a large volume of igneous rocks.
The mineralogy and structures in the metamorphic rocks document a crustal thickening event, caused by thick-skinned thrusting, that preceded and accompanied pluton emplacement, followed by later extension, crustal thinning and a second episode of pluton emplacement at lower pressures.
Early high pressure metamorphic minerals are overprinted by later lower pressure phases, documenting the uplift of deep seated rocks by thick-skinned thrusting followed by crustal thinning as the orogen evolved. During thinning large crustal blocks were tilted as recorded by steep ductile shear zones
and PT data that document differential uplift from east to west across these blocks. This was followed by a change to margin parallel translation with low pressure metamorphism limited to narrow contact aureoles around ongoing igneous activity. Through these studies and in collaboration with R. Butler who has studied the paleomagnetism of the rocks of the origin we have been able to document that proposed long distance (thousands of kilometers) migration of terranes northward along the Pacific coast of North America probably did not occur.
Studies of the chemistry of the 100 Ma to recent igneous rocks are being used to decipher the history magma origin through time and how this evolved as the orogen developed and changed.
Collaborative studies with colleagues in other institutions, particularly the University of Arizona, and the University of Vermont and through NSF funded initiatives such as ACCRETE provided access to a wide range expertise and opportunities for interaction.