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Bureau Seminar, February 7, 2014

The origin of ophiolite fore-arcs and their obduction.

Link to streaming video: available 02.07.2014 at 8:55am

Prof. John F. Dewey
Visiting Professor, JSG
Distinguished Emeritus Professor University of California, Emeritus Professor and Supernumerary Fellow, University College Oxford

I outline a simple model that solves the "ophiolite enigma", which is that ophiolite complexes sensu stricto must originate by organized sea-floor spreading that indicates an origin at oceanic ridges yet have (calc-alkaline and boninites characteristics that link them to the fore-arcs of oceanic arcs above subduction zones. The model was triggered by aspects of Hall's (2001) tectonic evolution model of southeast Asia, an analysis of the geology and evolution of the West Philippine Sea Plate and the Izu-Bonin Mariana Arc, and a global synthesis of ophiolites. Many ophiolites have complexly-deformed associated assemblages that suggest fracture zone/transform geology, which in turn has led to models for formation of some fore-arc ophiolites involving the nucleation of subduction zones on fracture zones/transforms. The model is that, following the nucleation of a subduction zone, commonly along oceanic transform/fracture zones a stable upper-plate ridge/trench/trench triple junction or upper-plate ridge-trench-transform plate geometry develops, with a ridge separating two hanging-wall plates, and allows the growth and lengthening of the arc-fore-arc to create a juvenile ophiolite fore-arc. The lengthening is parallel with the arc, generates a diachronous ophiolitic lithosphere in a suprasubduction zone forearc setting, is synchronous with the development of the rear-arc oceanic basin along the same ridge that could have large or small transform offsets, and allows for development of boninitic magmas and the basal subduction (not obduction) high-temperature-low pressure metamorphic sole typical of large obducted ophiolite complexes. During a juvenile arc's development, more than one back-arc spreading center may intersect the arc crust and transect the forearc to the trench creating several punctuated episodes of ophiolitic forearc basement generation and segments along a single continuous trench system. For upper plate ridge-trench-trench or ridge-trench-transform triple junctions, the simple plate kinematic geometry has no constraint or demand as to whether the upper plate ridge is creating a small back arc basin along which slab contaminated BABB erupt for most of its length or a main ocean basin along which typical MORB basalts erupt along most of its length.

The early Ordovician (c. 485 Ma) Bay of Islands Ophiolite Complex was obducted onto the Laurentian rifted margin as the fore-arc of an oceanic arc that collided with the margin during the mid-Ordovician (c. 470 Ma). The subduction zone was nucleated on an oceanic transform/fracture zone, part of whose remnants occur as the polyphase-deformed and intruded mafic and ultramafic rocks of the Coastal Complex. The ophiolite formed as a supra-subduction-zone fore-arc ophiolite at the spatial and temporal continuation of the ridge normal to the transform/fracture zone/subduction zone system and northeast of a trench/trench/ridge triple junction. A two-pyroxene garnet granulite/garnet amphibolite/epidote amphibolite mafic metamorphic sole at the base of the ophiolite was generated, roughly synchronously with the ophiolite, by the metamorphism of MORB mafic rocks in the descending slab at about 10kb and quickly attached to the base of the overlying ophiolite during slab flattening, and not by subduction zone extrusion. The metamorphic sole is not a metamorphic aureole at the base of a hot obducting ophiolite. Plate slip vector triangles around the triple junction, before collision and during obduction are constructed from the orientation of dykes in the sheeted complex and the trends of structures in the high to lower temperature parts of the sole and the obducted nappes of oceanic and continental margin rocks beneath the ophiolite. Linear structures in the sole trend north-north-west (the subduction direction); those in the greenschist-facies obducted oceanic and continental margin rocks trend west-south-west (the obduction direction).

About John Frederick Dewey
John Frederick Dewey obtained his first degree in 1958 from Queen Mary College London. In his PhD thesis (1960) at Imperial College London he investigated the Irish Caledonian mountain belt. He started his academic career with lectureships at the universities of Manchester and Cambridge. In 1970, he moved to the State University of New York at Albany. In 1982, he returned to the UK to become chair of the geological sciences department at the University of Durham, UK. In 1986 he moved to the equivalent position at Oxford. In 2001, he moved to the University of California at Davis, where he retired in 2006. Dewey is among the early proponents of plate tectonics. His life-long research interest is the question of how mountain ranges form. During a stay at the Lamont Geological Observatory in New York, he investigated the Appalachian/Caledonian belt, which is effectively the American side of the mountain range he studied in his PhD work. In more recent research, he has shown that mountains can build up much faster than geologists previously thought, taking as little as ten million years. He also studied the way heat is transferred in developing mountain ranges, which also happens faster than traditional models allowed. Dewey was elected a Fellow of the Royal Society in 1985 and became a member of the US National Academy of Sciences in 1997. In 1999 he was awarded the Wollaston medal of the Geological Society at London. As an emeritus professor, Dewey continues to explore mountains, but has also more time to spend with his model railway, which has been reported to include a geologically accurate representation of the Swiss Alps.


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