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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

Structure Research at Bryn Mawr College

Dr. Arlo B. Weil

Structural Geologist, Paleomagnetist

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.