Orogeny can be very short

Abstract

In contrast to continent͞continent collision, arc-continent collision generates very short-lived orogeny because the buoyancy-driven impedance of the subduction of continental lithosphere, accompanied by arc͞suprasubduction-zone ophiolite obduction, is relieved by subduction polarity reversal (flip). This tectonic principle is illustrated by the early Ordovician Grampian Orogeny in the British and Irish Caledonides, in which a wealth of detailed sedimentologic, heavy mineral, and geochronologic data pin the Orogeny to a very short Arenig͞Llanvirn event. The Orogeny, from the initial subduction of continental margin sediments to the end of postflip shortening, lasted Ϸ18 million years (my). The collisional shortening, prograde-metamorphic phase of the Orogeny lasted 8 my, extensional collapse and exhumation of midcrustal rocks lasted 1.5 my, and postflip shortening lasted 4.5 my. Strain rates were a typical plate-boundary-zone 10 ؊15 . Metamorphism, to the second sillimanite isograd, with extensive partial melting, occurred within a few my after initial collision, indicating that conductive models for metamorphic heat transfer in Barrovian terrains are incorrect and must be replaced by advective models in which large volumes of mafic͞ultramafic magma are emplaced, syn-tectonically, below and into evolving nappe stacks. Arc͞continent collision generates fast and very short orogeny, regional metamorphism, and exhumation.8 million year orogeny ͉ continent͞arc collision ͉ orogenic mechanisms

The Ayyubid Orogen: An Ophiolite Obduction-Driven Orogen in the Late Cretaceous of the Neo-Tethyan South Margin

“…Obduction-driven orogens, which habitually take a very short time to form (cf. Dewey 2005), may become totally obliterated by subsequent subduction-or collision-driven orogenies and their record may be misinterpreted as an earlier phase in the evolution of these later mountain ranges. The Ayyubids have thus long been thought of part of the Alpides.…”

The nature and origin of the Barrovian metamorphism, Scotland: diffusion length scales in garnet and inferred thermal time scales

“…This study suggests that the features that Ague & Baxter (2007) studied developed as the result of a late and localized thermal overprint on the regional metamorphic event and not the regional-scale thermal event itself. Using the Arrhenius parameters of Carlson (2006), the regional-scale thermal anomaly (and c. 1000 ìm Mn diffusion) will have developed over 10-30 Ma and disagreements between the duration of metamorphic heating and the duration of Grampian orogenesis (c. 12-18 Ma: Dewey & Mange 1999;Friedrich et al 1999;Oliver et al 2000;Dewey 2005) arise. The Arrhenius parameters of Chakraborty & Ganguly (1992) predict less retentive diffusion behaviour at sillimanite-grade conditions (T ¼ 670 8C, P ¼ 6.1 kbar) compared with those of Carlson (2006) (Table 2).…”

Petroleum generation and migration in the Cambro-Ordovician Laurentian margin succession of NW Scotland

“…However, several factors could have contributed to generation: (1) the pre-thrusting succession included a greater thickness of rocks now excised by thrusting; (2) as the source rocks probably lay to the east in deeper water facies where the section was thicker, generation could have occurred earlier than in the outcropping section; (3) enhanced heat flow pre-thrusting, indicated by magmatic activity and implicit in regional tectonic models, could have enhanced maturation; (4) additional heating could have occurred owing to structural burial during thrusting. These factors may have combined, so that flexural loading and greater burial in a source rock kitchen to the east, during Grampian and Scandian deformation (Dewey 2005), produced hydrocarbonbearing fluids that migrated westwards up-dip, especially when thrusting provided new migration pathways.…”

“…The total Laurentian Lower Palaeozoic sequence preserved is up to 1.5 km (Swett 1969). Structural burial during the Caledonian Orogeny (Moine Thrust imbrication dated at c. 435 -425 Ma; Freeman et al 1998;Dewey 2005;Goodenough et al 2011) may have been to a depth of ≥10 km (Coward 1983), and has left the succession thermally metamorphosed. Conodont alteration indices in the Durness region are about five, representing temperatures of up to 325°C (Johnson et al 1985;Laubach & Diaz-Tushman 2009), acritarch thermal indices suggest temperatures of about 150 -250°C, increasing northwards (Downie 1982), and illite crystallinity data along the whole outcrop indicate temperatures of 250 -350°C, also with higher degrees of thermal alteration in the north (Allison & Ferguson 1997).…”