Structure and Tectonics
Mary Hubbard

This story is expanded from the print version.

Developments in structural geology and tectonics have continued to encompass diverse arenas -- from experimental studies to field studies and theoretical studies. New technologies have achieved intriguing results in all of these areas. Even traditional field methods are incorporating computer-based mapping techniques, sometimes coupled with a GPS device. As a result, our ability to quantify different aspects of structural or tectonic processes has improved.

Over the past several years, an increasing number of studies have used thermochronological methods to constrain timing of tectonic processes via the relationship to cooling processes. One structural process that has been difficult to date directly is brittle faulting. But in the past year, S.C. Sherlock and R. Hetzel (Journal of Structural Geology (JSG), v. 23, n. 1, p. 33) were able to apply the laser-probe argon 40 and 39 method to pseudotachylite from a fault zone in Kenya in order to document that its formation was early Paleozoic and not rift-related. A number of criteria must be met before this method can be used to date pseudotachylite, of course, but their study will open the door to similar applications in other structural zones.

Many studies in the last few decades have focused on determining temperature and pressure conditions for geologic processes. Typically the thermobarometers used have been tied to chemical exchange during metamorphic reactions. In a very different approach to determine ambient temperature during faulting events, K.D. O'Hara (JSG, v. 23, n. 9, p. 1345) used the ratio of clasts to matrix in pseudotachylite. O'Hara proposed that this ratio is a function of the temperatures of the melt and the country rock and that by making an assumption about the melt temperature one can estimate the country-rock temperature at the time of the faulting event.

Not all recent structural studies required the use of pseudotachylite; in fact, some recent studies didn't involve rocks at all! Analogue experiments continue to provide insights into deformational processes. L. Arbaret and colleagues (JSG, v.23, n. 1, p. 113) used a ring apparatus filled with a transparent polymer and rigid polyethylene particles of various shapes and sizes to observe the rotational behavior of particles during simple shear. They concluded that many of their experiments matched theoretical models where elliptical shapes were used for the particles. Another unique analog experiment used the desiccation of a starch-water mixture to simulate joint formation (G. Müller, (Ibid., p. 45). Müller observed initial rupture velocities of 100 millimeters per second, followed by rupture through the specimen at rates of 10 millimeters per minute or less.

Numerical modeling provides another approach to studying deformation. E.O. Cristallini and R.W. Allmendinger (Ibid., p. 1883) created a 3-D model to look at variations along strike during trishear fault-propagation folding and during transpression. They modeled the transition from a blind thrust to an emergent thrust and observed differences between the geometry of growth and pre-growth strata following deformation. Ultimately the authors could compare model results to a field example in Utah and obtain an acceptable match.

High-precision GPS methods are now being used in a number of tectonically active regions. Results of one such study are presented by M.M. Miller and others (Tectonics, v. 20, n. 2, p. 161-176) for the Pacific Northwest. Data collected from the Pacific Northwest Geodetic Array (PANGA) delineate several kinematic zones. The northernmost zone is relatively stable while the central zone, primarily in western Washington and Oregon, shows motion related to oblique convergence of the Juan de Fuca plate. The southernmost zone shows northwestward movement of the Cascadia fore arc. Studies such as this enable us to take a detailed look at modern plate kinematics.

One of the new tools that is greatly enhancing thermochronologic studies is (U-Th)/He dating through use of apatite minerals. The method is becoming increasingly popular at the new labs sprouting up in the United States. In one recent study, P.D. Crowley and colleagues (Geology, v. 30, n. 1, p. 2) obtained (U-Th)/He ages on apatites from basement rocks in the Bighorn Mountains of Wyoming to better understand the exhumation history of the range. The distribution of the 65 million-year to 369 million-year ages that they obtained are consistent with the formation of the Bighorn Mountains as a doubly plunging anticline with folding of the basement and cover.

Structural geologists welcome the advances in new technology that allow us to come closer to understanding complexly deformed regions. Technological advances in related disciplines also brighten the future for continued interdisciplinary studies.

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Hubbard is department head and associate professor at Kansas State University. E-mail.

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