Hans Gerhard Avé Lallemant - US grants

The structural framework of the Brooks Range, Alaska has been the focus of recent research, and this renewal will build on this work. A number of fundamental problems will be addressed. The first is the formation of an apparent out of sequence thrust in the interior of the fold and thrust belt. Another is the timing and kinematic relationship of major extensional tectonism with progressive north-directed thrusting. Also to be studied are late-stage structures associated with antithetic shear and the formation of a large scale duplex in the center of the range. These are fall features of most fold and thrust belts, and results of this work will be applicable to other examples, as well s to the understanding of Alaskan geology.

This action provides partial participant support for invited speakers and student attendees of a Penrose Conference on Transpressional Tectonics of Convergent Plate Margins to be held in Bellingham, Washington, August 24-30, 1990.

The tectonic history of the Canada Basin is poorly understood and the Mesozoic history of the Arctic margin of Alaska & NW Canada is critical to this problem. Magnetic data from the oceanic crust of Canada Basin are ambiguous and cannot distinguish between (1) anticlockwise oroclinal rotation of the northern margin of Alaska away from the Canadian Arctic Islands about a pole of rotation new the MacKenzie delta, and (2) a major left-lateral transform fault to permit the southerly migration of northern Alaska. The two models predict different structural regimes for the eastern Brooks Range, MacKenzie delta, and Black River-Oglivie Mountains regions during the Mesozoic this project will address the problem by structural and kinematic studies along the Porcupine River fault system and adjacent areas. Spatial variations in the geometry of superposed structures will be documented from Precambrian through Cenozoic rocks, and ages of deformation all be established from stratigraphic and igneous intrusion relationships. The results of this study will improve our understanding of the tectonic evolution of the Canada Basin and the adjacent Arctic region, and will also have relevance to oil exploration in the area.

The uplift and exhumation of high pressure rocks in orogenic zones is a long-standing problem. A new hypothesis for the uplift and exhumation of blueschists and eclogites has been developed in this project on the basis of structural analysis of Late Cretaceous subduction complex in eastern Venezuela. The observation that these rocks had undergone large extensional strains parallel to the arcuate trend of the subduction zone and the volcanic arc led to a model in which oblique plate convergence changes required large arc-parallel extension and considerable forearc thinning. This renewal will test several aspects of this new model by dating the deformation sequence and determine the pressure-temperature history of particularly the lower grade rocks of the complex, and compare the results to the predictions of the model. Results are expected to test this possible mechanism of returning high pressure rocks to the earth's surface.

The Chortis block (Honduras, El Salvador and environs) is a microcontinental block at the northwestern end of the Caribbean plate which may have been transported east from southern Mexico during the Cenozoic opening of the Cayman Trough. However little is known about the tectonic history of this block. This project will gather kinematic data on Cenozoic faulting to help determine slip directions and to infer stress regime and estimate displacements to see if significant Cayman Trough offset is absorbed within the block. In an effort to place age constraints on the tectonic history, U-Pb, 40Ar/39Ar and fission track ages on granitoids and volcanic rocks will be carried out. These data will allow testing of the hypothesis that the Chortis block originated from the truncated southern margin of Mexico, and generally improve the understanding of the transport of microcontinental blocks along a large offset transform fault.

A X-ray facility consisting of a rotating anode X-ray generator and two different diffraction systems on opposite sides of the generator will be acquired with the funds from the Academic Research Infrastructure Program. One diffraction unit will be a 4-circle diffractometer for accurate lattice parameter and structural determinations of thin films, single crystals, and powder samples. A second diffraction unit will involve an advanced 2-dimensional array detector for small angle X-ray scattering from biological and thin film samples. The facility will be employed to study: 1) the structure of lanthanide endohedral fulleneres and fullerene-encapsulated metal clusters, the study of low-dimensional mixed-valence compounds associated with superconduct-ivity and and charge density waves, 3) various properties of the unique van der Waals C60 solids, 4) magnetic multilayer thin films, 5) diamond thin films prepared by chemical vapor deposition and ion- implantation techniques, 6) growth of thin ferroelectric films on crystalline and amorphous substrates, 7) genetic features of regulatory proteins and the biochemistry and biology of repressor proteins, 8) the thermodynamic properties of natural minerals. A modern X-ray facility with two types of diffractometers will be employed to study a diverse range of materials by scientists in the Chemistry, Electrical Engineering, Biosciences, Materials Science and Geology Departments. The materials studied will include fullerenes, diamond thin films, repressor proteins, magnetic multilayers, ferroelectric thin films, and natural minerals.

9506389 AveLallement This field-based study of the tectonically active area of Attu island in the Aleutian volcanic arc will collect detailed data on recently active brittle fault populations. The goal is to determine how to known oblique convergence across the Aleutian arc is partitioned into strike slip, thrust and extensional faults. The results of this study will significantly improve our understanding of oblique collisional orogens, and will aid in the understanding of active plate-boundary tectonic processes.

9714906 AveLallemant In circumstances where the convergence vector between two plates is oblique to the suture, deformation is partitioned into arc-parallel (strike-slip) and arc normal (thrust) components. The parameters controlling this partitioning are not well known, and several models exist. This research in the Aleutian arc takes advantage of a curved suture between plate where the vector varies smoothly from near O (orthogonal) to near 90o (pure strike-slip) to address some of the possible parameters. If the coupling between upper and lower plates remains constant, increasing arc-parallel extension in the upper plate with increasing obliquity should be observed. However, no arc-parallel extension need occur if coupling decreases with increasing obliquity. This project will employ GPS technology to determine translations of the islands along the Aleutian arc and structural analysis to determine operative slip and deformation response. Results should document partitioning of strain with increasing obliquity, identify operative slip mechanisms and allow testing of several hypotheses about partitioning in situation where convergent vectors are oblique to plate margins.

9706521 Sisson The uplift and exhumation of high-pressure metamorphic rocks related to subduction is a classic, unsolved geologic problem. High P/low T assemblages are generally thought to have formed a t great depth in subduction zones, but brought to the surface by a variety of possible mechanisms, resulting in two "types"- one characterized by retrograde, approximately isothermal decompression, and the other by a decompression path parallel to their prograde P/T path. In northern Venezuela there are two occurrences of High P/low T metamorphic belts of the same age and thought to be associated with the same subduction zone. However one is of the "Franciscan-type" and the other of the "Alpine-Type". This project will attempt to exploit this circumstance and isolate the mechanisms of uplift and ascent leading to the different types, and will involve extensive structural and petrologic study of each occurrence. Results have the potential for shedding considerable light on the uplift and exposure of rocks metamorphosed deep in subduction zones.

0003572
Levander

This is a project to study the Caribbean-South American oblique arc-continent collision zone using various geologic (mapping, structure), geochemical (Ar-Ar and U-Pb Geochronology), and seismic (active MCS with onshore/offshore recording using OBS instruments, passive array) techniques. There will also be a geodynamic modeling study that, very innovatively, includes the dynamics of crust-mantle interaction. The overall goal of the project is to understand further the geometry and chronology of a world class, arc-continent accretion event. Seosmic/geological cross-sections are to be derived for several transects across the orogen at different ages of collision. The group of investigators include highly capable seismologists in both active and passive seismology, as well as geologists familiar with the orogenic developments on land. The cooperation with Venezuelan scientists and students is excellent.
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Jadeitite-jadeite rock-is a rare material found at about ten locations worldwide, but in central Guatemala there are two distinct sources, on either side of the Motagua Fault Zone (MFZ) - the plate boundary zone separating North American and Caribbean plates. Jadeitite is sourced from serpentinites and associated with eclogite and blueschist, the markers of subduction zones. Jadeitites appear to represent high-pressure / low-temperature (HP/LT) crystallization of fluid derived from subducted ocean crust, without preservation of remnants of a protolith. Jadeitite petrogenesis represents an important geological process, but we lack a deep understanding of jadeitites, of their relationship to eclogites, blueschists, and host serpentinized peridotite, and of how they are exhumed. Recent dating of micas in HP/LT rocks show that remnants of two collisions, an 125-113 Ma Aptian one on the south side of the MFZ and a 77-65 Ma Maastrichtian. Not only is this a singular pairing of collisional belts, but the pairing of two jadeitite-bearing terranes. Moreover, the presence of abundant lawsonite eclogite south of the MFZ indicates very high pressure, low-temperature, wet processes that contrast with the more moderate mostly blueschist conditions north of the MFZ. These differing terranes offer an extraordinary opportunity to investigate the collisional environments and the processes that produce and exhume jadeitites, other HP/LT rocks, and their host serpentinites. It is anticipated that new data will enable a rigorous testing of the fluid crystallization hypothesis as well as assessment of the sources of fluid and solutes that crystallized jadeitite, altered blueschists and eclogites, and produced serpentinite and the conditions at which these multistage processes occurred.
The results of this three-year investigation will include (1) additional field studies to sample and study the local geology of the newly discovered high-P/T rock occurrences in Guatemala; (2) structural/tectonic studies to understand the distribution and mechanisms of tectonic emplacement; (3) characterization of the mineralogy and petrology of the high-P/T rocks including geothermobarometric and geochemical studies; (4) modeling fluid/rock interactions; (5) constraining the rock histories using U/Pb, Sm/Nd, Rb/Sr, Ar/Ar, and fission-track methods; and (6) investigation of Nd, Sr, Pb, and S isotope systematics by TIMS on separates and by microbeam techniques to infer protoliths and to investigate fluid sourcing, partitioning, and fractionation at high P/T. This information will be integrated into both tectonic and petrogenetic models for the evolution of the Guatemalan paleosubduction zones and the formation, in particular, of jadeitites. The broader impacts of this study include the collaboration of researchers at universities and museums, involvement of students, links with archaeological studies on Middle-American jade which is jadeitite, connections between Guatemalan and U.S. researchers, outreach to both the media and the public via interviews, articles and a Museum web-site, and planning for an exhibition that will feature the scientific dimensions of jade in relation to culture and procurement.

0607801
Levander

This award will support a two year extension of a currently funded CD project (BOLIVAR) which is investigating the processes of continental accretion in the Caribbean. Specifically, the project is investigating how arcs accrete to the northern edge of south America using various geologic (mapping, structure), geochemical (Ar-Ar and U-Pb Geochronology), and seismic (active MCS with onshore/offshore recording using OBS instruments, passive array) techniques. There is also a geodynamic modeling study that, very innovatively, includes the dynamics of crust-mantle interaction. The overall goal of the project is to understand further the geometry and chronology of a world class, arc-continent accretion event. Seismic/geological cross-sections are being derived for several transects across the orogen at different ages of collision. The group of investigators includes highly capable seismologists in both active and passive seismology, as well as geologists familiar with the orogenic developments on land. The cooperation with Venezuelan scientists and students is excellent.

In Venezuela, BOLIVAR has a formal multi-year counterpart, GEODINOS, funded by the Venezuelan government seismological organization FUNVISIS. With FUNVISIS, the PIs have completed a successful data acquisition effort consisting of active and passive land-marine seismic experiments, and numerous geologic/geochemical studies on the Venezuelan mainland and in the Leeward Antilles archipelago. Their study area is about the size of California and its continental margin, an area greater than 600,000 km 2 , and has an equally complex Late Mesozoic-Cenozoic history.