Saturday, December 25, 2010

Structural Geology of the Shallow Earth

Structural Geology of the Shallow Earth (<1000 km deep)
December 31, 2003
Structural geology has as its principal task the inference of the geometry and evolution of rock bodies (sedimentary, metamorphic, and volcanic strata and magmatic intrusives) contemporaneous with or subsequent to their deposition or emplacement. Traditionally, structural geology has been restricted to study of rock outcrops at the Earth’s surface. Over the past few decades, surface observations have been enhanced by incrementally higher resolution geophysical observations, especially reflection seismology at shallow depths and refraction and reflection tomography at depth.
Further, even with the geophysical observations, structural geology has been restricted to characterization of the Earth’s crust. Now, with progressively improved tomographic images of the whole Earth (especially the outer 1000 km), it is appropriate to begin the assessment of the structure and evolution of the shallow Earth, including the upper mesosphere.
Within the shallow crust, from surface and borehole observations, discontinuity characterization is based on lithology of sedimentary, metamorphic, and volcanic strata, especially those discontinuities observed between and within otherwise homogeneous rock bodies. Discontinuities include sedimentary depositional and volcanic emplacement surfaces, along with intrusive igneous contacts and faults. Seismic characterization of the shallow crust is largely limited to stratal-mimicking reflection events which may or may not resolve actual lithologic discontinuities. In addition velocity contrasts across faults and magmatic contacts (as well as lithologically distinct strata) can sometimes be resolved.
Until computing power began to allow it, resolution of discontinuities in the deeper crust and upper mantle has largely been restricted to nearly horizontal interfaces, especially the Mohorovicic discontinuity, the “low-velocity zone” within the upper mantle, and inferred steep gradients at ~410, ~660, and ~1000 km depth.
In plate tectonic context, the low-velocity zone is inferred to represent partially molten asthenosphere, with the upper boundary of the zone corresponding with the lower surface of the moving lithosphere. Thus, the top of the low-velocity zone is a discontinuity of active tectonic significance.
Zones of active seismicity delineate plate boundaries (especially at mid-ocean ridge and transform fault zones) and inferred subducting oceanic lithosphere. The inclined seismic zones do not directly indicate plate discontinuities, but, rather, appear to indicate deformation within the sinking lithoplate; discontinuities are inferred to parallel the seismic zones, separating the lithoplate from overlying and underlying asthenosphere and shallow mesosphere. Volcanic arcs are inferred to represent the “junction” of two discontinuities -- the upper surface of descending plate with the inferred boundary between overlying lithosphere and asthenosphere.
Lithosphere consists of both crust and shallow mantle. Thus the Moho occurs within lithoplate – except at mid-ocean ridges, where the Moho is the contact between volcanic-intrusive crust and zero-age oceanic asthenosphere. As spreading occurs, two plate components develop – intrusive and extrusive magmatic crust (emplaced at or very near the ridge crest) and underlying former asthenosphere. As the partially molten residual asthenosphere beneath the spreading center cools (top-down), it progressively solidifies, becoming part of the lithosphere. Thus the Moho has active tectonic significance close to the active seafloor spreading ridge crest and, possibly, beneath zones of active crustal extension (such as the Basin Range Province of the western United States). Equivalently, Moho corresponds with the shallow limit of the low-velocity zone (lithosphere-asthenosphere boundary) beneath ridges and rifts.
What of the deeper discontinuities (410, 660, and 1000 km)? Do they have tectonic significance, analogous to plate boundaries and surfaces? Or, do they represent only phase and/or chemical-petrologic discontinuities? Another way of expressing these questions is whether the layers between discontinuities behave in a tectonically coherent way. Or rather, does the mantle beneath the lithosphere behave in a predominantly “fluid” manner, as many numerical models imply?
Conventional structural analysis of rocks incorporates several approaches, depending on whether rock body behavior can be characterized as kinematically rigid or deformed internally. On a global scale, the remarkable plate approximation is its successful description of lithospheric motions in terms of kinematic rigidity. In general, the approximation assumes that plates do not deform internally, despite displacements between plates of thousands of kilometers. This does not imply that plates have a high dynamic rigidity, but, rather, that stresses are concentrated on plate boundaries. Where a plate experiences significant internal forces, it tends to fragment, forming two or more new plates.
Is there evidence for kinematic rigidity beneath the lithosphere? An affirmative answer is evident: there are several indicators of distinct reference frames below lithoplates. First, there are the magmatic (“hotspot”) traces existent on largely oceanic plates – for example, the Hawaiian-Emperor island seamount chain and the Kerguelen Plateau-Ninetyeast Ridge. Separate reference frames appear to exist beneath the plates of the Indian and Atlantic Ocean (except for the northernmost Atlantic) and beneath the Pacific Ocean. Largely successful kinematic/reconstruction models have been derived for each of the two regions (Atlantic-Indian: Müller et al., 1993; Pacific: e.g., Raymond et al., 2000), but the two reference frames are in apparent motion relative to one another (e.g., Gaina et al., 2000; Raymond et al., 2000).
In the hotspot reconstruction models, age progressions of inception of magmatism are largely consistent with the regional reference frames, although recurrent magmatism is observed along some traces (e.g., Pilger and Handschumacher, 1981). The reference frames could be interpreted to indicate the presence of melting anomalies (“hot spots”) within the upper mesosphere which do not move significantly relative to one another within the frame or coherent intraplate stress fields that are dominated by interaction of the lithosphere with the underlying asthenosphere and upper mesosphere.
References (added December, 2000)
Gaina, C., R.D. Müller, and S.C. Cande, 2000, Absolute plate motion, mantle flow, and volcanism at the boundary between the Pacific and Indian Ocean mantle domains since 90 Ma, in: History and Dynamics of Global Plate Motions, ed. M.A. Richards, R.G. Gordon, and R.D. van der Hilst, AGU Monograph #121, 189-210.

Müller, R. D., Royer, J.-Y., and Lawver, L. A., 1993, Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks, Geology, v. 21, p. 275-278.

Pilger, R. H., Jr., and Handschumacher, D. W., 1981, The fixed-hotspot hypothesis and origin of the Easter-Sala y Gomez trace, Geological Society of America Bulletin, 92, 437-446.

Raymond, C. A , Stock, J. M., and Cande, S. A., 2000, Fast Paleogene motion of the Pacific hotspots from revised global plate circuit constraints, in Richards, M. A., Gordon, R. G., van der Hilst, R. D. , eds., The history and dynamics of global plate motions, American Geophysical Union Geophysical Monograph 121, p. 359-375.
© 2010 Rex H. Pilger, Jr.

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