Papers by Richard Phillips
Geological Society, London, Special Publications, 2013
ABSTRACT Combined U-Pb, Lu-Hf and O isotope data of detrital zircons from the Devonian sandstone ... more ABSTRACT Combined U-Pb, Lu-Hf and O isotope data of detrital zircons from the Devonian sandstone of southern Libya provide important new boundary parameters for reconstruction of palaeosource areas and sediment transport and may lead to novel approaches to test current plate tectonic models, with important implications for our understanding of the evolution of northern Gondwana (in present-day coordinates) during the Palaeozoic. Detrital zircon U-Pb ages from Devonian sandstone of the eastern margin of the Murzuq Basin show four main age populations: 2.7-2.5 Ga (13%), 2.1-1.9 Ga (10%), 1.1-0.9 Ga (25%) and 0.7-0.5 Ga (46%). The ubiquitous occurrence of c. 1.0 Ga detrital zircons is characteristic of the Saharan Metacraton sedimentary cover sequence and provides new insights into palaeogeographic reconstructions of Gondwana-derived terranes in the Eastern Mediterranean and SW Europe. The Lu-Hf isotope data suggest that zircons crystallized within a narrow time interval from magmas with heterogeneous Hf isotope compositions. These magmas were derived by melting of pre-existing rocks, rather than being juvenile. The calculated Hf model ages range from 3.7 Ga to 1.3 Ga, with a major population between 2.8 Ga and 1.9 Ga, indicating prominent recycling of Archaean and Palaeoproterozoic crust.
Journal of African Earth Sciences, 2011
Other uses, including reproduction and distribution, or selling or licensing copies, or posting t... more Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited.
In situ-produced cosmogenic nuclei, made by cosmic ray induced nuclear reactions cumulatively on ... more In situ-produced cosmogenic nuclei, made by cosmic ray induced nuclear reactions cumulatively on exposed surfaces, are natural chronometers and valuable tools for environmental and geological research. Cosmogenic 36Cl (t1-2=3e5 yr) is dominantly produced in spallation reactions on Ca and K, and via neutron capture on 35Cl, and hence is applicable to a range of lithologies for studying events within the last 1 Myr or so. The different 36Cl production mechanisms result in versatility but also challenging data interpretation when unravelling the measured 36Cl concentrations. The main difficulty in utilising 36Cl for environmental and geological research arises from the stable isobar 36S. However, if high enough ion energies are available, these two isotopes can be separated based upon their different rate of energy loss in matter. This has typically required large (10-15 MV) legacy nuclear physics particle accelerators but recently it has been shown that sufficient separation can be achieved with much lower ion energies than before (~30 MeV); the detector resolution being improved by using uniform thin (~30 nm) Silicon rich Nitride membranes as a detector window to minimise energy losses and peak broadening. As a consequence, measurements can now be done with 5 MV, or even smaller, modern accelerator mass spectrometers utilising gas stripping to produce the highest possible quality beams. Accordingly a new class of commercial purpose-build 5-6 MV 36Cl-capable spectrometers is being deployed around the globe with additional measurement capacity greater than that of the installed base. This should increase accessibility and promote wider and more varied 36Cl use. However, laborious sample preparation chemistry and production rate uncertainties remain difficulties. An example 36Cl programme utilising the 5 MV accelerator mass spectrometer at SUERC will be presented. Our internal quality assurance program shows that no external uncertainty beyond 3% counting statistics is observed and Purdue PRIMELab Z93-0005 (nominally 1.20e-12 36Cl/Cl) AMS primary normalization standard is long-term consistent with K. Nishiizumi-provided secondary standard (nominally 5.0e-13 36Cl/Cl) used for monitoring. In addition to analytical quality of the analysis, monitored with the fore mentioned standards, we present results from an internal chemical standard, used to monitor sample preparation, as well as example data from a recent study of carbonate bedrock exhumation via active faulting.
The shallow subsurface structure of the 2009 April 6 M w 6.3 L'Aquila earthquake surface rupture ... more The shallow subsurface structure of the 2009 April 6 M w 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210 • and 228 • across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr -1 , consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be ∼33 ka. This compares with published values of 0.4 mm yr -1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr -1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr -1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard.
Geophysical Journal International, 2010
The shallow subsurface structure of the 2009 April 6 Mw 6.3 L'Aquila earthquake surface rupture a... more The shallow subsurface structure of the 2009 April 6 Mw 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210° and 228° across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr-1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be ~33 ka. This compares with published values of 0.4 mm yr-1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr-1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr-1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard.
Geophysical models that explore the deep rheology of continental-scale fault zones often lack rea... more Geophysical models that explore the deep rheology of continental-scale fault zones often lack realistic physical parameters that define the actual behaviour expected for a specific fault. This study aims to bridge the gap between geophysical- and geological-derived seismic models of continental scale fault zones by providing lithology and deformation state specific elastic properties from an exhumed ductile shear zone in north-central Turkey. This will allow better understanding of how major faults act at depth, and the degree at which strain partitions in the lower crust. The Uludağ Massif represents a mid-crustal section of the dextral strike-slip Eskişehir shear zone, which was active during the Oligocene and accommodated ~100km of displacement with a component of late oblique-normal slip. The exhumed Massif consists of high-grade metamorphic rocks belonging to the Uludağ Group, thought to be ‘post-Ordovician’ in protolith age, pierced by the Central and South Uludağ granites in ...
Understanding the behaviour of major continental-scale fault zones at depth, and in particular ho... more Understanding the behaviour of major continental-scale fault zones at depth, and in particular how the strain seen at the surface is accommodated in the lower ductile portion of the crust, is key to understanding how such faults will behave over the earthquake cycle. Whilst geophysical techniques provide useful information, they often lack realistic physical parameters that define the behaviour expected for a specific fault. The approach undertaken in this study is from a geological perspective and uses real rock data to model the seismic response of a ductile shear zone. A dense seismic array was installed in May 2012 over the North Anatolian Fault (NAF) close to site of the 1999 Izmit earthquake (M¬¬w 7.4) to attempt to model the deep structure of the fault. However, due to poor field exposures, an analogue for the NAF has had to be used. The field site for this project therefore is the Uludağ Massif, located ~100km to the south, which represents an exhumed mid-crustal 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....
Understanding the tectonic evolution of the Himalayan orogen requires knowledge of the mid crusta... more Understanding the tectonic evolution of the Himalayan orogen requires knowledge of the mid crustal deformation processes that occur during collision. We use variations in crystallographic preferred orientation (CPO) measurements and deformation microstructures observed in rocks from the metamorphic core of the orogen, the Greater Himalaya Sequence (GHS), to determine the kinematic evolution of the Annapurna-Dhaulagiri Himalaya.
Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained ... more Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained from microstructural analysis of exhumed terranes and shear zones. Here, we use Crystallographic Preferred Orientation (CPO) and Anisotropy of Magnetic Susceptibility (AMS) data to reveal the kinematic evolution of the metamorphic core of the Himalayan orogen, the Greater Himalayan Sequence (GHS). The Himalayan orogen is commonly explained with models of channel flow, which describe the GHS as a partially molten, rheologically weak mid crustal channel. Extrusion of the channel was facilitated by coeval reverse-and normal-sense shear zones, at the lower and upper channel margins respectively. Whilst many thermobarometric studies support the occurrence of channel flow, the spatial and temporal distribution of strain within the GHS is one aspect of the model that is yet to be fully resolved. We present a quantified strain proxy profile for the GHS in the Annapurna-Dhaulagiri region of central Nepal and compare our results with the kinematic predictions of the channel flow model. Samples were collected along a NS transect through the Kali Gandaki valley of central Nepal for CPO and AMS analysis. Variations in CPO strength are used as a proxy for relative strain magnitude, whilst AMS data provide a proxy for strain ellipsoid shape. Combining this information with field and microstructural observations and thermobarometric constraints reveals the kinematic evolution of the GHS in this region. Low volumes of leucogranite and sillimanite bearing rocks and evidence of reverse-sense overprinting normal-sense shearing at the top of the GHS suggest that channel flow was not as intense as model predictions. Additionally, observed EW mineral lineations and oblate strain ellipsoid proxies in the Upper GHS, indicative of three dimensional flattening and orogen parallel stretching, cannot be explained by current channel flow models. Whilst the results do not refute the occurrence of channel flow in the Annapurna-Dhaulagiri Himalaya, orogen parallel deformaiton may also play an important role during the evolution of the GHS. Such processes should be fully investigated to understand the role of orogen parallel deformation in the development of the Himalaya to further our knowledge of lithospheric deformation during orogenesis.
Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained ... more Knowledge of deformation processes that occur in the lithosphere during orogenesis can be gained from microstructural analysis of exhumed terranes and shear zones. Here, we use Crystallographic Preferred Orientation (CPO) and Anisotropy of Magnetic Susceptibility (AMS) data to reveal the kinematic evolution of the metamorphic core of the Himalayan orogen, the Greater Himalayan Sequence (GHS). The Himalayan orogen is commonly explained with models of channel flow, which describe the GHS as a partially molten, rheologically weak mid crustal channel. Extrusion of the channel was facilitated by coeval reverse-and normal-sense shear zones, at the lower and upper channel margins respectively. Whilst many thermobarometric studies support the occurrence of channel flow, the spatial and temporal distribution of strain within the GHS is one aspect of the model that is yet to be fully resolved. We present a quantified strain proxy profile for the GHS in the Annapurna-Dhaulagiri region of central Nepal and compare our results with the kinematic predictions of the channel flow model. Samples were collected along a NS transect through the Kali Gandaki valley of central Nepal for CPO and AMS analysis. Variations in CPO strength are used as a proxy for relative strain magnitude, whilst AMS data provide a proxy for strain ellipsoid shape. Combining this information with field and microstructural observations and thermobarometric constraints reveals the kinematic evolution of the GHS in this region. Low volumes of leucogranite and sillimanite bearing rocks and evidence of reverse-sense overprinting normal-sense shearing at the top of the GHS suggest that channel flow was not as intense as model predictions. Additionally, observed EW mineral lineations and oblate strain ellipsoid proxies in the Upper GHS, indicative of three dimensional flattening and orogen parallel stretching, cannot be explained by current channel flow models. Whilst the results do not refute the occurrence of channel flow in the Annapurna-Dhaulagiri Himalaya, orogen parallel deformaiton may also play an important role during the evolution of the GHS. Such processes should be fully investigated to understand the role of orogen parallel deformation in the development of the Himalaya to further our knowledge of lithospheric deformation during orogenesis.
Overview of combining regional strain-rate, slip-rate variability and stress transfer during faul... more Overview of combining regional strain-rate, slip-rate variability and stress transfer during fault interaction for seismic hazard assessment and understanding of continental deformation Abstract: Active faults experience earthquake rupture due to stress transfer from neighbouring earthquakes only if the fault in question is close to its failure stress. We lack knowledge of which faults are close to their failure stress and thus cannot interpret calculations of Coulomb stress transfer in terms of the probability of impending earthquakes. This presentation suggests that for active normal faults in central Italy whose geometry and slip-rates are well known, it is possible measure slip-rate variability and perhaps the elapsed time since the last earthquake(s), normalised to the fault slip-rates averaged over many earthquakes, using 36 Cl cosmogenic exposure dating, because these are proxies for how close a fault is to its failure stress. We will combine this with calculations of stress ...
The channel flow model for the Himalayan orogen suggests that the Greater Himalaya Sequence (GHS)... more The channel flow model for the Himalayan orogen suggests that the Greater Himalaya Sequence (GHS) represents a partially molten, rheologically weak, mid-crustal channel, bound above and below by rigid continental crust.
Understanding crustal processes that occur during continental collision requires knowledge of the... more Understanding crustal processes that occur during continental collision requires knowledge of the distribution of strain across shear zones and exhumed terranes. Variations in the strength of crystallographic preferred orientation (CPO) fabrics in tectonically deformed rocks provide a proxy for the relative strain magnitudes under which the CPOs formed. We present a quantified relative strain magnitude profile for the Greater Himalayan Sequence (GHS), which forms the metamorphic core of the Himalayan orogeny, derived from CPO data.
Journal of Sedimentary Research, 1999
Abstract Six steep (0.6-4.3 degrees), short (radial length 1.5-5 km), deeply incised Pliocene to ... more Abstract Six steep (0.6-4.3 degrees), short (radial length 1.5-5 km), deeply incised Pliocene to early Pleistocene paleofans and associated axial-fluvial strata are preserved adjacent to the footwall of the Palomas half graben in the southern Rio Grande Rift. Two different ...
Journal of African Earth Sciences, 2011
Other uses, including reproduction and distribution, or selling or licensing copies, or posting t... more Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited.
Geological Society, London, Special Publications, 2006
The channel flow model aims to explain features common to metamorphic hinterlands of some collisi... more 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 existence of well-preserved Holocene bedrock fault scarps along active normal faults... more ABSTRACT The existence of well-preserved Holocene bedrock fault scarps along active normal faults in the Mediterranean region and elsewhere suggests a dramatic reduction in rates of rock weathering and erosion that correlates with the transition from glacial to interglacial climate. We test and quantify this interpretation using a case study in the Italian Central Apennines. Holocene rates are derived from measurements of weathering-pit depth along the Magnola scarp, where previous cosmogenic 36Cl analyses constrain exposure history. To estimate the average hillslope erosion rate over ˜105 years, we introduce a simple geometric model of normal-fault footwall slope evolution. The model predicts that the gradient of a weathering-limited footwall hillslope is set by fault dip angle and by the ratio of slip rate to erosion rate; if either slip or erosion rate is known, the other can be derived. Applying this model to the Magnola fault yields an estimated average weathering rate on the order of 0.2-0.4 mm/yr, more than 10x higher than either the Holocene scarp weathering rate or modern regional limestone weathering rates. A numerical model of footwall growth and erosion, in which erosion rate tracks the oxygen-isotope curve, reproduces the main features of hillslope and scarp morphology and suggests that the hillslope erosion rate has varied by about a factor of 30 over the past one to two glacial cycles. We conclude that preservation of carbonate fault scarps reflects strong climatic control on rock breakdown by frost cracking.
Surface slip distributions for an active normal fault in central Italy have been measured using t... more Surface slip distributions for an active normal fault in central Italy have been measured using terrestrial laser scanning (TLS), in order to assess the impact of changes in fault orientation and kinematics when modelling subsurface slip distributions that control seismic moment release. The southeastern segment of the surface trace of the Campo Felice active normal fault near the city of L'Aquila was mapped and surveyed using techniques from structural geology and using TLS to define the vertical and horizontal offsets of geomorphic slopes since the last glacial maximum (15 ± 3 ka). The fault geometry and kinematics measured from 43 sites and throw/heave measurements from geomorphic offsets seen on 250 scarp profiles were analysed using a modification of the Kostrov equations to calculate the magnitudes and directions of horizontal principal strain-rates. The map trace of the studied fault is linear, except where a prominent bend has formed to link across a former left-stepping relay-zone. The dip of the fault and slip direction are constant across the bend. Throw-rates since 15 ± 3 ka decrease linearly from the fault centre to the tip, except in the location of the prominent bend where higher throw rates are recorded. Vertical coseismic offsets for two palaeo earthquake ruptures seen as fresh strips of rock at the base of the bedrock scarp also increase within the prominent bend. The principal strain-rate, calculated by combining strike, dip, slip-direction and post 15 ± 3 ka throw rate, decreases linearly from the fault centre towards the tip; the strain-rate does not increase across the prominent fault bend. The above shows that changes in fault strike, whilst having no effect on the principal horizontal strain-rate, can produce local maxima in throw-rates during single earthquakes that persist over the timescale of multiple earthquakes (15 ± 3 ka). Detailed geomorphological and structural characterisation of active faults is therefore a critical requirement in order to properly define fault activity for the purpose of accurate seismic hazard assessment. We discuss the implications of modelling subsurface slip distributions for earthquake ruptures through inversion of GPS, InSAR and strong motion data using planar fault approximations, referring to recent examples on the nearby Paganica fault that ruptured in the Mw 6.3 2009 L'Aquila Earthquake.
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Papers by Richard Phillips