December 2009 Lithosphere highlights

December 07, 2009

Boulder, CO, USA - Lithosphere articles examine the possibilities surrounding crustal melting during continental subduction; find evidence for igneous diapirism in the San Rafael Desert; date metamorphism in the Nashoba terrane; create a 3-D model of the central Australian lithosphere; determine how the weight of the Andes affects the continental crust; correlate the Mocha fracture zone with orogenic uplift in the Andes; and study pieces of the lower lithosphere as seen in mantle rocks once erupted in southwest Texas.

The consequences of crustal melting in continental subduction
Donna L. Whitney, University of Minnesota, Minneapolis, MN 55455, USA. Pages 323-327.

Most earth-science students are taught that the oceanic parts of tectonic plates subduct into the mantle but that continents do not subduct and that this difference explains why there is no very old oceanic crust but some continental crust is billions of years old. It has been known for the past 25 years, however, that the edges of continents may be pulled down into subduction zones to great depths, creating high-pressure minerals, such as (micro)diamonds. This new paper considers what happens if the subducted continental crust partly melts. How much melt is formed, and where does the melt go? Whitney et al. present ideas, based on computer models and field examples, for how the melted continental crust will contribute to the growth of mountain belts in the nonsubducting crust. This helps researchers understand why some mountains belts are very wide (Himalayas) but some are very narrow (Appalachians), and compares the evolution of "hot" mountain belts (western North America) with mountain belts that form in colder, thicker crust.

Evidence of small-volume igneous diapirism in the shallow crust of the Colorado Plateau, San Rafael Desert, Utah
Mikel Díez, University of Bristol, Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK. Pages 328-336.

The transport of magma through the upper crust by diapirism, governed by buoyancy forces and viscous behavior of wallrocks, is a controversial idea and has been disregarded by some authors. Díez et al. report geological and geophysical evidence from a remarkable outcrop in the Colorado Plateau, Utah, that indicates that igneous diapirism is a viable mechanism for magma transport through the upper crust. Igneous diapirism is then effective through initially brittle crust when magma interacts mechanically with wallrocks, reducing their strength and enhancing viscous deformation.

Refining temporal constraints on metamorphism in the Nashoba terrane, southeastern New England, through monazite dating
Misty Michele Stroud, University of Florida, Geological Sciences, 241 Williamson Hall, Gainesville, FL 32611, USA. Pages 337-342.

Electron-microprobe dating of monazite grains within high-grade mylonitic rocks of the Nashoba terrane in eastern Massachusetts provides new temporal constraints on metamorphism in southeastern New England. In situ dating of monazite grains from three fault zones has allowed the timing of multistage events to be discerned. Three distinct metamorphic events were detected in the Nashoba terrane. The first metamorphic event (M1) occurred from 435 to 400 Ma, with an average age of 423 Ma. A second metamorphic event (M2) occurred at ca. 390 Ma and was associated with widespread migmatization. A third metamorphic event (M3) occurred during the ca. 378-371 Ma time interval and was possibly associated with the Neoacadian orogeny. Intermittent monazite growth during the 360-305 Ma interval suggests that the main phase of metamorphism in the shear zones was complete, but the highly deformed fault zones acted as a conduit for fluid migration, which was responsible for the production of young monazite grains. By at least 345 Ma, the Nashoba terrane had cooled below the stability of sillimanite.

The architecture, kinematics, and lithospheric processes of a compressional intraplate orogen occurring under Gondwana assembly: The Petermann orogeny, central Australia
Alan Robert Alexander Aitken, Monash University, Geosciences, Bldg. 28, Melbourne, Victoria 3800, Australia. Pages 343-357.

Geophysical data is allied with geologic constraints to produce a three-dimensional model of the lithosphere of central Australia. This region was restructured during an unusual intracontinental mountain building event that occurred due to the assembly of the supercontinent Gondwana 550 million years ago. Lithospheric restructuring during this event include the transport of material away from a strong west Australia and towards a weaker east Australia, and also the uplift of a 20-km-thick wedge of mantle into the crust, causing the development of strong lithosphere in central Australia. These crustal movements demonstrate the importance of heterogeneous lithospheric strength in controlling the development of intracontinental mountain building and also indicate that leading models of Gondwana assembly require modification.

Curved Andes: Geoid, forebulge, and flexure
Clement G. Chase, University of Arizona, Geosciences, 1040 E 4th St., Tucson, AZ 85721-0077, USA. Pages 358-363.

The weight of the Andean mountain chain exerts a downward force on the continental lithosphere of South America that should cause a slight upward bend (flexure) east of the mountain front then a rapid downward bend under the mountains themselves. However, there is no clear topographic expression of an upward bend beneath the Amazon basin and southward due to sedimentary cover (and thick jungle). Using gravity observations from satellite orbits and filtering the resulting geoid, Chase et al. demonstrate that that the positive bend does exist and a simple three-dimensional mechanical model allows them to calculate a value for the lithospheric elastic thickness of about 50 km.

Collision of the Mocha fracture zone and a <4 Ma old wave of orogenic uplift in the Andes (36°-38°S)
Andres Folguera, Universidad de Buenos Aires, Ciencias Geologicas, Pab 2 de cdad Universitaria, Buenos Aires, 1428, Argentina. Pages 364-369.

The Southern Central and Northern Patagonian Andes (34 to 45 degrees South) have no indication of less than 2 Ma mountain building processes with the exception of the segment located between 36 and 38 degrees South. There, deformation started at 1.7-1.4 Ma, next to the arc zone. It is between these latitudes that mountain building processes less than 3.6 Ma old developed in the Pacific coastal region. Collision of the Mocha fracture zone explains the distribution, extent, and timing of less than 3.6 Ma contractional deformations, as well as incipient shallowing of the subducted Nazca plate beneath the South American plate.

Fluid-metasomatized mantle beneath the Ouachita belt of southern Laurentia: Fate of lithospheric mantle in a continental orogenic belt
Hobart Patrick Young, Rice University, Earth Science, MS 126, P.O. Box 1892, Houston, TX 77251-1892, USA. Pages 370-383.

The mantle, the layer of Earth that is compositionally distinct from the relatively thin crust above it, constitutes the bulk of a tectonic plate (the mechanical "lithosphere"), both beneath oceans and continents. In order to better understand the history of plate movements and the process whereby continents are convectively and chemically differentiated from bulk Earth, Young and Lee study pieces of the lower lithosphere (mantle) that are only rarely erupted at the surface by basaltic pipes, some of which contain diamonds. In this study, they present new chemical data of mantle rocks erupted in southwest Texas. The chemical composition of these samples indicates that the region was once a volcanic arc, such as the modern day Andes. They consider the history of plate movements there and possible scenarios for the emplacement of volcanic arc mantle lithosphere (the "mantle wedge"). They then discuss the implications of this history for models of lithospheric strength and the propensity for preservation of such material in a stable continental interior.
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