Abstract
The formation of low-angle detachments involves exhumation of previously ductile material and fault zone weakening. To better understand this relationship, we studied a deeply eroded metamorphic core complex, which formed in the core of the Bergen Arcs (W Norway) during Caledonian post-orogenic collapse. Multi-scale structural mapping in the Øygarden Complex constrains three structural levels characterized by localized shear (Upper Unit), distributed deformation (Middle Unit) and a migmatite double-dome (Lower Unit). All levels show retrogressive E-W stretching accompanied by extension-parallel recumbent folding, albeit, with opposing shear senses at upper and middle/lower levels. The systematic comparison of 23 shear zones constrains the ductile-to-brittle structural evolution. Initially, high temperatures and partial melting controlled pervasive deep crustal flow and ductile doming. During retrogressive shearing, lithological heterogeneity controlled strain localization and channelized fluid flow causing retrograde phyllosilicate growth. This established a feedback loop of fluid-flow, fabric weakening and progressive shear localization. The interconnection of inherited and newly formed weak, phyllosilicate-rich layers promoted the formation of bivergent detachments that rapidly exhumed a dome of previously ductile crust. Retrogressive weakening in a kilometer-wide ductile-to-brittle ‘processing zone’ may be essential for the formation of continental detachments.
Highlights
Detachments are ductile-to-brittle low-angle normal faults (Fig. 1) that record commonly 10s of kilometers of displacement (Armstrong, 1972; Axen, 2007; Buck, 1988; Lister and Davis, 1989; Wernicke, 1981)
We studied the deeply eroded footwall of a major continental detachment system in W Norway that formed during collapse of the Caledonian orogen (Andersen and Jamtveit, 1990; Fos sen, 2010; McClay et al, 1986; Norton, 1987; Osmundsen et al, 2005)
The Øygarden Complex (ØC) is in contact with remnants of Devonian supradetachment basins, uppermost parts of the orogenic wedge and various nappes of the Bergen Arc system (Fig. 3)
Summary
Detachments are ductile-to-brittle low-angle normal faults (Fig. 1) that record commonly 10s of kilometers of displacement (Armstrong, 1972; Axen, 2007; Buck, 1988; Lister and Davis, 1989; Wernicke, 1981). Even in exposed continental detachment systems, inadequate erosion levels or steep topographic relief (Fig. 1) often limit access to systematically study their ductile parts To address this issue, we studied the deeply eroded footwall of a major continental detachment system in W Norway that formed during collapse of the Caledonian orogen (Andersen and Jamtveit, 1990; Fos sen, 2010; McClay et al, 1986; Norton, 1987; Osmundsen et al, 2005). The so-called ‘strandflat’ morphology (Holtedahl, 1998) results in >1200 km of coastlines within an area of 75 km × 25 km, complemented by road sections along and across structural trends This situation provides ideal conditions for systematic structural mapping at multiple scales. Decollements are weak layer-parallel horizons localizing deformation, which can occur from the microscale (see examples below) to the scale of entire orogens (Fossen, 1992)
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