Convective removal of lower lithosphere beneath the Tibetan Plateau can account for a rapid increase in the mean elevation of the Tibetan Plateau of 1000 m or more in a few million years. Such uplift seems to be required by abrupt tectonic and environmental changes in Asia and the Indian Ocean in late Cenozoic time. The composition of basaltic volcanism in northern Tibet, which apparently began at about 13 Ma, implies melting of lithosphere, not asthenosphere. The most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere. The initiation of normal faulting in Tibet at about 8 (± 3) Ma suggests that the plateau underwent an appreciable increase in elevation at that time. An increase due solely to the isostatic response to crustal thickening caused by India's penetration into Eurasia should have been slow and could not have triggered normal faulting. Another process, such as removal of relatively cold, dense lower lithosphere, must have caused a supplemental uplift of the surface. Folding and faulting of the Indo-Australian plate south of India, the most prominent oceanic intraplate deformation on Earth, began between about 7.5 and 8 Ma and indicates an increased north-south compressional stress within the Indo-Australian plate. A Tibetan uplift of only 1000 m, if the result of removal of lower lithosphere, should have increased the compressional stress that the plateau applies to India and that resists India's northward movement, from an amount too small to fold oceanic lithosphere, to one sufficient to do so. The climate of the equatorial Indian Ocean and southern Asia changed at about 6–9 Ma: monsoonal winds apparently strengthened, northern Pakistan became more arid, but weathering of rock in the eastern Himalaya apparently increased. Because of its high altitude and lateral extent, the Tibetan Plateau provides a heat source at midlatitudes that should oppose classical (symmetric) Hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons. For the simple case of axisymmetric heating (no dependence on longitude) of an atmosphere without dissipation, theoretical analyses by Hou, Lindzen, and Plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal (monsoonlike) circulation than such a source centered on the equator, but only if heating exceeds a threshold whose level increases with the latitude of the heat source. Because heating of the atmosphere over Tibet should increase monotonically with elevation of the plateau, a modest uplift (1000–2500 m) of Tibet, already of substantial extent and height, might have been sufficient to exceed a threshold necessary for a strong monsoon. The virtual simultaneity of these phenomena suggests that uplift was rapid: approximately 1000 m to 2500 m in a few million years. Moreover, nearly simultaneously with the late Miocene strengthening of the monsoon, the calcite compensation depth in the oceans dropped, plants using the relatively efficient C4 pathway for photosynthesis evolved rapidly, and atmospheric CO2 seems to have decreased, suggesting causal relationships and positive feedbacks among these phenomena. Both a supplemental uplift of the Himalaya, the southern edge of Tibet, and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the Himalaya that, in turn, enhanced extraction of CO2 from the atmosphere. Thus these correlations offer some support for links between plateau uplift, a downdrawing of CO2 from the atmosphere, and global climate change, as proposed by Raymo, Ruddiman, and Froehlich. Mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts, but might also play an important role in altering regional and global climates.

Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon

The development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional-scale thermal regimes. These are: the total radiogenic heat supply within the thickened crust, the supply of heat from the mantle, the thermal conductivity of the medium and the length and time scales of erosion of the continental crust. The orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening, during which little temperature change occurs in individual rocks, followed by a lengthier phase of erosion, at the end of which the crust is at its original thickness. The principal features of pressure-temperature-time (PTt) paths followed by rocks in this environment are a period of thermal relaxation, during which the temperature rises towards the higher geotherm that would be supported by the thickened crust, followed by a period of cooling as the rock approaches the cold land surface. The temperature increase that occurs is governed by the degree of thickening of the crust, its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary. For much of the parameter range considered, the heating phase encompasses a considerable portion of the exhumation (decompression) part of the PTt path. In addition to the detailed calculation of PTt paths we present an idealized model of the thickening and exhumation process, which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path. An important feature of these PTt paths is that most of them lie within 50 °C of the maximum temperature attained for one third or more of the total duration of their burial and uplift, and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience. Considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm. A companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes. © 1984 Oxford University Press.

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Pressure-Temperature-Time Paths of Regional Metamorphism I. Heat Transfer during the Evolution of Regions of Thickened Continental Crust

This paper reports the results of a systematic geochemical study of intermediate and acid intrusive rocks from a number of continent-continent collision zones of Phanerozoic age. Four groups of intrusions can be recognized, each associated with a particular stage in the tectonic evolution of a collision zone. 

Pre-collision calc-alkaline (volcanic-arc) intrusions which are mostly derived from mantle modified by a subduction component and which are characterized by selective enrichments in LIL elements. 

Syn-collision peraluminous intrusions (leucogranites) which may be derived from the hydrated bases of continental thrust sheets and which are characterized by high Rb/Zr and Ta/Nb and low K/Rb ratios. 

Late or post-collision calc-alkaline intrusions which may be derived from a mantle source but undergo extensive crustal contamination and can only be distinguished from volcanic-arc intrusions by their higher ratios of Ta/Hf and Ta/Zr. 

Post-collision alkaline intrusions which may be derived from mantle lithosphere beneath the collision zones and which carry high concentrations of both LIL and HFS elements. 

The geochemical evolution of crustal melts within groups (ii) and (iii) can be viewed in terms of the dehydration reactions, volatile transfer and transient geothermal gradients that result from thrust tectonics in the zone of collision.

Geochemical characteristics of collision-zone magmatism

The thermal thickness of Precambrian lithosphere is modeled and compared with estimates from seismic tomography and xenolith data. We use the steady state thermal conductivity equation with the same geothermal constraints for all of the Precambrian cratons (except Antarctica) to calculate the temperature distribution in the stable continental lithosphere. The modeling is based on the global compilation of heat flow data by Pollack et al. [1993] and more recent data. The depth distribution of heat-producing elements is estimated using regional models for ∼300 blocks with sizes varying from 1°×1° to about 5°×5° in latitude and longitude and is constrained by laboratory, seismic and petrologic data and, where applicable, empirical heat flow/heat production relationships. Maps of the lateral temperature distribution at depths 50, 100, and 150 km are presented for all continents except Antarctica. The thermal thickness of the lithosphere is calculated assuming a conductive layer overlying the mantle with an adiabat of 1300°C. The Archean and early Proterozoic lithosphere is found to have two typical thicknesses, 200–220 km and 300–350 km. In general, thin (∼220 km) roots are found for Archean and early Proterozoic cratons in the Southern Hemisphere (South Africa, Western Australia, South America, and India) and thicker (>300 km) roots are found in the Northern Hemisphere (Baltic Shield, Siberian Platform, West Africa, and possibly the Canadian Shield). We find that the thickness of continental lithosphere generally decreases with age from >200 km beneath Archean cratons to intermediate values of 200±50 km in early Proterozoic lithosphere, to about 140±50 km in middle and late Proterozoic cratons. Using known crustal thickness, our calculated geotherms, and assuming that isostatic balance is achieved at the base of the lithosphere, we find that Archean and early Proterozoic mantle lithosphere is 1.5% less dense (chemically depleted) than the underlying asthenosphere, while middle and late Proterozoic subcrustal lithosphere should be depleted by ∼0.6–0.7%. Our results suggest three contrasting stages of lithosphere formation at the following ages: >2.5 Ga, 2.5–1.8 Ga, and <1.8 Ga. Ages of komatiites, greenstone belts, and giant dike swarms broadly define similar stages and apparently reflect secular changes in mantle temperature and, possibly, convection patterns.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900439

Thermal thickness and evolution of Precambrian lithosphere: A global study

A companion paper (England & Thompson, 1984a) investigates the pressure-temperature—time (PTt) paths followed by rocks undergoing burial metamorphism in continental thickening events. This paper discusses problems involved in inferring such paths from the petrological data available in metamorphic rocks and—once such paths are determined—how they may be interpreted in terms of the thermal budgets of metamorphism. Each of the principal facies series (glaucophane-jadeite, andalusite-sillimanite and kyanite-sillimanite) may be encountered by rocks involved in the thickening and erosion of continental crust in a regime of average continental heat flow. The inference of a minimum thermal budget required for a given metamorphism depends strongly on a knowledge of the PTt paths followed by rocks during the metamorphism. Discrimination between possible thermal regimes is greatly enhanced if portions of PTt paths, rather than single PT points, are available, and additional constraint is possible if these paths are supplemented by geochronological, structural and heat flow data. 1. I N T R O D U C T I O N One of the standard inverse problems of metamorphic petrology is to infer the thermal regime operating during a metamorphic episode from the mineral assemblages and compositions in rocks that reach the surface after the episode is finished. In fact, orogenic processes are sufficiently complicated, and petrological data sufficiently limited that no formal inverse is ever attempted; interpretation usually depends on the calculation of pressure-temperature— time {PTt) paths for model tectonic settings and on using these with metamorphic petrological data to give estimates of thermal regimes operating during metamorphism. The previous paper (England & Thompson, 1984a, hereafter called Part I) outlined some of the model pressure-temperature-time (PTt) paths that would be followed by rocks under a variety of conditions resulting from crustal thickening, and addressed the question of what class of information is recoverable from PT data obtained from metamorphic rocks. A major conclusion of Part I is that we expect the overall thermal development of a regional metamorphic terrain to depend on relatively few large scale parameters and, consequently, that if it is possible to extract information from mineral assemblages about regional PTt paths, we should be able to make estimates of the magnitudes of the parameters controlling the regional metamorphic budget—notably the total heat supply to the orogenic belt, the lengthscales of thickening, the length and time scales of erosion and the extent of magmatic involvement in the metamorphic episode. Smaller scale phenomena (intrusions, transposition of isograds by folding or faulting, etc.) will affect observations made in the field, but cannot be uniquely interpreted in terms of larger scale phenomena. I Journal of Petrology. Vol. 25. Part 4. pp. 929-955. I984| 930 A. B. THOMPSON AND P. C. ENGLAND In this paper we discuss the problems of relating petrological PT points or, more usefully, portions of PTt paths to regional thermal regimes. In principle such data, particularly in conjunction with heat flow, isotopic and structural studies, can provide valuable information on the tectonic setting and thermal budget of metamorphism. Unfortunately, it is often not possible to gather all the information necessary for a unique interpretation of a given terrain; nonetheless PTt data may be used to place constraints on metamorphic processes, and we hope that the considerations presented here will help in the interpretation of such data. 2. PT POINTS AND PTt PATHS Many of the inferences of conditions during the evolution of orogenic belts that have been made using PT data from metamorphic rocks are based on relatively simple conceptual models of the metamorphic environment and, while these models serve a useful purpose, it is important to recognize their limitations as well as their merits. In this section we consider how these conceptual models need modification in view of the nature of the PTt paths that are followed by rocks undergoing regional metamorphism. For consistency, most of the paths discussed here are taken from section 5 of Part I, but many of the arguments may readily be applied (with the appropriate adjustments) to other forms of PTt paths. The first set of problems encountered in inferring thermal and tectonic regimes from metamorphic data are concerned with deciding where on a PTt path the points that make up a PT array are recorded, and the rest of this section is largely concerned with them. We consider how the kinds of PTt paths calculated in Part I influence PT data recorded in mineral assemblages on the surface, through their effects on reaction kinetics, chemical diffusion and through the control exerted by fluids on the mineral assemblages preserved. In particular, we emphasize the means whereby information may be gained on parts of PT paths, rather than on discrete PT points. 2.1. Geotherms, facies series and PT paths The material of these papers concerns the perturbing and relaxation of temperatures in continental lithosphere, and the PT conditions experienced by rocks moving relative to these evolving geotherms. As the word geotherm is used in at least two distinct senses in the petrological literature it is worth including definitions here to avoid possible ambiguities. A geotherm is simply the relation between temperature and depth within the earth at any one time; this relation may be a steady state one or a transient one, but in either case its FIG. 1. Metamorphic mineral assemblages at the erosion surface in (a) are assigned through experimentally calibrated mineral reactions to metamorphic fades series (6) and fades (c). The equilibrium data in (b) are shown as examples only; the sources are given by Thompson & Thompson (1976, fig. 13) and by Thompson & Tracy (1979, fig. 2) and transferred to this figure using a crustal density of 2-8 Mg m~. The equilibria include two versions of the relative stability of Al2SiOj polymorphs (from Holdaway, 1971 and Richardson el a/., 1969; where andalusite = AND; kyanite = KYA; sillimanite = SIL), and the breakdown of albite (ALB) to jadeite (JAD) plus quartz (QTZ). The dehydration reactions produdng almost pure H2O (vapor = V) are experimentally investigated in the presence of excess water (onp x 1). Other abbreviations: muscovite (MUS), analdme (ANL), pyrophyllite (PYP), potash-feldspar (KSP). Mineral assemblages assigned to mineral facies (c) at the erosion surface in (a) are shown with normal type to distinguish them from facies representing present day conditions in the underlying crust, shown by italic type. The P-T locations of metamorphic facies (c) and facies series (6) are based upon correlation with experimental studies of many more mineral equilibria than shown here (for example see Fyfe el a/., 1978, fig. 6.8). Because of the transitional nature of mineral assemblages in the field and the displacement of calibrated reactions through crystalline solution in natural minerals, the P-T boundaries of facies and fades series are approximate and gradational. One common practice is the assigning of mineral assemblages at the erosion surface (solid points in (a)) to a steady-state geotherm, shown by the heavy curves in (6) and (c). The dashed lines crossing the geotherm in (c) represent schematic PTt paths experienced by rocks during exhumation (see Part I). •1 3 JO m C C C jo m I H m m C m m H X P R E S S U R E , kb < p » 2 .8 g c m \" 3 ) ? * * * PR ES SU R E , kb ( p -2 .8 g c m \" 3 ) 932 A. B. THOMPSON AND P. C. ENGLAND reference level is the earth's surface. A transient geotherm may be the result of transient heat sources, of motions due to erosion, overthrusting, rifting etc., or of thermal relaxation from a previously perturbed regime; all three of these cases occur during the evolution of the systems discussed in this paper. Individual PT points obtained from mineral assemblages are often interpreted in terms of geothermal gradients (Fig. 1); it should be clear from Part I that while an individual PT point may have been set on a geotherm, its relation to the heat source distribution giving rise to that geotherm is usually unclear. The problem becomes worse if an array of PT points from one terrain is to be interpreted; such arrays have been referred to, misleadingly, as 'metamorphic geotherms' (e.g. England & Richardson, 1977). In fact these arrays bear no relation to any one geotherm that existed during metamorphism, because they consist of a set of PT conditions inferred from assemblages that were preserved at different times and different places during metamorphism (e.g. England & Richardson, 1977, fig. 1; section 3.1 below). We prefer the designation 'piezothertnic array' (Richardson & England, 1979) for the set of peak metamorphic conditions experienced in a thickened pile, or 'PT array' for a suite of metamorphic conditions preserved in a metamorphic terrain. Certain rock types and minerals are often regarded as characteristic of distinct pressure (/>) and temperature (T) regimes, and the indications these give of changing metamorphic grade on a regional scale lie behind the facies series concept of Miyashiro (1961). The three principal facies series, glaucophane-jadeite (Gla—Jad), kyanite—sillimanite (Kya-Sil) and andalusite-sillimanite (And—Sil) are taken to be representative of, respectively, high P— low T, intermediate T and P, and high T-low P metamorphism. In addition, several workers (e.g. Heitanen, 1967) specify intermediate facies series (see Fig. 1). In principle this approach acknowledges only that the rocks involved have passed through some region of PT space that is characterized by the stability fields of the diagnostic minerals, although the facies series concept, in its application, is often used to infer a tectonic setting for the metamorphism. We shall refer in this paper to facies, but we 

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Pressure—Temperature—Time Paths of Regional Metamorphism II. Their Inference and Interpretation using Mineral Assemblages in Metamorphic Rocks

Metamorphic consequences of crustal eclogite production in overthrust orogenic zones

Summary. Existing surface heat flow determinations in north-east England indicate a difference in thermal structure between the Alston block of the North Pennines and the coastal regions. New heat flow determinations in the Askrigg block and on the coast support the original indications of higher heat flow in the positive blocks, which are underlain by 400 Myr old granites, than in their marginal sedimentary troughs along the present coastline. Although the two blocks are geophysically and geologically similar, surface heat flow in the Alston block is 30mW m-2 higher than it is in the Askrigg block 50km to its south. Heat flow refraction around high conductivity plutons with high heat production may significantly affect the interpretation of heat flow measurements made on a regional scale but in the present case its effect, if any, is to emphasize the heat flow difference between the two otherwise similar blocks. The chemical differences between the Wensleydale and the Weardale granites – cupolas of the granites underlying the Askrigg and Alston may be representative of the granites as a whole and the blocks respectively – difference in surface heat flows may result from a contrast in radiogenic heat production between the two granites extending to mid-crustal depths.

S. W. Richardson

Papers

Metamorphic consequences of crustal eclogite production in overthrust orogenic zones

Heat refraction and heat production in and around granite plutons in north-east England