The metamorphic sheet and underlying volcanic rocks beneath the Semail ophiolite in the northern Oman mountains of Arabia

Detailed mapping and structural analysis along the base of the Semail ophiolite thrust sheet in the northern Oman mountains has revealed a distinct and separate thrust sheet between the underlying marine sediments of the Hawasina Complex, and the overlying 12 km+ thick Semail ophiolite. This thrust sheet is termed the Haybi complex, and consists of sub-ophiolitic metamorphic rocks and serpentinite, alkaline and tholeiitic basalts (the Haybi volcanics), associated with mountain-sized "Exotic" limestones and an Upper Cretaceous sedimentary melange. The rocks of the Haybi complex are bounded by major thrust planes, the Semail thrust above and the Haybi thrust beneath, which truncate all schistosities, fold axes, imbricate thrust and associated features. The Haybi volcanics are mainly Triassic in age and form a substrate to or enclose large, isolated blOCKS of Permian or Triassic "Exotic" limestones, Although they have been considerably disrupted and imbricated during south-westward emplacement, intact sequences show that the lower part is composed mainly of alkaline pyroclastics and lavas, including ankaramites, nephelinites and trachytes, whereas the upper part is predominantly tholeiitic pillow lavas and breccias. Late sills of alkali pyroxenite, wehrlite and kaersutite gabbro intrude the tholeiitic volcanics in a few localities and have been dated as Turonian (Upper Cretaceous). Geochemical studies, particularly of "immobile" elements show that the lower volcanics and the late sills are strongly alkaline with high Ti, p, Zr and Nb contents, low Y/Nb ratios and steep LREE enriched rare earth patterns. They are typical "within-plate" alkaline magmas characteristic of continental rift zones and some ocean islands. Two types of tholeiites are recognised: a relative (to MORB) trace-element enriched "transitional" type which probably farmed in a transitional within-plate tectonic setting and a "depleted" type with the characteristics of islandarc tholeiites. The uppermost Haybi arc lavas are probably Middle Cretaceous in age (from radiolaria in interbedded cherts) and may be related to the initiation of a subduction zone in the Cretaceous. prior to ophiolite formation and emplacement. Metamorphic rocks showing an inverted metamorphic zonation from upper amphibolite facies immediately beneath the peridotite to greenschist facies at lower levels, outcrop discontinuously along the base of the Semail ophiolite thrust sheet. These metamorphic rocks show polyphase deformation, mylonitic fabrics, and have been disrupted. folded and imbricated, and in places form tectonic inclusions in a serpentinite melange. In the more intact sequences, garnet-clinopyroxene amphibolites, with rare hornblende and clinopyroxene-bearing marbles and banded quartzites occur at the higher levels whilst a wide range of meta-sedimentary and metabasaltic rOCKS occur in the greenschist facies. "Immobile" element and REE geochemistry supports field assumptions that the amphibolites were derived mainly from Haybi "transitional" (or MORS-type) volcanics. Protoliths of the meta-sediments include "Exotic" limestones, Mn-rich cherts and argillaceous turbidites probably derived from the Hawasina Complex. Low glaucophane content of amphiboles and low jadeite content of clinopyroxenes suggest relatively low pressures of crystallisation. The distribution coefficient KD for coexisting garnet and clinopyroxene suggests a temperature range of 670o to 750oC, the upper limit of the amphibolite facies. Residual heat from the recently-formed ophiolite provided the dominant heat source for metamorphism during the Turonian-Cenomanian (Upper Cretaceous) although frictional heating during thrusting could have supplemented this. Ophiolite emplacement in Oman is thought to be essentially a two-stage process. During initial displacement, high-temperature metamorphism occurred along the base of the fractured ophiolite forming the metamorphic sheet and successive tectonic slices were incorporated onto the base of the ophiolite, presumably by underthrusting. The metamorphism probably occurred along a shallowdipping subduction zone (dipping north-east) which was initiated during the formation of the Haybi arc lavas in the Middle Cretaceous. Final emplacement of the Semail ophiolite by gravity sliding or spreading was facilitated by a thin decollement layer of basal serpentinite along the Semail thrust plane which truncates all underlying structures. Since continent-continent collision has not occurred in this area of Tethys, the Oman ophiolite remains largely undeformed unlike the ophiolites along the Zagros suture zone of Iran or the Indus-Tsangpo suture zone in the Himalayas

National Resources Defense Council: Problem-Centered Strategic Thinking vs. Global Warming

In late 2000, the management and board of the Natural Resources Defense Council (NRDC) formally established global warming as their main priority. In January 2001, NRDC retained the Bridgespan Group to help define its options and develop a comprehensive strategy that would position the organization to maximize its impact. This white paper describes the process that NRDC underwent to determine which specific interventions the organization would support, what scale of effort would be required, how NRDC would accommodate the political realities inherent in the problem and how the NRDC would structure and manage its climate team to maximize effectiveness while minimizing internal cultural issue

Circuit Theory and Design

Contains research objectives

Through Einsteins Eyes

We have developed a relativistically accurate computer graphics code, and have used it to produce photo-realistic images and videos of scenes where special relativistic effects dominate, either in astrophysical contexts or in imaginary worlds where the speed of light is only a few metres per second

Circuit Theory and Design

Contains resesarch objectives.Lincoln Laboratory, Purchase Order DDL B-00368U. S. Air Force under Air Force Contract AF19(604)-7400U. S. NavyU. S. Arm

Exhumation history of the Higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: new constraints from fission track analysis

New apatite and zircon fission track data collected from two transects along the Dhauliganga and Goriganga rivers in the NW Himalaya document exhumation of the Higher Himalayan Crystalline units. Despite sharing the same structural configuration and rock types and being separated by only 60 km, the two study areas show very different patterns of exhumation. Fission track (FT) data from the Dhauliganga section show systematic changes in age (individual apatite FT ages range from 0.9 ± 0.3 to 3.6 ± 0.5 Ma, r 2 = 0.82) that record faster exhumation across a zone that extends from the Main Central Thrust to north of the Vaikrita thrust. By contrast, FT results from the Goriganga Valley show a stepwise change in ages across the Vaikrita thrust that suggests Quaternary thrust sense displacement. Footwall samples yield a weighted mean apatite age of 1.6 ± 0.1 Ma compared to 0.7 ± 0.04 Ma in the hanging wall. A constant zircon fission track age of 1.8 ± 0.4 Ma across both the footwall and hanging wall shows the 0.9 Ma difference in apatite ages is due to movement on the Vaikrita thrust that initiated soon after ∼1.8 Ma. The Goriganga section provides clear evidence for >1 Ma of tectonic deformation in the brittle crust that contrasts with previous exhumation studies in other areas of the high Himalaya ranges; these studies have been unable to decouple the role of climate erosion from tectonics. One possibility why there is a clear tectonic signal in the Goriganga Valley is that climate erosion has not yet fully adjusted to the tectonic perturbation