Granites and granite pegmatites composing the ∼ 2550‐Ma Qôrqut granite complex occur in a SSW‐NNE trending linear belt >150 km long extending through the Buksefjorden‐Ameralik‐Godthåbsfjord region of southern West Greenland. The main body of the complex crops out over a distance of 50 km from Ameralik to Kapisigdlit kangerdluat and reaches a maximum outcrop width of 18 km between Storø and Qôrqut. Around Qôrqut the complex comprises three main groups of granites: early leucocratic granites, various grey biotite granites, and late aplogranite‐granite pegmatites. Within the 1500‐m vertical section available in this area the complex has a tripartite structure comprising a lower zone dominantly of polyphase granite, an intermediate zone where country rock occurs as rafts in polyphase granite with a complex sheeted structure, and an upper zone dominantly of country rock sheeted by granite. Fifty‐two specimens of granite have been analyzed for major, minor, and some trace elements. Geochemical variation within the complex is consistent with either fractional crystallization or partial melting, but in both cases, feldspar + biotite must have been involved either as fractionating phases or as residual phases during melting to account for the trace element chemistry. Two possible models for the generation of the complex are either anatexis of granulite facies rocks in the lower crust following an influx of volatiles and heat from the mantle or melting at intermediate depths of amphibolite facies rocks with volatiles supplied by breakdown of hydrous phases.
Report of a Society Ordinary General Meeting held at Burlington House on 13 November 1985. The meeting was organized by Dr M. Brown and Dr H. Colley. With the success of the Specialist Groups and the decline of the ‘traditional’ OGM, the Society has sponsored several one-day thematic meetings either as OGMs or as SpecialScientific Meetings. Additionally, specialist or thematic meetings have reduced the opportunities for interdisciplinary research on a regional scale to be presented and discussed. These reasons prompted the convenors to propose a meeting on ‘Current research in regional geology’, with papers from academic institutes, the BGS and overseas. Speakers presented recent unpublished research results, and both the substantial attendance at the meeting and the lively discussion after each paper suggest that this style of OGM is very worthwhile. The BGS also displayed a number of poster contributions. An introduction to the meeting was given by M. Brown, who also shared the chair during the morning session with H. Colley. In the first paper, R. L. Johnson presented a general overview of the work undertaken by the BGS overseas, particularly in Africa. The most interesting type of project undertakh by the BGS in Africa under funding from the Overseas Development Administration is that involving research and development in which success is judged in terms of the prospecting licences sought at the end of a project! P. J. Treloar followed with a detailed account of recent research’in the Magondi (mid-Proterozoic) and Zambesi (Pan-African) mobile belts of NW and N
Research Article| September 01, 1993 Displacement history of the Atacama fault system 25°00′S-27° 00′S, northern Chile M. BROWN; M. BROWN 1Department of Geology, University of Maryland at College Park, Maryland 20742 Search for other works by this author on: GSW Google Scholar F. DIÁZ; F. DIÁZ 2Servicio Nacional de Geología y Minería, Casilla 10465, Santiago, Chile Search for other works by this author on: GSW Google Scholar J. GROCOTT J. GROCOTT 3School of Geological Sciences, Kingston University, Kingston-upon-Thames KT1 2EE, United Kingdom Search for other works by this author on: GSW Google Scholar Author and Article Information M. BROWN 1Department of Geology, University of Maryland at College Park, Maryland 20742 F. DIÁZ 2Servicio Nacional de Geología y Minería, Casilla 10465, Santiago, Chile J. GROCOTT 3School of Geological Sciences, Kingston University, Kingston-upon-Thames KT1 2EE, United Kingdom Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (9): 1165–1174. https://doi.org/10.1130/0016-7606(1993)105<1165:DHOTAF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation M. BROWN, F. DIÁZ, J. GROCOTT; Displacement history of the Atacama fault system 25°00′S-27° 00′S, northern Chile. GSA Bulletin 1993;; 105 (9): 1165–1174. doi: https://doi.org/10.1130/0016-7606(1993)105<1165:DHOTAF>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 SocietyGSA Bulletin Search Advanced Search Abstract In the Cordillera de la Costa of the central Andes in northern Chile, Mesozoic arc complexes are cut by a trench-parallel strike-slip fault system: the Atacama fault system. Brittle faulting in the Atacama fault system is superposed on steeply dipping foliations in ductile shear belts. Between 25°S and 27°S, the western part of the fault system was active in Early Cretaceous time as an upper amphibolite facies, down-to-the-east, dip-slip ductile shear zone. In the eastern part of the fault system, ductile deformation is of similar Early Cretaceous age but occurred under lower-grade metamorphic conditions at the greenschist/lower amphibolite facies transition. The mylonites in the eastern part of the fault system were formed by sinistral strike-slip displacement.The dip-slip and sinistral strike-slip displacements are contemporary with the development of a magmatic arc, and they imply that the tectonic environment in this part of the arc was transtensional. The ductile deformation was partitioned spatially into a dip-slip component associated with the emplacement of magmas and a sinistral strike-slip component.Brittle fault zones in the El Salado segment of the Atacama fault system define large-scale sidewall ripout structures. Subhorizontai slickenlines, ripout asymmetry, and S-C-type fabrics in fault gouge indicate that brittle deformation involved sinistral strike-slip displacements. The transition from ductile to brittle sinistral strike-slip displacements may have occurred due to cooling, in mid-Cretaceous time, when the magmatic arc was abandoned. 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.
Earth’s mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics1. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science1–4. Metamorphic rocks—rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)—record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth’s tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts5, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones5,6. Here we show that Earth’s modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago9–12) at the latest. Variability in Earth’s thermal gradients, recorded by metamorphic rocks through time, shows that Earth’s modern plate tectonics developed gradually since the Neoarchaean era, three billion years ago.
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