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, 94 (18), 9532-7

Buoyancy-driven, Rapid Exhumation of Ultrahigh-Pressure Metamorphosed Continental Crust

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Buoyancy-driven, Rapid Exhumation of Ultrahigh-Pressure Metamorphosed Continental Crust

W G Ernst et al. Proc Natl Acad Sci U S A.

Abstract

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 +/- 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over approximately 20 million years, rapid ( approximately 5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material-otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, H2O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Figures

Figure 1
Figure 1
Schematic diagram portraying the deep burial and thermal structure of a subducted microcontinent or continental salient (a), then decompression cooling of a rising slice of UHP quartzofeldspathic rock—not necessarily the complete section of sialic crust—accompanying steady-state subduction (b) [after Ernst and Peacock (12)]. During uplift of a thin UHP terrane (thickness somewhat exaggerated for clarity), cooling of the upper margin of the sheet takes place where it is juxtaposed against the shallower, lower temperature hanging-wall plate; cooling along the lower margin of the sheet takes place where it is juxtaposed against the lower temperature, subducting/refrigerating plate. Stages depicted are as follows: (a) prior to exhumation of the UHP complex; and (b) during exhumation of a thin (2–15 km thick) slice of the UHP complex. It is evident that tectonic exhumation of UHP continental slices requires erosive denudation and/or gravitational collapse as well as the presence of a sialic root still at depth. The resolution of forces acting on the sialic slab in stages a and b are discussed in the text. A, asthenosphere; L, lithosphere.
Figure 2
Figure 2
Modern analogue of continental collision and exhumation of a type A blueschist belt, modified after Osada and Abe (35), Charlton (33), and Maruyama et al. (36). The plate-tectonic setting and partly exposed HP blueschist belt (shaded pattern) are shown in a. The Australian continental crust is being subducted beneath the Timor–Seram segment of the Indonesian suture zone. The interpretive cross-section along line N-S is depicted in b. Geophysically documented slab break-off is shown as localized at the continent–ocean crustal interface (reasonable but not essential). The hypothesized buoyant exhumation of a slice of profoundly subducted continental crust (pattern of crosses) is a consequence of the contrasting densities of surrounding mantle peridotite above and below, and medial, sandwiched quartzofeldspathic sheet (Table 1). Shown are two possible contributions to the exhumation of UHP crustal rocks: (i) rebound of the subducted, ruptured lithosphere due to removal of load through decoupling of the dense, oceanic crust-capped (block pattern) lithospheric anchor; and (ii) buoyancy-driven return flow back up the subduction channel, reflecting increase in ductility and reduction in the relative importance of subduction-related shear forces. RF, reverse fault; NF, normal fault.
Figure 3
Figure 3
Stability fields for blueschists (shaded pattern), eclogites (stippled pattern), and adjacent metamorphic facies (unpatterned) [after Maruyama et al. (36)]. Light lines provide equilibrium curves for several univariant reactions, mid-weight lines indicate metamorphic–facies boundaries (actually multivariant zones of appreciable P—T width), and heavy lines show generalized P—T trajectories for specific, well-documented HP/UHP terranes. Light dashed line indicates the minimum melting curve for H2O-saturated granite. P–T trajectories for exhumation of the preserved UHP relics may be uncommon, and according to our tectonic scheme, would be more tightly constrained to retrace the prograde path in reverse (see text for discussion). Facies abbreviations: BS, blueschist; Zeo, zeolite; PP, prehnite-pumpellyite; PrA, prehnite-actinolite; GS, greenschist; AP, actinolite-calcic plagioclase; EA, epidote-amphibolite; AM, amphibolite; HGR, HP granulite; GR, granulite/pyroxene-hornfels; EC, eclogite; PG, pumpellyite-glaucophane subfacies; LG, lawsonite-glaucophane subfacies; EG, epidote-glaucophane subfacies. Phase abbreviations: Jd, jadeite; Qz, quartz; LAb, low albite; HAb, high albite.

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