Exhumation processes

UWE RING1, MARK T. BRANDON2, SEAN D. WILLETT3 & GORDON S. LISTER4 1Institut fur Geowissenschaften,Johannes Gutenberg-Universitiit,55099 Mainz, Germany 2Department of Geology and Geophysics, Yale University, New Haven, CT 06520, USA

3Department of Geosciences, Pennsylvania State University, University Park, PA I 6802, USA Present address: Department of Geological Sciences, University of Washington, Seattle, WA 98125, USA 4Department of Earth Sciences, Monash University, Clayton, Victoria VIC 3168,Australia

Abstract: Deep-seated metamorphic rocks are commonly found in the interior of many divergent and convergent orogens. Plate tectonics can account for high-pressure meta­ morphism by subduction and crustal thickening, but the return of these metamorphosed crustal rocks back to the surface is a more complicated problem. In particular, we seek to know how various processes, such as normal faulting, ductile thinning, and erosion, con­ tribute to the exhumation of metamorphic rocks, and what evidence can be used to distin­ guish between these different exhumation processes. In this paper, we provide a selective overview of the issues associated with the exhuma­ tion problem. We start with a discussion of the terms exhumation, denudation and erosion, and follow with a summary of relevant tectonic parameters. Then, we review the charac­ teristics of exhumation in differenttectonic settings. For instance, continental rifts, such as the severely extended Basin-and-Range province, appear to exhume only middle and upper crustal rocks, whereas continental collision zones expose rocks from 125 km and greater. Mantle rocks are locally exhumed in oceanic rifts and transform zones, probably due to the relatively thin crust associated with oceanic lithosphere. Another topic is the use of P-T-t data to distinguish between different exhumation pro­ cesses. We conclude that this approach is generally not very diagnostic since erosion and normal faulting show the same range of exhumation rates, reaching maximum rates of >5-10 km Ma-1 for both processes. In contrast, ductile thinning appears to operate at sig­ nificantly slower rates. The pattern of cooling ages can be used to distinguish between different exhumation processes. Normal faulting generally shows an asymmetric distri­ bution of cooling ages, with an abrupt discontinuity at the causative fault, whereas erosional exhumation is typically characterized by a smoothly varying cooling-age pattern with few to no structural breaks. Last, we consider the challenging problem of ultrahigh-pressure crustal rocks, which indi­ cate metamorphism at depths greater than 100-125 km. Understanding the exhumation of these rocks requires that we firstknow where and how they were formed. One explanation is that metamorphism occurred within a thickened crustal root, but it does seem unlikely that the crust, including an eclogitized mafic lower crust, could get much thicker than c. 110 km while maintaining a reasonable Moho depth ( <70 km, assuming that the seismic­ ally definedMoho would be observed to lie above the eclogitized lower crust). Diamond­ bearing crustal rocks cannot be explained by this scheme. The alternative is to accrete the upper 10-40 km of lithospheric mantle into the orogenic root. This scenario will provide sufficientpressures for both coesite- and diamond-bearing eclogite-facies metamorphism, while maintaining a reasonable Moho depth ( <70 km) and reasonable mean topography (::;3 km). We speculate that the detachment and foundering of the mantle root may con­ tribute to the exhumation of any crustal rocks contained within the mantle root.

Exhumation is a generic term describing the faulting, ductile thinning, and erosion. These pro­ return of once deep-seated metamorphic rocks to cesses are important, not only for the exhumation the Earth's surface. Field geology is, by definition, that they cause, but also for their influenceon the the geology of exhumed rocks. In fact, most of our formation of orogenic topography and the contri­ understanding of crustal deformation and meta­ bution to production of synorogenic sediments. morphism is based on studies of exhumed rocks. Exhumation can occur in virtually any geo­ Exhumation occurs by three processes: normal logical setting, regardless of age or tectonic

RING, U., BRANDON, M. T.,W ILLETT, S. D. & LISTER, G. S. 1999. Exhumation processes. Jn: RING, U.,BRANDON, M. T., LISTER, G. S. & WILLETT, S. D. ( eds) Exhumation Processes: Normal Faulting, Ductile Flow and Erosion. Geological Society, London, Special Publications, 154, 1-27. 2 U. RING ET AL. regime. Even in the earliest studies of alpine tec­ has been much debate recently about the possi­ tonics, erosion was recognized as an important bility of buoyant rise of high-pressure and ultra­ process for unroofing the internal metamorphic high-pressure quartzofeldspathic rocks, an idea zones of convergent mountain belts. Early that has close similarity to the diapiric model for geologists observed that mountainous regions gneiss dome emplacement (Calvert et al. this eroded faster than adjacent lowlands, and that volume). ancient mountain belts were commonly flanked Our obj ective is to provide a selective review by thick synorogenic deposits that could be of the exhumation problem. We focus on five traced by provenance to erosional sources topics: (1) a review of the terminology used to within the orogen. discuss exhumation and its relationship to oro­ The term 'tectonic denudation' (Moores et al. genesis, (2) identificationof tectonic parameters 1968; Armstrong 1972) made its way into the relevant to the exhumation processes, (3) a literature with the discovery of metamorphic summary of how exhumation varies as a func­ core complexes in the Basin-and-Range tion of tectonic setting, (4) the critical review of province of western United States. Early evidence that might be diagnostic of specific workers recognised that normal faulting (Fig. 1) exhumation processes, and (5) a discussion of was capable of unroofing mid-crustal rocks, and the origin and exhumation of ultra-high­ that the hallmark of this type of exhumation was pressure metamorphic rocks, which represent a the 'resetting' of footwall rocks to a common iso­ particularly challenging example of deep topic age. In fact, we now understand that the exhumation. common isotopic age is caused by rapid cooling as the hanging wall is stripped away. This obser­ vation has lead to the widely held view that rapid Terminology cooling is a diagnostic feature of tectonic The exhumation problem is surrounded by a exhumation. Work over the last ten years has confusing and inconsistent terminology, which demonstrated that exhumation by normal fault­ can leave even simple concepts, such as uplift ing often occurs in convergent as well as diver­ (England & Molnar 1990) and extension gent orogens. (Wheeler & Butler 1994; Butler & Freeman A third exhumation process is ductile thinning 1996), difficult to follow. In this section, we (Fig. 1), which can contribute to unroofing of examine the terminology and provide some metamorphic rocks. This idea was at the centre simple definitionsand suggestions for consistent of the debate about diapiric emplacement of usage. migmatites and gneiss domes (Ramberg 1967, The term orogen has broadened over the 1972, 1980, 1981). In this sense, diapiric years, and now is commonly used to refer to any emplacement of a pluton can also be viewed as a mountainous topography at the Earth's surface type of exhumation, given that the pluton is resulting from localized deformation. This usage 'exhumed' by thinning of its cover. The role of includes convergent orogens like the European ductile thinning has received less attention than Alps and the Cascadia accretionary wedge of other exhumation mechanisms, but it appears to northwestern North America, and divergent be important in some cases. For example, there orogens like the Basin-and-Range province and

Normal faulting

Ductile exhumation path flow

Fig. 1. Schematic illustration of the three exhumation processes: normal faulting, ductile flow, and erosion. Normal faulting covers both brittle normal faulting in the upper crust and normal-sense ductile shear zones in the deeper crust. Ductile thinning refers to wholesale vertical shortening in the orogenic wedge. The circle shows an undeformed particle accreted at the base of the wedge, which becomes deformed (indicated by ellipses) along the exhumation path. EXHUMATION PROCESSES 3 the East African rift. Orogens are commonly of convergent orogens, such as the Apennines of described as compressional and extensional, Italy (Royden 1993), the Betic Cordillera of which emphasises the horizontal stress regime Spain (Platt & Vissers 1989, Vissers et al. 1995), within the orogen. Unfortunately, these terms and the Himalayas (Burchfiel& Royden 1985). are not complementary, given that compression Plate boundary terminology provides a better refers to stress and extension to strain. More classification of an orogen. The terms conver­ importantly, the simple reference to a dominant gent, divergent and translational refer to the horizontal strain ignores the fact that some kinematic relationship of the plates that bound orogens are characterized by coeval horizontal the orogen, and carry no implications about the extension and horizontal contraction of differ­ dominant state of stress or strain within the ent levels within the same orogen (mixed-mode orogen. For example, one can describe the flow field in Fig. 2). Such mixed-mode defor­ Apennines as a convergent orogen, even though mation fieldshave been recognized in a number the deformation fieldis mixed with some parts of

a

flow lines

b

Thickening flow field

c

Thinning flow field

Fig. 2. Schematic illustrations of flowfields for three end-member convergent wedges (modified from Feehan & Brandon 1999). (a) A mixed-mode flowfield characterized by horizontal contraction at the base of the wedge and vertical contraction near the top. This situation was proposed by Platt (1987). The dashed line marks the level where the principle strain-rate directions reverse. (b) A thickening flowfield is characterized by converging flowlines and widespread horizontal contraction. (c) A thinning flowfield is characterized by diverging flow lines, which indicates widespread vertical contraction and horizontal extension. Note that horizontal extension can be achieved by either ductile flowor normal faulting. 4 U. RING ET AL. the orogen showing horizontal contraction and frequently used to refer to the removal of others, horizontal extension. material at a particular point on the Earth's Horizontal extension is commonly taken as surface. This distinction is not universally evidence of tectonic exhumation. The implicit accepted, but we think it is useful and encourage assumption is that horizontal extension indicates its adoption. Walcott (1998) emphasized this vertical shortening, which in turn indicates tec­ usage in his synthesis of the Southern Alps tonic thinning of the overburden. There are two orogen of New Zealand. The Southern Alps are problems with this linkage of horizontal exten­ characterized by a large gradient in surface sion to tectonic exhumation. The firstis that the erosion rates across the orogen, from c. 1 km strain geometry is not always as assumed. For Ma-1 on the eastern dry side of the range to instance, broad shear zones at translational plate c. 10 km Ma-1 on the western rainy side of the boundaries are characterized by plane strain range. Most particle paths in the orogen have a with the maximum extension and maximum significant westward component of motion, shortening directions lying in the horizontal, and causing them to move horizontally through the little to no strain in the vertical. The result is orogen from the slowly eroding east side to the little to no vertical thinning or thickening of the rapidly eroding west side. As stated by Walcott overburden. In another example, Ring & (1998) (our italics): 'In two dimensions the Brandon (this volume) show that the Franciscan amount of rock removed by erosion at a spatial subduction complex was shortened by about point on the Earth's surface may be vastly 30% in the vertical, but that shortening was bal­ greater than the exhumation experienced by the anced by a nearly equal volume strain. The rock and is better referred to as denudation or result was that the other principal strains were erosion'. nearly zero. This example shows that vertical England & Molnar (1990) made the useful dis­ thinning can occur without any appreciable hori­ tinction between surfa ce up lift, meaning the ver­ zontal extension. tical motion of the Earth's surface relative to sea This ambiguity can be avoided by focusing on level, and rock uplift,meaning the vertical motion the vertical strain - either brittle or ductile - of rock relative to sea level. Erosion and denuda­ when assessing the role of tectonic exhumation. tion are then defined as the difference between In this regard, the terms thickening and thinning rock uplift and surface uplift at a single spatial are unambiguous descriptions of the change in point. Since erosion is measured in the vertical, it overburden thickness caused by vertical strain. is probably best viewed as a flux (i.e., velocity Vertical thickening causes tectonic burial and normal to the approximately horizontal surface vertical thinning, tectonic exhumation. of the Earth) rather than a true velocity. This There has been some discussion about the usage is consistent with estimates of drainage­ suitability of exhumation as a term for unroofing scale erosion rates, which are calculated by divid­ of rocks. In a strict sense, exhumation means 'to ing sediment yield by drainage area. dig up or disinter' (Summerfield& Brown 1998). As used here, exhumation is measured by As such, it refers to the motion between a rock integrating the difference between the rock relative to the Earth's surface (England & velocity and surface velocity while following the Molnar 1990). An alternative term is denuda­ rock along its material path ( cf. England & tion, which has a general meaning 'to make bare' Molnar 1990). Barometers and ther­ and a geological meaning 'to expose rock strata mochronometers provide information about by erosion'. Denudation and erosion are, in fact, exhumation since they tell us about the unroof­ widely used synonyms, although there is a ten­ ing history specific to a rock sample. For the dency to restrict denudation to mean erosion at simple case of a vertical particle path, exhuma­ the regional scale (Summerfield& Brown 1998). tion and erosion are equivalent, but this situ­ The use of denudation in a more generic sense ation cannot be expected in general. When has clear precedent given the term tectonic exhumation is divided by time, the resulting denudation, which means to unroof by normal exhumation rate can be viewed as a spatially and faulting (Moores et al. 1968; Armstrong 1972). temporally averaged erosion rate, but this In our review of the literature, exhumation equivalence should be viewed with caution. The and denudation are used almost interchange­ approximation becomes increasingly flawed as ably, although a subtle distinction is sometimes the closure depth for the relevant barometric made about the frame of reference. Exhumation and thermochronometric data increases. For is used to refer to the unroofinghistory of a rock, instance, cosmogenic isotopes provide estimates defined by the vertical distance transversed by of exhumation rates within meters of the surface the rock relative to the Earth's surface. On the (Cerling & Craig 1994), which means that the other hand, denudation, and erosion as well, are measured rates are essentially equal to erosion EXHUMATION PROCESSES 5

rates. Low-temperature thermochronometers a have deeper closure depths, and thus provide a more averaged estimate of the erosion rate. For instance, the apatite (U-Th)/He ther­ mochronometer (Lippolt et al. 1994; Wolff et al. 1997) has a very low closure temperature (c. 75°C), which implies closure depths of 2-3 km, assuming slow erosion rates. This depth will be even shallower in areas of fast erosion because of the upward advection of heat caused by erosion. Apatite fission-track ages have a higher closure temperature (c. l10°C) and deeper closure depths ( <4-5 km), so exhuma­ tion rates determined from those data will have a more approximate relationship to surface erosion rates. Zircon fission-track ages (c. 240°C) and 40 Ar-39 Ar ages (>300°C) have greater closure temperatures, and an even more b distant relationship to surface erosion rates. To summarize, we recommend the following definitions. Exhumation: the unroofing history of a rock, as caused by tectonic and/or surficialprocesses. Erosion: the surficial removal of mass at a spatial point in the landscape by both mechanical and chemical processes. Denudation: the removal of rock by tectonic c and/or surficial processes at a specified point at or under the Earth's surface.

a

Fig. 4. Schematic illustration of convergent wedges showing the relationship between Vc, Va, Vs and the S point. The vector for Va shows the case for subduction-zone advance where Va > 0. (a) Extreme case where both lithospheric plates are completely accreted into the orogen. Vs and the S point are b undefinedin this case. (b) Subduction-related accretionary wedge. The S point is at a depth of about 30 km. Incoming crustal section is about 6 km thick. (c) Convergent continental wedge. The S point is at a depth of about 120 km, which means that imbrication and accretion include both crust and lithospheric mantle from the underthrust plate.

Orogenic deformation and tectonic models Fig. 3. Schematic illustration of divergent settings. (a) Symmetric rifting, resulting from coaxial Plate-tectonic theory dictates that the velocity extension of the lithosphere. (b) Asymmetric rifting, field at the Earth's surface is dominated by the due to non-coaxial deformation of the lithosphere. motion of rigid plates. Deformation occurs 6 U. RING ET AL. mainly at the velocity gradients within the zones downing plate (Fig. 4b) . In this case, the orogen that definethe plate boundaries. To a firstorder, is the accretionary wedge, which grows slowly by orogens are either divergent (Fig. 3) or conver­ accretion of sediments. Intermediate to these gent (Fig. 4) depending on the sign of the rela­ end-members are relatively small convergent tive velocity, Vc, defined here to be positive for orogens that form by accretion and contraction convergence. of sediment, crustal basement rocks, and some­ Divergent orogens display considerable com­ times lithospheric mantle as well (Fig. 4c) . Well­ plexity depending on the symmetry of internal studied examples include the European Alps, structure (Fig. 3). In continental settings, diver­ the Pyrenees, the Apennines, and the arc­ gence results in symmetric or asymmetric continent collision of Taiwan. stretching and rifting. Symmetric stretching can For a subduction/accretion model, there is result in significant amounts of ductile thinning. always a point or, more precisely, a locus of A more asymmetric style of stretching is influ­ points at the base of the orogen, that mark the enced by the development of strongly localized lower limit of mass transfer (i.e. accretion) normal faults, which impart a sense of vergence across the plate boundary (Willett et al. 1993; to the deformation. In both cases, the defor­ Beaumont et al. 1994; Waschbusch & Beaumont mation, whether brittle or ductile, involves a 1996). Below this point, which we denote as S, strong component of horizontal extension and subduction of the downgoing plate occurs tectonic denudation of the footwall beneath beneath a discrete shear zone. The velocity of S, major normal faults. designated as V0, describes retreat or advance Similarly, convergent orogens show a broad of the subduction zone. As shown in Fig. 4, our diversity depending on both the boundary con­ convention is to define Vc and Va relative to a ditions and the internal response. The general fixedoverriding plate, with positive towards the expectation is that convergence causes pervasive overriding plate. A positive Va implies advance horizontal contraction throughout the orogen. of the subduction zone towards the overriding However, horizontal extension has been recog­ plate; a negative Va represents slab rollback and nized as an important feature in a number of motion of the subduction boundary away from convergent orogens. For instance, in the special the overriding plate. Waschbusch & Beaumont case of a retreating plate boundary (Royden (1996) used a slightly different convention. In 1993; Waschbusch & Beaumont 1996), oceanic their notation, Vc = Vp- VR and Va = Vs- VR, subduction is commonly characterized by hori­ where Vp and VR are the absolute velocities of zontal contraction in the accretionary wedge and the subducting (pro-) and overriding (retro-) simultaneous horizontal extension within the arc plates, and V5 is the horizontal migration veloc­ and back arc. ity of the subducted slab relative to the deep The complexity of deformation within con­ mantle. vergent orogens, as well as the diversity of tec­ Subduction-zone advance (Va > 0) describes tonic style and exhumation processes, has led to the motion of the S point towards the overrid­ considerable effort in describing and modelling ing plate. Advance must be accompanied by these orogenic systems. Much of the deforma­ contraction of the overriding plate. A modern tional response to convergence is dictated by example is western South America where how much of the lithosphere is involved in the crustal thickening and growth of the Andean deformation. At one limit, the entire lithosphere orogenic belt has occurred without any signifi­ of one or both bounding plates is forced to con­ cant accretion from the subducting Nazca plate. tract and thicken (Fig. 4a). Whole-lithosphere Without accretion, shortening of the South thickening has been proposed as an important American plate must be accompanied by east­ feature of large-scale continental collisions, such ward motion of the Nazca slab (Pardo-Casas & as the India-Asia collision (England & McKen­ Molnar 1987; Isacks 1988; Pope & Willett 1998). zie 1982; England & Houseman 1986). England Note that outward growth of an orogen is a & Houseman (1986) have concluded that tec­ natural consequence of the addition of mass by tonic exhumation will occur when the gravi­ accretion and will occur independent of the tationally unstable mantle lithospheric root velocity of S. As a result, the advance of the detaches, but they argue that the orogen other­ retro-side deformation front does not require wise shows no strong tendency for vertical thin­ the advance of the underlying subduction zone. ning during convergence. When Va < 0, the S point migrates away from The other end member for convergent the overriding plate, defining the case of sub­ orogens is oceanic subduction zones, which are duction retreat. Slab retreat or rollback has long characterized by accretion of a thin layer, often been recognized as an important factor influenc­ limited to only the sedimentary cover of the ing the style of deformation in convergent EXHUMATION PROCESSES 7 orogens (Uyeda & Kanamori 1979; Dewey 1980; zones, frontal accretion will also result in hori­ Jarrard 1986; Royden & Burchfiel 1989; zontal contraction in the accreted material. Waschbusch & Beaumont 1996). Examples of They also show that when rates of accretion are retreating convergent orogens include the low or nil, subduction-zone retreat can cause Mariana margin in the western Pacific and the mixed contraction and extension in pre-exist­ Hellenic margin of Greece. In an oceanic ing rocks that border the subduction zone. A setting, slab retreat leads to extension of the fast rate of erosion in the Olympic Mountains upper plate and formation of a back-arc basin. In of the Cascadia fore-arc high has caused pro­ a continental setting, the consequences are less nounced vertical extension in the most deeply clear, but as the slab retreats away from the exhumed part of the accretionary wedge overriding plate, the position of active contrac­ (Brandon et al. 1998; Brandon & Fletcher tion migrates away from the upper plate, cre­ 1998). In contrast, a fast rate of underplating ating additional space to accommodate accreted combined with a slow rate of erosion should material. If retreat creates space faster than cause horizontal extension in the upper rear accretion can fill it, then the upper plate might part of the orogen (Platt 1986, 1993; Brandon & respond by horizontal extension. Fletcher 1998). A possible example might be The deformation within a convergent orogen the Apennine thrust belt, which shows active is also controlled by the distribution of fluxes extension in the internal part of the orogen around the orogen. Accretion and surface (Elter et al. 1975; Patacca et al. 1993). Fast erosion control the fluxes into and out of the underplating might be expected given that the orogen. Accretionary fluxesgenerally enter into Apennines saw a transition over the last 5 Ma the orogen on its pro-side, either by frontal from oceanic subduction to subduction of the accretion ( offscraping) or basal accretion passive margin of the Adriatic continental (underplating). Accretion can also occur on the block (Dewey et al. 1989). An alternative expla­ retro-side of the wedge (Willett et al. 1993), but nation (Royden 1993) is that extension is the fluxes are usually much smaller than those caused by subduction-zone retreat. on the pro-side, except for the case of subduc­ tion-zone advance. A significant erosional flux only occurs when there is extensive subaerial What gets exhumed? topography, and when that topography has suf­ Exhumation occurs at a variety of tectonic set­ ficient runoff to develop an integrated river tings (Fig. 5), but mainly at oceanic rifts and system to carry out eroded material. transform faults, continental rift zones, subduc­ As a general rule, frontal accretion and tion zones, and at continent-continent collision erosion both tend to promote horizontal con­ zones. Here we summarize the types of rocks traction across an orogen. A fast rate of frontal that are exhumed in these settings. In general, accretion will tend to cause pervasive contrac­ we findthat each tectonic setting has a maximum tion throughout the orogen. This case is nicely exhumation depth, with oceanic rifts and trans­ illustrated by the strain calculations in Dahlen forms showing the shallowest exhumation & Suppe (1988). Waschbusch & Beaumont depths (c. 10 km) and continental collision (1996) show that at retreating subduction zones, the deepest (>125 km).

oceanic divergent oceanic convergent continental convergent continental divergent (e.g. Atlantic) (e.g. Crete, Franciscan) (e.g. Alps, Himalayas) (e.g. Galicia, Cyclades)

Fig. 5. Idealized cross section showing depth range of metamorphism in ocean-convergent, continental­ convergent, and continental-rift and oceanic-rift settings. 8 U. RING ET AL.

Oceanic rifts and transforms blueschist and eclogite from c. 50 km depth are locally exposed (Schliestedt et 1987; Okrusch This setting is charaterized by shallow exhuma­ al. & Brocker 1990). Divergence started there tion from depths of c. 1 0 km. Given the thin crust sometime after the middle Oligocene for oceanic lithosphere, this depth is sufficientto (Raouzaios et 1996; Thomson et 1998; Ring expose the mantle, as evidenced by spectacular al. al. et al. 1999), but yet we know that much of the local exposure on the seafloor of the serpen­ exhumation of the Cycladic high-pressure rocks tinized peridotites. In the modern oceans, occurred earlier, during the Eocene and Early exhumed mantle is most commonly found along Oligocene, shortly after the rocks were sub­ long-offset transform faults and 'under-fed' ducted and accreted (Avigad et 1997; Ring et ridge crests. The exposed peridotites are plagio­ al. al. 1999). Forster & Lister (this volume) argue clase- and spinel-bearing, indicating shallow that deep exhumation in the Cyclades is a result mantle rocks. of multiple episodes of normal faulting, with any Exhumed mantle has also been observed in single event involving only a modest amount of the deep ocean in association with extreme exhumation. rifting. Off the Spanish/Portuguese coast Collectively, these examples suggest that indi­ between the supposedly oceanic crust (Grau et vidual episodes of continental rifting will al. 1973) of the Iberian abyssal plain and the exhume rocks from depths no greater than Galicia margin, serpentinized peridotite derived c. 25 km. Nevertheless, it may be difficult to tell from the upper mantle is largely buried beneath whether or not tectonic exhumation was caused sediments but also crops out locally (Boillot et al. by divergent plate motions (rifting) or by syn­ 1980). The Galicia margin is made up of a convergent extension, as highlighted by the dis­ number of tilted blocks formed during continen­ cussion between Andersen (1993) and Fossen tal rifting. According to Boillot et al. (1980), the (1993) concerning exhumation in the Norwegian serpentinized peridotite is thought to be the Caledonides. result of serpentinite diapirism and tectonic unroofing of mantle rocks along the rift axis of the margin just before sea-floor spreading Subduction zones started between Galicia and Newfoundland. Subduction zones expose a wider variety of The setting would thus be transitional from con­ deeply exhumed rocks. In a number of cases, tinental to oceanic rifting. the metamorphic grade is no greater than Ophicalcites record ancient examples of blueschist facies, indicating exhumation from exhumed oceanic lithosphere. An ophicalcite is depths of about 30-40 km, which is the max­ a sedimentary breccia made up of mafic and imum thickness of modern subduction-related ultramafic clasts set in a pelagic carbonate accretionary wedges. matrix (e.g. Lemoine 1980; Bernoulli & Weissert The Mariana convergent margin (Fryer 1996) 1985). They are thought to form in intra-oceanic displays an intriguing example of deep exhuma­ settings. The clasts provide a clear record that all tion. Cold intrusions of serpentinite are found as levels of the underlying oceanic lithosphere conical volcano-like features littering the were exposed at the seafloor. seafloorof the Mariana forearc. Rare blueschist minerals are locally found in association with these serpentinite diapirs. For this case, exhuma­ Continental rifts tion of mantle and high-pressure rocks is The rift shoulders of the East African rift expose thought to be driven by the buoyancy caused by mainly middle to lower crustal Precambrian serpentinization of mantle peridotites. Serpen­ gneisses, but those rocks are known to have been tinization can cause a decrease in density from mostly exhumed prior to the onset of Cenozoic 3300 kg m-3 to as low as 2600 kg m-3 (-22%; rifting. The rift process itself appears to have Christensen & Mooney 1995). caused only minor exhumation. Continental Eclogite-facies mafic rocks (i.e. eclogites) rifting in the Basin-and-Range province of have been recognized at many ancient subduc­ western North America loca1ly resulted in tion zones. These rocks may occur as isolated exhumation of the upper and middle crust (e.g. blocks, in association with lenses and blocks of Applegate & Hodges 1995). However, despite serpentinized peridotite. The evidence usually large-magnitude extension (about 100%, Wer­ indicates that the eclogites were severely dis­ nicke et al. 1988), no high-pressure metamorphic rupted and dismembered after metamorphism. rocks (i.e. >10 kbar) are found at the surface. Pressure estimates indicate exhumation from 50 Deeper exhumation in continental rift zones km and deeper. The Central Belt of the Francis­ has occurred in the Cyclades of Greece, where can subduction complex of coastal California EXHUMATION PROCESSES 9 serves as a classic example of disrupted eclogites interesting example is the Zermatt-Saas zone of in a subduction zone setting. The 'knockers' are the Swiss-Italian Alps (e.g. Bearth 1956, 1967, made up of typical oceanic basalts. Some are sur­ 1976), which consists of metabasalts, metacherts, rounded by an actinolitic rind, indicating that and ultramafite. Metamorphic assemblages indi­ they resided for some time in a serpentinite cate both high pressure and ultrahigh-pressure matrix (Coleman & Lanphere 1971; Moore conditions (locally >100 km depth). The litho­ 1984). logical association suggests a subduction-zone The hanging wall of the subduction zone is, at environment, with initial accretion of crust and least in some cases, invoked as a source for these mantle from an oceanic lower plate, followed by deep rocks (e.g. Coleman & Lanphere 1971; low-temperature/high-pressure metamorphism. Platt 1975; Moore 1984 for the Franciscan sub­ However, age constraints (Reddy et al. 1998) duction complex). The reason is that modern suggest that exhumation occurred during conti­ subduction zones have well-defined Benioff nental collision, which distinguishes these rocks zones indicating that the lower-plate mantle is from subduction-zone eclogites, like those of the subducted, not accreted. This observation sug­ Franciscan complex. gests that eclogite blocks were formed by accre­ The most challenging examples of deeply tion of maficcrust from the subducting slab into exhumed rocks are ultrahigh-pressure (UHP) the mantle of the overriding plate. The exhuma­ metamorphic rocks, which are usually found tion of these rocks remains poorly understood, exhumed in continental collisional zones. These although the observation of exhumation of high­ rocks were first discovered in the Cima-Lunga pressure metamorphic minerals in the Mariana nappe (Ernst 1977; Evans & Trommsdorff 1978), forearc suggests that serpentinite diapirs might the Dora-Maira massif (Chopin 1984), and the be involved in exhumation of eclogite blocks as Zermatt-Saas zone (Reinecke 1991) of the Alps, well. and the Western Gneiss region of Norway (Griffin 1987; Smith& Lappin 1989). They are now known from a number of collisional Continental collision zones orogens (Coleman & Wang 1995). Ultrahigh­ The internal zones of collisional belts expose the pressure rocks are continental or oceanic crustal widest variety of exhumed rocks. Schist and rocks that were metamorphosed within the stab­ gneiss from the upper and middle crust are com­ ility field of coesite or diamond (Schreyer 1995; monly exhumed in this setting. Some convergent Coleman & Wang 1995). The exposure of these orogens, such as the Delamerian orogen of south rocks at the Earth's surface demonstrates that Australia, the Mount Isa orogen of northeast continental and oceanic crust can be subducted Australia, and the Rocky Mountains of the to depths > 100 km and then returned to the western US, are almost entirely made up of surface. A common debate is whether or not upper and middle crustal rocks and generally UHP rocks formed within highly overthickened lack exposed high-pressure granulite, blueschist, crust or within the mantle (see section on UHP or eclogite (i.e. rocks characterized by metamor­ rocks below). phic pressures <10 kbar). We suspect that high­ Garnet peridotites are also exposed in several pressure rocks are formed in these settings but collisional orogens such as the Alps ( Alpe the exhumation processes operating there were Arami, Ernst 1977; Evans & Trommsdorff 1978) somehow unable to bring those rocks to the and the Betic-Rif orogen (e.g. Ronda and Beni surface. Bousera peridotites, Loomis 1975; Pearson et al. Other collisional belts show clear evidence of 1989). For the Ronda and Beni Bousera peri­ crustal thickening and deep exhumation during dotites, graphite pseudomorphs after diamond orogenesis, as indicated by the general occur­ indicate depths >125 km (Pearson et al. 1989; rence of metamorphic rocks from > 40 km depth. Tabit et al. 1990). The initial stages of exhuma­ A classic example is the Sesia zone in the Italian tion are attributed to Mesozoic rifting (Vissers et Alps (Compagnoni & Maffeo 1973), which con­ al. 1995). However, the Alpe Arami peridotite tains high-pressure continental crustal rocks that was subjected to metamorphism at depths of were metamorphosed at the base of a thickened >70-100 km during the Alpine orogeny (e.g. crustal root, and then subsequently exhumed. Ernst 1977; Evans & Trommsdorff 1978; Becker Pressure estimates indicate metamorphism at 1993), which would have postdated Mesozoic c. 70 km, which is consistent with the maximum rifting. The presence of deep-seated ultramafic Moho depths in modern orogens (e.g. Meissner rocks at the Earth's surface indicates that 1986; Christensen & Mooney 1995). orogenic wedges must have sufficient upward Less common within collisional orogens are flowto overcome the negative buoyancy of these deeply exhumed oceanic assemblages. An rocks. 10 U. RING ET AL.

(]eneralrer.narks rate of mechanical erosion of the drained part of the continents is c. 0.052 km Ma-1. However, Our summary suggests a counter-intuitive result: more important is the fact that recorded erosion continental rift zones seem to have only modest rates reach local values of 5-13 km Ma-1 (e.g. potential for deep exhumation, whereas conti­ Southern Alps of New Zealand and the Taiwan nental collision zones seem to have the greatest Alps). In fact , 2% of the drained area of the con- potential. We see no simple explanation for this . tinents has erosion rates > 0.5 km M a- 1 . I t IS result. important to stress that these rates repr se t Another highlight is the evidence from the � � drainage-scale averages. Thus, local rat�s wtthm Mariana subduction zone indicating that some a drainage could be much greater. ErosiOn rates high-pressure metamorphic rocks might be associated with warm-based alpine glaciers, such brought back to the surface by buoyancy-driven as those of southern Alaska, have rates of exhumation, triggered by sepentinization of 1-100 km Ma-1 (Hallet et al. 1996). It is interest­ upper-plate mantle. If such serpentinization ing to speculate that an increase in Alpine does occur beneath the forearc region, then we glaciation, caused by global cooling, incr�ased will probably need to reconsider what the precipitation, and/or growth of mountamous seismic Moho means in those settings. Serpen­ topography, might play a major role in exhum­ tinization can cause a decrease in compressional ing metamorphic rocks and in limiting the wave velocities from about 7.7-8.2 km s-1 to maximum height of mountains. The important 5.3-5 . 5 km s-1 (Christensen & Mooney 1995). conclusion is that relative to tectonic exhuma­ tion, which may be no greater than c. 5-10 km Ma-l, surficial erosion can locally be a very fast Diagnostic features of different process. . exhumation processes Fast eroding regions tend to be mountamous, tectonically active, and wet. Conversely, arid cli­ A difficult question in most orogenic belts, mates tend to have slow erosion rates regardless especially the older ones, is the relative contri­ of the amount of topography. At present, arid butions of different exhumation processes. landscapes cover about one-third of the co ti­ Here, we assess features that might be diagnos­ . � nents (p. 278 in Bloom 1998). And h1gh tic of erosion, normal faulting, and ductile thin­ plateaux, such as Tibet and the Altiplano in the ning. We also evaluate some problems with quantifying the rates of these exhumation pro­ cesses. Erosion in the World's River Basins (Data from Milliman and Syvitski, 1992)

Erosion 10000

The large volumes of detrital deposits found >:- adjacent to almost all convergent contine�tal 8 8 1000 orogens provides ample evidence that erosiOn c is a significant exhumation process. The � Po::: Himalayan foreland and the offshore Bengal Q 0 100 and Indus fans preserve an important record of ·;;:: erosional exhumation of the Himalayas � :> that the voluminous Songpan-Ganzi flysch was --< produced during exhumation of the Dabie Shan ultrahigh-pressure rocks. None of this evidence requires that erosion is the sole exhumation 20 40 60 80 100 process or even the dominant process, but clearly erosion cannot be ignored as a contribut­ Cumulative Probability (% area) ing factor. Fig. 6. Average erosion rate from the world's river Erosion is often assumed to be a fairly slow basins plotted against cumulative probability (data exhumation process, but there is little support compiled from Milliman & Syvitski 1992). Th� for this view. Figure 6 summarizes Milliman & diagram shows that mechanical erosion is typ1cally Syvitski's (1992) compilation of modern sedi­ slow (weighted mean rate c. 0.05 km Ma-1), but in ment yield from a global distribution of280 river some orogens, mechanical erosion can be extremely drainages. The conclusion is that the average fast with rates locally exceeding 10 km Ma-1. EXHUMATION PROCESSES 11

Andes, serve as examples of mountainous land­ range; Johnson 1997 for the Betic Cordillera of scapes with little to no erosion. The transition southern Spain; Brandon et al. 1998 for the from an arid to a humid climate appears to be Olympic Mountains). marked by a dramatic increase in erosion rates, Our conclusion is that erosion cannot be primarily due to the enhanced transport capacity excluded as an exhumation process on the basis that comes with a well-drained landscape. of rate alone. For modern orogens, lack of sub­ Drainage-scale erosion rates are commonly aerial relief (e.g. submarine accretionary wedge) assumed to scale with precipitation or runoff, or an arid climate or pronounced rain shadow but regional compilations do not provide clear (e.g. Tibet) would qualify as credible arguments support for this inference (e.g. Pinet & Souriau for generally slow erosion rates, but these argu­ 1988; p. 134 in Allen 1997). In other words, ments are fairly difficult to apply to ancient set­ erosion rates do not appear to be strongly influ­ tings. Isopach analysis provides a direct measure enced by climate as long as the climate is not of erosional exhumation, although it is com­ arid. The rate of tectonically driven uplift monly found that synorogenic sediments end up appears to be a much more important factor in being dispersed to great distances from the controlling local erosion rates. Recent work by orogen. In the Himalayan system, the dispersal Burbank et al. (1996) and Hovius et al. (1997) is over distances of thousands of kilometres, have called attention to the importance of reaching distant ocean basins and subduction bedrock landslides in producing transportable zones. The reason for the wide dispersal is that a material in well-drained tectonically active land­ collisional foreland basin can never hold more scapes. In this type of setting, the rate of erosion than about half of the sediment produced by is probably directly driven by the rate of the tec­ erosional lowering of the orogen (England tonically-driven uplift, given the usual con­ 1981). Wide dispersal makes it difficult, even in clusion that in well-drained humid landscapes, the best-preserved orogens, to make a useful river incision keeps pace with tectonic uplift. comparison between the volume of eroded sedi­ An important question is whether modern ment and the depth of exhumation. erosion rates are representative of long-term Another factor that has to be taken into erosion rates. We ignore human effects here; account in subduction zone settings is the recy­ most compilations of modern erosion rates have cling of eroded material back into the subduc­ included corrections to remove this factor. tion zone. In a similar manner, sedimentary Perhaps the most important factor influencing basins adjacent to collision orogens might be erosion rates on the short-term is climate­ overridden and hidden underneath large-slip induced changes in vegetation cover, which con­ faults. trols the amount of regolith and soil that can be Another method that has much promise for stored on the hill slopes (Bull 1991 ). A loss of distinguishing between normal faulting and ero­ vegetation cover can result in very high short­ sional exhumation is the use of isotopic cooling term erosion rates, but those rates can only be ages in synorogenic sediments (e.g. Garver et al. sustained for a short period of time until the this volume). Detrital ages provide information stored regolith is striped away. The major about the lag-time distribution of the sediment, climate cycles have periods ranging from 21 to where lag time is defined as the difference 100 ka, with the 100 ka glacial cycle being especi­ between the cooling age of a detrital grain and ally pronounced during the Quaternary. In this its depositional age. For instance, the growth context, long-term erosion rates can be usefully and decay of an erosion-dominated orogen defined as the average rate of erosion for a should show a parallel decrease and increase in period of time >100 ka, to ensure that 'short­ lag time for the sediment shed from the orogen. term' climate variations are averaged out. The changes observed in the lag-time distri­ Long-term erosion rates can be estimated by bution will be delayed relative to the actual reconstructing eroded geological features, by growth and decay of the topography because isopach analysis of synorogenic sediments, and some time is needed to erode away rocks that by thermochronologic dating of exhumed have pre-orogenic cooling ages after initial oro­ bedrock or detrital grains in synorogenic sedi­ genic uplift, and to erode away rocks that have ments. A number of studies have documented syn-orogenic cooling ages during the post­ long-term syn-orogenic erosion rates ranging orogenic decay of the topography. In contrast, from 1 to 15 km Ma-1 (e.g. Kamp et al. 1989 for after orogenic uplift has stopped normal faulting the Southern Alps of New Zealand; Copeland & should be marked by the rapid appearance of Harrison 1990 for the Himalayas; Burbank & sediment with short lag time, and then long Beck 1991 for the Salt Range of Pakistan; periods of time where the lag time increases at Fitzgerald et al. 1995 for the central Alaska the same rate as stratigraphic age. These two 12 U. RING ET AL. features of the lag-time evolution mark the fact process. Exhumation due to normal faulting that normal faulting brings hot rocks to the commonly results in abrupt breaks in the surface without any need for erosion. Further­ cooling-age pattern with younger ages in the more, if normal faulting is rapid, much of the footwall of the normal fault (Fig. 7a) (Wheeler upper crust will be reset to a common isotopic & Butler 1994). Johnson (1997), Foster & John cooling age. Brandon & Vance (1992) reported (this volume) and Thomson et al. (this volume) an example like this for sediments shed from the provide examples from the Betic Cordillera, the Eocene metamorphic core complexes in the Basin-and-Range province and Crete, respec­ Cordilleran thrust belt of eastern Washington tively. In contrast, erosional exhumation should State (USA) and southeast British Columbia be characterized by a relatively smoothvariation (Canada). Sandstone samples spanning from the in cooling ages across the eroded region (Fig. Eocene through the Early Miocene show peaks 7b ). Brandon et al. (1998) report a useful in the fission-track grain-age distribution for example for the erosional exhumed fore-arc detrital zircons that remain fixed at c. 43 and high above the Cascadia subduction zone. c. 57 Ma. These peaks were called static peaks Attenuation of stratigraphic or metamorphic because they did not move with depositional units, by itself, is not diagnostic of tectonic thin­ age. They can be related to the slow erosion of ning by normal faulting (Wheeler & Butler 1994; the Eocene core complexes after they formed. Ring & Brandon 1994; Ring 1995; Butler & Freeman 1996) (Fig. 8). Thrust faults can thin a stratigraphic or metamorphic section when that Normal faulting section is tilted rearward prior to thrusting. In There is abundant evidence that normal faulting this case, the thrusts can cut down-section in the aids the exhumation of metamorphic rocks. The direction of transport, while cutting up towards Basin-and-Range province (Armstrong 1972; the Earth's surface as thrust faults should. Crittenden et al. 1980; Davis 1988; Foster & John In some cases thrusts may be rotated by this volume), the Aegean (Lister et al. 1984; Thomson et al. this volume; Forster & Lister this a volume), the Betic Cordillera (Platt & Vissers Exhumation bv normal faulting 1989; Vissers et al. 1995) and the Alps (Manck­ telow 1985; Selverstone 1985; Behrmann 1988; fault� Ratschbacher et al. 1991; Ring & Merle 1992; / / Reddy et al. 1999) are well-documented settings _ / ..-/ / / --- / / where normal faulting contributed to exhuma­ ..-/ / / _/ - / / tion. - / / - ..-/ - - / / ///..-/ The most commonly cited evidence for - /// / normal faulting is the presence of 'younger over ..-/ / older' or 'low-grade on high-grade' tectonic con­ depth contours prior to exhumation tacts, where large faults, with low to moderate dips, have placed younger above older rocks or low-grade rocks on high-grade rocks, and in the process have cut out a significant thickness of b Exhumation by erosion stratigraphic or metamorphic section. A more diagnostic feature for large-slip normal faults is the juxtaposition of a ductily deformed footwall with a brittlely deformed hanging wall (e.g. Lister & Davis 1989; Foster & John this volume). The evolution of superimposed sedimentary basins can be used to determine the relative dis­ placement of footwall and hanging wall. These 'piggy-back' style basins also provide a record palaeohorizontal, which can be used to restore Fig. 7. Exhumation pattern as a function of process. underlying faults back to their original orien­ Dashed lines show depth contours prior to exhumation. (a) Normal faulting will create an tation. This information, together with the shear asymmetric exhumation pattern. Cooling ages will sense of a fault, is essential for resolving the get younger as one approaches the normal fault (see nature (reverse or normal in relative sense) of a Foster & John and Thomson et al. this volume). (b) fault. Exhumation by erosion alone will commonly result in The pattern of cooling ages in the field can a broadly domed pattern, which should vary also provide clues about the exhumation smoothly, independent of faults and local structures. EXHUMATION PROCESSES 13

a a fault has to be related to the P-T-t evolution in its foot- and hanging wall at the time of tec­ tonic transport, or displacement along a fault relative to the palaeosurface at the time of fault activity has to be demonstrated in order to dis­ tinguish between normal faults and thrusts ( cf. Wheeler & Butler 1994). A problem for quantifying the relative contri­ c. 5 km bution of normal faulting to exhumation of deep-seated rocks is that the total offset and original dip of most crustal-scale normal faults, especially those involving deep crustal rocks, are typically poorly resolved. The offset is com­ monly approximated with barometric estimates from above and below the fault. In this case, it has to be demonstrated that faulting is actually contemporaneous with the pressure break across the fault. Reddy et al. (1999) report an example from the Zermatt-Saas zone where the b break in metamorphic pressure across a shear zone allowed them to constrain a total of 30 km of exhumation by normal faulting within 9 Ma, indicating an average exhumation rate of 3.5 km Ma-1. Only a few studies have attempted to c. 5 km measure slip rates for normal faults. The great­ est reported slip rates come from some exten­ sional detachments in the Basin-and-Range province and are of the order of 7-9 km Ma-1 (Davis & Lister 1988; Spencer & Reynolds 1991; Foster et al. 1993; Scott et al. 1999; Foster & John this volume). There is growing evidence that many of these 'high-speed' extensional Fig. 8. Examples of attenuation of metamorphic section by contractional faults. (a) Normal nappe detachments had an initially gentle dip (<30°) sequence characterized by the tectonic juxtaposition (Scott & Lister 1992; Foster & John this of higher-grade rocks over lower-grade rocks. Nappe volume). Other estimates for exhumation by sequence is further affected by post-nappe folding. extensional detachments in the Basin and Re-imbrication of the thrust and fold sequence; the Range indicate lower rates of about 1-2 km re-imbricating contractional fault climbs upwards Ma-1 (see review in Foster & John this volume). towards the foreland, but cuts downward through the Average rates from other orogens appear to be tilted section resulting in faulted contacts where generally slow: (1) <2 km Ma-1 for a 10 Ma lower-grade rocks are thrust over higher-grade rocks. interval for the Tauern window in the Eastern Such a multiphase contractional history is commonly Alps (Frisch al. 1998) and, (2) 0.6-0.9 km found in the internal zones of orogens. (b) et Contractional fault cutting down through an uplifted Ma-l and <0.6-0.7 km Ma-1, respectively, for a metamorphic section, while cutting upward relative 20 Ma interval for the Menderes Massif in to the foreland in the direction of tectonic transport, western Turkey (Hetzel et al. unpublished data; and thereby attenuating metamorphic or structural Ring unpublished data). section. Normal faults in long-lived continental rifts also appear to have relatively low slip rates and slip generally occurred along steeply dipping folding, block tilting or differential exhumation (c. 60°) normal faults. In the Miocene to Recent until their geometry classifies them as normal East African rift system, apatite fission-track faults at the front of a thrust culmination (e.g. ages are, in general, not reset by the young Price 1981). Conversely, normal faults are some­ rifting events (Foster & Gleadow 1996), which times domed by isostatic rebound due to unload­ indicates a slip rate for the normal faults of <0.3 ing of their footwalls until the fault locally has a km Ma-l. At the Livingstone escarpment of the thrust sense of offset (Buck 1988; Lister & Davis northern Malawi sector of the East African rift 1989). Therefore, the kinematic development of system, the Miocene to Recent offset at the 14 U. RING ET AL. escarpment yields similar low slip rates (Ring wedges from western North America is less than unpublished data). half that indicated by the estimated finitevertical The exhumation rate during normal faulting is shortening in the rock (Feehan & Brandon 1999; a function of both slip rate and fault dip. A Ring & Brandon this volume). It follows that normal fault with a 30° dip that slipped at a rate huge vertical strains are needed for ductile thin­ of 7-9 km Ma-1 would exhume rocks at a rate of ning to make a significant contribution to 3.5-4.5 km Ma-1. 1f this detachment had an orig­ exhumation. Vertical contraction on the order of inal dip as gentle as 20°, it would exhume rocks 70% has indeed been documented, for example, at a rate of 2.4-3.1 km Ma-1. It is important to by Norris & Bishop (1990) and Maxelon et al. note that most slip rates are averaged over rela­ (1998) from the interior of the Otago accre­ tively long periods of time (see cited time inter­ tionary wedge, exposed on the South Island of vals above for the Tauern window and the New Zealand. Dewey et al. (1993) also reported central Menderes Massif). It is conceivable that vertical contraction on the order of 70--80% from normal faulting occurred in pulses at faster rates. the Western Gneiss region. The studies by None the less, the long-term average appears to Feehan & Brandon (1999) and Ring & Brandon have been relatively slow. (this volume) show that ductile thinning oper­ ated at low rates of <0.3 km Ma-1. Recent model­ ling by Platt et al. (1998) in the Betic Cordillera Ductile thinning argues for vertical shortening of 75% and ductile Penetrative deformation fabrics present in most thinning operating at a rate of 4.5 km Ma-1 • exhumed mountain belts indicate that ductile Platt (1993) pointed out that thrusting alone flow is an important process. This process can can not tectonically exhume rocks. This is cer­ either aid or hinder exhumation, depending tainly true if thrusting is confined to a thin zone upon whether ductile flow causes vertical thin­ at the base of a sequence of nappes, which is ning as associated with the formation of a sub­ commonly the case in the upper crust. However, horizontal foliation, or vertical thickening as in the internal zones of convergent orogens, associated with the formation of a subvertical nappes usually show a pervasive degree of foliation (Fig. 2b and c). The presence of a sub­ internal deformation and generally flat-lying horizontal foliation is generally diagnostic of foliations. The foliations show that nappe stack­ ductile thinning. The general observation of sub­ ing was associated with pronounced vertical horizontal foliations in the internal zones of thinning, which would ultimately contribute to many orogens shows that ductile thinning com­ exhumation of the nappes. In this regard, it is monly aids exhumation (Selverstone 1985; interesting to note that high-pressure belts, as a Wallis 1992, 1995; Wallis et al. 1993; Platt 1993; rule, occur above lower grade units indicating Mortimer 1993; Ring 1995; Krabbendam & that the overburden of the high-pressure nappes Dewey 1998; Ring et a!. 1999). Ductile thinning must have been reduced prior to their emplace­ by itself cannot fully exhume rocks and an ment above the lower pressure units. Pervasive additional exhumation process is required to ductile flow in the hanging wall associated with bring rocks back to the Earth's surface (Platt et thrusting of the high-pressure nappe might have al. 1998; Feehan & Brandon 1999). aided the exhumation of the latter. Note that In the simplest case, where exhumation is vertical ductile thinning might be coupled with entirely controlled by ductile thinning, exhuma­ erosion and/or normal faulting in upper parts of tion by ductile thinning is given by the average the nappe pile. stretch in the vertical because this tells how much the vertical has changed in thickness. However, if exhumation occurs by additional processes as P- T-t data and exhumation processes well, it is more difficult to quantify the contri­ bution of ductile thinning to exhumation. In this P-T paths case, the vertical rate at which a rock moved The shape of P- T paths is sometimes used to through its overburden and the rate of thinning distinguish exhumation processes. The shape of of the remaining overburden at each step along a P- T path is typically difficult to resolve, the exhumation path have to be considered especially the youngest part of the path, which (Feehan & Brandon 1999). The general con­ is probably the most diagnostic of process (J. clusion is that the contribution of ductile thinning Selverstone, pers. comm. 1996). Nonetheless, to exhumation will always be less than that esti­ fast initial exhumation rates of deeply buried mated from the vertical stretch only. Simple one­ rocks should generally result in near-isothermal dimensional calculations show that the decompression. Such P-T paths are known, for contribution of ductile thinning in convergent instance, from the garnet-oligoclase-facies EXHUMATION PROCESSES 15

rocks of the Southern Alps of New Zealand rocks causes upward advection of heat and thus (Holm et al. 1989; Grapes 1995) and the Mulha­ retards cooling and promotes isothermal cen nappe of the Betic Cordillera (e.g. Gomez­ decompression. The result is a relatively high Pugnaire & Fernandez-Soler 1987) (paths 1 and thermal gradient and compressed isotherms in 2 in Fig. 9). The Southern Alps were dominantly the upper crust (Fig. 10). In this situation, rocks exhumed by erosion and the Mulhacen nappe is can quickly move through the closely spaced thought to be exhumed primarily by normal isotherms without being much exhumed. The faulting. Nonetheless, the shapes of the P-T closure-temperature concept (Dodson 1973) paths from both units are virtually indistin­ indicates that a radiogenic isotopic system will guishable (Fig. 9). record the time when a dated mineral cools below its effective closer temperature. However, there is no 'closure pressure'. Thus, T-t paths for cases of near-isothermal decompression A number of isotopic studies have used evi­ typical for fast exhumation (path 3 in Fig. 9), it dence of rapid cooling to argue for rapid is difficult to estimate the depth where the iso­ exhumation (e.g. Copeland et al. 1987; Zeck et topic system closed. As a result, exchumation al. 1992). Rapid exhumation of deep-seated rates will be poorly resolved. Only hairpin P-T

T (°C)

6 0.. � ---c _____2o,o ____ ��---4o,o ______o,o ____� -.--�-s o,o __r--.

, , ' ', ' \ \ \ \ \ \ \ \ 30 \ '

40

50

60 �� Q) 0 70

hypothetical particle (Fig. 1 0)

Fig. 9. Selected P- Tpaths from exhumation settings that are controlled (1) mainly by erosion, e.g. Southern Alps of New Zealand (Holm et a!. 1989; Grapes & Wantanabe 1995), and (2) mainly by crustal extension, e.g. Mulhacen nappe of Betic Cordillera in southern Spain (Gomez-Pugnaire & Fernandez-Soler 1987). Path (3) shows a hypothetical isothermal decompression curve, which illustrates the problem of applying the 'closure­ temperature concept' for estimating exhumation rates. Dark grey dot shows P-T evolution of the hypothetical particle shown in Fig. 10. The hairpin shape of path ( 4) from the EasternBelt of the Franciscan subduction complex of California (e.g. Ernst 1993) is probably caused by slow exhumation in a subduction-zone setting. 16 U. RING ET AL. paths like that for curve 4 in Fig. 9 from the Reddy et al. (1999) showed that mmtmum Franciscan subduction complex have cooling pressure estimates for phengite crystallization rates that can be simply related to exhumation show no correlation with age within a shear zone rates. This situation seems to hold only when in the hanging wall of the Zermatt-Saas exchumation rates are petrology and iso­ but there seems to be a growing consensus that topic dating. Alternatively, dating of the crystal­ several processes, operating at different levels lization age of minerals such as phengite, from in the lithosphere, are involved. The evidence which minimum metamorphic pressure can be suggests that UHP metamorphism mainly obtained (Massone 1995), may provide a means occurs within collisional orogens, but where for constraining a P-t history. within the orogen? More specifically, does

initial thermal gradient 12°/km basal thermal gradient 30°/km basal thermal gradient 30°/km

0 0 0 .... ooc ooc .... ooc 200oc 200°C

Isotherms 400°C 400°C 20- 200°C 20 600°C 20 600°C

400°C • 40- 40 40

Depth 800°C aoooc (km) 600°C

60 60 60-

particle at 82.5 Ma, particle at 65 Ma, aoooc • i.e. for last 17.5 Ma: i.e. for last 17.5 Ma: hypothetical particle exhumation rate of 2.0 mm/a, exhumation rate of 2.0 mm/a, 80- at 100 Ma 80 cooling rate 5. 1 °/Ma 80 cooling rate 42.9°/Ma

Fig. 10. Illustration of possible effects of rapid exhumation. A P-T path for the hypothetical particle considered in this example is shown in Fig. 9. (a) Depth arrangement of isotherms during rapid underthrusting of a particle to a depth of 70 km, assuming an initial thermal gradient of 12oc km-1. (b) Accretion is followed by exhumation at a constant rate of 2 km Ma-1, which initiates a new, relatively steep thermal gradient (i.e. rapid exhumation results in a very modest cooling rate of soc Ma-1 averaged over the first17.5 Ma of exhumation). The development of the isotherms in this example has been modelled using a one-dimensional steady-state model where accretion is balanced by erosion. The model assumes that temperatures at the surface and at the base of a layer remain fixedat their set values of 0 oc and 840 oc, respectively. These set values can be viewed as describing a basal geotherm of 30°C km-1. The thermal diffusivity ( K) in our example is 32 km2 Ma-1, which is typical for collisional belts such as the Himalayas (Zeitler 1985). For model details, see appendix of Brandon et al. (1998). (c) Final exhumation of particle at the same rate as in (b), but average cooling rate is now more than eight times higher as in (b) (43°C Ma-1, averaged over 17.5 Ma). EXHUMATION PROCESSES 17 metamorphism occur within the crust or the lithospheric mantle (Fig. lla). Deeper litho­ mantle? spheric mantle does not thicken and is thus pas­ Ernst et al. (1997) argues that ultrahigh­ sively depressed or subducted into the pressure metamorphism is caused by subduction asthenosphere. UHP metamorphism is inferred of continental crust along an oceanic subduction to take place beneath the Moho, within the zone. The continental crust is embedded in the mantle part of the orogenic root. Some type of oceanic lithosphere, so that slab pull is able to fault imbrication is required to interleave conti­ drag the crust down to depths of > 100-125 km. nental and oceanic crust rocks within this mantle The crust somehow detaches and returns to the root. This seems plausible because the litho­ surface, driven mainly by its strong positive spheric mantle would be expected to behave in a buoyancy relative to the surrounding mantle. brittle fashion given the relatively low tempera­ This interpretation is certainly plausible, but it tures of typical UHP metamorphism (<800 to relies heavily on the early presence of an oceanic 900°C). subduction zone to account for deep burial and To explore the implications of the model, we metamophism. Early subduction of oceanic assume the following simplified scenario. An lithosphere is supported by the occurrences of incoming continental platform with an initial ophiolitic rocks in some ultrahigh pressure crustal thickness he = 40 km and a mean eleva­ metamorphic terrains (i.e. Zermatt-Sass zone tion z = 0 km (sea level), and a thickened crust described above). However, ultrahigh-pressure He= 70 km within the orogen. The 70 km thick­ metamorphic rocks are generally found in ness would be representative of the Moho in a Mediterranean-style continental collision zones collisional orogen (Christensen & Mooney with little to no evidence of a pre-collisional 1995). The initial thickness of the lithospheric magmatic arc (Ernst et al. 1997). Thus, one is left mantle is set at hm = 100 km (Molnar et al. 1993). to question if there was a well-organized pre-col­ The crust and lithospheric mantle are con­ lisional oceanic subduction zone during UHP strained to thicken by the same amount, metamorphism. This problem has caused us to = Sv Helhe = oHm/ohm = 1.75 consider if it might be possible to form ultrahigh­ pressure metamorphic rocks within the colli­ assuming the representative values above for He sional orogen itself, and without appealing to an and he. Assuming isostatic equilibrium, we can early oceanic subduction zone. predict the maximum thickness of the orogenic root,

Orogenic root models (He+ oHm) = Sv (he + ohm), The pressures associated with UHP meta­ the mean elevation of the orogen above sea morphism seem incompatible with metamor­ level, phism in thickened continental crust of an z = ((Pa e)/ ) he -1 , average crustal density (2830 kg m-3, Chris­ [ - P Pa + ((Pa - Pm)lpa) ohm] (Sv ) tensen & Mooney 1995). Assuming a reasonable and the pressure at the base of the orogen, metamorphic temperature of 800°C, this crust P = e e would have to be 100 km thick to produce the hase (P h + Pm ohm) Sv g, 29 kbar conditions needed to convert quartz into where g is the acceleration of gravity. coesite and a thickness of > 120 km, correspond­ These results are illustrated in Fig. 11b-d, ing to pressures of >35 kbar, to stabilize using Sv = 1.75. About 10-40% of the incoming diamond. At 100 km crustal thickness, the lithospheric mantle would have to be accreted to resulting orogenic topography would have a the orogenic root to get pressures sufficient for mean elevation of > 10 km, which is twice the UHP metamorphism. The resulting mean eleva­ height of the highest orogenic topography on tion would be c. 3 km. For this model, the present Earth. It seems unlikely that such high metamorphic pressure needed for UHP meta­ topography could be supported for any geologi­ morphism is provided by the negative buoyancy cally reasonable length of time (e.g. Bird 1991) . of the mantle portion of the root. Note, however, Some other explanation is needed. We explore that the amount of interleaved crustal rocks in two ideas (Figs 11 and 12), both of which invoke the mantle part of the root must remain small to a thicker and denser orogenic root, either due to retain sufficient negative buoyancy within the the inclusion of lithospheric mantle (e.g. Molnar root. et al. 1993) or eclogitized lower crust (Dewey et If this explanation is correct, we are still left to al. 1993). explain how the crustal rocks separate from the For the first option, the orogenic root is made mantle part of the root and return to the surface. up of crust and a fraction o of the incoming Houseman et al. (1981) and Molnar et al. (1993) 18 U. RING ET AL.

a

thickened crust

He, Pc = 2830 kg m-3

thickened lith?spheric mantle

oHm, Pm = 3300 kg m-3

undeformed lithospheric mantle

250 Fig. 11. A thickened lithospheric mantle model for the origin of UHP metamorphic rocks. (a) The �E'225 Q)� orogenic root is formed by accretion of both crust c: - 200 -5o and a specified fraction of lithospheric mantle. ·- 0 175 ..c: .... Average densities are from Christensen & Mooney - (.) E ·;:: 150 (1995), Molnar et al. (1993) and Houseman & Molnar :::1 Q) 0) 125 (1997). (b-d) The results of isostatic calculations for ·-E 0 X ._ the maximum thickness, basal pressure and elevation Cll 0 100 �-0 of the orogen. This calculation assumes uniform 75 thickening of the crust and accreted lithospheric mantle by a factor Sv == 1.75, which gives a final 75 :0 crustal thickness of 70 km, comparable to maximum Q)� rn- Moho depths in modern collisional orogens. Note cu .c- 0 that mean elevation is greatest when no lithospheric cae 50 mantle is accreted because of the greater density of ....Q) .� c: the lithospheric mantle relative to asthenospheric :::1 Q) f/)0) rn o mantle...... Q) ..... 0 a...... 0 25

c:- 4 o E :t=i ..:::&:. ca- > c: 3 Q) Q) (i) 0) c: 0 2 (1j 0 Q).,_ 2 0

0 �--��--�-L� 0 1 0 20 30 40 50 60 70 80 90 1 00

Accreted fraction of lithospheric mantle (%)

have argued that the gravitationally unstable Raleigh-Taylor instability (see Molnar et al. lithospheric mantle in the root will ultimately 1993 for a recent analysis). A similar rise time is detach and sink into the asthenosphere. The rise probably associated with separation of the more time for this detachment process depends on buoyant UHP crustal rocks from the mantle time constants for thermal relaxation of the root lithosphere (Wallis et al. 1998). Buoyant rise or and for viscous flow associated with the diapirism may account for how UHP rocks reach EXHUMATION PROCESSES 19

upper crust hue = 25 km, Puc = 2750 kg m·3

- lower crust thickened upper crust h1c = 15 km, Pic = 3000 kg m·3 / = 2750 kg/m Puc 3, Hue / -1- / eclogite-facies lower crust Pe = 3500 kg/m3, H1c

undeformed lithospheric mantle

Pm = 3300 kg m-s, hm

Fig. 12. An eclogitized lower crust model for the origin of UHP metamorphic rocks. See text for details. the base of the crust, perhaps at the same time as extreme case where the crust above the Moho is the mantle root descends into the astheno­ formed from the upper crust by itself. Hue is set sphere. This interpretation remains speculative to 70 km, which we use again as our representa­ but provides a stimulating view of how UHP tive depth for Moho in a collisional orogen. The crustal rocks might start to make their way back upper and lower crust are assumed to thicken by to the surface. the same amount, which means that We consider a second option, that eclogization = = = 2.8 of the mafic lower crust provides the thickness Sv Huclhuc H1clh1c and pressure needed for UHP metamorphism assuming the representative values for Hue and (Fig. 12). According to Christensen & Mooney hue· Isostatic balance gives the following equa­ (1995), the lower crust of the continents, from 25 tions for the maximum thickness of the orogenic to 40 km on average, is mainly maficin composi­ root: tion with a density of2900--3100 kg m-3. At depths (Hue + Hlc) = Sv (hue + hlc), >c. 50 km, corresponding to pressures >c. 14 kbar, these rocks would be converted to eclogite with mean elevation: an average density (Pe) of c. 3500 kg m-3 and P­ Z = [ ( (Pa -Puc) I Pa) hue + ( (Pa - Plc) Pa) hlc] wave velocities of c. 8 km s-1 (see Christensen I & - 1 Mooney 1995, Table 4, p. 9775). In this case, the (Sv ) - ((Pe - Plc) I Pa) Sv hlc Moho would no longer mark the top of the and pressure at the base of the orogenic root: mantle, but rather the transition into eclogite­ Pbase = [(hue Puc) + (hlc Pe)] Sv g. facies maficcrustal rocks (Dewey et al. 1993). The high density cited above only applies to eclogite, For this model, our equations predict an oro­ and not to the more silicic coesite- and diamond­ genic root that is 112 km thick with an average bearing rocks that have come to definethe UHP elevation of only 1.5 km and a maximum pres­ metamorphic problem. The UHP silicic rocks are sure at the base of the root of 33 kbar. The pres­ not eclogites. None the less, they belong to the sure is sufficient for coesite stability but higher pressure part of the eclogite facies, and can diamond would only be stable if temperatures be referred to as the coesite or diamond subfacies. were well below 700°C. A thicker orogen would As in the previous interpretation, we must argue be needed to account for the full range of pres­ that the eclogite-rich crustal root would contain a sures observed for UHP metamorphic suites but small fraction of structurally interleaved silicic we cannot see how this model could be used to UHP metamorphic rocks. generate a thicker orogen without making the To explore the implications of the eclogite Moho deeper than 70 km. model, we again start with a 40 km thick crust For this model, the orogenic root is composed with a mean elevation at sea level, an upper entirely of crust, but the ecologitized lower crust crustal thickness hue = 25 km, and a lower mafic is gravitationally unstable and thus prone to crustal thickness h1c = 15 km. We examine the detachment as described above for the 20 U. RING ET AL. thickened lithospheric mantle model. A critical units, which were capable of cooling down hun­ problem is that the crustal root described here dreds of square kilometre-sized nappes quickly. may be so weak that it could not persist for any Rapid exhumation and quick cooling might have significant time before detaching and sinking prevented melting. However, the initial size of into the asthenosphere. Lithospheric mantle is an UHP nappe is critical in this regard. stronger and should be able to persist in a thick­ (3) Is the preservation of ultrahigh-pressure ened form for a longer period of time. Given this assemblages in continental basement consistent factor, plus the greater range of possible meta­ with dry metamorphic conditions during morphic pressures, the thickened lithospheric exhumation? A relatively dry granitic basement mantle model seems to provide a better expla­ rock (e.g. Dora Maira Massif, Western Gneiss nation for the origin of UHP metamorphism. region) typically contains about 2 wt% struc­ Note however that neither model is exclusive turally bound water. To transform dry mineral and that a mafic lower crust would become assemblages to high- and ultrahigh-pressure eclogitized if it was thickened to depths assemblages, one has to hydrate the rocks to sta­ >c. 50 km. bilize minerals like talc and phengite in such rocks. The presence of a fluidphase during ultra­ high-pressure metamorphism has also been Other unresolved issues inferred from mass-transfer processes and fluid­ There are many additional unresolved issues inclusion evidence (Schreyer 1995; Harley & regarding ultrahigh-pressure rocks, some of Carswell 1995; Phillipot et al. 1995). Oxygen­ which are listed here. isotope studies in the ultrahigh-pressure rocks of (1) What is the thickness of UHP units? It has Dabie Shan, China, show that the granitoids been proposed that ultrahigh-pressure rocks are where heavily hydrothermally altered by near­ typically found as relatively large (i.e. up to hun­ surface waters before metamorphism (Rumble dreds of km2) internally coherent nappes, which et al. 1998). This summary suggests that the con­ are only a few kilometres thick (Ernst et al. tinental basement is in general not dry before 1997). Such a statement is certainly true for the being converted to an ultrahigh-pressure rock. well-mapped ultrahigh-pressure Brossasco­ The preservation of ultrahigh-pressure assem­ Isasca unit of the Dora Maira Massif which is blages during exhumation would then suggest sandwiched between lower pressure nappes that fluidwas channelized in shear zones. Local­ (Chopin 1984; Chopin et al. 1991). However, in ized deformation and fluid flow may have other UHP nappes the regional extent and the allowed the preservation of ultrahigh-pressure thickness of the nappes are poorly constrained. assemblages in shielded blocks. For example, diamond-bearing eclogites in the (4) How much of the exhumation history of an Erzgebirge of eastern Germany apparently rep­ ultrahigh-pressure terrain is preserved in its resent very small lenses (< hundreds of metres in present structural and stratigraphic setting? Stan­ diameter) within high-pressure gneiss (Massone dard structural fieldstudies, in conjunction with 1999). The average thickness of UHP nappes is P-Twork, are capable of constraining aspects of crucial for understanding their thermal history the last 20-30 km of the exhumation path. Van and also their rheologic behaviour during der Klauw et al. (1997) and StOckhert & Renner exhumation. (1998) demonstrated, for instance, that quartz (2) What is the relationship of ultrahigh-pres­ microfabrics in UHP rocks record greenschist­ sure metamorphism to magmatism? The high facies deformation. Structures that formed at radiogenic heat production typical of continen­ deeper crustal levels are commonly thought to tal rocks should lead to thermally-induced have been highly or completely obliterated. The melting since ultrahigh-pressure metamorphism preserved high- or even ultrahigh-pressure is generally at conditions above the 'wet' solidus deformation relics are hard to relate to mapable for granitic melts (Huang & Wyllie 1975). The large-scale nappe contacts or accretion-related local presence of melt has been discussed by structural discontinuities. Schreyer et al. (1987), Schreyer (1995), Phillipot A remarkable feature of at least some UHP (1993) and Sharp et al. (1993) but clear evidence nappes is the lack of pronounced deformation. for melting remains scarce. Most people argue The virtually undeformed Variscan Brossasco that the exhumation path of ultrahigh-pressure granite of the Dora Maira Massif preserves its rocks is characterized by cooling during decom­ original igneous texture and its intrusive relation­ pression (Roberto Compagnoni, pers. comm. ship with a surrounding metasedimentary unit 1996, 1997; Ernst et al. 1997). The exhumation­ (Biino & Compagnoni 1992). In Dabie Shan, related cooling suggest that the ultrahigh-pres­ some of the rocks preserve a pre-metamorphic sure rocks were thrust onto fairly cold foreland hydrothermal alteration by near-surface meteoric EXHUMATION PROCESSES 21 waters (Rumble et al. 1998). In both cases, ultra­ slip rate of 3 km Ma-1 for a 20°-dipping normal high pressure metamorphism caused virtually no fault (which equals an exhumation rate of 1 km change to rock texture or isotopic composition. Ma-l), and a rate of 1 km Ma-1 for ductile thin­ St6ckhert & Renner (1998) show that the unde­ ning would combine to give a total exhumation formed Brossasco granite indicates that differen­ rate of 3 km Ma-1. The fast net rate is not useful tial stress was too low to cause plastic flowduring in distinguishing between different exhumation burial, accretion and exhumation. The only evi­ processes. dence for significant deformation by dislocation Fast exhumation rates inhibits fast cooling. creep under ultrahigh-pressure conditions comes However, fast cooling may commonly follow from ultrahigh-pressure eclogite from the rapid exhumation because of the upward advec­ Zem1att-Saas zone ( omphacite microstructures tion of heat. There appears to be a serious need reported by van der Klauw et al. 1997). to constrain well-defined T-t and especially P-t (5) Are there transient accelerations in the rate paths for exhumed rocks. of the processes involved in exhumation (e.g. Hill (2) How important is tectonic exhumation? et al. 1995) ? Many of the arguments that concern Tectonic exhumation, especially in the form of the sustainability of topography and the mainte­ normal faulting has been recognized as a nance of steady-state geotherms, critically common factor in continental orogenesis. A depend on the time constants of the processes widely held view is that early crustal thickening involved. For example, it is relatively easy to in convergent continental orogens will generally produce a depressed geotherm in the overriding lead to normal faulting and crustal thinning. The plate, above a subduction zone, as the result of firstsalient problem is to diagnose unequivocally large-scale overthrusting of a back-arc basin, horizontal crustal extension in orogens. Other providing an alternative explanation for the syn­ problems associated with normal faulting chroneity of ophiolite emplacement and the for­ appear to be the depth range of normal faults mation of eclogites and blueschists (see Rawling and the maximum throw on normal faults. & Lister this volume). Since relatively thick The few data on ductile thinning suggest that crust might exist for less than 1 Ma, rapid oscil­ it is a slow exhumation process. More quantita­ lations in tectonic mode may provide an expla­ tive work on ductile thinning, especially in con­ nation for many of the paradoxes outlined in the tinental collision zones, is needed to discussions above. demonstrate that this process contributes in a significant way to the exhumation of metamor­ phic rocks. Concluding remarks - outstanding (3) What is the role of erosion? Erosion appears to be able to operate at very fast rates, problems perhaps as high as 15 km Ma-1, given sufficient In most active mountain belts, erosion and tec­ precipitation, steep terrain, and comparable tonism are dynamically coupled to the point uplift rates. There is no reason to indicate that where it may be difficult to separate cause and these rates could not be sustained for long effect. This makes it difficult to distinguish periods of time, as long as uplift rates continued between different exhumation processes. to match erosion rates and climate conditions Nonetheless, exhumation typically occurs by remained favourable for fast erosion. Alpine multiple processes and there is a need to quan­ glaciation appears to be the most aggressive tify the relative contributions of the agent of erosion and one that is particularly sen­ different exhumation processes, using inform­ sitive to global climate. In this regard, the rela­ ation from metamorphic petrology, isotope tively high sediment production rate of the thermochronology, structural and kinematic Quaternary may be the result of a cooler climate analysis, synorogenic stratigraphy, geomorphol­ and more extensive alpine glaciation. An ogy, and palaeo-elevation analysis. outstanding problem is that much of our current To highlight some of our conclusion, we ask understanding of erosion rates is based on rela­ the following four questions. tively short records. There is a serious need for (1) How diagnostic are exhumation rates fo r better long-term estimates using sediment distinguishing exhumation processes? We inventories or thermochronometry. believe that exhumation rates alone cannot dis­ (4) Where do UHP rocks fo rm and how are tinguish between exhumation processes. Like­ they exhumed? Ultrahigh-pressure rocks occur wise, net exhumation rates do not supply much only in collisional belts, but they otherwise information on the rates of specific processes. appear to form below the seismically determined The following example may help to illustrates Moho. Coesite-bearing ultrahigh-pressure rocks this point: an erosion rate of 1 krn Ma-1, a fast can form in the root of a highly overthickened 22 U. RING ET AL. crust when large parts of the more mafic lower BERNOULLI, D. & WEISSERT, H. 1985. Sedimentary crust have been eclogitized. We have shown an fabrics in Alpine ophicalcites, South Pennine example where eclogitized lower crust would be Arosa Zone, Switzerland. Geology, 13, 755-758. placed beneath the Moho in which case the crust BIINO, G. & COMPAGNONI, R. 1992. Very-high pressure metamorphism of the Brossasco coronite meta­ might be about 110 km thick (with the Moho at a granite, southern Dora Maira Massif, Western depth of c. 70 km). The predicted mean elevation Alps. Schweizerische Mineralogische und Petro­ would only be about 1.5 km. Ultrahigh-pressure graphische Mitteilungen, 72, 347-363. rocks may also form within a mantle-rich oro­ BIRD, P. 1991. Lateral extrusion of lower crust from genic root. The involvement of lithospheric under high topography in the isostatic limit: mantle limits the mean elevation of the orogen to Jo urnal of Geophysical Research, 96, about 3 km. The question that remains unan­ 10275-10286. swered is how do these rocks make their way BOILLOT, G., GRIMAUD, S., MAUFFRET, A., MOUGENOT, back to the surface? D., MERGOIL-DANIEL, J., KORNPROBST, J. & TORRENT, G. 1980. Ocean-continent boundary off the Iberian margin: a serpentinite diapir west of This introduction benefited from comments by the Galicia Bank. Earth and Planetary Science Christopher Beaumont, Soren Dtirr, Gary Ernst, Letters, 23-34. Thomas Flottmann, Jessica Graybill, Kit Johnson, 48, BLOOM, 1998. 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