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Structural Geology
Mary Hubbard

The field of structural geology has experienced exciting advances in a number of different areas. Two themes have dominated the literature in the last year: time and factors that influence the location and style of deformation, such as rock rheology and the presence of melts. New technological advances have greatly enhanced geologists' understanding of the relationship between time and deformation processes. Some of these advances have improved our ability to determine the timing of ancient deformational activity while other new methods facilitate determination of locations and rates of ongoing crustal deformation. Studies of rock rheology have spanned the size-range from nanoscale to mountain belt-scale and have highlighted just how deformable our "rigid" lithospheric plates are.

Researchers have known for the last several decades that deformation and metamorphism occur over time rather than during distinct events (England and Thompson, Journal of Petrology, v. 25, p. 894; Burton and O'Nions, Earth and Planetary Science Letters, v. 107, p. 649; Maddock, Journal of the Geological Society of London, v. 149, p. 249; Vance, Geological Journal, v. 30, p. 241). Researchers also recognize that many rocks have experienced several phases of these processes throughout their history. Some of the challenges in structural geology have been to determine the length of time over which these processes have acted and even when phases of deformation occurred. Geologists recently began utilizing the electron microprobe for compositional and age mapping of in-situ monazites (Williams and Jercinovic, Journal of Structural Geology, v. 24, p. 1013). This powerful technique can be combined with detailed microstructural analysis to refine interpretations of the timing of fabric development. Though this method must still be used in conjunction with other geochronological techniques, there is enormous potential for determining details of the structural history of a region even if multiple deformational phases have occurred.

Modern deformational processes have also been the focus of recent research developments through the integration of geodetic methods such as Global Positioning System (GPS) and interferometric synthetic aperature radar (InSAR) with structural studies. This research directly benefits earthquake hazard research. An example of the application of GPS studies to tectonic research is the refinement of microplate or crustal block geometry in the Mediterranean region (Oldow et al., Geology, v. 30, p. 779). Adria is a crustal block located at the boundary between the Eurasian and African plates. Oldow et al. used GPS measurements to identify differing rates of current motion in the eastern versus western parts of Adria. They have proposed that Adria represents two fragments, with the eastern fragment moving northward relative to a fixed Eurasian reference at much faster rates than the western fragment.

The InSAR method is a wonderful complement to GPS measurements in that it also can be used to identify regions of crustal movement. InSAR uses radar data acquired from satellites over an interval to detect a change in the location of individual points. This method can detect horizontal or vertical movements and can even detect strain accumulation in a region prior to an earthquake event. Pollitz et al. (Science, v. 293, p. 1814) used InSAR to detect accelerated strain following the 1999 Hector Mine earthquake in California. They concluded that the observed uplift patterns related to mantle flow following stress drop during the earthquake event. Wicks et al. (Journal of Geophysical Research, v. 106, p. 13769) used InSAR to detect subsidence in the Coso Range of California. These authors propose that the subsidence is due to the leaking of magmatic fluids in a deep reservoir.

Clearly both GPS and InSAR can be used to detect regions of differential motion and deformation, but to understand why deformation has occurred in a particular place, geologists must turn to the rocks themselves. Recent studies have focused on the types of geologic situations that may influence deformation. One factor is the presence of melts. Rushmer (Tectonophysics, v. 342, p. 389) conducted melt experiments on biotite-bearing gneiss and concluded that there was little volume increase during melt formation. She suggests that the minimal dilation may lead to the trapping of melts in the lower crust, thus forming a weak layer that could localize deformation during orogenesis.

Other approaches to factors influencing deformation have focused on rocks at the very small scale. Herwegh and Kunze (Journal of Structural Geology, v. 24, p. 1463) studied calc-mylonites from the Alps and determined that the presence of nano-scale organic particles altered the way the rock deformed, primarily by inhibiting recrystallization. Layers without the organic particles were able to recrystallize during deformation. This study shows that even seemingly minor factors can influence how rocks deform.

Methods in geochronology, geodesy, geochemistry, petrology, and microstructural analysis are developing rapidly and will continue to advance the field of structural geology. Our ability to integrate multiple disciplines and make observations at all scales is maximizing the information we can extract from the past and present deformational record.

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Hubbard is head and associate professor in the Department of Geology at Kansas State University. E-mail: mhub@ksu.edu.

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