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Department of Geology
Bryn Mawr College
101 North Merion Avenue
Bryn Mawr, PA 19010
Phone: (610) 526-7392
Fax: (610) 526-5086

Tectonics Research at Bryn Mawr College


Dr. Arlo B. Weil

Structural Geologist, Paleomagnetist

Lifecycles of Supercontinent


Precambrian continental reconstructions have recently become the subject of renewed interest following the proposal that all major continental blocks were part of a long-lived late Proterozoic supercontinent: Rodinia. While the existence of a major long-lived (~2500-500 Ma) Proterozoic supercontinent had earlier been advocated on the basis of paleomagnetic data, the more recent reconstructions of a short-lived Rodinia have largely been based on geological evidence linking truncated Meso-Proterozoic mobile belts. In the latter scenario the assembly of Rodinia is marked by Grenville-aged deformation (circa 1.1 Ga) on the margins of Laurentia, East Gondwana, Amazonia and Baltica, with the western margin of Laurentia facing East Antarctica in the so-called SWEAT or AUSWUS connection. Breakup and redistribution of the continental elements of Rodinia seems to have been initiated at c. 750 Ma with the separation of East Gondwana from the western margin of Laurentia. This rifting event and subsequent drift of the rifted elements eventually led to the amalgamation of greater Gondwana at ~550 Ma. (image from http://www.scotese.com/)

Dr. Weil's research is focused on the lifecycle of the proposed Rodinia supercontinent - its amalgamation and breakup - and on the supercontinent's paleogeography. In the absence of preserved oceanic lithosphere and marine magnetic anomaly records for any time prior to the Jurassic, paleomagnetic data provide the only quantitative means to infer ancient continental paleogeography. For Laurentia, the centrally positioned craton in Rodinia, there is a paucity of high quality paleomagnetic data for the Late Proterozoic, and additional data are sorely needed to define its paleogeography and tectonic history throughout this time period.

Current research is focused on several sequences of Proterozoic sedimentary and igneous rocks in the southwestern U.S. including: 1) the Grand Canyon Supergorup, Arizona, and 2) the Uinta Mountain Group, Utah.

 

Grand Canyon, Arizona.


Unita Mountains, Utah.

Pangea Amalgamation


The modern ocean seafloor, which dates back to at most the early Jurassic, carries a magnetic reversal record that provides robust evidence for the configuration of Pangea just prior to its break up, i.e., Pangea A type. This configuration is in agreement with paleomagnetic data from all of Pangea's major blocks for the latest Triassic through Early Jurassic. However, the Permian and Triassic paleomagnetic data from Gondwana and Laurussia show an appreciable mismatch (overlap) in a Pangea A fit. Consequently, if the paleomagnetic data are representative of Gondwana and Laurussia's position for the Permian and Triassic, they necessitate approximately 2000 km of continental overlap in a Pangea A fit, which is impossible. To get around this dilemma, paleomagnetists have suggested: 1) unrecognized magnetic overprints; 2) a tighter fit between the northern and southern continents (Pangea A2); 3) the existence of a significant non-dipole component of the geomagnetic field; or 4) a more north-easterly, non-overlapping position of Gondwana (Pangea B). Although the more easterly position of Gondwana in Pangea B remedies the overlap observed in Late Paleozoic and Mesozoic paleomagnetic data for the Pangea A-type fit, it also requires a 3500 km megashear to accommodate the dextral translation needed to arrive at a Pangea A-type fit by the Latest Triassic. This dextral translation from Pangea B to Pangea A is thought to have taken place in the Permian possibly extending into the Triassic.

The enormous length and the irregular plate-boundary geometry required by Gondwana and Laurussia's configuration in Pangea B would result in a recognizable fault system with regions of transpression and transtension. However, there is little geologic evidence for an extensive dextral fault system during the time of Pangea amalgamation. On the contrary, most geologists envision a more or less north-south (in present-day coordinates) collision in which the northern margins of Africa and South America collided along the southern margin of Europe and eastern margin of North America, respectively. This model results in a Pangea A-type configuration, negating the need for the controversial Pangea B to A megashear.

Some of Dr. Weil's research reports regional paleostress directions derived for the Late Carboniferous and Permian from analysis of the Cantabria-Asturias Arc's deformation history. The position of the CAA along the ancient plate boundary between Laurussia (North America, Europe and Northern Asia) and Gondwana (Africa, Antarctica, Australia, South America, and India) is ideal to study plate interaction during Pangea's formation. The structural and paleomagnetic results from the CAA indicate two main tectonic phases: 1) collision and subduction (east-directed polarity) in the hinterland of the Cantabria-Asturias regions in Westphalian and Stephanian times, followed by 2) Gondwana's northward migration and collision with Iberia, Variscan Europe, and ultimately Laurentia in Permian times. This tectonic model requires that final Variscan deformation experienced by northern Iberia has a remote paleo-stress field oriented north-south in present-day coordinates and NNE-SSW in paleogeographic coordinates parallel to the originally linear belt. This tectonic scenario contradicts the transpressive WNW-ESE paleo-stress field in present-day coordinates and NW-SE paleo-stress field in paleogeographic coordinates for Iberia that is required in the hypothetical Pangea B to Pangea A dextral megashear model. Thus, this megashear is not supported by results from the CAA due to the obliquity of the observed paleo-stress field with the inferred paleo-stress field for dextral megashear, and the Pangea B configuration is rejecteted.