Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 A history of the Asian monsoon and its interactions with solid Earth tectonics in Cenozoic South Asia PETER D. CLIFT1* & A. ALEXANDER G. WEBB2 1 Department of Geology and Geophysics, E235 Howe-Russell, Louisiana State University, Baton Rouge, LA 70803, USA 2 Department of Earth Sciences and Laboratory for Space Research, The University of Hong Kong, Pokfulam Road, Hong Kong, China P.D.C., 0000-0001-6660-6388 *Correspondence: pclift@lsu.edu Abstract: Although there is some evidence for an Eocene monsoon, the most important intensification of rainfall appears to start at c. 24 Ma in the Early Miocene. Many palaeoceanographical proxies for monsoon intensity are linked to wind and do not correlate well with humidity of the continental climate over tectonic timescales. Rainfall peaked in the middle Miocene (c. 15 Ma) with strong drying after 8 Ma. This timing does not correlate well with either initial uplift of the Tibetan Plateau or with the retreat of shallow seas from central Asia. The c. 24 Ma onset of strengthening rainfall is associated with the initiation of rapid erosion and cooling of Himalayan metamorphic rocks. The progressive detachment of the subducting Indian lithosphere from the eastern and western syntaxes at c. 25 Ma to the east-central Himalaya at c. 13–11 Ma would have produced corresponding propagation of rising Himalayan topography following release of the weight of the detached slab. Rapid uplift of the Himalayan barrier, blocking moisture-laden winds, is considered the most likely trigger for a stronger summer monsoon in South Asia, which in turn allowed faster erosion and exhumation of the Greater Himalaya after 24 Ma. It has been recognized for some time that the tectonic evolution of the solid Earth has important impacts on the enveloping atmosphere and oceans. The uplift of mountain belts and the opening of ocean gateways and basins has controlled water and air circulation patterns over large expanses of the globe. In turn, this has affected regional climate (Molnar et al. 1993). It is easy to understand why the uplift of high mountain belts or plateaus might disrupt atmospheric circulation (Manabe & Terpstra 1974; Kitoh 2004). Likewise, the opening and closure of oceanic gateways can divert warm currents from flowing from equatorial regions into high latitudes, affecting regional and global climate over million-year and greater timescales (von der Heydt & Dijkstra 2006; Egan et al. 2013). The Asian monsoon system is proposed to be one of the most dramatic examples of such interactions between the solid Earth and the atmosphere. Monsoon intensity is hypothesized to have changed in the aftermath of the collision that began between India and Eurasia at about 50– 60 Ma (Najman et al. 2010; DeCelles et al. 2014; Hu et al. 2015; Hua et al. 2016), and which continues to the present day. Climate–tectonic interactions, however, produce feedbacks in both directions. It has been recognized that surface processes might have important impacts on the development of the solid Earth. Erosion, particularly linked to rainfall and potentially focused by rivers, glaciation and/or orography, may remove significant amounts of rock in active mountain belts where tectonically driven rock uplift is occurring (Koons et al. 2002; Malavieille 2010). This process allows deep-buried units (e.g. high-grade metamorphic rocks) to be brought to the surface. Because erosion and sediment transport are closely linked to climatic variability, it is understood that changes in rainfall patterns should have an important impact on the timing and geometry of mountain belt structure (Sinclair et al. 2005; Wobus et al. 2005). One of the most well-known examples of such possible interactions lies in Cenozoic Asia. The Asian monsoon system has long been associated with the uplift of high topography in the Himalaya and Tibetan Plateau (Kutzbach et al. 1993; Molnar et al. 1993), although to the present time we lack the data necessary to fully test many of the climate–tectonic models that have been advanced. The channel-flow model is a popular one that tries to link the monsoon and the tectonics of the Himalaya. Focused erosion along the southern edge of the Tibetan Plateau is proposed to allow the unroofing of high-grade rocks in the Greater Himalaya and control the location of the major bounding thrusts in From: TRELOAR, P. J. & SEARLE, M. P. (eds) Himalayan Tectonics: A Modern Synthesis. Geological Society, London, Special Publications, 483, https://doi.org/10.1144/SP483.1 © 2018 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB the ranges (Beaumont et al. 2001). There is, however, no agreement as yet about the validity of such models and other favour alternatives, such as thrust wedges (Robinson et al. 2003; Vannay et al. 2004) and a lack of correlation between climate belts and structures (Burbank et al. 2003). Testing between such conceptual models has been challenging because in order to demonstrate such linkages we need to know the history of tectonics of the mountain belt, the development of high topography through time and space, as well as the varying intensity of the Asian monsoon itself over tectonic, million-year timescales. This latter aspect has been made more difficult because of a dearth of well-dated complete sedimentary sections that could be analysed, as well as a poor understanding about what exactly makes a good monsoon proxy in the first place (Clift & Plumb 2008; Sun et al. 2010; Caballero-Gill et al. 2013; Clift et al. 2014; Ramesh et al. 2017). Is the monsoon most closely identified with rainfall, seasonality or wind intensity? There is also the additional problem of separating global from regional effects. The Cenozoic is marked by longterm global cooling, culminating in the Pleistocene ice ages (Zachos et al. 2001); and although this longterm trend is itself plausibly linked to chemical weathering of the Himalayas (Raymo & Ruddiman 1992), it is not in itself an aspect of the Asian monsoon. In any case, debate continues about the cause of the drawdown in atmospheric CO2, with the largescale burial of organic matter, especially in the Bengal Fan, being highlighted as a primary control (Derry & France-Lanord 1997; Galy et al. 2007). Recent scientific drilling in the Indian Ocean and in the marginal seas of East Asia now holds out the possibility of acquiring some of the records that will allow climatic–tectonics models to be tested, at least in certain areas and over certain critical time intervals. In this review we examine the history of the Asian monsoon and evidence for the feedbacks between the solid Earth and the climatic systems over tectonic timescales. The South Asian monsoon Asia is not the only continent to have a monsoon system. Most large land masses have seasonal climates that are linked to large-scale pressure differences in the atmosphere between the continental interior and surrounding oceans (Webster et al. 1998; Wang & Ding 2008). The Asian monsoon is, however, the most intense of these, probably because Asia itself is the largest continental mass and because the centre of this continent is taken up by unusually high topography, spanning the Tibetan Plateau and further north into Mongolia (Fig. 1). The summer monsoon essentially reflects the consequence of the low-pressure system that develops in Central Asia during the boreal summer as a result of heating by the sun compared to cooler conditions over the Indian and Pacific oceans (Wang 2006). The monsoon itself is typically divided into two regional systems: a South Asian monsoon whose moisture is derived from the Indian Ocean; and an East Asian monsoon, which draws on water sources in the South China Sea and the Pacific Ocean. While the South Asian monsoon migrates across the Indian subcontinent from the Bay of Bengal until encountering the barrier of the Himalayas, the East Asian monsoon instead starts in SE Asia and moves towards the NW controlled by the location of the Westerly Jet that flows to the south of the Tibetan Plateau during the boreal winter (Schiemann et al. 2009) (Fig. 1). The winter monsoon represents the reverse of this situation, in which a high-pressure system develops in the continental interior and cold dry winds blow out of the deep interior (Siberian High) carrying wind-blown dust during the winter dry season. It is this seasonality that is a particular hallmark of monsoon activity. The concept of a global monsoon has become popular in recent years (Wang & Ding 2008), and there is no denying that the Asian monsoon is linked both to monsoons in Africa and in Australia. Nonetheless, in this study we focus particularly on the evolution of climate in South Asia and assess its links to the history of the Himalaya. Oxygen isotope data from the Burma Basin in Myanmar now imply a seasonal monsoon climate extending back to the Late Eocene (c. 34 Ma), essentially soon after the start of the India–Asia collision (Licht et al. 2014). This may not be as surprising as it might initially seem when one considers the evidence for there being a significant area of high topography, at least in southern Tibet, prior to the collision with India that might have supported a stronger monsoon (Kapp et al. 2007). Models for the existence of a precollision ‘Lhasaplano’ plateau are now supported by crustal thickness estimates (Zhu et al. 2017), combined carbon and oxygen isotopes (Ding et al. 2014), as well as clumped isotope data (Ingalls et al. 2017). Evidence for an Eocene monsoon also comes from the recognition of aeolian deposits from the Xining Basin of NW China (Fig. 1) linked to desertification and the action of winter monsoon winds (Licht et al. 2014). This interpretation supports the earlier recognition of gypsum deposits from this same area that were laid down in ephemeral lakes and whose accumulation has been linked to a wetter environment during the Eocene (DupontNivet et al. 2007). The climate in northern China appears to have dried after c. 34 Ma, probably related to a global environmental change linked to the onset of large-scale Antarctic glaciation around that time. Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS Fig. 1. Shaded topographical and bathymetric image of the regions discussed in this paper, as well as the location of the primary atmospheric belts, the Westerly Jet and the Intertropical Convergence Zone (ITCZ) which help to control the distribution of monsoon rainfall. The map was partly produced from GeoMapApp (Ryan et al. 2009). NP, Nanga Parbat; ZB, Zhada Basin; PB, Pulan Basin; TB, Thakkola Basin; GB, Gyirong Basin. Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 Variable monsoon Dry Weakening Monsoon P. D. CLIFT & A. A. G. WEBB Monsoon Monsoon Tropical not monsoonal Smectite/Kaolinite 12 Seasonal 8 4 Tropical More leaching (a) Clay mineralogy 0 5 0 0.7 15 20 25 30 15 20 25 30 Chemical weathering, K/Al Less chemical weathering (b) 10 K/Al 0.9 1.1 5 10 Carbon isotopes 23 C13 21 More seasonal 0 (c) MMCO 1.3 19 17 15 C4 C3 13 0 5 10 15 20 25 30 5 10 15 20 25 30 5 10 15 20 25 30 10 15 20 25 30 More clastic flux to sea Ti/Ca Site 1148 (d) Mass flux, Ti/Ca 0.15 0.10 0.05 0 Less chemical weathering 0 (e) CRAT 0.5 0.9 Less seasonal climate Hematite/Goethite (f) Hematite/Goethite 4 2 0 -2 0 5 Xining–Minhe Basin Central China (g) Pollen Proxies from Central Asia Picea pollenites Spruce Pinus pollenites Pine Potomogeton Pondweed Nitraria Shrub Ephedripites gymnosperm >40% upper plot lower plot 20-40 11-19 7-10 >30% Pl. 0 20-30 15-20 10-15 Plio. 3-6 1-2 <1% 4-10 2-4 1-2 <1% L. Miocene Mid. Mioc. 5 10 Transition to Humid Climate Humid flora Arid flora E. Miocene 15 Age (Ma) 20 Oligocene 25 30 Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS Whether there was an Eocene monsoon or not remains controversial because of the limited number of data that support such a model and the recognition that the data from Myanmar come from close to the coast, where a wet environment might have been expected whether there was a strong monsoon or not in a low-latitude setting (Wang & Ho 2002; Boos & Kuang 2010). What is better defined is the Miocene climatic evolution, because of sedimentary records in the Himalayan foreland basin, as well as marine records from the Indian Ocean and the South China Sea. The Oligocene remains obscure because of a large unconformity in the foreland basin that means there is no record of weathering and erosion onshore for that time period (Najman 2006). So far, scientific ocean drilling has yet to sample siliciclastic sediments of this age in the Indian Ocean submarine fans either, although drilling during IODP Expedition 354 in the Bay of Bengal did penetrate Oligocene carbonate sequences (France-Lanord et al. 2016). Other proxy records indicative of early monsoon strengthening include the history of wind-blown dust, loess, accumulation in northern China. Records in the western Loess Plateau extended the start of this activity to before c. 22 Ma (Guo et al. 2002). If loess sedimentation is connected to winter monsoon intensity over long periods of geological time, as is generally believed, then this would imply that the present atmospheric system dates from that time. In turn, this Early Miocene strengthening is consistent with the compilation of palynology data from across China that also argues for a reorganization of climatic belts around the start of the Miocene. These data imply that the present large-scale environmental distribution was established around that time, with wet conditions towards the SE and drier conditions in the west (Sun & Wang 2005). Such a pattern contrasts with the earlier more zonal patterns that are generally inconsistent with a strong monsoon. Other support for a marked change in monsoon strength at the start of the Miocene comes from sedimentary archives of erosion and chemical weathering (Fig. 2). However, the relationship between monsoon intensity and chemical weathering, as well as physical erosion, can be complicated, despite the fact that weathering rates generally increase as humidity and temperature increase (West et al. 2005). Nonetheless, there is evidence to suggest that increasing precipitation along the Himalayan front has resulted in faster erosion (Clift et al. 2008b). On steep mountain topography, where landsliding can occur, the growth of vegetation and its role in reducing erosion is less important than in flood plains (Burbank et al. 1993). Likewise, on the periphery of the monsoon phases of wettening, climate has tended to lead to a greater intensity of chemical weathering, which can be preserved in the deep-sea sediments. Records of chemical weathering from the South China Sea have been used to argue for a strengthening of, at least, the East Asian monsoon after around 24 Ma when chemical weathering intensity increased, reaching a peak in the middle Miocene at Ocean Drilling Program (ODP) Site 1148 (Fig. 2e & f) (Clift et al. 2008b, 2014). Figure 2b shows a decrease in chemical alteration of the sediment deposited on the southern continental margin of China, as proxied by K/Al after 15 Ma. This trend was linked to a steady decrease in global temperatures over the same area following strong weathering during the Mid Miocene Climatic Optimum (MMCO) (Wan et al. 2009). Analysis of spectral reflectance data has been able to constrain the relative abundance of hematite, which is most readily formed in warmer, drier environmental conditions, and goethite, which is associated with wetter, colder conditions. Values of hematite/goethite are at a maximum during the middle Miocene (Fig. 2f), suggestive of more seasonal conditions (i.e. with a dry season and, therefore, more hematite) that then reduced, at least into the Pliocene, indicative of a weakening monsoon (Clift et al. 2014). Another spectral proxy CRAT, which compares the relative abundance of chemicalweathering-derived clays to those linked to physical erosion (chlorite/chlorite + hematite + goethite), demonstrates a progressive strengthening of weathering and thus summer monsoon intensity from 23 to c. 15–12 Ma, tracking the long-term evolution in hematite/goethite (Fig. 2e & f). The apparently dry anomaly at 17–16 Ma is linked to the less seasonal, although warm, wet conditions of the MMCO (Wan et al. 2009). As in SW Asia, there is a gradual drift in southern China towards a greater abundance of C4 vegetation associated with a drier environment (Jia et al. 2003) (Fig. 2c). Erosional fluxes, which can be constrained by quantifying the delivery of siliciclastic sediment to the continental margins of SE Asia, have increased at the same time that these Fig. 2. Compilation of some of the more robust erosion and weathering proxies spanning 25 Ma at ODP sites 1146 and 1148 in the South China Sea. (a) Kaolinite/(illite + chlorite) from Wan et al. (2007). (b) K/Rb as a measure of chemical weathering intensity from the data of Wei et al. (2006). (c) Carbon isotopes from black carbon from ODP Site 1148 representing regional terrestrial biomass (Jia et al. 2003). (d) Scanned Ti/Ca from ODP Site 1148 (Hoang et al. 2010) with the vertical scale expanded to eliminate the influx from Taiwan since 5 Ma. (e) The CRAT proxy of Clift et al. (2008b) tracking the relative influence of chemical weathering v. physical erosion. (f ) Hematite/goethite from Clift (2006). (g) Pollen proxies from Central Asia (Sun & Wang 2005). Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB Age (Ma) 0 5 G. Bulloides (%) Pleist. Pliocene 10 L. Miocene 15 25 20 M. Miocene E. Miocene Ol. 20 (a) 15 ODP Site 730: G. bulloides Oman margin 10 5 Start of record 0 -15 Pedogenic carbonates Himalaya foreland (b) 13C -10 -5 C3 dominant: Wetter 0 Potwar Plateau NW India C4 dominant: Drier 5 1 (c) 18O 2 3 Hotter 4 Colder MMCO Global seawater 5 75 0.15 (d) 0.17 Drier 65 0.19 60 Chemical Weathering Indus Marine A1 Well, Arabian Sea 55 Sedimentation Rate (1000s of km3/myr) Wetter (e) 200 Start of record Sedimentation Rate Indus Fan 0.21 Faster erosion Slower erosion 150 100 50 Ar-Ar Mica Cooling Ages in the Greater Himalayas Normalized Probability (f) Central and Eastern All Greater Himalayas AFT Greater Himalayas Ar-Ar 0 5 10 15 Western Eroded Greater Himalayas Greater Himalayas Ar-Ar 20 25 K/Al CIA 70 Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS chemical weathering indices strengthened, also suggesting stronger precipitation starting around the beginning of the early Miocene (Clift 2006). It is noteworthy that similar trends in the delivery of sediment were also noted, at least over long tectonic timescales, in the western Himalaya where offshore industrial seismic surveys have allowed large-scale long-term mass budgets for the Indus submarine fan to be developed (Clift 2006). The fact that this reconstruction (Fig. 2e) also parallels the regional budget for deltas in SE Asia, such as the Pearl River delta, suggests that some common mechanism has been controlling mass flux in both places. The most obvious conclusion is that this is the Asian monsoon, especially because the Pearl River Basin does not drain high topography in Central Asia but is limited to southern China and therefore ought not to be affected so strongly by mountain-building or tectonic processes. As such, this implies that changes in weathering in this river basin should be dominantly climatically controlled (Clift et al. 2008b). Interestingly, the chemical weathering and erosion budgets both indicate a strengthening after 24–23 Ma and a weakening of the monsoon after around 12 Ma (Figs 2 & 3). The late Miocene history of the monsoon is controversial because this is the time period for which the most number of proxy records exist, some of which are not in accord with each other. For many years it was accepted that the South Asian monsoon strengthened after around 8 Ma. There is little doubt that there was a strengthening of oceanic upwelling offshore Arabia starting at that time (Kroon et al. 1991; Prell et al. 1992). Modern upwelling in that region is associated with summer monsoon winds driving surface waters offshore which induces upwelling of the deeper nutrient-rich waters that allow certain planktonic microfauna to thrive (e.g. the foraminifer Globigerina bulloides) (Curry et al. 1992). It was the initial increase in G. bulloides at 8 Ma that was one of the first indicators of an intensified monsoon (Kroon et al. 1991), a finding that has been confirmed by subsequent work (Gupta et al. 2015) (Fig. 3a). The onset of oceanic upwelling at 8 Ma also correlated with a change in flora in the Himalayan foreland basin. Carbon isotope data indicate a reduction of woody C3-dominated assemblages, while C4 grasslands expanded (Quade et al. 1989; Singh et al. 2011) (Fig. 3b). In northern China this time was also the moment that the Red Clay Formation began to accumulate in the Loess Plateau region (An et al. 2000; Sun et al. 2006; Nie et al. 2016), although recent improved age models now suggest that some of the sections may be older (18–12 Ma) (Zhao et al. 2017) or younger than previously recognized (c. 5.2 Ma) (Anwar et al. 2015). Because the Red Clay is considered to be a chemically weathered form of the Quaternary loess (Ding et al. 1997), an aeolian sediment transported by winter monsoon winds, its appearance was also linked to changes in the climate from some older state into something close to the modern, presumably more monsoonal, conditions. However, the timing of loess sedimentation need not be diagnostic of monsoonal conditions because recent work suggests that the start of accumulation is largely a function of tectonic processes providing permanent accommodation space, not a change of climate (Wang et al. 2017). Furthermore, it is now recognized that intensified oceanic upwelling may have begun much earlier, around 12.9 Ma. This age is derived both from the abundances of G. bulloides on the Arabian margin (Gupta et al. 2015) (Fig. 3a) and also from the development of a highly asymmetrical carbonate platform in the Maldives, whose geometry is determined by the prevailing summer winds. Recent dating of this platform by International Ocean Discovery Program (IODP) Expedition 356 argues for a change in the windstrengthened direction at that time (Betzler et al. 2016), rather earlier than originally envisaged. The mismatch between the oceanic wind-based records and rain-related erosion and weathering records results in an apparent paradox. Certainly, at the present day, both wind and rain are associated with a strong summer monsoon. However, it must to be recognized that some of these proxies track humidity or seasonality, while others are more closely associated with wind strength. These two need not necessarily be associated with one another in a simple fashion over long periods of geological time. If the palynology, erosion and weathering proxies point to an Early Miocene monsoon strengthening, then how do we reconcile these with evidence for Mid and Late Miocene change? Global cooling during the Cenozoic (Zachos et al. 2001) also needs to be accounted for because chemical weathering Fig. 3. Comparison of climate, erosion and exhumation proxies in the Himalaya. (a) Abundance of G. bulloides at ODP Site 730 on the Oman margin as a proxy for the summer monsoon wind strength (Gupta et al. 2015). (b) Carbon isotope character of pedogenic carbonate in Pakistan as an indicator of dominant vegetation in the Potwar Plateau of Pakistan (Quade et al. 1989) and NW India (Singh et al. 2011). (c) Global seawater temperatures from Zachos et al. (2001). (d) Degree of chemical alteration of sediments on the Indus continental shelf at Indus Marine A-1 as measured by K/Al and the Chemical Index of Alteration (CIA) (Clift et al. 2008b). (e) Rates of sediment supply to the Arabian Sea calculated from regional seismic data (Clift 2006). (f ) Exhumation rates of the Greater Himalaya tracked by bedrock Ar–Ar dating (Clift et al. 2008b) and foreland basin sediment (Szulc et al. 2006). Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB intensity is strongly linked to temperature, as well as to humidity. One solution to this apparent mismatch is that reducing chemical weathering intensity is largely driven by the long-term cooling of Earth rather than changes in seasonal summer rainfall (Clift et al. 2014). In contrast, erosion rates are much more closely linked to precipitation and are less affected by changes in temperature. There now seems little doubt that the Asian climate dried after 8 Ma. Stable oxygen isotope data linked to the intensity of rainfall and preserved in fossil mammal teeth and bivalve shells indicate that a seasonal (i.e. monsoonal) climate has existed in SW Asia at least since 10.7 Ma, but that the intensity of rainfall reduced after 7.5 Ma (Dettman et al. 2001). Figure 4 shows this in the smaller differences between dry and wet season δ 18O values measured in shells recovered from foreland basin Siwalik sedimentary rocks of this age. δ 18O values in rainfall are controlled by the source of the water and the intensity of the precipitation. Such a development implied the opposite climatic trend from that inferred from the wind-based proxies from the Arabian continental margin, but is consistent with the concept of an Early Miocene strengthening if the rainfall was already strong by 10.7 Ma. Drying after 8 Ma is also consistent with the C3 to C4 transition in vegetation noted above (Fig. 3b), with grassland favoured by drier conditions. The Plio-Pleistocene monsoon appears to have been quite variable. The loess is considered one of the best repositories of monsoon variability over this time period. Magnetic data, in particular, are a proxy for summer monsoon intensity because of their link to the formation of iron-rich soils during times of wetter environment and stronger chemical weathering. In contrast, the abundance of aeolian dust (>19 µm) can be used as a proxy for winter monsoon intensity (An et al. 2000). Figure 5 shows how the evolution of the East Asian monsoon has been reconstructed based on records from the Chinese Loess Plateau. From 3.6 to 2.6 Ma both winter and summer monsoons appear to intensify, while after that time the summer monsoon weakened somewhat, while remaining quite variable. The winter monsoon, however, continued to strengthen with strong cyclicity linked to glacial cycles. In general, colder conditions tend to foster stronger winter monsoons and weaker summer monsoons, which are modulated mostly by solar insolation (Clemens & Prell 1991; Clemens et al. 1991). This variation is less important for the purpose of this study because the duration of each of the dry and wet phases is sufficiently short (<100 kyr) to be irrelevant to the long-term tectonic processes that we focus on in this review. In summary, the Asian monsoon may have been active during the Eocene and then dried into the Oligocene, although the evidence to support this early Calculated water 18 O 10 Dry Season Wet Season 5 0 -5 -10 -15 2 3 4 5 6 7 8 9 10 11 12 Age (Ma) Fig. 4. Diagram from Dettman et al. (2001) showing the changing seasonality of rainfall in Pakistan. The plot shows the estimated δ 18O for surface waters during dry and wet seasons derived from the analysis of mollusc shells. Using maximum and minimum δ 18O values for shell carbonate and temperatures based on palaeobotanical data, water δ 18O is calculated using a fractionation relationship for molluscan aragonite. Estimated temperatures are different for the three shaded areas: the oldest employs 20–32°C; the middle uses a range of 17–32°; and the youngest interval uses 14–34°C. Error bars represent a ±5°C uncertainty in the temperature used. SMOW is standard mean ocean water. Reproduced with permission of the Geological Society of America. Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS Magnetic susceptibility (10-8 m3 kg-1) 0 0 100 200 Strata Age (Ma) 2 4 6 8 10 50 30 >19 µm grain-size fraction (%) Zhaojiachuan section Loess Palaeosols Bedrock Fig. 5. Terrestrial records from the Chinese Loess Plateau modified from An et al. (2001). The shaded zones at 3.6–2.6 Ma indicate a time when the monsoon wind strengthened and became much more variable. The stratigraphy and time series of magnetic susceptibility (thin line) and the >0.19 mm grain-size fraction (thick line) are from the Zhaojiachuan section on the Loess Plateau. The chronology is based on the polarity boundary ages, and was obtained by interpolation from a sedimentation model using the >19 mm grain-size fraction and a magnetic susceptibility model in the basal part where grain-size data are not available. The white, grey and dark shaded patterns in the simplified stratigraphy column represent loess or loess-like sediment, palaeosols and bedrock, respectively. Reproduced with permission of Springer Nature. development is presently sparse. There is a much clearer history of intensification following the start of the Miocene (c. 24 Ma), with rainfall in South Asia reaching its peak between around 15 and 12 Ma. After this time the continental climate dried at the same time that temperatures fell over the Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB long term. Drying was particularly noteworthy after about 8 Ma, when wind strength increased. Since the onset of the northern hemisphere glaciation at c. 3 Ma, monsoon strength has varied over short orbitally modulated timescales (41 and 100 kyr), with strong summer rains during interglacial times. Himalayan tectonics Models that seek to explain the processes that control Himalayan mountain building have followed a number of paths over the years. In this collisional environment, compressional faulting has long been recognized. However, a significant development in the understanding of Himalayan strain accommodation was the discovery of the South Tibet Fault. This was initially interpreted as a low-angle normal fault driving some of the exhumation of amphibolitefacies Himalayan rocks (Herren 1987) (Fig. 6), resulting in it commonly being referred to as the South Tibet Detachment (Burchfiel et al. 1992). Orogenic wedge models have been proposed to explain the tectonic structure of the Himalayan ranges (Robinson et al. 2003, 2006; Vannay et al. 2004) that are essentially imbricated stacks of offscraped parts of the Indian Plate subducting below Tibet. In contrast, many structural geologists now subscribe to the ‘channel-flow model’ in which a ductile middle crust underlying southern Tibet exhumed towards the surface between the South Tibet Fault and the Main Central Thrust (Fig. 6) (Beaumont et al. 2001), at least between around 23 and 17 Ma (Searle & Szulc 2005; Godin et al. 2006; Hodges 2006). In this model, the gravitational potential of the Tibetan Plateau is released by extrusion towards the south modulated by focused erosion along the southern edge of the plateau. Removal of shallow layers allows deeper buried rocks to come to the surface. Such a model implies a simple linkage between surface processes modulated by monsoon intensity and the location of tectonic structures. This relationship is largely paralleled by observations in the modern mountains, suggesting a correlation between seismic activity and rainfall (Wobus et al. 2003). Compilations of eroded mass flux to the marine basins of the Indian Ocean, which represent the bulk of the erosional flux from the mountains since the start of collision (Fig. 3e), are also broadly in accord with this type of model. Periods of fastest exhumation were also times of rapid erosion and, furthermore, correlate with times of wettest climate, although all of these processes outlast the 17 Ma end of most proposed channel-flow periods (Clift et al. 2008b). In their synthesis of the erosion and weathering data from marine basins, Clift et al. (2008b) proposed that the exhumation of the bulk of the Himalayan amphibolite-facies rocks was largely triggered by a changing climate to more monsoonal, more erosive, wetter conditions starting around the beginning of the Miocene, even if the reason for this change in climate was not understood. Although this observation was consistent with the channelflow model, it is not uniquely tied to this. Many mountain belts have been modelled to behave as a wedge of imbricated rock whose taper is controlled by a variety of factors including dip of the detachment, basal friction and rate of accretion of new material (Davis et al. 1983). Indeed, focused erosion applied to a wedge-type model would also result in focused exhumation and roofing of deep buried rocks along a well-defined belt in the way that it can be observed in the present Himalaya (Konstantinovskaia & Malavieille 2005). Clift et al. (2008b) advanced a model in which exhumation of the bulk of the Himalayan amphibolite-facies rocks was essentially synchronous along strike during the strong monsoon period of the early and middle Miocene. More recently, it is recognized that exhumation and associated structural development is, in fact, diachronous along the mountain front. Webb et al. (2017) demonstrated that the timing of cessation of motion along the South Tibet Fault is oldest near the western (Nanga Parbat) and eastern (Namche Barwe) syntaxes but youngs towards the centre of the mountain belt, especially towards those regions around the east-central Himalaya (c. 90° E). Interestingly, this trend is also mirrored by the age of rapid erosion preserved in the foreland basin within the Siwalik Group. Zircon fission-track and mica Ar–Ar data that capture the time of cooling in the sources show that there is a clear younging towards the centre of the mountain belt, especially when viewed via the richer datasets collected along western side. A wave of rapid cooling migrated from the western Himalayas at 25– 20 Ma eastwards towards the east-central Himalaya, such that rapid exhumation can be inferred across the east-central part of the range after c. 15 Ma (Fig. 7) (see also the detailed discussion in the supplementary material of Webb et al. 2017). Doubts about the applicability of the channelflow model have, moreover, been raised by tectonic synthesis (Yin 2006) that documented how the Tethyan and Lesser Himalaya are in contact with one another in the western Himalaya (Fig. 6). If the flow in the hypothetic channel had truly been driven by focused erosion combined with the gravitational potential of the plateau, then this arrangement should not be observed because the crystalline rocks of the Greater Himalaya should separate these sequences. Yin (2006) postulated that this arrangement occurs because for most of its evolution, the South Tibet Fault could have been active as a backthrust, which branched northwards from the Main Central Thrust and accommodated the emplacement of the Himalayan Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS Fig. 6. Simplified geological map of the Himalaya modified from Webb et al. (2017). This map emphasizes certain elements of the Himalayan tectonostratigraphic framework over others in order to better illustrate evidence associated with slab dynamics interpretations. Namely: (1) the Kailas Formation in southern Tibet is illustrated (in as small a fashion as possible to be just visible, but yet larger than actual size) because this unit is speculated to reflect relative slab rollback in the late Oligocene (by DeCelles et al. (2011)); (2) the fault pattern in the east-central Himalaya emphasizes late normal and out-of-sequence thrust faults which may reflect a wedge extrusion system developed in response to the last phase of proposed slab detachment that occurred in this region; (3) the common division of the ‘Lesser Himalayan strata’ and the ‘Sub-Himalayan strata’ is replaced by a combined unit termed ‘post-slab detachment accretionary belt’ in order to emphasize that these are simply pre- and syn-orogenic strata deformed in a single fold–thrust system, with no particular emphasis assigned to any fault that happens to juxtapose these strata at the surface. Further explication of these details can be found in Webb et al. (2017). Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB Longitude (a) 72˚ 83˚ 76˚ 77˚ 87˚ 92˚ Zircon fission track 0 5 10 15 20 25 30 35 40 45 50 (b) Ar-Ar Muscovite 0 5 10 15 20 25 30 35 40 45 50 Age (Ma) Fig. 7. Compilation of detrital thermochronology results from the Himalayan foreland basin, modified from Webb et al. (2017). Detrital thermochronology involves sampling sedimentary materials and acquiring cooling ages from detrital components in order to constrain the cooling history of the sediment source regions. In (a) and (b) 40Ar/39Ar muscovite and zircon fission-track zircon results are plotted, respectively, for dates younger than 50 Ma. 40Ar/39Ar muscovite ages date the cooling of muscovite crystals below 425 ± 25°C (Harrison et al. 2009), whereas fission-track zircon ages date the cooling of zircon crystals below 240 ± 30°C (Hurford 1986; Bernet & Garver 2005). The data are shown using the kernel density estimation (KDE) methodology, which plots the detrital dates as a set of Gaussian distributions (Vermeesch 2012). This approach allows the age ranges and abundances of different detrital age populations to be compared: peaks in the curves represent peaks in the detrital age populations. For these plots, the population for a single sample is shown as a curve, sample longitude is keyed to a colour spectrum and the depositional age is shown via the squares at the young (left) terminations of the curve. These data provide an approximation of the cooling experienced by adjacent Himalayan regions. Signals in the Himalayan foreland basin can be complicated by river-sediment transport along the range trend, since not all river systems transport sediment perpendicularly away from the mountains and thus might not only represent cooling and exhumation over the limited extent of the range immediately adjacent to the sampling location. A general trend appears in these data: peaks in the cooling age populations appear to young to the east from 25–20 to 10–8 Ma. This is consistent with an eastwards-migrating pulse of hinterland cooling and erosion during this period. Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS crystalline core at depth. The simple triangle-zone geometry for the leading edge of the crystalline rocks which is predicted by this model has been confirmed via structural and metamorphic mapping along the length of the range (Webb et al. 2007, 2011a, b; Leger et al. 2013; He et al. 2015, 2016). The continued burial of the leading edge of the crystalline core in various parts of the Himalaya today provides a strong limit for the contention that the South Tibet Fault exhumed this leading edge in the early Miocene (Yin 2006). Despite ongoing dispute concerning these developments (e.g. Khanal et al. 2015; Soucy La Roche et al. 2016), this evidence suggests that an alternative mechanism was responsible for the development of the crystalline core of the orogen; a proposal supported by the observation that the Greater Himalaya are often imbricated (Carosi et al. 2010; Martin et al. 2010; Corrie & Kohn 2011; Larson et al. 2015; Wang et al. 2016), which is again inconsistent with the idea of a ductile channel bounded by the South Tibet Fault and Main Central Thrust above and below. Nonetheless, if we are to test for the influence of climate and surface processes in controlling Himalayan tectonics, then a detailed record of the exhumation of deep buried metamorphosed units is essential because this is the primary way in which the monsoon can influence the solid Earth. Triggers of monsoon intensification Why would the Asian monsoon intensify in the region of the Himalaya throughout the Early and Middle Miocene? If the intensity of the monsoon is truly modulated by the height of the Tibetan Plateau, then the strengthening of the summer monsoon rainfall might be related to either a rapid uplift of the plateau at this time or a significant growth in its lateral extent. Neither of these explanations appears likely. The plateau was probably high before or during the early stages of the India–Asia collision (Harris 2006; Kapp et al. 2007) and palaeoaltitude reconstructions also support the idea that at least central and southern Tibet were close to modern elevations well before the intensification of the monsoon (Rowley & Currie 2006; Rowley & Garzione 2007; Currie et al. 2016; Ingalls et al. 2017), and >4.6 km in central Tibet at least by the Late Oligocene (DeCelles et al. 2007; Quade et al. 2011; Ding et al. 2017). Studies of the plateau are often hampered by the fact that the age range of the preserved stratigraphy is relatively young, which precludes us from knowing precisely when the modern topography was achieved, only that it was present by a certain time, usually in the late Miocene. There is increasing evidence that even along its eastern flanks significant topography had started to develop during the Paleogene (Wang et al. 2012; Tang et al. 2017) and was not delayed until the Late Miocene (c. 10 Ma), as previously suggested (Clark et al. 2005). Some climate models argue that monsoon intensity would have increased once the plateau had reached half of its present extent (Huber & Goldner 2012) or height (Prell & Kutzbach 1992), and it is conceivable that a plateau that began to expand after the start of collision at about 60–50 Ma could have reached a critical size at around the beginning of the Miocene. Such a proposal, while plausible, is, however, rather difficult to demonstrate. Because the intensity of the Asian monsoon is critically dependent on the generation of a lowpressure system over the continent during the summer, the extent of shallow seas in Central Asia has been invoked as being a key control. Ramstein et al. (1997) have linked the progressive retreat of the shallow Paratethys from the area of the modern Tarim Basin towards the Caspian Sea to the intensification of the summer monsoon. The removal of water from this area allows the land surface to heat up to a much greater extent during the summertime, resulting in strengthened monsoon circulation. New age controls concerning the retreat of the sea stress that this is a progressive process starting in the late Eocene (Bosboom et al. 2011), but not one that can easily be linked to a rapid intensification of the type seen from around the beginning of the Miocene. That is not to say that the Paratethys retreat is not important, only that it is unlikely to be the primary trigger to monsoon intensification. An alternative suggestion was made by Armstrong & Allen (2011), who instead favoured the idea that it was the northwards drift of India that was instrumental in pushing the Himalaya into a band of high rainfall, essentially the boreal summer extent of the summer monsoon, as represented by the Inter Tropical Convergence Zone (ITCZ: Fig. 1). In this model the rainfall was always strong along the ITCZ, so that the intensification of rainfall on the mountains was mostly related to their movement relative to a more stable climatic belt. Even so, the ITCZ would have been affected by climatic changes within Central Asia and, in particular, the intensification of atmospheric low-pressure systems during the boreal summer, which would have drawn the ITCZ further north into the continental interior. Recent climate models now suggest that the latitude of the Tibetan Plateau, as much as its elevation, was critical to the strength of the East Asian monsoon (Zhang et al. 2018). The progressive drift from 11° to 25° N in the Eocene to 26°–40° N today is predicted to be a major influence on the intensifying rainfall in eastern Asia. The concept that the height of the Himalayan barrier was particularly important in ponding hot air south of the Himalaya was a major advance in our Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB understanding of this part of the wider Asian monsoon system (Boos & Kuang 2010). In contrast, climate modelling indicates that the presence of the central and northern Tibetan Plateau and the Tian Shan increases summer precipitation in East Asia, but drives an anticyclonic wind anomaly that suppresses summer precipitation in northern India (Tang et al. 2013). This increases the chances for the East and the South Asian monsoons to evolve independently. There is, however, little evidence concerning the evolution of altitude in the Himalaya, even less so than that for the Tibetan Plateau. One of the few lines of evidence we have concerns the area around Mount Everest where high altitude is inferred from fluid-infiltration studies that imply a similar altitude to the range in this part of the Himalaya since c. 17–15 Ma, when the South Tibet Fault was still active, but provide no constraints about earlier times (Gébelin et al. 2013, 2017). There are, however, some indications that the Himalaya may have become progressively higher, starting around the end of the Oligocene and culminating around c. 15–12 Ma. This relates to the phenomenon of slab break-off, which was proposed by DeCelles et al. (2002) on the basis of tomographical seismic images that show the presence of a detached lithospheric slab below India (Van der Voo et al. 1999; Replumaz et al. 2010). The slab appears increasingly deep from 90° E westwards to 75° E (Replumaz et al. 2010; Capitanio & Replumaz 2013), a pattern that Replumaz et al. (2010) interpreted to indicate that the slab detached from the orogen progressively, starting in the west at c. 25 Ma and finishing in the east at c. 10 Ma, with continued northwards motion of the orogen relative to the sinking slab leading to the current configuration. If this subducted portion of the Indian Plate did break off, then the removal of this significant gravitational weight would have led to a dynamic rebound of the Himalayas (Husson et al. 2014). The gravitational weight is largely in the form of the mantle lithosphere not the continental crust itself; yet, analogue modelling now demonstrates that continental lithosphere is capable of subduction even when it is not attached to an oceanic slab as must have been the case prior to the break-off (Replumaz et al. 2016). Replumaz et al.’s (2010) estimates for the age of slab break-off are based on geometrical reconstruction of the kinematics of the detached slab and overriding orogen – a direct approach dependent on the quality of the seismic tomographical imagery. In addition, a host of predicted tectonic consequences provide multiple opportunities to indirectly constrain the timing of slab detachment. For example, it has been proposed that the occurrence of alkaline magmatism across southern Tibet might be linked to a slab break-off event (Pan et al. 2012; Zhang et al. 2014; Guo et al. 2015). Recent compilations noted that this magmatism is oldest closest to the syntaxes at c. 30–25 Ma and youngs towards the east-central part of the mountain belt at c. 15–8 Ma, and therefore might indicate the propagation of a slab tearing from the two ends of the Himalaya towards the (eastern) middle of the range (Zhang et al. 2014; Leary et al. 2016). The timing of sedimentation of the Kailas Formation and along-strike equivalents in the Indus-Yarlung Suture Zone itself (Fig. 6) is also consistent with an eastwards propagation of slab rollback and break-off (DeCelles et al. 2011, 2018; Leary et al. 2016). In this model slab rollback would drive extension and basin formation, although it should be noted that there is no accord about the tectonic origin of the suture zone basins, with some considering them to be flexural foreland basins on the north side of the proto-Himalaya (Aitchison et al. 2002; Wang et al. 2015). If the aforementioned along-strike age trends in (a) the rapid cooling preserved in the sedimentary record and (b) the South Tibet Fault motion cessation record changes in Himalayan orogenic wedge dynamics in response to slab detachment, then these likewise serve as proxies for the timing of slab detachment. The cessation of the South Tibet Fault motion is the most tightly constrained aspect, from c. 26– 24 Ma at the ends of the range to c. 13–11 Ma in the 90° E vicinity of western Bhutan (Webb et al. 2017). The coincidence of these activities is supportive of a linkage between deep lithospheric processes and the development of the mountain chain and shallow crustal levels. Furthermore, Webb et al. (2017) pointed out the fact that imbrication of deeply buried crystalline sheets continues for much longer in the east-central part of the mountain chain, which would have been the last place to have been affected by slab break-off. Shallowing of the subducting slab is a natural consequence of break-off and might be expected to have terminated the deep tectonic imbrication, explaining why this is more advanced in the east-central Himalaya and less well developed towards the syntaxes. Correspondingly, earlier transitions to frontal accretion/relatively shallow duplexing are predicted for the western Himalaya and far-eastern Himalaya v. the east-central Himalaya. Finally, there is ample evidence for there being a major unconformity in the foreland basin, which might be another consequence of the break-off event, although this is not well constrained partly as a result of the terrestrial character of the sediment infilling the foreland basin that bracket this as Oligocene and pre-dating c. 20 Ma (Najman 2006). Nonetheless, the distinction between the c. 2.5 km-thick Lower Miocene Dharamsala Group sediments of the western Himalaya v. the relatively thin (c. 1 km), sparse sedimentary layers of this age in the east-central Himalaya (e.g. cf. DeCelles et al. Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS 1998 v. Powers et al. 1998) may point to along-strike sedimentation differences that could correspond to the proposed propagation of slab detachment. The critical factor in relating this break-off model to monsoon intensity is the idea that the Himalaya themselves might have increased in altitude by about 1 km in response to slab detachment, as indicated by dynamic deflection calculations (Husson et al. 2014). Because climate models indicate a roughly linear relationship between the intensity of summer monsoon rainfall and the height of the Himalayan barrier (Ma et al. 2014), the break-off and resultant rapid rise of the Himalaya might be anticipated to have strengthened the South Asian monsoon. The proposed slab detachment timing aligns with the timing of the monsoon intensification from c. 24 to c. 15 Ma, and thus could potentially explain why the monsoon increased in strength so much after the start of the Miocene. Of course, this may only apply to the South Asian monsoon system, with the East Asian monsoon being more critically dependent on the elevation and extent of the northern Tibetan Plateau and Tian Shan (Tada et al. 2016). Nonetheless, the modelling of Boos & Kuang (2010) certainly shows the effects of Himalayan topography on some portions of the East Asian monsoon, especially the region of the Pearl River Basin that is the source of key monsoon records in the South China Sea. Furthermore, the onset of desertification in the Tarim Basin is consistent with the idea that the northern Tibetan Plateau, as well as the Tian Shan, had begun to uplift by around 20–25 Ma (Zheng et al. 2015), which in turn correlates with the intensification of the East Asian monsoon around the same time, at least according to the geochemical records in the South China Sea (Clift et al. 2014). The South Asian monsoon can be seen more directly as being the product of solid Earth lithospheric processes, which then impact the climate system and result in feedback in terms of intensified erosion along the southern flank of the Himalaya. This erosion would help to translate the rapid uplift in response to slab detachment into exhumation of deep-seated rocks. Although the concentration of rainfall is largely a function of topography, which is in turn linked to the structure of the fold belt with ramps on thrusts generating topographical ridges, the strength of the monsoon is also a significant control on erosion and exhumation. Studies on millennial timescales have shown how erosion patterns are linked to monsoon intensity (Clift et al. 2008a). Over tectonic timescales such a linkage would result in long-term exhumation being partially controlled by the monsoon climate. Furthermore, the broad-scale correlation of regional humidity, erosion intensity and exhumation rates of the sources does imply that there are significant feedbacks from the climate to the mountain belt. The timing of the tectonic and climatic events appears to be broadly consistent with such a model. The weakening of the South Asian monsoon after c. 15–10 Ma, and especially towards the Pliocene based on the marine records for chemical weathering and erosion (Figs 2 & 3), might reflect diminishment of Himalayan topography, which might not be as powerfully raised by erosion–orogenic wedge feedbacks as it was by uplift in response to slab detachment. Despite earlier suggestions that east–west extension might have caused a lowering of the plateau, there is not much evidence for this process having a big impact on the average altitude on a regional scale. In SW Tibet, oxygen isotope data indicate a reduction in altitude of as much as 1.5 km since the end of the Miocene (Saylor et al. 2009), consistent with clumped isotope analyses of the same area that indicates at least 1 km of altitude loss (Huntington et al. 2015). At the same time monsoon rains weakened in that area (Saylor et al. 2016), but whether this is more widespread or even linked to the changing altitude is unclear the present time. In any case, the start of extension and potential loss of altitude dates back to c. 18 Ma (Coleman & Hodges 1995; Williams et al. 2001), somewhat before the drying of the Asian climate after c. 8 Ma, even if the process may have accelerated in the Late Miocene (Harrison et al. 1992). Evidence from sedimentary basins in southern Tibet is supportive of the idea of accelerating extension in the Late Miocene because of the widespread preservation of sedimentary rocks in east–west extensional basins starting at that time: after 9.2 Ma in the Zhada Basin (Saylor et al. 2009, 2010); after 8.7 Ma in the Pulan Basin (Murphy et al. 2002); after 11 Ma in the Thakkola Basin; and 11 Ma (Garzione et al. 2000, 2003) and after 10.8 Ma in the Gyirong Basin (Xu et al. 2012). The effect of a lower Himalayan barrier would work in conjunction with the weakening anticipated as a result of global cooling after the MMCO. Separating the two influences is not easy. The weakening of the monsoon after the MMCO (Clift et al. 2008b) starts earlier than the rapid extension and proposed fall in Tibetan and Himalayan elevation after 11 Ma, and in turn this faster extension is somewhat older than the drying of the climate seen after 8 Ma too. Nonetheless, the global compilation of Zachos et al. (2001) indicates a faster fall in global temperatures after 6 Ma, when the West Antarctic Ice Sheet became permanent, a little younger than the drying of the Indian climate inferred from the Himalayan foreland basin (Quade et al. 1989). Regional plant and mammal fossil records point to overall cooler conditions in southern China and northern India in the Late Miocene, which is likely to be linked to a strong winter monsoon (Quan et al. 2012; Zhang et al. 2012). Wind strength is believed to have increased regionally and probably Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 P. D. CLIFT & A. A. G. WEBB Fig. 8. Schematic three-dimensional diagram showing the lateral propagation of slab detachment from both west and east across the Himalayan system from Webb et al. (2017). Shaded red colours represent the upper surface of the descending Indian Plate. Slab detachment affects topographical evolution by releasing the vertical traction excited by the subducting slab, thereby releasing the dynamic deflection. This results in a wave of uplift from the edges towards the centre of the chain. Reproduced with permission of the Geological Society of America. globally at that time (Peterson et al. 1992; Gupta et al. 2015; Tang et al. 2015; Betzler et al. 2016), with the strong winter monsoon driving down temperatures across Eurasia. During the Pleistocene, glacial conditions largely correlate with a weak summer monsoon because cold winters in Eurasia reduce solar insolation of this continent (Cheng et al. 2006; Liu et al. 2006; Rohling et al. 2009). A cooling Late Miocene Eurasia would be expected to contribute to a weaker summer monsoon, along with any tectonically induced reduction in the average Tibetan/Himalayan elevation. Conclusions Early attempts in the early 1990s to link the tectonic evolution of the India–Asia collision and the intensification of the monsoon focused on rapid uplift of the Tibetan Plateau at around 8 Ma, around the same time that the Asian monsoon was postulated to have intensified. Since that time, significant amounts of data now provide a more detailed picture. The monsoon is much older than originally believed, initiating in the Eocene and likely to have strengthened substantially from the start of the Miocene into the Middle Miocene. It is now clear that proxies for oceanic upwelling that largely track the intensity of winds do not necessarily correlate with proxies for continental humidity. Rates of chemical weathering and continental erosion increased significantly during the Early and Middle Miocene in South Asia, probably as a result of strengthening of the rainfall against the rising Himalayan mountain front. Subsequently, the climate dried from the Middle Miocene at least until the Pliocene, so that the ‘8 Ma transition’ previously highlighted is essentially one of drying rather than strengthening monsoon rainfall. This is consistent with the palaeovegetation data. which indicate a large-scale switch from tree-dominated C3 flora to C4 grassland flora. Understanding that the South Asian monsoon is largely the product of the barrier effect produced by the Himalaya provides a new explanation for why the summer rainfall would have strengthened so much throughout the Early and Middle Miocene. There are no clear data that demonstrate that the Tibetan Plateau either got much higher or wider at this particular time. There is, however, abundant geological evidence that the plateau was already high, at least in its southern parts, soon after the start of collision if not before. In contrast, Early Miocene uplift of the northern plateau and the Tian Shan would have been an appropriate trigger not only for Downloaded from http://sp.lyellcollection.org/ by guest on July 19, 2018 MONSOON INTERACTIONS WITH SOLID EARTH TECTONICS the desertification of the Tarim Basin, but also the strengthening of the East Asian monsoon. Tomographical images of a detached lithospheric slab, together with a younging of alkaline magmatism, duration of Greater Himalayan tectonic imbrication and the cessation of motion on the South Tibet Fault, now suggest that a progressive tearing of the subducting slab (Fig. 8), which started near the Himalayan syntaxes prior to c. 25 Ma, had migrated into the central part of the mountain belt by c. 15 Ma. The loss of the gravitational weight of the lithospheric slab from the subducting Indian Plate not only explains the change in the tectonics across the Himalaya but also provides a simple explanation for the raising of the Himalayan barrier and thus the strengthening of the monsoon during the Early Miocene. In this scenario, solid Earth tectonic forces are responsible for the strengthening of the summer monsoon rains in South Asia after c. 24 Ma, while it is the resultant stronger erosion that explains the rapid cooling and unroofing of the high-grade metamorphic and igneous units that followed. The Himalaya–monsoon climate–tectonic system remains the world-type example of such interactions. Future work on these issues must now focus on the transition at the start of the monsoon intensification and especially in the Oligocene, the sedimentary record of which is missing from onshore but may now be at least partly addressed with new oceanic drilling records. Coupled with this improved erosion budget for the Indian Ocean, submarine fans may test whether erosion in the mountains truly correlates with cooling and exposure of deep-buried, highgrade metamorphic rocks, which is critical to these hypotheses. P. Clift thanks the Charles T. 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