The Cenozoic evolution of the Himalaya-Tibet Plateau, dictated by the India-Asia convergence, remains a subject of substantial ambiguity. Here, a thermo-mechanical model is used to show the critical controls of decelerating convergence on the formation and stabilization of distinctive tectonic structures during prolonged collision. At high constant convergence rates, similar to the late Paleogene India-Asia motions, the lower plate crust is injected beneath the overriding crust, uplifting a plateau, first, then is exhumed towards the orogeny front. Conversely, low constant convergence rates, similar to the Neogene India-Asia motions, induce crustal thickening and plateau formation without underplating or exhumation of incoming crust. Strikingly, models simulating the decelerating India-Asia convergence history portray a dynamic evolution, highlighting the transitory nature of features under decreasing convergence, as the orogen shifts to a new equilibrium. In the transitional phase, the slowing of convergence decreases basal shearing and compression, leading to extension and heating in the orogen interiors. This allows diapiric ascent of buried crust and plateau collapse, as accretion migrates to a frontal fold-and-thrust belt. The models provide insights into the multi-stage evolution of the long-lived Himalayan-Tibetan orogeny, from fast early growth of the Tibetan Plateau, through its transient destabilisation and late-stage internal extension, behind the expanding Himalayan belt.

Exposed sections of accretionary orogens allow reconstruction of their tectonic evolution. Most commonly, orogens are characterised by two-dimensional shortening perpendicular to the orogenic front. We describe the mid-crustal section of the back-arc of the early Paleozoic Famatinian accretionary orogen, exposed in the Sierra de Quilmes. Here crustal deformation evolved from a typical two-dimensional shortening with tectonic transport towards the west, to a non-coaxial constrictional strain with a southward tectonic transport parallel to the orogen. During the early phase of deformation, HT-LP metamorphic complexes were juxtaposed by west-directed thrusting on remarkably thick shear zones forming a thrust duplex. Deformation of the buried footwall complex continued after the exhumed hanging wall ceased to deform. We suggest that the thermally-weakened footwall complex responded by initiating a phase of south-verging thrusting, parallel to the orogen, associated with strong constriction, associated with L-tectonites, and sheath folds. This late phase of deformation defines a non-coaxial constrictional regime characterized by simultaneous east-west and vertical shortening and strong north-south, orogen-parallel stretching. Titanite ages and Zr-in-titanite thermometry demonstrate that this back-arc remained above 700 °C for 120 Ma between 500 and 380 Ma. Combined with regional geology, the new data suggest that west-verging thrusting interrupted an early, back-arc extensional phase, and lasted from ~ 470 to 440 Ma, and that footwall constriction and south-verging thrusting continued for another 40 to 60 Ma. The Famatinian back-arc exposed in Sierra de Quilmes thus is an example of how shortening and orogenic growth in a hot orogen was counterbalanced by lateral flow.

It is increasingly recognised that most sheet-like igneous intrusions such as sills and dykes have segmented, rather than planar margins. The geometry of these segments and their connectors can provide insights into magma propagation pathways and host-rock deformation mechanisms during their emplacement. Here we report the results of scaled laboratory experiments on the emplacement of shallow-crustal, saucer-shaped sills with a focus on their propagation and segmentation. Visco-elasto-plastic Laponite RD® (LRD) and Newtonian paraffin oil were used as analogues for layered upper crust rocks and magma, respectively. Our results indicate that: 1) experimental saucer-shaped intrusions are highly segmented with marginal lobes and fingers; 2) the evolution and geometry of marginal segments and their connectors are different within the horizontal inner sill and the inclined outer sill; and 3) the bimodal nature of segment aspect ratios is linked to propagation of the inner sill along a horizontal host-rock interface versus interaction of the inclined outer sill with a homogenous upper layer. Measurements of inlet magma pressure and structural analysis suggest that marginal finger and lobe segments propagate in a repetitive sequence that starts with segmentation, followed by merging of segments and new growth of fingers/lobes. Based on the 3D geometry of segments, we suggest that sill segmentation is linked to smaller scale visco-plastic instabilities that occur within the inner sill and large scale mixed mode (I+III) fracturing during the inclined sheet propagations.

We investigate the conditions under which saucer-shaped sills form through the upper crust and their geometries. We performed a series of scaled laboratory experiments that employ visco-elastic-plastic Laponite RD® (LRD) gels to model upper crustal rocks, and Newtonian paraffin oil as the magma analogue. Both homogenous and layered analogue upper crust is considered. In homogenous 3 wt. % LRD, the injected oil formed a saucer-shaped intrusion with the shortest inner sill observed among all of the experiments. Saucer-shaped sills always formed in experiments with a two-layer upper crust. These experiments show sharp transitions from an inner flat sill to outer inclined sheets, which are characterised by non-planar margins. The experimental results show that: (1) the transition from an inner flat sill to outer inclined sheet occurs when the sill radius to overburden depth ratio (r/H) is between 0.5 and 2.5; (2) the inclined sheets propagate upwards with angles, θ = 15° to 25°; (3) the ratio of the Young’s modulus (E*) between the layers controls when the inner flat sill to outer inclined sheet occurs; and (4) irregular finger-like and/or lobe segment geometries form at the propagating tip of the intrusion. The results also suggest that there is no strict requirement for high horizontal stresses to form natural saucer-shaped sill geometries. We conclude that the layered visco-elastic-plastic crustal analogues better represent natural, complex saucer-shaped sill geometries. Furthermore, the observed sharp transitions between inner and outer sills are compatible with brittle-elastic fracture mechanisms operating at the intrusion scale.