Plate margin within the Tethys-Pacific Wilson cycle with respect to the onset of subduction and its interruption by accreting island arcs. The proposed science is considered to be important because:South AmericanThe main aims of the study are: i) to determine the timing of amalgamation of western Pangea and its effect on rock uplift and exhumation along the relict South American Plate margin, ii) to determine the timing of the disassembly of western Pangea, the opening of the western Tethys Ocean and the timing of crustal anatexis, and to iii) constrain the evolution of the
A) The Northern Andean Segment (north of 5°S) provides an extremely well exposed section of rocks that permit the processes of continent-continent and continent-oceanic arc collision and accretion to be studied, and hence any concluding models will represent useful analogues for interpreting geological observations in other locations where continental plates have grown via the collision and accretion of oceanic and continental crust, e.g. the northern and eastern margins of Australia.
B) The significant hydrocarbon reserves of the north-western Amazon basin, along with extensive porphyry related metal deposits within the Andes render the Northern Andean Segment economically important, although they are under-exploited due to a paucity of knowledge of the geological evolution of the region.
A wide disparity exists in the documented evolution of the Palaeozoic-Mesozoic rocks within Ecuador and Colombia, despite the fact that they continuously crop-out along-strike, throughout both countries. These differences render it difficult to compare geochronological and stratigraphic information from different regions and hence coherent models for the Palaeozoic - early Cretaceous evolution of the entire Northern Andean Segment have not been proposed. Extensive mapping by the British Geological Survey during 1986-1992 divided the Palaeozoic - Mesozoic rocks of Ecuador (Eastern Cordillera) into four N-S trending terranes (Litherland et al., 1994). However, more recently Pratt et al. (2005) proposed an autochthonous geological model for the same rocks. Palaeozoic - Mesozoic rocks of the Central Cordillera of Colombia are thought to form a single allochthonous terrane (the Tahami Terrane; Toussaint and Restrepo, 1994; Figure 1), which accreted against the Precambrian shield during the Cretaceous. These contrasting models form the core of the proposal and originate because of a paucity of quantitative data, resulting in few temporal constraints.
Ar) thermochronological methods will be used to extract plausible thermal history paths from Precambrian gneisses, Paleozoic metasedimentary rocks, Triassic migmatites and Jurassic granitoids to assess the pre-Andean exhumation history of the NW SOAM Plate margin during continental disassembly, and subsequent active margin processes such as island arc accretion.39Ar/40This proposal requests funds to complete the post-graduate studies of Ryan Cochrane, which requires analyses of samples already collected from well-exposed sections of Precambrian - Mesozoic rocks within Ecuador and Colombia, and funds for one additional field season. Geochronological (ID-TIMS and LA-ICPMS), geochemical (XRF and ICP-MS) and isotopic tracing (Hf and Nd) data will be used to construct a temporal and tectonic framework for several sections. High temperature (>350°C; apatite and titanite U-Pb; hornblende and white mica
The acquired data and interpretations will be used to develop a model for the Palaeozoic - early Cretaceous geological evolution of the NW SOAM plate margin. We intend to obtain data from both Ecuador and Colombia to test the likelihood of diachroneity during the amalgamation and disassembly of Pangea, as well as during the interruption of the Pacific active margin by the collision of an intra-oceanic arc system.