Petrologic and structural investigations in a polydeformed terrane in southwestern New Hampshire show that more than one stage of noncoaxial folding has significantly affected the metamorphic history of this region. Detailed examinations of both isograd-isotherm patterns and mineral reaction histories within pelitic rocks suggest that pre-existing, nearly horizontal isotherms were folded during an early stage of folding about north-south axes. After folding, thermal relaxation resulted in cooling in the anticlinal portions of the early folds and heating in the synclinal regions. This process of isotherm folding and re-equilibration was repeated during a later stage of east-west folding, resulting in complex isograd patterns and mineral reaction histories. Detailed mapping of isograd and isotherms in the pelitic rocks of this region shows a complex patten. High grade and high temperature assemblages occur along early and late synclinal axes, with the highest grade assemblages occurring at the intersection of the two synclinal axes. Conversely, the lowest grade assemblages are found at the intersection of the early and late anticlinal folds. Four different types of fold intersections result from the two stages of noncoaxial folding. Pelitic rocks at each of these four different fold intersections show different and contrasting mineral reaction histories. At the intersections of early and late synclines the rocks show evidence for continuous heating, while continuous cooling trends are seen at intersections of early and late anticlines. Complex reaction histories are observed at the intersections of early synclines and late anticlines and early anticlines and late synclines, which show heating-cooling and cooling-heating trends, respectively. These results show that over small areas of a metamorphic terrane different samples may show widely different P-T paths. Therefore, without careful structural analysis and thorough sampling, tectonic interpretations based solely on paths determined from a few widely scattered samples may lead to erroneous conclusions.
Research Article| October 01, 1995 Fluids and the Heart Mountain fault revisited Alexis S. Templeton; Alexis S. Templeton 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar James Sweeney, Jr; James Sweeney, Jr 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Hans Manske; Hans Manske 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Jennifer Fox Tilghman; Jennifer Fox Tilghman 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Adam Violich; Adam Violich 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar C. Page Chamberlain C. Page Chamberlain 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Search for other works by this author on: GSW Google Scholar Author and Article Information Alexis S. Templeton 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 James Sweeney, Jr 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Hans Manske 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Jennifer Fox Tilghman 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Adam Violich 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 C. Page Chamberlain 1Department of Earth Sciences, Dartmouth College, Hanover, New Hampshire 03755 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1995) 23 (10): 929–932. https://doi.org/10.1130/0091-7613(1995)023<0929:FATHMF>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Alexis S. Templeton, James Sweeney, Hans Manske, Jennifer Fox Tilghman, Adam Violich, C. Page Chamberlain; Fluids and the Heart Mountain fault revisited. Geology 1995;; 23 (10): 929–932. doi: https://doi.org/10.1130/0091-7613(1995)023<0929:FATHMF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Stable isotopic results from the Heart Mountain fault in northwestern Wyoming show that fluids migrated along the detachment during faulting. We report herein calcite (δ18O and δ13C values in fault breccias, shifted by as much as −15‰o (relative to SMOW) and −5‰o (relative to PDB), respectively, which demonstrate focusing of meteoric waters along the detachment. The isotopic depletions systematically increase toward the northwestern margin of the fault terrane, where Absaroka intrusive centers may have provided a source region for hydrothermal fluids. In addition, isotopic disequilibrium between vein and wallrock samples and enhanced 18O and 13C depletions in lower-plate calcite veins suggest that the fluids were externally derived and migrated upward to the detachment. We advocate a model in which Heart Mountain faulting and syntectonic fluid flow occurred beneath a continuous allochthon. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Pressure-temperature (P-T) paths observed in pelitic schists on either side of the Main Mantle Thrust in northern Pakistan record the dynamics of the collision between the Kohistan Island-Arc and Indian plate. Geothermometry studies, mineral reaction textures, and thermodynamic modeling of zoned garnets suggest that the rocks in the Kohistan Arc and the Nanga Parbat–Haramosh Massif experienced different pressure-temperature histories as a result of imbrication of these two terranes during thrusting. Rocks in the Kohistan Arc followed decreasing pressure-temperature paths, with early garnet growth occurring at high pressures (9.5 kbar) and later garnet growth at lower pressures (8.5 kbar). Conversely, rocks in the Nanga Parbat–Haramosh Massif record an increasing P-T path history. The early P-T history within the massif was at low pressures (4.0 kbar) and low temperatures (450°C). Later, both pressure and temperature increased to a maximum of 7.5 kbar and 580°C. The contrasting P-T paths observed within these two terranes provide evidence for overthrusting of the Kohistan Arc over the Nanga Parbat–Haramosh Massif along the Main Mantle Thrust.
ABSTRACT The pressure‐temperature and temperature‐time paths derived for rocks in the Kohistan arc and adjacent Nanga Parbat‐Haramosh massif record the dynamics of the collision between the island arc and the Indian plate. Studies of P‐T‐t paths show that the Kohistan arc was thrust over the Nanga Parbat‐Haramosh massif at least 25 Ma ago, but not more than 30–35 Ma ago. Rocks in the Kohistan arc followed decreasing pressure paths, with the early metamorphism beginning at high pressures (9.5 kbar) and later metamorphism occurring at 8.0 kbar. In contrast, rocks in the Nanga Parbat‐Haramosh massif (Indian plate) experienced increasing pressure and temperature paths. Prior to thrusting, the massif was at low pressures (4.0 kbar) and low temperatures (450°c). Later, the pressure and temperature increased to 8 kbar and 580°c. The authors interpret the convergence (to approximately the same pressure and temperature) of the P‐T paths in the two terranes as being the result of thrusting and thermal equilibration between the thrust sheets. 40 Ar/ 39 Ar cooling ages of hornblendes and other geochronological data suggest that the time of peak metamorphism and hence the completion of thickening was approximately 30–35 Ma ago. Temperature‐time paths show that after thrusting, during the period 25–10 Ma, the Kohistan arc and Nanga Parbat‐Haramosh massif were uplifted at similar rates (0.5 km Ma). However, in the past 10 Ma the Nanga Parbat‐Haramosh massif has been uplifted more rapidly than the adjacent Kohistan arc. Rapid uplift has been accommodated by late faults along the edge of the massif.
Is erosion important to the structural and petrological evolution of mountain belts?The nature of active metamorphic massifs colocated with deep gorges in the syntaxes at each end of the Himalayan range, together with the magnitude of erosional fluxes that occur in these regions, leads us to concur with suggestions that erosion plays an integral role in collisional dynamics.At multiple scales, erosion exerts an influence on a par with such fundamental phenomena as crustal thickening and extensional collapse.Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.
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