Rachel Walcott

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Stress fields of ancient seismicity recorded in the dynamic geometry of pseudotachylyte in the Outer Hebrides Fault Zone, UK

Journal of the Geological Society

Heterogeneous sequences of exhumed fault rocks preserve a record of long-term evolution of fault ... more Heterogeneous sequences of exhumed fault rocks preserve a record of long-term evolution of fault strength and deformation behaviour during prolonged tectonic activity. Along the Outer Hebrides Fault Zone (OHFZ), UK, numerous pseudotachylytes record palaeoseismic slip events within sequences of mylonites, cataclasites and phyllonites. To date, the kinematics and controls on seismicity within the long active history of the OHFZ have been poorly constrained. Additional uncertainties over the relative location of a meteorite impact and possible pre-OHFZ brittle faulting also complicates interpretation of the diffuse seismic record. This study presents kinematic analyses of seismicity in the OHFZ, combining observations of offset markers, en-echelon injection veins, and injection vein geometry to reconstruct slip directions and

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Supplementary tables 1–4. Stress fields of ancient seismicity recorded in the dynamic geometry of pseudotachylyte

Supplementary tables 1–4.

Rupture geometries in anisotropic amphibolite recorded by pseudotachylytes in the Gairloch Shear Zone, NW Scotland

Scottish Journal of Geology, 2019

Rupture geometries in anisotropic amphibolite recorded by pseudotachylytes in the Gairloch Shear

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Squeezing river catchments through tectonics: Shortening and erosion across the Indus Valley, NW Himalaya

Geological Society of America Bulletin, 2016

Tectonic displacement of drainage divides and the consequent deformation of river networks during... more Tectonic displacement of drainage divides and the consequent deformation of river networks during crustal shortening have been proposed for a number of mountain ranges, but never tested. In order to preserve crustal strain in surface topography, surface displacements across thrust faults must be retained without being recovered by consequent erosion. Quantification of these competing processes and the implications for catchment topography have not previously been demonstrated. Here, we use structural mapping combined with dating of terrace sediments to measure Quaternary shortening across the Indus River valley in Ladakh, NW Himalaya. We demonstrate ~0.21 m k.y. -1 of horizontal displacement since ca. 45 ka on the Stok thrust in Ladakh, which defines the southwestern margin of the Indus Valley catchment and is the major back thrust to the Tethyan Himalaya in this region. We use normalized river channel gradients of the tributaries that drain into the Indus River to show that the lateral continuation of the Stok thrust was active for at least 70 km along strike. Shortening rates combined with fault geometries yield vertical displacement rates that are compared to time-equivalent erosion rates in the hanging wall derived from published detrital 10 Be analyses. The results demonstrate that vertical displacement rates across the Stok thrust were approximately twice that of the time-equivalent erosion rates, implying a net horizontal displacement of the surface topography, and hence narrowing of the Indus Valley at ~0.1 m k.y. -1 . A fill terrace records debris-flow emplacement linked to thrust activity, resulting in damming of the valley and extensive lake development. Conglomerates beneath some of the modern alluvial fans indicate a northeastward shift of the Indus River channel since ca. 45 ka to its present course against the oppo site side of the valley from the Stok thrust. The integration of structural, topographic, erosional, and sedimentological data provides the first demonstration of the tectonic convergence of drainage divides in a mountain range and yields a model of the surface processes involved.

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Constraints on the kinematics of post-orogenic extension imposed by stretching lineations in the Aegean region

Tectonophysics, 1998

New kinematic data from shear zones of Tertiary age in the metamorphosed basement of the Aegean r... more New kinematic data from shear zones of Tertiary age in the metamorphosed basement of the Aegean region, in combination with published kinematic, lithological and palaeomagnetic data, have enabled us to determine a detailed kinematic evolution of the region during post-orogenic extension in the Aegean. Three distinct stages are recognised. Firstly, extension of both the western and eastern Aegean started in the Late Oligocene-Early Miocene (¾36-25 Ma ago), initially in an ¾023º direction. Stage two started shortly after this with the division of the Aegean crust into the West Aegean Block (a coupled assemblage of small blocks) bound on its northwestern and southeastern margins by the Scutari-Pec Line and Mid-Cycladic Lineament (fracture zones), respectively. Between 25 Ma and ¾3 Ma the West Aegean Block underwent ¾30º clockwise rotation, whilst the eastern Aegean underwent, on average, ¾19º anticlockwise rotation. Rotation of the West Aegean Block as a semi-coherent block was aided by the relative 'coolness' of the block compared to the 'warmer' eastern Aegean region. As the crust thinned and rotated, the NW-SE-trending faults which characterise the West Aegean Block were cross-cut by E-W-trending faults. In the latest Miocene-Pliocene, the thermal front migrated westwards into the West Aegean Block and the Mid-Cycladic Lineament ceased activity. The final kinematic stage marked the start of the subdivision of the West Aegean Block into two portions in the Pliocene. The southeastern portion (NW Cycladic area) coupled with the SW Cycladic area to form a Central Aegean Block that started to translate southwest. The northern and western portion rotated a further ¾10-15º clockwise whilst the southeastern rim rotated anticlockwise. These data demonstrate that Aegean post-orogenic extension is not dominated by a single block rotation or radial spreading event, but is characterised by the sequential formation and reformation of lithospheric-scale blocks that have developed in response to large-scale partitioning of rotational strain.

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Occurrence and mechanisms of pseudotachylyte formation in the Gairloch Shear Zone

The Gairloch Shear Zone, a NW-SE band of localised Laxfordian to Grenvillian brittle-ductile shea... more The Gairloch Shear Zone, a NW-SE band of localised Laxfordian to Grenvillian brittle-ductile shearing, contains examples of supposed pseudotachylyte veins associated with late-stage brittle 'crush' belts within the Loch Maree Group supracrustals and Lewisian gneiss of NW Scotland. Whilst these are previously described and dated in the literature, uncertainty remains as to whether all mapped occurrences are pseudotachylyte, and additionally what mechanisms might trigger and control the formation and distribution of pseudotachylyte during movement episodes. A series of potential pseudotachylyte veins were sampled in the Gairloch area from the 'crush belts' observed by previous workers to lie along lithological boundaries. Veins were identified by injection geometries, fracture associations, distinct vein margins and dark glassy matrix, with subsequent microscopic analysis allowing assessment of how well these accepted criteria can identify pseudotachylyte in the field....

Far-Field Deformation Resulting from Rheologic Differences Interacting with Tectonic Stresses: An Example from the Pacific/Australian Plate Boundary in Southern New Zealand

Geosciences, 2014

The Miocene in Southern New Zealand was dominated by strike-slip tectonics. Stratigraphic evidenc... more The Miocene in Southern New Zealand was dominated by strike-slip tectonics. Stratigraphic evidence from this time attests to two zones of subsidence in the south: (a) a middle Cenozoic pull-apart basin and (b) a regionally extensive subsiding lake complex, which developed east and distal to the developing plate boundary structure. The lake overlay a block of crust with a significantly weak mid-crustal section and we pose the question: can rheological transitions at an angle to a plate boundary produce distal subsidence and/or uplift? We use stratigraphic, structural and geophysical observations from Southern New Zealand to constrain three-dimensional numerical models for a variety of boundary conditions and rheological scenarios. We show that coincident subsidence and uplift can result from purely strike-slip boundary conditions interacting with a transition from strong to weak to strong mid-crustal rheology. The resulting pattern of vertical displacement is a function of the symmetry or asymmetry of the boundary conditions and the extent and orientation of the rheological transitions. For the Southern New Zealand case study, subsidence rates of ~0.1 mm/yr are predicted for a relative plate motion of 25 mm/yr, leading to ~500 m of subsidence over a 5 Ma time period, comparable to the thickness of preserved lacustrine sediments.

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Universality and variability in the 2D form of drainage basins: a study of active orogens and passive margins

ABSTRACT Identifying the controls on basin morphology is needed to understand how landscapes evol... more ABSTRACT Identifying the controls on basin morphology is needed to understand how landscapes evolve under changing conditions. Some important morphological and topological relationships such as Hack&#39;s and Horton/Tokunagas&quot; laws suggest that river basins, irrespective of the climatic, tectonic and/or biogeographic conditions under which they developed, are likely to have statistically predictable planform morphologies. Indeed empirical studies of the spacing between basins that drain the main divide of some active and inactive orogens suggest that this may be the case. In such situations major basins typically have an outlet spacing that is approximately half of the distance between the outlet and the main divide. However, notable deviations from this outlet spacing relationship have also been documented. We therefore address the question: what controls the planform morphology or &quot;packing arrangement&quot; of river basins (of all scales) on the flanks of orogens? To answer this question we conducted a study of basin morphology from both active mountain belts (e.g. Himalaya and Andes) and from morphological similar, but tectonically very different, passive margins (e.g. southeast Africa). We specifically investigate the relationship between basin outlet spacing and other two dimensional geometric properties such as basin length, main valley length and basin area. We show that if the rate of relative displacement between the main divide and outlet is slower than the rate of river erosion then, river basins, irrespective of size, tend towards an approximately diamond shaped packing arrangement in which the basin width is approximately half that of basin length. However, where relative displacement between the main divide and outlet has exceeds the rate of river erosion, such as in the western Himalaya, rivers are forced into an alternative, and possibly unstable packing arrangement.

The Impact of Vegetation on the Long-term Landform Evolution of the Chilean Coastal Ranges

ABSTRACT We assess the topographic response to vegetation density along a 1600 km climatic transe... more ABSTRACT We assess the topographic response to vegetation density along a 1600 km climatic transect ranging from hyper-arid to humid temperate conditions along the Chilean coastal ranges. Using a new method of statistical analysis of topography we provide insights into the relationship between vegetation and sediment transport. Specifically, we find the following relationship between vegetation density (or its absence) and landscape form: (1) where vegetation is currently absent (&lt; 26°S) topographic form is insensitive to aspect; (2) in regions with moisture limited vegetation cover, aspect is an important parameter determining hill slope steepness on a local and regional scale; (3) where vegetation cover is currently aspect-insensitive (&gt; 38°S) the steepness of hillslopes is only aspect-sensitive where it isotropic (plutonic) lithologies. We conclude that equilibrium hillslopes that are vegetated are steeper on average than barren hillslopes within the same climatic zone. This implies that hillslopes whose vegetation density is decreased or disturbed by climate change or human activity will remain out of equilibrium until either the mean slope is lowered or vegetation density is restored.

The long term impact of vegetation on the topography of the Chilean Coastal Ranges

ABSTRACT The 1600 kilometre long Chilean Coastal Range varies in climate from hyper-arid in the n... more ABSTRACT The 1600 kilometre long Chilean Coastal Range varies in climate from hyper-arid in the north to humid-temperate in the south. Traversing this range is a series of east-west trending valleys. The vegetation density within many of these valleys varies with the aspect of the underlying topographic surface. Previous studies have shown that the aspect sensitivity of vegetation density in regions with a Mediterranean climate is due directly or indirectly to the difference in insolation of north and south facing slopes. Thus, the north-south trending Chilean Coastal Range provides a unique opportunity to assess the impact that life has on topography, or more specifically, the systematic response of topography to changes in vegetation density. In this study, we use a novel application of statistical topographic analysis to provide insights into the relationship between vegetation, topography and sediment transport. Specifically, we observe the following relationships between vegetation density and topographic form. Firstly, where vegetation is currently absent (in the hyper-arid climatic zone north of 26 degrees South) the mean angle of hillslopes appears to be insensitive to the aspect of the hillslope. In contrast, in the Mediterranean climatic zone between 26 deg S and 38 deg S hillslope angles depend, on a local and on a regional scale, on the aspect of the hillslope. Finally, we observe that where vegetation density does not vary with the aspect of the hillslope (i.e. in the humid-temperate climatic zone south of 38 deg S), aspect-sensitive hillslope steepness is only detected in areas that are underlain by largely isotropic, plutonic lithologies. These observations lead us to conclude that within the same climatic zone, all else being equal, equilibrium hillslope that are densely vegetated will tend to be steeper than hillslopes which are less densely vegetated or barren. Thus, hillslopes which have suffered an increase or decrease in vegetation density will remain out of equilibrium until either the mean hillslope angle is lowered or vegetation density is restored.

Influence of lithology on hillslope morphology and response to tectonic forcing in the northern Sierra Nevada of California

Journal of Geophysical Research: Earth Surface, 2013

Many geomorphic studies assume that bedrock geology is not a first-order control on landscape for... more Many geomorphic studies assume that bedrock geology is not a first-order control on landscape form in order to isolate drivers of geomorphic change (e.g., climate or tectonics). Yet underlying geology may influence the efficacy of soil production and sediment transport on hillslopes. We performed quantitative analysis of LiDAR digital terrain models to examine the topographic form of hillslopes in two distinct lithologies in the Feather River catchment in northern California, a granodiorite pluton and metamorphosed volcanics. The two sites, separated by <2 km and spanning similar elevations, were assumed to have similar climatic histories and are experiencing a transience in landscape evolution characterized by a propagating incision wave in response to accelerated surface uplift c. 5 Ma. Responding to increased incision rates, hillslopes in granodiorite tend to have morphology similar to model predictions for steady state hillslopes, suggesting that they adjust rapidly to keep pace with the incision wave. By contrast, hillslopes in metavolcanics exhibit high gradients but lower hilltop curvature indicative of ongoing transient adjustment to incision. We used existing erosion rate data and the curvature of hilltops proximal to the main channels (where hillslopes have most likely adjusted to accelerated erosion rates) to demonstrate that the sediment transport coefficient is higher in granodiorite (8.8 m 2 ka À1 ) than in metavolcanics (4.8 m 2 ka À1 ). Hillslopes in both lithologies get shorter (i.e., drainage density increases) with increasing erosion rates.

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New approaches to analysing landscape development: Some contributions from Edinburgh

Scottish Geographical Journal, 2008

The Institute (formerly Department) of Geography of the University of Edinburgh has been active i... more The Institute (formerly Department) of Geography of the University of Edinburgh has been active in the rejuvenation of the sub-discipline of geomorphology over the past two decades with its emphasis on addressing macroscale questions of long-term landscape development. In addition to interdisciplinary collaborations in the fields of low-temperature thermochronology and numerical landscape modelling, researchers there have played a key role

Fluid inclusion evidence for widespread shallow hydrothermal activity in South Victoria Land, Antarctica

New Zealand Journal of Geology and Geophysics, 1992

Sparse but widespread veins cut Paleozoic basement, overlying Devonian‐Triassic Beacon sediments ... more Sparse but widespread veins cut Paleozoic basement, overlying Devonian‐Triassic Beacon sediments and Jurassic Ferrar Dolerite in South Victoria Land. The hydrothermal activity was mainly fracture controlled, and resulted in formation of well‐defined steeply dipping veins. The fluids that produced these veins were variable in composition, ranging from very low salinity water, through low‐moderate salinity CO2‐bearing water, to CaCl2‐NaCl‐bearing brine. Mineral

Tectonic controls on the evolution of the Clutha River catchment, New Zealand

New Zealand Journal of Geology and Geophysics, 2012

A synthesis of published information on mountain uplift and river capture in Otago suggests that ... more A synthesis of published information on mountain uplift and river capture in Otago suggests that the Clutha River catchment has evolved westwards and expanded since the Pliocene. River capture events that facilitated catchment expansion are indicated by sediment provenance, drainage geometry and freshwater fish genetics. The catchment has been partly confined by NWtrending ranges and the Southern Alps to the west, and drainage geometry was disrupted by subsequent growth of NE-trending ranges. Examination of crustal-scale deformation via an established numerical model, which portrays the Otago Schist basement as rheologically weak compared to adjacent greywacke-dominated structural blocks, suggests that uplift geometry was controlled by the inherited Cretaceous boundary between these crustal blocks. Pre-existing faults had relatively minor effects on uplift geometry. A low-relief corridor between Canterbury and Southland permitted genetic connections of freshwater fish populations through to B1 Ma, to the west of the developing Clutha catchment.

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Using hilltop curvature to derive the spatial distribution of erosion rates

Journal of Geophysical Research: Earth Surface, 2012

Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical ... more Erosion rates dictate the morphology of landscapes, and therefore quantifying them is a critical part of many geomorphic studies. Methods to directly measure erosion rates are expensive and time consuming, whereas topographic analysis facilitates prediction of erosion rates rapidly and over large spatial extents. If hillslope sediment flux is nonlinearly dependent on slope then the curvature of hilltops will be linearly proportional to erosion rates. In this contribution we develop new techniques to extract hilltop networks and sample their adjacent hillslopes in order to test the utility of hilltop curvature for estimating erosion rates using high‐resolution (1 m) digital elevation data. Published and new cosmogenic radionuclide analyses in the Feather River basin, California, suggest that erosion rates vary by over an order of magnitude (10 to 250 mm kyr−1). Hilltop curvature increases with erosion rates, allowing calibration of the hillslope sediment transport coefficient, which ...

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Scale dependence of hypsometric integrals: An analysis of southeast African basins

Geomorphology, 2008

Previous studies have shown that the hypsometry of individual drainage basins is sensitive to bot... more Previous studies have shown that the hypsometry of individual drainage basins is sensitive to both tectonic uplift rates and variations in erosional resistance of different lithological units. However, basin hypsometry has also been found to be related to the size, the shape and relief of the sampled area as well as factors such as the type of dominant erosional process. This study extends the evaluation of the controls on a basin's hypsometry by assessing factors that might influence the spatial distribution of hypsometric integrals at different scales. The method is applied to catchments draining the southeast margin of southern Africa (KwaZulu-Natal region) because of the region's rather uniform runoff, a rather well-constrained denudational history and a relatively simple coastward sequence of bedrock geology. Several sets of basins were extracted from an SRTM-derived 90 m-resolution DEM: one set of main basins (those that have outlets at sea level) and six sets of sub-basins of different size based on Strahler order. The mean area of the subbasins sampled ranged from 9 km 2 for Strahler order 2 through to 3215 km 2 for Strahler order 6. In contrast to previous studies data from this study fail to show a correspondence between hypsometric integral and indices of basin dimension, such as basin area and basin relief. Rather basin hypsometry in SE Africa varies spatially and with scale. In particular, a mountain-range scale, positive hypsometric integral anomaly situated between 10 and ∼100 km of the coast line was observed but only in basins of Strahler order 5 and less. This anomaly is compatible with moderate crustal displacement focussed midway between the coast and the Drakensburg Escarpment. Some intermediate scale, positive hypsometric integral anomalies (in basins of Strahler order 4 and less) was also observed. In some cases, these anomalies correspond to differences in erosional resistance among major lithological groups. Finally, numerous local (basin-scale) positive hypsometric integral anomalies (in basins of Strahler order 3 and less) were also observed. In many cases, these latter anomalies are related to local differences in lithological resistance (e.g. the presence of dolerite sills and dikes). The analysis of the spatial variation of hypsometric integrals in basins of different size ranges is therefore a useful first-step in assessing factors that control landscape development.

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The Alpine evolution of Thessaly (NW Greece) and late Tertiary Aegean kinematics

ABSTRACT Vita. Thesis (doctoral)--Universiteit Utrecht, 1997. Includes bibliographical references... more

Universality and variability in basin outlet spacing: implications for the two‐dimensional form of drainage basins

Basin Research, 2009

ABSTRACTIt has been observed that the distance between the outlets of transverse basins in orogen... more ABSTRACTIt has been observed that the distance between the outlets of transverse basins in orogens is typically half of the distance between the main divide and the range front irrespective of mountain range size or erosional controls. Although it has been suggested that this relationship is the inherent expression of Hack's law, and/or possibly a function of range widening, there are cases of notable deviations from the typical half‐width average spacing. Moreover, it has not been demonstrated that this general relationship is also true for basins in morphologically similar nonorogenic settings, or for those that do not extend to the main drainage divide. These issues are explored by investigating the relationship between basin outlet spacing and the 2‐dimensional geometric properties of drainage basins (basin length, main valley length and basin area) in order to assess whether the basin outlet spacing‐range width ratio is a universal characteristic of fluvial systems. We exam...

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Deformed river basins of the Himalaya

Identification of the controls on basin morphology in mountain belts is needed to understand how ... more Identification of the controls on basin morphology in mountain belts is needed to understand how landscapes evolve under changing conditions. Although river basins vary enormously in area, many of their morphological relationships, such as Hack's law, are scale invariant irrespective of mountain type. This suggests that, in most mountain belts, the fundamental process(es) that control basin morphology are also scale invariant and therefore largely insensitive to variations in tectonic activity. However, river basins in the Himalaya are anomalously wide when compared with basins developed on the flanks of other semi-linear ranges. We present a detailed study of Himalayan river basin morphology to determine how the evolution of this orogen may have influenced the shape of these unusual basins. We investigate, in particular, the statistical geometric properties of basins, such as the length, width and area of basins, with respect to the scale and the location of the basin within the mountain belt. Our results show that the anomalously wide basins found over much of the Himalaya have a limited scale range and distribution. These data therefore provide an indication of the significant control that the evolution of this mountain range has had on basin morphology at the local scale. The fact that these catchments have departed from what is perceived as a stable scaling relationship implies that, while their rivers can incise at a rate broadly comparable to the rate of rock uplift, their drainage divides can not migrate fast enough to reconfigure in response to tectonic shortening. As a result, long-term crustal shortening has significantly deformed the river network within the central and western Himalaya.

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Critical thresholds controlling basin morphodynamics; a study of southern Tibetan-Himalayan basins

It has long been recognised that mountain ranges tend to develop watersheds which have predictabl... more It has long been recognised that mountain ranges tend to develop watersheds which have predictable planform aspect ratios aligned perpendicular to the strike of the range. However, there is one notable exception - the Himalaya - where drainage basins appear to be anomalously wide. In this paper we analyse the spatial and scalar dependence of catchments draining the southern Tibetan

Rachel Walcott

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