Crustal versus mantle core complexes

Abstract

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Contrary to what is commonly believed, it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface. In models, convex upward detachments result from a rolling hinge process. A C C E P T E D M A N U S C R I P TMantle core complexes develop in either the oceanic lithosphere, at slow and ultra-slow spreading ridges, or in continental lithospheres, whose initial Moho temperature is lower than 750°C, with "sub-Moho mantle-dominated" strength profiles. It is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at ACCEPTED MANUSCRIPTA C C E P T E D M A N U S C R I P T 2 lithosphere scale without major and permanent detachment, except if strong strain localization could occur in the lithosphere mantle. Distributed crustal extension, by upper crust faulting above a décollement along the ductile crust increases toward the rift axis up to crustal breakup.Mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis, during increasing extension.

Deep Crustal Flow Within Postorogenic Metamorphic Core Complexes: Insights From the Southern Western Gneiss Region of Norway

“…A magnitude smaller are the culminations in between the Devonian basins (Figure b). While the entire WGR can be seen as one giant MCC (Andersen & Jamtveit, ; Krabbendam & Dewey, ; McClay et al, ), we find it useful to apply the MCC concept (Brun et al, ; Platt et al, ; Whitney et al, ) also to the second‐order antiformal culminations in the footwall of the NSDZ.…”

“…Postorogenic metamorphic core complexes (MCCs) show that the crust reequilibrates after orogeny and an entirely new crustal template may be created (e.g., Brun et al, ; Buck, ; Coney, ; Osmundsen et al, ; Platt et al, ; Vanderhaeghe & Teyssier, ; Whitney et al, ). Since the classic paper by Block and Royden (), it seems widely accepted that MCCs form by viscous crustal flow as a mode of isostatic compensation in response to upper crustal thinning by extensional faulting.…”

Tectonic Inheritance Following Failed Continental Subduction: A Model for Core Complex Formation in Cold, Strong Lithosphere

“…For core complexes formed in hot, thick crust, the necessary mechanical decoupling between the upper and lower crust develops naturally due to the sharp strength contrast between the strong brittle upper crust and the weak, ductile lower crust (e.g., Brun et al, ; Labrousse et al, ; Rosenbaum et al, ; Whitney et al, ). These conditions are common in wide, thick orogenic systems which develop laterally pervasive layers of weak ductile lower crust with thickness of km to 10s of kilometer (e.g., Wu et al, ; Wu & Lavier, ).…”

“…In the upper plate, a wedge of brittlely distended upper crust is displaced laterally away from that dome and from upwelling ductile material beneath, along a weak detachment fault (e.g., Lister & Davis, ). Although MCCs typically form under specific crustal conditions during extension of warm, mechanically weak crust, these conditions can arise from a variety of tectonic settings, and thus, MCCs are found in both oceanic and continental crust within oceanic spreading centers, continental margins, continental rifts, and continental collision zones (e.g., Baldwin et al, ; Brun et al, ; Platt et al, ; Whitney et al, ). Examples of collision‐related MCCs include the Aegean Cyclades (Jolivet et al, ), the gneiss domes of Papua New Guinea (Baldwin et al, ; Little et al, ), and the Himalayan Gurla Mandhata (Murphy & Copeland, ).…”

“…In this case, the LANF may have been reactivated from a preexisting low‐angle structure, such as a thrust fault (e.g., Singleton et al, ), or it potentially may have nucleated in intact rock at a primary low dip, with the latter phenomenon being harder to explain mechanically (e.g., Collettini & Sibson, ; Sibson, ). An alternative concept is that most LANFs initiate as high‐angle normal faults through the brittle crust and are later rotated back via a “rolling‐hinge” process to low angles at shallow depths as a result of tectonic unloading and consequent flexural and isostatic footwall uplift, possibly aided by focused lower crustal flow or asthenospheric upwelling (e.g., Brun et al, ; Lavier et al, , ; Lister & Davis, ; Platt et al, ). Although paleomagnetic evidence of footwall rotation has confirmed the rolling‐hinge mechanism for oceanic MCCs (e.g., Garcés & Gee, ), the origin and development of continental LANFs is still debated, in part because it is difficult to demonstrate geologically that large horizontal axis rotations have affected deeply exhumed metamorphic rocks and in part because the wide range of lithospheric conditions that may exist prior to continental extension can lead to diverse tectonic histories that are less uniform and more complex than those of relatively young oceanic lithosphere.…”