Dr Sara Polanco

We investigate how the mechanical properties of intra-oceanic arcs affect the collision style and associated stress-strain evolution with buoyancy-driven models of subduction that accurately reproduce the dynamic interaction of the lithosphere and mantle. We performed a series of simulations only varying the effective arc thickness as it controls the buoyancy of intra-oceanic arcs. Our simulations spontaneously evolve into two contrasting styles of collision that are controlled by a 3% density contrast between the arc and the continental plate. In simulations with less buoyant arcs (15¿31¿km; effective thickness), we observe arc-transference to the overriding plate and slab-anchoring and folding at the 660¿km transition zone that result in fluctuations in the slab dip, strain-stress regime, surface kinematics, and viscous dissipation. After slab-folding occurs, the gravitational potential energy is dissipated in the form of lithospheric flow causing lithospheric extension in the overriding plate. Conversely, simulations with more buoyant arcs (32¿35¿km; effective thickness) do not lead to arc-transference and result in slab break-off, which causes an asymptotic trend in surface kinematics, viscous dissipation and strain-stress regime, and lithospheric extension in the overriding plate. The results of our numerical modeling highlight the importance of slab-anchoring and folding in the 660¿km transition zone on increasing the mechanical coupling of the subduction system.

Cratons are ancient regions of relatively stable continental fragments considered to have attained long-term tectonic and geomorphic stability. Low-temperature thermochronology data, however, suggest that some cratons have experienced discrete Phanerozoic heating and cooling episodes. We report apatite fission track, and apatite and zircon (U-Th)/He low-temperature thermochronology data from the Archean Pilbara craton and adjacent Paleoproterozoic basement, NW Australia. Inverse thermal history simulations of this spatially extensive data set reveal that the region has experienced ~50¿70°C cooling, which is interpreted as a response to the unroofing of erodible strata overlying basement. The timing of cooling onset is variable, mainly ~420¿350¿Ma in the southern and central Pilbara-eastern Hamersley Basin and ~350¿300¿Ma in the northern Pilbara, while the westernmost Pilbara-central Hamersley Basin does not record a significant Paleozoic cooling event. These differences are attributed to variations in sedimentary thickness and proximity to adjacent rift basins, which lack Archean age zircons in their Paleozoic strata. The onset of Paleozoic cooling coincides with the timing of the episodic intraplate late Ordovician-Carboniferous Alice Springs Orogeny. This orogeny is thought to have resulted from far-field plate margin stresses, which in turn caused the opening of the adjacent Canning Basin, to the north and east of the craton. We propose that basin development triggered a change of base level, resulting in denudation and the crustal cooling event reported here. Our results provide further evidence for the transmission of far-field forces to cratons over hundreds of kilometers and support the view that cratons have experienced geomorphic changes during the Phanerozoic.

Few modern sediment dispersal pathways predate the breakup of Pangea. This suggests that river lifespan can be controlled by continental assembly and dispersal cycles, with the longest-lived river systems present during supercontinent regimes. Based on the strikingly similar age spectra and Hf isotopic array extracted from Paleozoic to early Mesozoic sedimentary sequences from the Paleo-Tethyan margin basins, we argue that a long-lived supercontinental- scale system, with headwaters originating in Antarctica, flowed northward to finally debouch on the margin with the Paleo-Tethys Ocean. Channel-belt thickness scaling relationships, which provide an estimate of drainage area, support the notion that this was a supercontinental-scale system. Sediments were eroded from Proterozoic orogenic belts and flanked resistant kernels of Archean cratons. Remnants of this system, which can still be traced today as topographic lows, controlled post-breakup drainage patterns in Gondwanan fragments in Western Australia. We conclude that supercontinental regimes allow sediment dispersal systems to be long-lived, as they provide both an abundant sediment supply, due to erosion of large-scale, collision-related internal mountain systems, and a stable, large-scale configuration that lasts until breakup.

Downstream variation in grain size associated with changes in river pattern is a topic that interests multiple disciplines. How grain size varies between adjacent reaches with strongly contrasting river pattern is an outstanding question. This study presents a combined field and numerical modelling investigation of a river with a downstream planform change from single channel to anabranching, where the planform is controlled by a change in underlying lithology. This approach enabled exploration of the controls on sedimentology in a river for which there is very limited opportunity to collect flow and sediment transport data. This study shows that the surficial grain size decreases as a result of the downstream change in planform. This is because of a decrease in flow velocity and shear stress associated with a decrease in channel depth related to the planform change. Channel geometries in both the field and modelling data fit into distinct groups based on channel depth, the deepest being the single channel reach and the shallowest being the anabranching. This downstream reduction in channel dimension (depth) is caused because the total discharge is split from one channel into multiple channels. The coarsest grain sizes (cobble) are deposited at the terminus of the single channel and in the distributary channels; anabranching channels contain sand-size sediments. This study shows that, in a transition from single channel to anabranching, the channel dimensions decrease as the number of channels increases, resulting in a decrease in bed shear stress and the fining of bed material downstream.

River avulsion is a complex multistage process, yet the causes of river avulsions are not well understood. Using four decades of satellite, climatic, and discharge data, we show that on the Magdalena River, Colombia, the frequency of crevasse splay and avulsion triggering increases during La Niña climatic events. Field investigation of an ongoing partial avulsion event that started in 2007 shows that it occurred due to the combined effects of climate and tectonic forcing. The avulsion event started with the formation of a crevasse splay, followed by maintenance and expansion of one crevasse channel, in-filling of a lake, and development of a network of new channels. Based on topographic, bathymetric, and sedimentologic data, we determined that the location of the avulsion point is related to changes in the longitudinal profile caused by a fault; while on a reach scale, it is linked to the location of the channel's thalweg. We also show that while there is enough gradient to drive the avulsion, because crossvalley slope is larger than down-valley slope, levee height is not higher than channel depth, a criterion that has been proposed to be a necessary factor for avulsion. We show that the avulsion was triggered by frequent and highdischarge events related to the 2007-2008 and 2010-2011 extreme La Niña events. Our field data suggest a trade-off between avulsion setup and trigger, and that the necessary normalized superelevation needed to drive avulsion may decrease with an increase in the magnitude of the trigger. Moreover, our results provide insights into how global-scale climatic processes trigger avulsion, which is useful for hazard mitigation and for the interpretation of avulsive deposits preserved in the stratigraphic record.

Anabranching rivers are defined as a system of multiple channels separated by stable alluvial islands. While substantial progress has been made in developing a physics-based understanding of what drives the differences between meandering and braided river channels, anabranching rivers have not been well-integrated into these models. Here, we propose that alluvial bar growth on the floodplain may be a precursor for the formation of anabranching rivers. Alluvial bar growth strongly depends on the aspect ratio of the flow (width divided by depth) and rivers with wide floodplains have flows with a large aspect ratio. Consistent with this idea, remotely sensed measurements of floodplain width of four rivers from different climatic and tectonic settings show that anabranching river patterns are often associated with relatively wide floodplains. To explore the physics behind that empirical relationship we carried out two sets of morphodynamic numerical simulations using boundary conditions from field-scale modern anabranching rivers spanning different climatic and geologic settings as well as hypothetical floodplain widths. Results from the simulations show that, for a given flood discharge, widening the floodplain changes the river pattern from single channel to multi-threaded with mobile bars to multi-channeled with immobile islands. Multi-channeled patterns arise because the emergence of bars causes flow bifurcation. Subsequent bifurcation instability leads to the emergence of multiple stable channels. As the channels increase their cross-sectional area, shields stresses on intervening bars are reduced until the bars stabilize into islands. We suggest that the presence of stable islands allows vegetation to grow or cohesive sediment to accumulate leading to enhanced channel bank strength and a stable anabranching pattern. Our results suggest that alluvial bar growth can be a precursor to formation of anabranching rivers. Compared with field measurements our simulations accurately predict the number of active channels to within ~ 20% which seems to support the idea that some anabranching rivers may originate from alluvial bar growth.

The type section of the late Ediacaran (ca 565 Ma) Bonney Sandstone in South Australia provides an opportunity to interpret a succession of Precambrian clastic sediments using physical sedimentary structures, lithologies and stacking patterns. Facies models, sequence stratigraphic analysis, and process-based architectural classification of depositional elements were used to interpret depositional environments for a series of disconformity-bounded intervals. This study is the first detailed published work on the Bonney Sandstone, and provides additional context for other Wilpena Group sediments, including the overlying Rawnsley Quartzite and its early metazoan fossils. Results show that the ~300 m-thick section studied here shows a progressive change from shallow marine to fluvially dominated sediments, having been deposited in storm-dominated shelf and lower shoreface environments, lower in the section, and consisting primarily of stacked channel sands, in a proximal deltaic environment near the top. Based on the degree of influence of wave, tidal or fluvial depositional processes, shallow marine sediments can be classified into beach, mouth bar, delta lobe and channel depositional elements, which can be used to assist in predicting sandbody geometries when only limited information is available. Sediments are contained within a hierarchical series of regressive, coarsening-upward sequences, which are in turn part of a larger basin-scale sequence that likely reflects normal regression and filling of accommodation throughout a highstand systems tract. Paleogeographic reconstructions suggest the area was part of a fluvially dominated clastic shoreline; this is consistent with previous reconstructions that indicate the area was on the western edge of the basin adjacent to the landward Gawler Craton. This research fills in a knowledge gap in the depositional history of a prominent unit in the Adelaide Rift Complex and is a case study in the interpretation of ancient deposits that are limited in extent or lacking diagnostic features.