Abstract Scaled analogue experiments with layered brittle and ductile materials have been used to simulate the development of listric growth-fault and expulsion rollover systems during gravitational spreading of a passive margin sedimentary wedge detached on salt. The experiments were performed with varying sedimentation patterns and rates to simulate different depositional scenarios. Deformation monitoring with 3D optical image correlation techniques was used to quantify the 3D surface evolution and strain history of model structures. Our results indicate that rollover structure kinematics is strongly coupled to sedimentation patterns and rates. Whereas differential loading governs the margin-scale state of stress and extensional spreading in the experiments, more localized feedback between the dynamic depositional systems, fault-controlled subsidence, and salt mobilization control the strain history of local fault structures. This is reflected in the characteristic succession of extensional structures that evolve from symmetrical grabens through early, mature and late (collapsed) basinward listric growth-fault and rollover systems into landward listric growth-fault and rollover systems. A lack of sedimentation enhances reactive diapir rise and passive diapirism, whereas low sedimentation rates favour development of long-lived basinward listric growth-fault or expulsion rollover systems. Conversely, high sedimentation rates lead to the development of landward listric growth-fault and rollover systems.

Abstract The channel flow model aims to explain features common to metamorphic hinterlands of some collisional orogens, notably along the Himalaya–Tibet system. Channel flow describes a protracted flow of a weak, viscous crustal layer between relatively rigid yet deformable bounding crustal slabs. Once a critical low viscosity is attained (due to partial melting), the weak layer flows laterally due to a horizontal gradient in lithostatic pressure. In the Himalaya–Tibet system, this lithostatic pressure gradient is created by the high crustal thicknesses beneath the Tibetan Plateau and ‘normal’ crustal thickness in the foreland. Focused denudation can result in exhumation of the channel material within a narrow, nearly symmetric zone. If channel flow is operating at the same time as focused denudation, this can result in extrusion of the mid-crust between an upper normal-sense boundary and a lower thrust-sense boundary. The bounding shear zones of the extruding channel may have opposite shear sense; the sole shear zone is always a thrust, while the roof shear zone may display normal or thrust sense, depending on the relative velocity between the upper crust and the underlying extruding material. This introductory chapter addresses the historical, theoretical, geological and modelling aspects of channel flow, emphasizing its applicability to the Himalaya–Tibet orogen. Critical tests for channel flow in the Himalaya, and possible applications to other orogenic belts, are also presented.

Abstract The principle of channel flow as defined in fluid dynamics has been used in continental geodynamics since the 1980s. The basic equations for one-dimensional flow introduced to geologists by Turcotte and Schubert were further developed by several research groups to meet the needs of specific studies. The most substantive differences among numerical models are results of different solutions for flow in crust, developed for different boundary conditions. The concept of channel flow has met with strong opposition and criticism from geophysicists and modellers. Although it is difficult to prove unambiguously that there is an active weak channel, it is still the most successful model to explain and predict the tectonics, metamorphism and exhumation of high-grade terranes in some orogens. Moreover, the concept of channel flow has stimulated novel approaches to the study of both the tectonics and metamorphism of large, hot orogens and the interaction between tectonic and surface processes.

Abstract Numerical models for channel flow in the Himalayan–Tibetan system are compatible with many tectonic and metamorphic features of the orogen. Here we compare the provenance of crustal material in two channel flow models (HTI and HT111) with observations from the Himalaya and southern Tibet. Thirty million years after the onset of channel flow, the entire model crust south of the India–Asia suture still consists only of ‘Indian’ material. The model Greater Himalayan Sequence (‘GHS’) is derived from Indian middle crust originating < 1000 km south of the initial position of the suture, whereas the Lesser Himalayan Sequence (‘LHS’) is derived mainly from crust originating > 1400 km south of the suture. Material tracking indicates little or no mixing of diverse crustal elements in the exhumed region of the model ‘GHS’, which is derived from originally contiguous materials that are transported together in the top of the channel flow zone. These results are compatible with provenance data indicating a clear distinction between GHS and LHS protoliths, with the GHS originating from a more distal position (relative to cratonic India) than the LHS. In model HT111, domes formed between the suture and the orogenic front are cored by ‘Indian’ middle crust similar to the ‘GHS’, consistent with data from the north Himalayan gneiss domes. Material tracking shows that plutons generated south of the suture should have ‘Indian’ crustal signatures, also compatible with observations. Model ‘GHS’ pressure–temperature–time (P-T-t) paths pass through the dehydration melting field between 30 and 15 Ma, consistent with observed leucogranite ages. Finally, exposure of mid-crustal ‘GHS’ and ‘LHS’ material at the model erosion front is consistent with the observed appearance of sedimentary detritus in the Lesser Himalaya. We conclude that channel flow model results are compatible with provenance data from the Himalaya and southern Tibet.

Abstract We summarize our results from Bhutan and interpret the Greater Himalaya Sequence (GHS) of Bhutan, together with a portion of the underlying Lesser Himalaya Sequence, in the context of recently published channel flow models. For the GHS rocks now exposed in Bhutan the depth for beginning of muscovite dehydration melting (approximately 750°C at 11 kbar) and associated weakening of these rocks is constrained by geobarometry to be at about 35–45 km. The location of initial melting was down-dip and over 200 km to the north of Bhutan. Melt was produced and injected into ductilely deforming metamorphic rocks as they were extruded towards the south between the Main Central Thrust (MCT) and the South Tibetan Detachment zones. The lateral flow of low viscosity rocks at these depths occurred under southern Tibet between 22 Ma and 16 Ma. Subsequently, the channel rocks decompressed from 11 to 5 kbar (from 35 km to a depth of 15 km), but maintained high temperatures, between about 16 Ma and 13 Ma. The data from Bhutan are consistent with channel flow models if there were several pulses of channel flow. The first, between 22 and 16 Ma, produced the rock seen in the lower half of the GHS of Bhutan. A second pulse, which is cryptic, is inferred to have led to the uplift and exhumation of the MCT zone. A third, in central Bhutan, is exposed now as the hanging wall of the Kakhtang thrust, an out-of-sequence thrust that was active at 12–10 Ma. The latter two pulses likely broke around a plug at the head of the first pulse that was formed where the melt in the channel had solidified.