Geology topics

R. Meissner

Publications and source records attributed to R. Meissner.

4 recordsLinked to original sources

USGS2006

Continental crustal evolution observations

How has the continental crust evolved? What are the primary processes responsible for its composition, structure, and mode of deformation? What role do fluids play in deep crustal processes? In the last dozen years, geophysicists have obtained images of the deep continental crust that can be used to examine these questions and refine geologic models of crustal evolution. In this report we summarize recent progress in geophysical studies of the deep continental crust and highlight some of the more important implications of deep crustal processes.

Eos, Earth and Space Science News↗

USGS2006

Speculations on continental crustal evolution

The evolution of the continental crust is a topic that has challenged Earth scientists since the earliest hypotheses of crustal evolution were put forth by such luminaries as Hutton, the 18th century Scottish scientist, and later by Stille (Germany), Argand (France), and Dana (United States). Recent geophysical observations provide important constraints on hypotheses of crustal evolution, and the most important of these observations are reviewed in a companion paper [ Mooney and Meissner , 1991], henceforth referred to as Paper 1. In this article we briefly speculate on crustal evolution using both geological and geophysical data as guidelines. For the past 25 years, the basic framework for models of the evolution of the Earth's crust and lithosphere has been plate tectonics. This framework has been particularly successful in explaining the processes that form and modify the oceanic crust but has had somewhat more limited success in its application to the continental crust. Some of the basic tenets of plate tectonics, as applied to the continental crust, are listed below. Continental masses coalesce and disperse as ocean basins open and close in the Wilson cycle.

Eos, Earth and Space Science News↗

USGS2002

Seismic anisotropy and mantle creep in young orogens

Seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere. We discuss the origin of anisotropy at various depths, and relate it to tectonic stress, geotherms and rheology. The anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine, and may result from ductile deformation, dynamic recrystallization or annealing. Anisotropy beneath young orogens has been measured for the seismic phase Pn that propagates in the uppermost mantle. This anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event, and thus provides information on the process of mountain building. Whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts, Pn anisotropy is aligned parallel to the structural axis. We interpret this to indicate mountain-parallel ductile (i.e. creeping) deformation in the uppermost mantle that is a consequence of mountain-perpendicular compressive stresses. The preferred orientation of the fast axes of some anisotropic minerals, such as olivine, is known to be in the creep direction, a consequence of the anisotropy of strength and viscosity of orientated minerals. In order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal 'escape' (or 'extrusion') tectonics to the uppermost mantle. We present rheological model calculations to support this hypothesis. Mountain-perpendicular horizontal stress (determined in the upper crust) and mountain-parallel seismic anisotropy (in the uppermost mantle) require a zone of ductile decoupling in the middle or lower crust of young mountain belts. Examples for stress and mountain-parallel Pn anisotropy are given for Tibet, the Alpine chains, and young mountain ranges in the Americas. Finally, we suggest a simple model for initiating mountain parallel creep.

Geophysical Journal International↗

USGS1998

Weakness of the lower continental crust: A condition for delamination, uplift, and escape

We discuss three interconnected processes that occur during continental compression and extension: delamination of the lower crust and sub-crustal lithosphere, escape tectonics (i.e., lateral crustal flow), and crustal uplift. We combine calculations of lithospheric viscosity–depth curves with geologic observations and seismic images of the deep crust to infer the mechanisms controlling these processes. The basic driving force for delamination is the negative buoyancy (in some regions) of the continental lower crust and sub-crustal lithosphere with respect to the warm, mobile asthenosphere. A phase transformation in the lower crust from mafic granulite facies to eclogite may be important for providing negative buoyancy. Where negative buoyancy exists, the onset of delamination is mainly a question of the presence of a suitable decoupling zone between the denser lithosphere and the lighter upper and middle crust. We estimate the depth to potential decoupling zones by calculating lithospheric viscosity–depth curves based on reasonable geotherms and models of lithospheric composition. Low-viscosity zones occur at three depths: (1) at the base of the felsic (upper) crust; (2) within the lower crust; and (3) several tens of kilometers below the Moho. The commonly observed absence of a high-velocity (>6.8 km/s) lower crustal layer beneath extended crust may be explained by delamination wherein decoupling occurs at the top of the lower crust. In addition to being zones of potential decoupling, crustal low-viscosity zones are avenues for lateral crustal flow, a process that is often referred to as crustal escape (e.g., eastern Tibetan Plateau). The third process addressed here, crustal uplift, is mainly found in compressional environments and can be related to mature (i.e., complete or nearly complete) delamination and/or a thick low-viscosity lower crust. Mature delamination generates crustal uplift as the sinking, dense lithosphere is replaced by the mobilized hot asthenosphere. A very different mechanism of uplift is associated with some continental high plateaus, where a high convergence rate and the lateral intrusion of cold, rigid shield crust into warm, low-viscosity orogenic crust acts like a solid piston moving into hydraulic fluid. The displacement of the low-viscosity crustal `fluid' generates broad plateau uplifts. Modern examples are the intrusion of the Indian shield into the Tibetan Plateau and the Brazilian shield into the Andes. All of these processes, delamination, tectonic escape, and uplift are interconnected and are related to weakness in the lower crust during continental compression and extension.

Tectonophysics↗