Abstract The Andes, the world's largest non-collisional orogen, is considered the paradigm for geodynamic processes associated with the subduction of an oceanic plate below a continental plate margin. In the framework of UNESCO-sponsored IGCP 586-Y project, this Special Publication includes state-of-the-art reviews and original articles from a range of Earth Science disciplines that investigate the complex interactions of tectonics and surface processes in the subduction-related orogen of the Andes of central Chile and Argentina ( c. 27–39°S). This introduction provides the geological context of the transition from flat slab to normal subduction angles, where this volume is focused, along with a brief description of the individual contributions ranging from internal geodynamics and tectonics, Quaternary tectonics and related geohazards, to landscape evolution of this particular segment of the Andes.

Abstract Apatite (U–Th)/He thermochronology from palaeosurface-bounded vertical transects collected in deeply incised river valleys with >2 km of relief, as well as geomorphic analysis, are used to examine the timing of uplift of the Frontal Cordillera and its relation to the evolution of the proximal portions of the Andean foreland between 32° and 34°S latitude. The results of apatite (U–Th)/He (AHe) analyses are complex. However, the data show positive age-elevation trends, with higher elevation samples yielding older AHe ages than samples at lower elevation. Slope breaks occur at c. 25 Ma in both profiles, separating very slow cooling and or residence within a partial retention zone (slope of c. 10 m/Myr) at the highest elevations from a slope of c. 60–100 m/Myr cooling rate at lower elevations. The older AHe ages suggest either (1) minimal burial of the Frontal Cordillera and/or (2) significant pre–middle Miocene local relief. Geomorphic analysis of the adjacent, east-draining Río Mendoza and Río Tunuyán catchments reveals a glacial imprint to the landscape at elevations above 3000 m, including greater channel steepness and lower profile concavities developed during glacial erosion. Detailed analysis of headwall heights provides evidence of ongoing rock uplift along the entire eastern flank of the Frontal Cordillera and in the eastern flank of the Principal Cordillera south of the slab dip transition.

Published paleomagnetic data from well-dated sedimentary rocks and lavas from the Lhasa terrane have been reevaluated in a statistically consistent framework to assess the latitude history of southern Tibet from ca. 110 Ma to the present. The resulting apparent polar wander path shows that the margin of the Lhasa terrane has remained at lat ~20° ± 4°N from ca. 110 to at least 50 Ma and has drifted northward to its present latitude of 29°N since the early Eocene. This latitude history provides a paleomagnetically determined collision age between the Tibetan Himalaya and the southern margin of Asia that is ca. 49.5 ± 4.5 Ma, if not a few millions of years earlier after considering reasonable estimates for shortening within the suture zone. This collision occurred at lat ~21° ± 4°N, or perhaps ~2° lower if an average-size forearc is considered. These paleomagnetic data indicate that at most, only 1100 ± 560 km of post–50 Ma India-Asia convergence was partitioned into Asian lithosphere. The lower bound of these paleomagnetic estimates is consistent with the magnitude of upper crustal shortening and thickening within Asia calculated from structural geologic studies. Thus, a substantial amount of the shortening within, and therefore surface uplift of, the Tibetan Plateau predates the Tibetan Himalaya–Lhasa collision. These conclusions suggest that the Tibetan Plateau is similar to the Altiplano of the Andes, in that most of the plateau developed at subtropical latitudes above an oceanic sub-duction zone in the absence of a continent-continent collision. A direct implication of these findings is that 1700 ± 560 km or more post–50 Ma India-Asia convergence was partitioned into the lower plate of the orogenic system (i.e., units of Indian affinity). Recent paleomagnetic and plate tectonic analyses suggested significant extension of Greater India lithosphere after breakup from Gondwana but prior to collision with the southern margin of Asia. Cretaceous extension within Greater India was inferred to open an oceanic Greater India Basin, which would have maintained a deep tropical water mass along the southern edge of greater Asia throughout most of the Paleogene. We suggest ways in which future climate models can incorporate this paleogeography to more accurately explore how Paleogene atmospheric processes interact with or are modified by the juxtaposition of a tropical ocean basin and the high uniform topography of the Tibetan Plateau.

In the easternmost Himalaya and southeastern Tibet, the Namche Barwa–Gyala Peri massif and adjacent Lhasa block host some of the Earth’s most active geologic processes and extreme topography. Synthesis of U-Th/He and Ar-Ar thermochronology, anatectic history, seismicity, and structural geology shows the important role that surface processes have played in this region in both local and orogen-scale crustal dynamics. Basement rocks of the massif underwent an episode of metamorphism, partial melting, and focused deformation that began ca. 10 Ma and likely remains active due to thermally mediated feedbacks between these processes and erosion. Strong differential rock uplift at Namche Barwa established the immense Namche Barwa knickzone on the Yarlung Tsangpo River, which has been stabilized through coupling between erosion driven by high stream power and localized deformation. This knickzone has maintained a high secondary base level of ~3000 m for the upper Yarlung Tsangpo watershed and so has shielded a large region of southeastern Tibet from excavation by the river, which in turn could alter the morphology and so the dynamics of the eastern Himalayan orogenic wedge. The landscape evolution of the southeast Lhasa block involved slow regional unroofing or incision in the Neogene, a significant pulse of ~5 km of rapid exhumation from ca. 10 to 5 Ma, and since then a great reduction in exhumation started once the Namche Barwa knickzone on the Yarlung Tsangpo was established. The low-relief high-elevation surface in the area is a relatively young feature, developed after the rapid 10–5 Ma exhumation pulse.

The Lhasa and Qiangtang terranes of Tibet collided following Late Jurassic–Early Cretaceous consumption of oceanic lithosphere along the intervening Bangong suture zone. This continental collision led to the development of the south-directed, northern Lhasa thrust belt that is exposed ~1200 km along strike in central Tibet. We conducted geologic mapping and stratigraphic and geothermochronologic studies in the Duba region of the northern Lhasa terrane, located ~250 km northwest of the city of Lhasa. In the Duba region, granites were emplaced into the mid-crust between 139 and 121 Ma and subsequently exhumed and juxtaposed against Cretaceous strata between 105 and 90 Ma in the footwall of an interpreted passive roof thrust system. We suggest that this structural style dominates the Cretaceous–early Cenozoic evolution of the northern Lhasa thrust belt and provides an explanation for the scarcity of basement rock exposures in the Lhasa terrane despite >50% upper crustal shortening. Furthermore, we highlight similarities between the collision-related northern Lhasa and Tethyan Himalayan thrust belts, both of which are bound by sutures and associated with underthrusting of lower plate lithosphere.

The Triassic Qiangtang metamorphic belt in central Tibet consists of eclogite- and blueschist-bearing tectonic mélange exposed in an intracontinental setting. Study of the belt has yielded insight about the tectonic history and crustal architecture of the Qiangtang terrane. Weakly deformed mafic blocks exhibiting high-pressure, low-temperature metamorphic mineral assemblages are exposed within a greenschist facies sedimentary-matrix mélange over an east-west distance of ~600 km and north-south distance of ~150 km. Everywhere it has been mapped, the Qiangtang mélange is exposed in the footwalls of Late Triassic–Early Jurassic domal, low-angle normal faults. The hanging walls of these normal faults are composed of late Paleozoic to early Mesozoic metasedimentary and crystalline rocks; similar lithologies are exposed in footwall mélange rocks. High-pressure metamorphism occurred during Middle Triassic time. Regional geological mapping and provenance of detrital zircon crystals from late Paleozoic to Mesozoic supracrustal rocks of the Qiangtang terrane indicate that Qiangtang crust exposed on the north and south sides of the metamorphic belt are of Gondwanan affinity. This is at odds with the widely held belief that the Qiangtang metamorphic belt marks a suture between a Gondwanan affinity southern and/ or western Qiangtang terrane and a Cathayasian affinity northern and/or eastern Qiangtang terrane. Furthermore, metasedimentary rocks exposed within the metamorphic belt yield detrital zircon age probability distributions consistent with derivation from Qiangtang terrane supracrustal strata and Paleo-Tethys affinity rocks exposed north of the Qiangtang terrane. These data, along with the significant north-south surface exposure of the metamorphic belt, suggest that a large part of the lower to middle crust is composed of silica-rich metasedimentary-dominated mélange rock that formed during Middle Triassic southward subduction along the Jinsha suture on the northern edge of the Qiangtang terrane. The implied crustal structure, which is also defined by geophysical experiments, provides an explanation for the apparent deviation from Airy isostasy from the Lhasa terrane to the Qiangtang terrane. We suggest that the replacement or partial incorporation of mafic crystalline middle to lower crust with less dense metasedimentary mélange rock led to a significant contribution of Pratt isostasy across central Tibet.

Studies of the pre-Cenozoic geologic history of the Tibetan Plateau provide important constraints on the timing and spatial variability of crustal thickening and resulting topographic uplift. Here we present new 1:100,000-scale structural mapping and U-Pb detrital zircon analyses from the Domar fold-thrust belt in the western Qiangtang terrane to constrain the history of crustal thickening in this portion of the Tibetan Plateau. We find that (1) Paleozoic strata of the Domar area were shortened prior to deposition of Permian units; (2) the youngest strata in the area are Late Jurassic to Early Cretaceous in age, rather than middle Cretaceous or Cenozoic, as previously interpreted; and (3) the youngest strata record tectonism synchronous with south-directed thrusting in the Domar fold-thrust belt, with no evidence of significant shortening during the Cenozoic India-Asia collision. Together, our results suggest that the majority of the shortening of this region occurred during the middle Mesozoic. In particular, Late Jurassic to Early Cretaceous formation of the Domar fold-thrust belt likely resulted from underthrusting of the northern Lhasa terrane beneath the southern margin of the Qiangtang terrane during the middle Mesozoic Lhasa-Qiangtang collision along the Bangong-Nujiang suture. These findings add to a growing body of geologic evidence indicating that the Tibetan Plateau had already undergone significant shortening, crustal thickening, and likely rock uplift prior to the India-Asia collision.

Northeastern Tibetan Plateau growth illuminates the kinematics, geodynamics, and climatic consequences of large-scale orogenesis, yet only recently have data become available to outline the spatiotemporal pattern and rates of this growth. I review the tectonic history of range growth across the plateau margin north of the Kunlun fault (35°–40°N) and east of the Qaidam basin (98°–107°E), synthesizing records from fault-bounded mountain ranges and adjacent sedimentary basins. Deformation began in Eocene time shortly after India-Asia collision, but the northeastern orogen boundary has largely remained stationary since this time. Widespread middle Miocene–Holocene range growth is portrayed by accelerated deformation, uplift, erosion, and deposition across northeastern Tibet. The extent of deformation, however, only expanded ~150 km outward to the north and east and ~150 km laterally to the west. A middle Miocene reorganization of deformation characterized by shortening at various orientations heralds the onset of the modern kinematic regime where shortening is coupled to strike slip. This regime is responsible for the majority of Cenozoic crustal shortening and thickening and the development of the northeastern Tibetan Plateau.

Spatiotemporal patterns of Cenozoic deformation along the margins of the Tibetan Plateau can provide key evidence with which to investigate the mechanisms of continental deformation and plateau growth as well as their impact on regional climate. Along the northeastern margin of the Tibetan Plateau, Cenozoic deformation and regional aridification have been attributed to the upward and outward growth of the plateau. Analysis of stratigraphic and stable isotopic data shows that, in early to middle Miocene time, intracontinental mountain ranges subdivided a broad foreland basin, which developed on the northern margin of the Tibetan Plateau shortly after collision between India and Eurasia, into smaller intramontane basins. Stratigraphic and stable isotope data collected from a number of subbasins along the northeastern Tibetan Plateau, spanning as much as 30 m.y. in age and ranging to 3 km in thickness, reveal a pattern of deformation and basin isolation that began ca. 22 Ma with the initial unroofing of the eastern Laji Shan near the town of Minhe and partially separated the Xining basin from the Hualong, Linxia, and Xunhua basins to the south. Westward paleoflow indicators on the eastern margin of the Guide basin indicate that the Zamazari Shan had attained topographic relief by ca. 20 Ma and separated the Guide basin from the Jian Zha, Hualong, and Xunhua basins to the east. Deformation of the Laji Shan–Jishi Shan progressed to the south, deforming the Jishi Shan ca. 13 Ma and separating the Linxia Basin from the Hualong and Xunhua basins to the west. Final separation between the Jian Zha and Xining basins occurred at 10–8 Ma with the growth of the western Laji Shan and Riyue Shan. Unique stable isotope records reflect the different hydrologic and tectonic settings of each basin and highlight the importance of local climate conditions in each basin. However, ca. 14 Ma all basins underwent a synchronous change in climate toward more arid conditions, as indicated by a gradual to abrupt positive shift in δ 18 O values. This climate event corresponds with aridification events to the west near the Qaidam basin and may be related to the reorganization of vapor transport pathways around a growing eastern Tibetan Plateau.