A compilation of age, Sr, Nd and O isotopic analyses of plutonic rocks from the western North American Cordillera shows that the high-flux pulses of magmatism that make up most of the exposed North American arcs are dominated by upper plate lithospheric mass. Oceanic crustal (basalts and sediments) melting cannot be a principal mechanism responsible for arc magmatism. Mass balance calculations show that no more than 50% of that mass can be mantle-derived.

Megathrust faults marking the subduction of oceanic plates are fundamental features of Earth tectonics ? without them, the recycling of plates and plate tectonics, as we know it would not exist. In addition, these faults have a significant societal relevance since movement along these faults generates devastating earthquakes. One of the most important tasks in solid Earth Sciences is to be able to understand the mechanical and thermal processes that

Ulyana N. Horodyskyja, Cin-Ty Aeolus Leea, , and Mihai N. Duceab aRice University, Department of Earth Science, MS-126, 6100 Main St., Houston, TX 77005, United States bDepartment of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St., Tucson, AZ 85721, United States Received 18 September 2006; revised 1 February 2007; accepted 2 February 2007. Editor: R.W. Carlson. Available online 11 February 2007. Abstract Some insights

It has been suggested that during collision events leading to crustal thickening and formation of eclogite or through underplating and fractionation of basaltic magma at the base of the crust, the mafic/ultramafic root of the continental crust will become denser than the underlying upper mantle. This density instability will lead to delamination that occurs during continental collision and drives lower crust recycling into the mantle. Lower crustal

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Abstract: Surface exposures as well as deep-crustal and upper-mantle xenoliths constrain the composition of the lithospheric column beneath the Sierra Nevada mountain range (California) as it resulted from the generation of the Mesozoic Sierra Nevada batholith (SNB). After the cessation of magmatism at similar to 80 Ma, the SNB consisted of a similar to 30 to 35 km thick granitic crust underlain by a batholithic "root," a similar to 70 km thick sequence

Apatite and zircon (U-Th)/He ages from a 100-km long range-perpendicular transect in the northern Sierra Nevada, California, are used to constrain the exhumation history of the range since ca. 90 Ma. (U-Th)/He ages in apatite decrease from 80 Ma along the low western range flanks to 46 Ma in the higher elevations to the east. (U- Th)/He ages in zircon also show a weak inverse correlation with elevation, decreasing from 91 Ma in the west to 66 Ma

Recent studies of the upper mantle above the flat slab in central Chile and Argentina indicate the seismic velocity structures in this area are very different from those found in subduction zones with "normal", steeper slab geometries. The mantle above the horizontal section of the flat slab is characterized by low P wave velocities, high S wave velocities, and low Vp/Vs ratios. As the slab begins to transition to a more normal dip to the south,