Ductile extrusion in continental collision zones: ambiguities in the definition of channel flow and its identification in ancient orogens

Abstract

Field characteristics of crustal extrusion zones include: high-grade metamorphism flanked by lower-grade rocks; broadly coeval flanking shear zones with opposing senses of shear; early ductile fabrics successively overprinted by semi-brittle and brittle structures; and localization of strain to give a more extensive deformation history within the extrusion zone relative to the flanking regions. Crustal extrusion, involving a combination of pure and simple shear, is a regular consequence of bulk orogenic thickening and contraction during continental collision. Extrusion can occur in response to different tectonic settings, and need not necessarily imply a driving force linked to mid-crustal channel flow. In most situations, field criteria alone are unlikely to be sufficient to determine the driving causes of extrusion. This is illustrated with examples from the Nanga Parbat-Haramosh Massif in the Pakistan Himalaya, and the Wing Pond Shear Zone in Newfoundland.In this paper we review channel flow and crustal extrusion in relation to field geology. Our aim is to provide a critical evaluation of whether the predictions of channel flow models are testable using field observations, and also to consider the relevance of channel flow in relation to existing models of crustal extrusion. We illustrate the inherent difficulty and ambiguity of applying the channel flow concept with field examples from both active and ancient orogens (the Nanga Parbat-Haramosh Massif, Pakistan Himalaya, and the Wing Pond Shear Zone in the Appalachian orogen, Newfoundland). General aspects of orogenic thickening and crustal extrusion are presented first, followed by more specific discussion of channel flow and the nature and limitations of possible diagnostic characteristics.

Exhumation of a Variscan orogenic complex: insights into the composite granulitic–amphibolitic metamorphic basement of south‐east Corsica (France)

“…A detailed review of possible exhumation mechanisms of deeply buried continental rocks is beyond the scope of this work. The data on the Corsican gneisses are consistent with an exhumation controlled by tectonic channel flow at the bottom of the upper plate, enhanced by the thermal weakening of the hot lower crust (Engi et al, 2001;Schulmann et al, 2005;Chatzaras et al, 2006;Jones et al, 2006). These models allow exhumation of deeply buried rocks in a compressive setting and explain the stacking of rocks with different P-T-t histories in the same tectonic complex.…”

Channel flow, ductile extrusion and exhumation in continental collision zones: an introduction

“…This situation may have been achieved in the two Himalayan 'syntaxes': the Nanga ParbatHaramosh massif (e.g. Craw et al 1994;Zeitler et al 2001;Butler et al 2002;Koons et al 2002;Jones et al 2006) and the Namche Barwa massif (e.g. Burg et al 1997Burg et al , 1998Burg 2001;Ding et al 2001).…”

Ediacaran intracontinental channel flow

“…Here, the recognition of channel fl ow must combat defi ciencies such as incomplete outcrop, poorly preserved boundary conditions, and a lack of transient geodynamic data (Jones et al, 2006). Nevertheless, if primary orogenic structures remain intact, deep dissection potentially allows a complete crustal section to be exposed, and preserved orogenic channels should be identifi able using diagnostic fi eld-based criteria (cf.…”

“…Nevertheless, if primary orogenic structures remain intact, deep dissection potentially allows a complete crustal section to be exposed, and preserved orogenic channels should be identifi able using diagnostic fi eld-based criteria (cf. Godin et al, 2006;Jones et al, 2006). These include: 1.…”