Paleomagnetism is the study of the Earth's ancient magnetic field as it is recorded in the rock record. Understanding the past configuration of the Earth’s magnetic field allows geologists to accurately date rock sequences, reconstruct continental paleogeography, and quantify absolute rotations and tilts of large rock masses.
The Paleomagnetic laboratory at Bryn Mawr College was founded in 2004 with the help of a National Science Foundation Equipment grant awarded to Dr. Arlo Weil, funds from the department of Geology at Bryn Mawr and funds from the Provost office at Bryn Mawr College. The main function of the lab at Bryn Mawr is to investigate past continental configurations, especially the amalgamation and dispersal of large supercontinents, to help understand the mechanics and kinematics of fold-thrust belt formation, to better understand the processes responsible for remagnetizing carbonate and redbed rocks, and the anisotropy of magnetic susceptibility of crustal shear zones. The lab is available for use to other interested academics.
Life Cycles of Supercontinents
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.
Dr. Weil's research is focused on the life cycle 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 Supergroup, Arizona, and 2) the Uinta Mountain Group, Utah.
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 paleostress 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 paleostress field in present-day coordinates and NW-SE paleostress 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 paleostress field with the inferred paleostress field for dextral megashear, and the Pangea B configuration is rejected.
Kinematics and Mechanics of Curved Fold-Thrust Belts
Nearly a century ago geologic observers had already recognized the importance and sought the meanings of oroclines - or curved mountain belts. In 1955, S.W. Carey first coined the term orocline (Greek for the words mountain and bend) to represent, "an orogenic system that has been flexed in plan to a horse-shoe or elbow shape." In this original definition the word orocline was representative of belts, originally linear, that later experienced a curvature. The term has grown in recent years to incorporate all orogenic belts that have either primary or secondary induced curvature.
It was Carey's opinion that oroclines were one of the most intriguing tectonic features on Earth, and that they could, if understood, provide a key to the evolution of continents, and integrate all other structural features of the Earth into a coherent pattern. Although Carey appeared a little zealous in his assessment of the importance of oroclines, I do agree with his conviction that oroclines are one of the more fascinating tectonic features on Earth. Seen in its map view, almost every orogenic belt has some degree of curvature. Consequently, understanding the origin as well as development of these unique features are of fundamental concern.
A large part of Dr. Arlo Weil's research is focused on understanding the kinematics and mechanics of forming highly curved mountain belts. He uses both classical structural geology field techniques as well as detailed paleomagnetic and rock magnetic analyses. Presently Dr. Weil's research is focused on The Ibero-Armorican Arc in southwestern Europe, The Sevier thrust-belt in the Rocky Mountain states, and portions of the Appalachian Orogen in eastern North America.
Orogenic Related Carbonate Remagnetization
The final amalgamation of Pangea during the late Paleozoic Variscan-Alleghanian orogeny is widely recognized as having caused global-scale remagnetizations. Although mostly reported in limestones, this event affected many types of sedimentary rocks in all of Pangea's major blocks, including, but not limited to, North America, Europe, Asia, Africa, and Australia. The ubiquity of these remagnetizations has led to considerable rock magnetic research focused on the possible cause(s) and carrier(s) of this pervasive event. Two main mechanisms have been proposed for the remagnetization of Paleozoic limestones: 1) the acquisition of a thermoviscous remanent magnetization (TVRM) caused by burial and prolonged exposure to elevated temperatures, and 2) the acquisition of a secondary chemical remanent magnetization (CRM) through magnetic mineral growth activated by basinal brines and other orogenic fluids. It is now widely believed that secondary CRMs are the cause of most Paleozoic carbonate remagnetizations, and that TVRMs are unlikely given the relatively low burial temperatures determined for carbonates studied (< 250o C). Although the ubiquity of this process is widely accepted, the mechanism for the remagnetizations, and its relationship with orogeny and migration of orogenic fluids, is still not fully understood. To further our understanding, details of mineralogy and genesis of CRM carriers must be determined. Ultimately, a better understanding of the origin and distribution of NRM carriers will allow comparison between affected Paleozoic carbonates from varying localities, to determine whether they have acquired similar CRMs in response to the Late Paleozoic Variscan-Alleghanian orogeny, and, as a result, whether remagnetized carbonates share a common rock magnetic signature, remagnetization history and consequently a similar rock-fluid interaction.
To better understand the origin and global predominance of the Late Paleozoic remagnetization event, and its proposed "fingerprint" on carbonates, detailed studies are underway on several Paleozoic remagnetized carbonates. To characterize the distribution of crystal morphology and granulometry, and determine in which minerals the magnetic remanence resides, rock magnetic properties of whole rock chips are being compared with those of magnetic extracts and "non-magnetic" residue. These rock magnetic properties include hysteresis parameters, demagnetization of 3-D isothermal remanent magnetization (IRM), acquisition of IRM, saturation IRM (SIRM), and anhysteretic remanent magnetization (ARM), and low-temperature demagnetization. To describe the morphology and chemical composition of the magnetic grains present, scanning electron microscopy (SEM) is being used on magnetic extract and thin sections.
Collectively, the rock magnetic and SEM results should provide identification of the mineralogy and grain size of remanence carriers in remagnetized carbonates, and determination of the source for the rock magnetic "fingerprint" found in these carbonates. Ultimately, these results will be used to elaborate on the mechanism of carbonate remagnetization and on the relationship between remagnetization events, regional deformation, orogenic fluids and global tectonic events.