Divergent plate motion drives rapid exhumation of (ultra)high pressure rocks

Abstract

Exhumation of (ultra)high pressure [(U)HP] rocks by upper-plate divergent motion above an unbroken slab, first proposed in the Western Alps, has never been tested by numerical methods. We present 2D thermo-mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate, followed by upper-plate divergent motion away from the lower plate. Results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of (U)HP rocks directly to the Earth's surface, without the need for significant overburden removal by erosion. Model exhumation paths are fully consistent with natural examples for a wide range of upper-plate divergence rates. Exhumation rates are systematically higher than the divergent rate imposed to the upper plate, and the modeled size of exhumed (U)HP domes is invariant for different rates of upper-plate divergence. Major variations are instead predicted at depth for differing model scenarios, as larger amounts of divergent motion may allow mantle-wedge exhumation to shallow depth under the exhuming domes. The transient temperature increase, due to ascent of mantle-wedge material in the subduction channel, has a limited effect on exhumed continental (U)HP rocks already at the surface. We test two examples, the Cenozoic (U)HP terranes of the Western Alps (continental upper plate) and eastern Papua New Guinea (oceanic upper plate). The good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre-Cenozoic (U)HP terranes where the geologic record of exhumation is only partly preserved.

The Exhumation of Subducted Oceanic‐Derived Eclogites: Insights From Phase Equilibrium and Thermomechanical Modeling

“…Besides, a discontinuity of the oceanic crust would promote the detachment of oceanic material from the slab to the channel. The rollback of subducted plate and the movement of the overriding plate away from the trench provide ample space for exhumation, as already discussed by Malusà et al (, ) and Liao et al (). The exhumed HP‐UHP terrane is exposed in the form of a mixture of several metamorphic rock types in our models.…”

“…Meanwhile, the thickness of subduction channel increases rapidly to 15–20 km (Figure b). Besides, Liao et al () indicate that the motion of the plate away from trench also creates some space for facilitating the exhumation of HP‐UHP rocks. However, this phenomenon is not observed in our models.…”

“…More than ten mechanisms have been proposed for the exhumation of (U)HP metamorphic rocks in continental subduction zones, based on geological research and numerical thermomechanical modeling; they are (1) channel flow (Burov et al, ; Cloos & Shreve, ; Gerya et al, ), (2) diapiric exhumation (Ellis et al, ; Hall & Kincaid, ; Little et al, ; Liu et al, ; Sizova et al, ), (3) a coexistence of channel flow and diapiric exhumation (Liu et al, ), (4) slab breakoff (Davies & Blanckenburg, ; Duretz et al, , ; Ernst et al, ; Kaneko, ; Warren et al, ), (5) slab rollback (Brun & Faccenna, ; Husson et al, ), (6) upper plate divergent motion (Liao et al, ; Malusà et al, , ), (7) microplate rotation (Wallace et al, ; Webb et al, ), (8) multistage exhumation (Burov et al, ; Li & Gerya, ; Warren et al, ), (9) wedge‐like extrusion (Chemenda et al, ; Ernst, ; Li et al, ; Maruyama et al, ; Stöckhert & Gerya, ; Warren et al, ; Yamato et al, ), (10) exhumation through overthrusting (Chemenda et al, ; Jolivet et al, ; Li et al, ), (11) slab extraction (Froitzheim et al, ), (12) extensional collapse (e.g., Platt, ; Thompson et al, ), and so on. Besides, some mechanisms, such as strain weakening, melt weakening, hydration weakening, grain size reduction, and the development of a foliation, may weaken the underthrusting crust and therefore facilitate the exhumation of subducted material.…”

“…Space in the subduction channel is crucial for the exhumation of (U)HP rocks. The motion of the upper plate away from trench (Liao et al, ; Malusà et al, , , ) and the rollback of the lower plate (Brun & Faccenna, ; Husson et al, ; Jolivet et al, ; Lister & Forster, ) lead to removal of a tectonic lid and create ample space to promote (U)HP rocks exhume to surface. Many popular models are fixed‐boundary synconvergent exhumation models, such as channel flow, which need two classical way to remove the overlying rocks pile and exhume (U)HP rocks to surface (Malusà et al, ; Malusà & Fitzgerald, ), including erosional exhumation and tectonic exhumation (Agliardi et al, ).…”

“…Various factors influencing the exhumation of oceanic‐derived eclogite have been considered previously (Brun & Faccenna, ; Ernst et al, ; Gerya et al, ; Guillot et al, ; Jolivet et al, ; Li & Gerya, ; Liao et al, ; Liu et al, ; Malusà et al, , ; Wallace et al, ; Warren et al, , ; Webb et al, ; Yamato et al, ). They may be put into three categories, ordinary controlling factors (e.g., slab geometry, slab age, convergence rate, channel thickness, and effective viscosity), dynamic evolution factors (e.g., the thermal evolution of the subduction zone and the weakening of subducted crust), and significant changes of the tectonic setting (e.g., slab breakoff, slab rollback, trench retreat, and upper plate divergent motion).…”