Late Orogenic Heating of (Ultra)High Pressure Rocks: Slab Rollback vs. Slab Breakoff

Elena Sizova,Taras Gerya,Shah Wali Faryad,Christoph Hauzenberger,Harald Fritz

doi:10.3390/geosciences9120499

Elena Sizova, Taras Gerya + Show 3 more

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https://doi.org/10.3390/geosciences9120499

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Abstract

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.

Highlights

Modern and ancient continental collision zones have been thoroughly studied both geologically and numerically, even for modern examples, we often cannot explain all geological data by theoretically predicted tectono-magmatic models [1]
We present a special type of continental collision numerical experiments
The evolution of experiment R1 is represented in Figure 3, where the continental collision (Figure 3a,b) results in the exhumation of (U)HP metamorphic rocks (Figure 3c–e), accompanied by the continental slab retreat causing asthenospheric upwelling and partial heating of the exhumed crustal material (Figure 3d,e)

Summary

Introduction

Modern and ancient continental collision zones have been thoroughly studied both geologically and numerically, even for modern examples, we often cannot explain all geological data by theoretically predicted tectono-magmatic models [1]. A possible complication recorded in collisional orogens is a two-stage metamorphic evolution of (ultra)high pressure ((U)HP) metamorphic rocks that were first exhumed from mantle depths to the middle–lower crust by slight cooling and later underwent heating before they arrived at the surface. Rocks subjected to these two events are usually recorded by a β-shape. If the temperature of the second event was high and lasted for a long time period, the minerals and their textures from the previous stages might have been obliterated, and only the evidence of the last stage of metamorphism was preserved In this case, different apparent pressure–temperature (P–T) loops might be constrained, which are usually related to different thermal gradients and tectonic settings. Overprinting metamorphic events with distinct thermal gradients and P–T–time (t) paths have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian

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