Elsevier

Tectonophysics

Volume 631, 15 September 2014, Pages 212-250
Tectonophysics

Invited Review
Rheological and geodynamic controls on the mechanisms of subduction and HP/UHP exhumation of crustal rocks during continental collision: Insights from numerical models

https://doi.org/10.1016/j.tecto.2014.04.033Get rights and content

Highlights

  • We study continental subduction and UHP exhumation using advanced numerical models.

  • Continental subduction is mainly a transient process requiring strong mantle lithosphere rheology

  • During convergence, UHP exhumation occurs at subduction phase.

  • UHP exhumation is a poly-phase process driven by viscous buoyancy drag.

  • UHP exhumation is favored in slow convergence settings.

Abstract

While subduction of crustal rocks is increasingly accepted as a common scenario inherent to convergent processes involving continental plates and micro-continents, its occurrence in each particular context, as well as its specific mechanisms and conditions is still debated. The presence of ultra-high pressure(UHP) terranes is often interpreted as a strong evidence for continental subduction (subduction of continental crust) since the latter is seen as the most viable mechanism of their burial to UHP depths, yet if one admits nearly lithostatic pressure conditions in the subduction interface (or "channel"). The presumed links of continental subduction to exhumation of high- and ultra-high-pressure (HP/UHP) units also remain a subject of controversy despite the fact that recent physically consistent thermo-mechanical numerical models of convergent processes suggest that subduction can create specific mechanisms for UHP exhumation. We hence review and explore possible scenarios of subduction of continental crust, and their relation to exhumation of HP and UHP rocks as inferred from last generation of thermo-mechanical numerical models accounting for thermo-rheological complexity and structural diversity of the continental lithosphere. The inferences from these models are matched with the petrology data, in particular, with P–T–t paths, allowing for better understanding of subtleties of both subduction and burial/exhumation mechanisms. Numerical models suggest that exhumation and continental subduction are widespread but usually transient processes that last for less than 5–10 Myr, while long-lasting (> 10–15 Myr) subduction can take place only in rare cases of fast convergence of cold strong lithospheres (e.g. India). The models also show that tectonic heritage can play a special role in subduction/exhumation processes. In particular, when thicker continental terrains are embedded in subducting oceanic plate, exhumation of UHP terranes results in the formation of versatile metamorphic belts and domes and in series of slab roll-back and exhumation events with remarkably different P–T–t records.

Introduction

Continental subduction and the mechanisms of formation and exhumation of UHP rocks are two enigmatic processes that are closely linked together (e.g., Ernst, 2001). From geodynamic point of view, the occurrence of HP–UHP rocks raises two types of questions related to the mechanisms of their burial, and to those of their return to the surface. So far the occurrence of HP–UHP rocks in zones of continental convergence is most often interpreted as evidence for subduction (e.g., Hacker and Gerya, 2013, Kylander-Clark et al., 2011, Smith, 1984). In most cases it is supposedly linked to subduction of passive margins and early stages of intercontinental collision associated with subduction of continental lithosphere (Burov et al., 2001, Burtman and Molnar, 1993, Yamato et al., 2007). In other cases UHP exhumation is produced during late stages of oceanic subduction, during the transition from oceanic subduction to continental collision (Hacker and Gerya, 2013, Angiboust et al., 2009) or when small thick continental terrains (i.e., buoyant microcontinents) embedded in “normal” lithosphere are forced down together with the subducting plate and exhumed as a consequence of slab roll-back (Ernst, 2001, Brun and Faccenna, 2008, Husson et al., 2009, Tirel et al., 2013). More specific mechanisms linked to subduction have also been inferred, yet, for the moment, without quantitative match with in-situ P–T-paths. These include slab “eduction” (Andersen and Austrheim, 2008, Duretz et al., 2012), “subduction erosion” leading to diapiric rise of UHP terranes (Gerya and Stöckhert, 2006, von Huene et al., 2004), and foundering of orogenic roots (Hacker and Gerya, 2013).

It is worth mentioning that alternative “non-lithostatic” interpretations for the occurrence of UHP terranes exist (e.g., Mancktelow, 1995, Mancktelow, 2008, Petrini and Podladchikov, 2000, Schmalholz and Podladchikov, 2013, Schmalholz et al., 2014), suggesting that non-lithostatic overpressure of different nature may alter pressure levels recorded by the UHP material by up to a factor of 2, so that the UHP rocks may be formed at about 50 km depth and hence do not need to be transported to such great “subduction” depths (> 100–200 km) inferred from the lithostatic hypothesis. It is hence argued that the occurrence of UHP material does not present evidence for continental subduction and, by extrapolation, that the latter might not exist, or at least is not needed for explanation of the occurrence of the UHP terranes. The overpressure models inherently imply that P–T–t data are of limited use as markers of dynamic processes. Indeed, if subduction does not take place, then overpressure can be build up in the compressed media at levels determined exclusively by the local yield strength and loading conditions, which depend on many uncertain factors such as rheological properties, fluid content, porous pressure, etc., resulting in almost + 100% error on depth estimations. Alternatively, full-scale subduction models (e.g., Li et al., 2010, Toussaint et al., 2004a) predict no significant non-lithostatic pressures in non-locked subduction channel (< 20% below 50 km depth) allowing for reliable interpretation of P–T/P–T–t data. It should also be emphasized, that the “non-lithostatic“ models do not explain why the same P/T gradients are found for both oceanic and continental HP/UHP rocks in a given setting (~8-10°C/km; e.g., Agard et al., 2001) and amongst the various continental subduction regimes (Agard and Vitale-Brovarone, 2013), nor why continental HP/UHP material was only found in places where subducted, dense (oceanic) slab material is imaged by tomography at depths.

At the outcome, elucidation of the mechanisms of formation and exhumation of the UHP rocks is of utmost importance both for understanding the mechanisms of continental convergence (e.g. subduction versus pure shear or folding) and for evaluation of the degree of utility of the petrology data for constraining geodynamic processes.

Indeed, “anti-subduction” models of UHP rock formation (e.g., Petrini and Podladchikov, 2000, Schmalholz et al., 2014) arise from reasonable doubts in physical plausibility of crustal subduction in continental settings where slow convergence rates and high buoyancy of continental crust are largely unfavorable to subduction processes. Subduction is only one of the four possible mechanisms of accommodation of tectonic shortening (Fig. 1): pure-shear thickening; simple shear subduction or underplating; folding (Burg and Podladchikov, 2000, Cloetingh et al., 1999), and gravitational (Raleigh–Taylor (RT)) instabilities in thickened, negatively buoyant lithosphere (e.g., Houseman and Molnar, 1997) dubbed here “unstable subduction.” Whereas in oceans subduction is a dominating mode of accommodation of tectonic compression, in continents all of the above scenarios can be superimposed to some degree. For instance, “megabuckles” created by lithospheric folding (Burg and Podladchikov, 2000) can in theory localize and evolve into mega-thrust zones or result in the development of Rayleigh–Taylor (RT) instabilities. On the other hand, RT and boudinage instabilities leading to slab-break-off may also occur in subducting lithosphere leading to interruption of the subduction process (Pysklywec et al., 2000).

However, it is not only the presence of UHP material but also a host of structural (e.g., Hacker et al., 2006) and geophysical data (e.g., Ford et al., 2006, Handy et al., 2010, Tetsuzo and Rehman, 2011, Zhang et al., 2009) that provide a support for the idea that continental subduction takes place at some stages of continental convergence (e.g., Toussaint et al., 2004a, Toussaint et al., 2004b). For example, at least 700 km of Indian continental crust are “missing” from surface since India–Asia collision (e.g., DeCelles et al., 2002), and hence had to be buried in some way at depth. Physical conditions for subduction include (1) presence of sufficient far-field slab-pull/push forces, (2) weak mechanical coupling between the upper and lower plates (i.e., weak subduction interface) and (3) sufficient mechanical strength of the lower plate assuring preservation of its geometric and mechanical integrity during subduction. In oceans, additional strain localization and plate weakening mechanisms are needed for subduction initialization and for downward bending of strong lithosphere when it slides below the upper plate (Cloetingh et al., 1982, McAdoo et al., 1985, Watts, 2001). Enhanced pre-subduction bending of the lithosphere is possible due to inelastic flexural weakening, that is, ductile yielding and “plastic hinging” produced by high flexural stresses near the peripheral bulge (Burov, 2010a, Burov, 2011, Burov and Diament, 1995, McAdoo et al., 1985). Flexural weakening of oceanic lithosphere is amplified by pressure reduction due to pore fluids and rheological softening due to metamorphic reactions, e.g., serpentinization, produced by fluids penetrating in normal faults created by tensional flexural strains in the uppermost parts of the peripheral bulge (e.g., Angiboust et al., 2012, Faccenda et al., 2009a, Hacker et al., 2010, Kylander-Clark et al., 2011, Ranero et al., 2003). In continental settings, subduction initialization is actually less problematic since the continental lithosphere follows the path opened by the preceding oceanic subduction.

Since the slab pull/push forces can be directly estimated from gravitational force balance (e.g., Bott, 1993), the most uncertain conditions here refer to the mechanisms of weakening of the subduction interface and to the preservation of slab strength (and integrity) during subduction. The former seem to be influenced by metamorphic processes, at least in two aspects: one concerning the role of the metamorphic materials in enabling subduction processes, and the other concerning the capacity of the lithosphere to transport crustal rocks — future high-pressure metamorphic materials — to a great depth. As mentioned above, it is generally agreed, based both on models and observations, that oceanic subduction is possible due to lubrication of the subduction interface by serpentinized mantle layer formed along crust–mantle interface, and due to mechanical weakening resulting from reactions with free and hydrous fluids released or absorbed during metamorphic phase changes (e.g., Angiboust et al., 2012, Faccenda et al., 2009a, Hacker et al., 2010, Kylander-Clark et al., 2011, Ranero et al., 2003). In continents, the governing weakening mechanisms are not well established but the presence of thick, relatively weak and rheologically stratified crust appear to be of primary importance (e.g., Burov et al., 2001, Yamato et al., 2008). Strength reduction and density changes due to metamorphic transforms in LP–HP range and the associated partial melting should also play a certain role (Yamato et al., 2008), but the impact of UHP transforms on subduction may be of minor importance (Toussaint et al., 2004a), specifically because some UHP transforms occur during the exhumation stage only and therefore cannot influence slab behavior at most crucial initial subduction stage (Peterman et al., 2009). Preservation of slab integrity is a major problem for continental subduction, since continental convergence occurs at much slower rates than in oceans. In the case of oceanic subduction (at rates of 5–15 cm·yr 1), the slab has no time to heat up due to the thermal diffusion from the surrounding asthenosphere. As a consequence, it loses its strength only at a great depth. In continents, convergence rates are much slower, sometimes not exceeding a few mm·yr 1. Under these conditions, the lithosphere may heat up, thermally weaken and drip-off before it reaches the UHP depth (e.g., Yamato et al., 2008).

Oceanic subduction has numerous lines of direct evidence such as Benioff zones, straight kinematic inferences from paleomagnetic data, relatively “sharp” tomographic images and gravity anomalies. For continental subduction, on the other hand, the corresponding observational data is much more “blurred”, such that probably one of the most straightforward evidences for continental subduction refers to the presence of HP and UHP continental material in convergence zones (e.g., Ernst, 2010, Guillot et al., 2000, Guillot et al., 2001, Guillot et al., 2009, Hacker et al., 2006, Lanari et al., 2012, Maruyama et al., 2010, Sanzhong et al., 2009). The high- to ultrahigh-pressure (HP/UHP) metamorphic belts are believed to be witnessing subduction processes as the exhumed continental blocks appear to bear an overprint of the subduction record as they return to surface (e.g. Diez Fernández et al., 2012, Hacker et al., 2010, Ring et al., 2007, Zhang et al., 2009). This evidence can be preserved in small and disconnected lenses (eclogite blocks in blocks in a quartzofeldspathic matrix, see Hacker et al., 2006 for review), as mineral relicts within a dominant low- to medium-pressure metamorphic matrix (e.g., Guillot et al., 2009), or as relatively large HP/UHP units (e.g., Yamato et al., 2008). If one assumes lithostatic P–T conditions commonly inferred for subduction zones, then UHP material should have been buried to depths of 100–170 km and brought back to the surface. Consequently, if the UHP depth estimates are valid (e.g., Spear, 1993), the HP/UHP rocks can be regarded as passive markers of continental subduction and their P–T–t paths can be used for reconstruction of subduction dynamics and of the conditions at the subduction interface. Under these assumptions, detailed studies of HP/UHP rocks can provide constraints on thermo-mechanical processes in subduction zones (Coleman, 1971, Ernst, 1973, Ernst, 2010). These data provide insights on exhumation mechanisms as well, since different processes and contexts potentially result in different styles of deformation and, hence, in different exhumation P–T–t paths. In particular, based on the analysis of metamorphic data (e.g., Ernst, 2010) it has been suggested that two main types of continental convergence can be distinguished: fast “Pacific underflow”, where continental subduction is preceded by that of thousands of km of oceanic lithosphere, and slow “Alpine closure” of an intervening oceanic basin leading to short-lived continental subduction soon followed by pure shear collision. It is indeed possible to make a disctinction (Agard and Vitale-Brovarone, 2013) between 1) long-lived subduction (~20-40 Myr) of large-scale continental fragments, containing both lower and upper crustal material, exhumed from UHP (and HP) depths (Hacker et al., 2006) and 2) limited volumes of essentially upper crustal material exhumed over shorter time-scales (5-10 Myr), either from HP/UHP depths (Alps, Himalaya; Guillot et al., 2009) or only HP depths (continental subduction beneath obducted ophiolites: Oman, New Caledonia; Agard et al., 2010; aborted collision: Corsica; punctuated exhumation of microcontinents: Aegean, Turkey; Brun and Faccenna, 2008). It has also been pointed out that the exhumed HP–UHP complexes display low-aggregate bulk densities (e.g., Ernst, 2010), while the exhumation rates in some cases largely exceed the convergence rates (e.g., Yamato et al., 2008), jointly suggesting a buoyancy-driven ascent mechanism.

Large-scale nappe stacking and folding, and other complex deformation processes occurring at the subduction interface largely distort kinematic imprint of subduction (e.g. Diez Fernández et al., 2012, Tirel et al., 2013), hence justifying a numerical modeling approach for decrypting and matching structural and metamorphic observations. For this reason, in recent approaches, the data from HP and UHP rocks are treated in conjunction with synthetic P–T–t paths predicted from thermo-mechanical numerical models of convergent processes. This provides validation of the inferred concepts of convergent dynamics and thermo-mechanical properties of oceanic and continental subduction zones (e.g., Li and Gerya, 2009, Yamato et al., 2007, Yamato et al., 2008). Since the mechanisms of continental convergence and exhumation of HP/UHP material are still very much in debate, and the ideas on the interpretation of metamorphic data and on the mechanisms of convergence require further investigation. In particular, for each given context it should be demonstrated, in an independent way, that: (1) continental subduction is a viable mechanism of accommodation of tectonic shortening; and (2) it is possible to propose a particular mechanism of HP/UHP exhumation compatible both with the P–T–t data and with the proposed subduction dynamics.

According to observations (e.g., Diez Fernández et al., 2012, Ernst, 2010) and recent modeling results (e.g., Burov and Yamato, 2008, Li and Gerya, 2009, Yamato et al., 2008), the mechanisms of exhumation of deeply buried, HP-LT continental units and collision are versatile and in general poly-phase. However, it comes out from regional-scale numerical experiments that continental subduction provides a physically most consistent background for formation and exhumation of the HP/UHP material. The numerical models also reproduce the observations suggesting that exhumation of the UHP material goes by context-dependent multi-stage mechanisms. In particular, exhumation of the UHP material from depths in excess of typical crustal depths (40–50 km) may occur by Stokes flow mechanism at a high rate controlled by buoyancy and viscosity of the matrix, while, when the UHP material reaches 40–50 km depth it is more slowly dragged to the surface within the accretionary prism, or by simple shear upward sliding of semi-brittle crustal slices and large multi-kilometer scale segments (Burov et al., 2001, Angiboust et al., 2009).

Whatever is the mechanism of UHP exhumation, one can conclude that in case of stable subduction, the subduction interface should be devoid of significant deviations from lithostatic pressure conditions (Burov et al., 2001, Burov and Yamato, 2008, Li et al., 2010). In case of stable subduction, only small under-pressures and overpressures (20%, or < 0.3 GPa) may be produced in the subduction interface at depths below 40–50 km (Burov and Yamato, 2008, Li et al., 2010, Toussaint et al., 2004a, Toussaint et al., 2004b), even though the surrounding lithosphere constituting the upper and lower wall of the interface may experience pressure deviations of up to 50% of lithostatic level (e.g., Li et al., 2010, Toussaint et al., 2004a). The in-wall over- and under pressures are basically caused by bending stress concentrations, yet, these zones do not belong to the subduction interface walls and do not participate in the exhumation turn-over (Burov and Yamato, 2008). Consequently, all studies converge to the point that if subduction takes place, the UHP P–T–t data can be decoded in terms of exhumation depth within 10–20% accuracy using lithostatic pressure gradients.

By now, a large number of modeling studies have investigated various factors influencing subduction processes (e.g., Chemenda et al., 1995, Chemenda et al., 1996, Doin and Henry, 2001, Gerya et al., 2002, Arcay et al., 2005, Pysklywec, 2006, Gray and Pysklywec, 2010, Gray and Pysklywec, 2012, Li and Gerya, 2009, Li et al., 2010, Pysklywec et al., 2000, Sizova et al., 2012, Sobouti and Arkani-Hamed, 2002, Warren et al., 2008a, Warren et al., 2008b, Yamato et al., 2007, Yamato et al., 2008). However, not all of the existing models are sufficiently consistent. The analog models are largely inadequate because of impossibility to incorporate phase changes, rheological simplifications, absent or poorly controlled thermal coupling (not mentioning that it is practically impossible to extract PT paths from these models). The numerical models are often limited by simplified visco-plastic rheologies or by the rigid top/“sticky air” upper-boundary condition, which is widely used instead of the paramount free-surface boundary condition. The use of rigid-top upper-boundary condition forces stable subduction (Doin and Henry, 2001, Sobouti and Arkani-Hamed, 2002), attenuates pure shear, cancels folding and does not allow for consistent prediction of topography evolution. Many models also do not incorporate surface processes which are key forcing factors of continental collision (e.g., Avouac, 2003, Avouac and Burov, 1996, Burov, 2010b, Burov and Toussaint, 2007, Toussaint et al., 2004b) and an integral part of the final stages of exhumation. Some studies also force a specific convergence mode, in particular, subduction, via prescription of favoring boundary conditions, for example, by putting an additional boundary condition (e.g., “S-point”) inside the model (e.g., Beaumont et al., 1996, Beaumont et al., 2000). Some other models favor pure shear collision by including a weak zone in the plate shortened in the direction opposite to the pre-imposed mantle flow (Pysklywec et al., 2002). Some older codes operating in deviatoric stress formulation (e.g., Navier–Stokes approximation) had specific problems with accurate evaluation of total pressure needed for tracing of P–T–t conditions and correct account for brittle deformation (this problem was fixed in most of the recent codes). Even though some earlier modeling studies (Burg and Gerya, 2005, Burov et al., 2001, Gerya et al., 2002, Toussaint et al., 2004a, Toussaint et al., 2004b) have considered phase changes, fully coupled models with progressive phase changes directly derived from thermodynamic relations have emerged only few years ago (Francois et al., submitted for publication, Li and Gerya, 2009, Li et al., 2010, Li et al., 2011, Stöckhert and Gerya, 2005, Yamato et al., 2007, Yamato et al., 2008).

Summarizing the requirements to the new generation of numerical models of collision and exhumation, we therefore can note that they should: (1) consider the entire regional context, i.e. encompass lateral spatial scales from 1500 km and vertical scales from 400 km; (2) allow for all modes of deformation, (3) account for viscous-elastic–plastic rheology and thermal evolution, (4) be thermodynamically coupled, i.e. account for phase changes (and ideally also for fluid circulation), (5) account for surface processes and free-surface boundary condition (or at least incorporate “sticky air” approximation of the free surface), and (6) provide an accurate solution for total pressure and report P–T paths based on the dynamic total pressure, rather than based on depth.

It is hence evident that a joint approach considering subduction processes in direct relation to exhumation and formation of HP/UHP material is the most promising one for understanding both the mechanisms of continental convergence and of exhumation.

The goal of this paper is therefore multi-fold: we start from discussing different concepts linking continental convergence with formation and exhumation of UHP terranes. We then discuss physical and rheological conditions allowing for subduction in continental settings, with a specific focus on the conditions allowing for preservation of slab integrity. We next revise the conditions for HP/UHP exhumation. We finally link all processes together attempting to obtain better insights on the mechanisms of continental convergence, and, by proxy, of formation and exhumation of the HP/UHP material.

Section snippets

Non-lithostatic models of formation and exhumation of UHP rocks during continental collision

As mentioned, several alternative mechanisms have been proposed both to explain the mechanics of continental convergence, and formation and exhumation of HP/UHP material. Their common feature refers to the idea that the UHP material comes from shallower depths than is commonly inferred from the assumption of the lithostatic pressure gradient. This implies a presence of static or dynamic overpressure during UHP rock formation which can be created by different mechanisms most of which are

Preservation of slab integrity as paramount condition of subduction

Subduction implies preservation of slab integrity, hence small bulk deformation of the lower plate during convergence: a subducting plate bends without significantly changing its length and thickness. The slab should also provide an efficient stress guide for push/pull forces that drive subduction. To meet the above conditions, the lithosphere has to preserve sufficient mechanical strength as it sinks into the asthenosphere. Otherwise it would stretch or thicken, break-off, stagnate or drip-off

General concepts

Apart of the role of metamorphic rocks as of markers of subduction processes, it is also expected that metamorphic changes, specifically those leading to the formation of weak and/or denser rocks such as schists and eclogites, provide important controls on subduction interface dynamics, largely due to their weakening and lubricating effect, and also, in case of large quantities, due to their high density. The UHP rocks are considerably denser than the surrounding matrix and hence would not flow

Successful numerical models of continental subduction and HP/UHP exhumation

We next discuss the lower and upper bounds on the parameters controlling continental subduction and thus HP-UHP rocks exhumation. We assess various factors controlling continental collision/subduction by using state-of-the art numerical thermo-mechanical models coupled with thermodynamic processes. In these models, density and other physical properties of the material are computed by minimization of free Gibbs energy as function of P–T conditions (e.g., Connolly, 2005) and re-iterated back to

Discussion and conclusions

  • (1)

    The numerical experiments discussed here show that subduction processes can result in the formation and exhumation of HP/UHP terranes for physically reasonable parameter ranges. When subduction takes place, the UHP P–T–t data can be decoded in terms of the exhumation depths within 10–20% accuracy from lithostatic pressure gradients. Yet, the presence of UHP rocks can be regarded as solid evidence for subduction only in association with subduction-compatible P–T/P–T–t data and structural

Acknowledgments

We are very much thankful to the both reviewers, Boris Kaus and Brad Hacker for their very helpful comments and suggestions. Boris Kaus is also thanked for many useful corrections concerning manuscript wording and style. Different parts of the MS have benefited from discussions with T. Gerya, S. Wolf and B. Huet. This publication was supported by the Advanced ERC Grant Rheolith.

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