Rebecca Anne Jamieson

Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation

Recent interpretations of Himalayan–Tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the Tibetan plateau, and that ductile extrusion of high-grade metamorphic rocks between coeval normal- and thrust-sense shear zones can explain exhumation of the Greater Himalayan sequence. Here we use coupled thermal–mechanical numerical models to show that these two processes—channel flow and ductile extrusion—may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low-viscosity material. Our models provide an internally self-consistent explanation for many observed features of the Himalayan–Tibetan system.

Crustal flow modes in large hot orogens

Abstract Crustal-scale channel flow numerical models support recent interpretations of Himalayan—Tibetan tectonics proposing that gravitationally driven channel flows of low-viscosity, melt-weakened, middle crust can explain both outward growth of the Tibetan Plateau and ductile extrusion of the Greater Himalayan Sequence. We broaden the numerical model investigation to explore three flow modes: homogeneous channel flow (involving laterally homogeneous crust); heterogeneous channel flow (involving laterally heterogeneous lower crust that is expelled and incorporated into the mid-crustal channel flow); and the hot fold nappes style of flow (in which mid-/lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid-crust). The three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material. The hot fold nappe mode is driven tectonically by, for example, collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows. The heterogeneous mode combines tectonic and gravitationally driven flows. Preliminary results also demonstrate the existence and behaviour of mid-crustal channels during advancing and retreating dynamical mantle lithosphere subduction. An orogen temperature—magnitude (T-M) diagram is proposed and the positions of orogens in T-M space that may exhibit the flow modes are described, together with the characteristic positions of a range of other orogen types.

Barrovian regional metamorphism: where’s the heat?

Abstract Coupled thermal-mechanical models of convergent orogens offer a novel way to investigate the interactions between heat and tectonics that lead to regional metamorphism. In this study, the effects of different distributions of heat-producing material in the crust and upper mantle on crustal thermal histories and deformation fields are investigated. The models involve subduction-driven collision with moderate convergence and erosion rates. For models involving standard continental crust, where heat production is initially concentrated in the upper crust, P-T-t paths do not intersect the field of typical Barrovian P-T conditions. However, heat-producing material can be tectonically redistributed, for example, by subduction of crustal rocks to upper mantle depths, or by formation of thick accretionary wedges or continental margin sequences during convergence. Models that include a wedge of heat-producing material in the upper mantle generate high temperatures in the lower crust and upper mantle that lead to a change in orogenic style; radioactive heating of partially subducted crustal material on time scales of 10–30 Ma yields temperatures high enough for partial melting. However, crustal P-T-t paths are unlikely to intersect the Barrovian field unless erosion or convergence rates change. Models that include a crustal-scale region with moderate, uniform heat production, simulating a large accretionary wedge or tectonically thickened continental margin sequence, generate P-T-t paths that intersect the Barrovian field. However, as convergence proceeds, the heat-producing region is deformed, eroded, and reduced in volume, so that the model orogen begins to cool down after about 20 Ma. The model results provide an explanation for many first-order tectonic and metamorphic features of small orogens, including metamorphic styles ranging from blueschists to the Barrovian series to granulites, late-orogenic granitoid magmatism, and the crustal-scale tectonic features associated with regional metamorphic belts. We conclude that the thermal state of an orogen is controlled by the evolving competition between cooling by subduction and radioactive heating within the deforming orogen.

On the origin of orogens

In order to understand how orogens “work,” a quantitative approach demonstrating proof of concept is essential. Our goal is to reconcile the diverse array of tectonic features observed in natural orogens in the context of “working” numerical models that are consistent with both the underlying physics and first-order geological constraints. We present a simple conceptual temperature-magnitude (T-M) framework for orogenesis in terms of the progression from small-cold to large-hot orogens, and we use forward numerical models to test hypotheses corresponding to specific stages along the T-M spectrum. Small-cold orogens are analyzed using crustal-scale singularity ( S ) point models, in which suborogenic mantle lithosphere is kinematically subducted beneath crust that deforms by critical wedge mechanics. The transition from oceanic subduction to continental collision, and the subsequent evolution of large-hot orogens, has been investigated using both crustal- and upper-mantle–scale models, the latter including dynamic subduction of suborogenic mantle lithosphere. Large-hot orogens with thick crust are characterized by elevated plateaus with a strong superstructure underlain by hot, weak, lower-crustal infrastructure. Beneath plateaus, tectonic processes are dominated by ductile flow of weak crust in response to differential pressure, while plateau flanks form external thrust-sense wedges. We discuss four topical issues in orogenic tectonics, including the response of the suborogenic mantle lithosphere to convergence, the interaction of climate and tectonics, the current debate concerning wedge versus channel-flow models to explain the Himalayan-Tibetan system, and the interpretation of metamorphic architecture in terms of orogenic processes. We conclude that collisional orogenesis is driven largely by subduction and accretion of material at convergent margins, accompanied by shortening, thickening, and heating of deformed crust.

Transect across the northwestern Grenville orogen, Georgian Bay, Ontario: Polystage convergence and extension in the lower orogenic crust

The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso-proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late-stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon-parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

Tectonic assembly of inverted metamorphic sequences

Inverted metamorphic sequences are characterized by peak metamorphic temperatures that increase structurally upward, with isograds that are typically parallel to associated thrust faults. A fully coupled thermal-mechanical model for convergent orogens shows how an inverted sequence can be tectonically assembled in a crustal-scale ductile shear zone at moderate to high rates of synorogenic erosion. Model inverted sequences form by tectonic juxtaposition of points with widely differing initial positions that reach peak temperatures at different times and in different places within the model orogen, and thus do not represent metamorphic or thermal gradients. Inverted crustal isotherms are not required to produce model inverted isograds. Model results agree well with metamorphic pressure-temperature data from the Main Central Thrust zone of the central Nepal Himalayas.

Provenance of the Greater Himalayan Sequence and associated rocks: predictions of channel flow models

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 (HT1 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 midcrustal ‘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.

The orogenic superstructure-infrastructure concept: Revisited, quantified, and revived

The historical superstructure-infrastructure concept (S-I) expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels. Two-dimensional thermal-mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening (phase 1), thermal relaxation and rheological weakening (phase 2), and ductile flow at depth (phase 3). Results predict an upper-crustal superstructure, dominated by early steep structures, separated across a subhorizontal high-strain zone from a ductile infrastructure with late gently dipping structures; this is consistent with observations from the western Superior Province. These models can account for contrasts in structural style, metamorphic grade, seismic reflectivity, and age between upper- and lower-crustal levels. In contrast to conventional thrust-tectonics models, the revived S-I model shows how young structures can form beneath older ones during progressive convergence, thereby encouraging reassessment of standard seismic reflection interpretations.

Last gasp of the Grenville Orogeny; thermochronology of the Grenville Front tectonic zone near Killarney, Ontario

We present U-Pb (titanite, zircon) and

Trace element mobility in the mylonite zone within the ophiolite aureole, St. Anthony Complex, Newfoundland

^{40}Ar/^{39}Ar