Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids

Abstract

Collision tectonics and the associated transformation of continental crust to high-pressure rocks (eclogites) are generally well-understood processes, but important contradictions remain between tectonothermal models and petrological-isotopic data obtained from such rocks. Here we use 40Ar-39Ar data coupled with a thermal model to constrain the time-integrated duration of an orogenic cycle (the burial and exhumation of a particular segment of the crust) to be less than 13 Myr. We also determine the total duration of associated metamorphic events to be approximately 20 kyr, and of individual heat pulses experienced by the rocks to be as short as 10 years. Such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events (earthquakes). Such events triggered transient heat advection by hot fluid along deformation (shear) zones, which cut relatively cool and dry subducted crust. In contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust, our non-steady-state cold-crust model satisfactorily explains several otherwise contradictory geological observations.

Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction–collision system, Bergen Arcs, Norway

“…Isotopic equilibration between fluids and rock followed by consumption of the fluid by the eclogitization reactions ) in tandem with only partial eclogitization of the granulites, indicates that the total volume of fluids has been small relative to the rock volume. In line with the conclusions of and in contrast to the inferences of Camacho et al 2005, in such a scenario the potential of the fluids to advect heat and matter on a regional scale into the granulite would be minor to negligible.…”

“…In places, granulite facies ultramafic, olivine-rich rocks occur as m-scale lenses. They preserve their granulite facies mineralogy, even in eclogitized matrix (Kühn et al 2000, Camacho et al 2005. We sampled pristine quartz mangerite (Taulsvågen, Radøy) for zircon U-Pb analysis (Samples HA693, HA696; see Appendix for detailed description).…”

On the significance of short-duration regional metamorphism

“…These new approaches must allow assessment of the veracity of claims of extremely short-duration metamorphism that have been made on the basis of diffusion geospeedometry (e.g. Camacho et al 2005;Ague & Baxter 2007;Viete et al 2011b;Spear et al 2012;Spear 2014). …”

“…Moreover, examples of temporal non-uniformity in rates of metamorphism have been related to extremely short-duration thermal instabilities (tectonometamorphic episodicity) within collisional orogens and subduction zones (e.g. Wijbrans & McDougall 1986, 1988Keay et al 2001;Kohn 2004;Camacho et al 2005;Ague & Baxter 2007;Pollington & Baxter 2010;Viete et al 2011bViete et al , 2015Dragovic et al 2012Dragovic et al , 2015.…”

“…England et al 1992;Treloar 1997;Stüwe 1998;Burg & Gerya 2005) heat production, or fluid (e.g. Camacho et al 2005;Dragovic et al 2012Dragovic et al , 2015, magmatic (e.g. Baxter et al 2002;Brouwer et al 2004;Reverdatto & Polyansky 2004;Viete et al 2013) or tectonic (e.g.…”