This page is currently (and probably always will be) under construction. Having just recently "landed" in the wonderful Appalachians, there has been a lot of material to digest! My students and I are actively involved in the Acadian and Taconian/pre-Taconian time slices of the orogen. We are also investigating the origin and significance of the "shatter zone" around the Cadillac Mountain pluton on Mount Desert Island and a number of other pluton-related problems. Northern New England is also a splendid locality for studying microstructural processes, and I am involved in several cool projects with colleagues like Dave West, Charlie Guidotti and Mike Williams. Below are brief descriptions of two major projects in the northern Apps.
Acadian Studies (in collaboration with Dave West and Charlie Guidotti)
Convergent orogens are dynamic, long-lived tectonic environments that provide the main setting for Phanerozoic continental growth by two fundamentally different processes: (a) lateral accretion of terranes and (b) vertical addition of mantle-derived magmas. Evolution and growth of an orogen involves complementary relationships between thermal structure and rheological properties; thermal structure affects the rheology and therefore deformation of an orogen, but the style of deformation and crustal thickening affects the thermal structure. A clearer understanding of this tectonothermal evolution requires knowledge of both the relative and absolute temporal relationships between deformation and peak regional metamorphism in an evolving orogen. However, such studies are made difficult by a number of factors that, to a greater or lesser extent, are common in orogens worldwide and lead to ambiguities. These factors include: (1) polymetamorphism; (2) polydeformation; (3) lack of continuity in age and composition of rocks across strike along orogen-scale transects; (4) lack of continuity in exposed crustal level across strike; and (5) lack of absolute age control on pluton-driven thermal events. Ambiguity is compounded if the deformation, metamorphism and magmatism reflect multiple orogenic events. These factors and associated ambiguities are minimized along our proposed transect in Maine.
The Appalachians are a classic mountain range formed by three widely-accepted convergent orogenic events: (1) the Taconian from 455 to 442 Ma; (2) the Acadian from 423 to 385 Ma; and the Alleghanian from 280 to 260 Ma (temporal ranges from Robinson et al., 1998). The Acadian orogeny caused the most widely distributed metamorphism and plutonism in New England, but 35 years after the first plate-tectonic interpretation of the Appalachians (Wilson, 1966) the tectonothermal evolution of the Acadian orogeny remains enigmatic. Recent studies by Dwight Bradley and colleagues, Peter Robinson and colleagues, Dyke Eusden and colleagues, and Bob Tucker and colleagues, suggest that the Acadian orogen migrated from SE to NW across Maine and adjacent parts of New England and Canada. This model is based largely on sedimentological and paleontological data tracking the foreland basin position, combined with new U/Pb zircon ages on numerous plutons tracking the locus of magmatism. Our preliminary observations bear on this model of orogen migration.
Reconnaissance microstructural work at the SE end of our transect indicates that peak regional metamorphism was synchronous with more than one foliation-forming event and that peak metamorphic mineral growth was followed by continued intense regional deformation (check out these great syntectonic microstructures). In stark contrast, our work at the NW end of the transect indicates that peak regional metamorphism occurred very late in the deformational history and peak metamorphic mineral growth was only locally followed by weak regional deformation, the age of which is currently unknown (compare the above syntectonic microstructures with these, which show peak regional metamorphism occurred very late in the deformation history, and was followed by local pluton-related deformation and metamorphism). Thus, as Acadian deformation migrated to the NW, deformation and metamorphism were apparently protracted in the hinterland (SE) and episodic in the foreland (NW). This provides an unusual opportunity to evaluate, in detail, the complementary evolution of deformation and metamorphism across an orogen.
The work we are conducting includes microstructural, petrological and chronological analyses, and will complement existing stratigraphic, paleontological and U/Pb geochronological data bearing on Acadian evolution. We will concentrate almost exclusively on Silurian-Devonian rocks, thus avoiding complications associated with pre-Acadian tectonothermal events. Moreover, the textural and mineralogical features required for our approach are best developed in these Silurian-Devonian strata. Our results will be used to: (1) construct pressure-temperature-time-deformation paths for metamorphic rocks across a well-preserved portion of the Acadian orogen; (2) quantitatively resolve the ages of overprinting metamorphic events in these rocks in order to differentiate regional thermal events from later, relatively local pluton-related thermal pulses; (3) evaluate the absolute timing relationships between deformation and metamorphism as a function of position within the Acadian orogen; (4) construct a conceptual tectonothermal model for the evolution of the Acadian orogen that can be compared with other orogenic belts and ultimately used in geodynamic models; and (5) clarify the transition between orthogonal and oblique convergence late in the Acadian orogeny as recorded in the SE part of the transect.
Taconian and pre-Taconian studies
Much of Earth's continental crust comprises Precambrian through Cenozoic orogens, where preexisting rocks have been variably modified by metamorphism and deformation. Owing to the association between collisional tectonic events and mountain building, this long record of orogenesis provides the most accessible means of reconstructing paleotectonic settings and events that have shaped our planet. Although the Appalachian mountains are widely used to illustrate Earth's tectonic evolution and associated orogenesis, collisional processes and events that drove the Ordovician Taconian orogeny in the Québec-Maine segment of the northern Appalachians remain enigmatic. Deformational features in this region cannot be satisfactorily related to a specific convergent setting or collisional event(s) because the nature of the eastern (present coordinates) Laurentian margin during the early Paleozoic is poorly understood. Interpretations are divided as to whether the orogeny was caused by ophiolite emplacement, island-arc collision, or microcontinent collision. Because the Taconian orogeny initiated the Appalachian mountain-building cycle, understanding its architecture and evolution is essential for understanding the Appalachians as a whole.
The core of Chris Gerbi's PhD thesis will involve investigations of the structural, metamorphic, and temporal relationships among rock units that make up the Chain Lakes massif and Boil Mountain ophiolite in order to test competing models and hypotheses for the Taconian orogeny in the northern Appalachians. The approach will be multidisciplinary, including mapping, structural, microstructural, metamorphic, and geochronological studies. Integrated field and laboratory studies will establish: (a) the relative and absolute timing of metamorphic and deformational events within the Chain Lakes massif, (b) the crystallization age of the mafic portion of the Boil Mountain ophiolite, (c) the age and kinematics of ophiolite emplacement, (d) whether mafic volcanic rocks near the ophiolite represent a genetic part of the ophiolite, and (e) the age of possibly rift-related mafic dikes in the Chain Lakes massif. The geochronology portion of the project will include in-situ U-Pb analyses of monazite to provide high temporal resolution for metamorphic events, small-number multigrain U-Pb analyses of zircon or baddeleyite from gabbro to determine the crystallization age of the ophiolite, U-Pb analyses of zircon to establish the age of the dikes, and laser step- heating 40Ar-39Ar analysis of muscovite to determine the minimum ophiolite emplacement age. Results will be compared with existing interpretations of the Thetford Mines ophiolite and overlying units in southern Québec, the Maquereau Dome in the Gaspé Peninsula, and the Dashwoods Subzone in Newfoundland in order to evaluate temporal and spatial relationships among these potentially correlative units. Because the extents and boundaries of terranes are fundamental components of the tectonic models, testing the terrane relationships will test the models. Chris's work will help to clarify the nature of the Taconian orogeny in the northern Appalachians, and will bear on conceptual models describing Iapetan paleogeographic reconstruction and collisional accretionary processes. See Chris's web page for details.