Professor of Geophysics Supervisors: PhD Advisors - Bob White, Richard Hobbs and Postdoctoral adviser Alan Levander Address: OES, NOCS, European Way Southampton SO14 3ZH UK
As part of U.K./Indonesian/U.S. project CRUST (Collaborative Research into Understanding Sumatran... more As part of U.K./Indonesian/U.S. project CRUST (Collaborative Research into Understanding Sumatran Tectonics), the National Science Foundation funded one week of multichannel seismic acquisition during July 2008. Using the German research vessel Sonne, we acquired 25 reflection profiles using an array of 12 G- guns totaling 5420 cu. in recorded on a 192 channel, 2.4 km hydrophone streamer. These profiles, totaling ~1250 km in length, image the southern portion of the rupture area of the December 26, 2004 Mw 9.3 earthquake, in the vicinity of the epicenter. Nineteen profiles image the subducting ~70 myr old Indian Plate crust as it enters the Sunda Trench and underthrusts the resultant accretionary prism. A clear negative-polarity décollement lies close to the top of the oceanic plate, and numerous faults cut the extremely thick (>5.5 km) incoming sediments. These sediments originated as part of the Bengal Fan and may arrive at the Trench having already undergone significant dewatering and consolidation. The accretionary prism consists largely of landward vergent thrusts for the outer ~40-45 km, followed by a marginal plateau up to 70 km wide within which vergence appears to switch from landward to seaward, and an inner prism ~50 km wide exhibiting primarily seaward vergence and allowing for lower acoustic penetration. There is no evidence for a megasplay fault similar to that observed in the Nankai Trough near the Kii Peninsula. Strata from the incoming plate are well preserved and minimally deformed in sections several kilometers thick as far inboard as the landward edge of the marginal plateau. Significant for tsunamigenesis, the suggested strength of the incoming section inherent in these observations supports models involving updip rupture propagation that appears to reach the Trench. Six profiles image the forearc basin and the structural boundary between the inner prism and the basin, which we interpret as the West Andaman Fault. This structure is near vertical and thought to be a strike-slip response of strain-partitioning in this obliquely convergent system. Several basin unconformities are present, as well as debris flows that together may yield insight into margin history and past seismic events.
The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosp... more The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interf...
Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreadi... more Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.
ABSTRACT Fluids in subduction zones can influence seismogenic behaviour and prism morphology. The... more ABSTRACT Fluids in subduction zones can influence seismogenic behaviour and prism morphology. The Eastern Makran subduction zone, offshore Pakistan, has a very thick incoming sediment section of up to 7.5 km, providing a large potential fluid source to the accretionary prism. A hydrate-related bottom simulating reflector (BSR), zones of high amplitude reflectivity, seafloor seep sites and reflective thrust faults are present across the accretionary prism, indicating the presence of fluids and suggesting active fluid migration. High amplitude free gas zones and seep sites are primarily associated with anticlinal hinge traps, and fluids here appear to be sourced from shallow biogenic sources and migrate to the seafloor along minor normal faults. There are no observed seep sites associated with the surface expression of the wedge thrust faults, potentially due to burial of the surface trace by failure of the steep thrust ridge slopes. Thrust fault reflectivity is restricted to the upper 3 km of sediment and the deeper décollement is non-reflective. We interpret that fluids and overpressure are not common in the deeper stratigraphic section. Thermal modelling of sediments at the deformation front suggests that the deeper sediment section is relatively dewatered and not currently contributing to fluid expulsion in the Makran accretionary prism.
ABSTRACT The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and rece... more ABSTRACT The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and recent, smaller earthquakes also suggest seismicity on the megathrust; however, its historical record is limited and hazard potential enigmatic. We have developed a 2-D thermal model of the subduction zone. The results are twofold: (1) The thick sediment cover on the incoming plate leads to high (~150°) plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a shallow depth, and (2) the shallow dip of the subducting plate leads to a wide potential seismogenic zone (up to ~350 km). Combining these results with along strike rupture scenarios indicates that Mw8.7-9.2 earthquakes are possible in the seaward Makran subduction zone. These results have important earthquake and tsunami hazard implications, particularly for the adjacent coastlines of Pakistan, Iran, Oman, and India, as the Makran has not been previously considered a likely candidate for a Mw > 9 earthquake.
ABSTRACT The Makran subduction zone is the widest accretionary prism in the world (~400km), gener... more ABSTRACT The Makran subduction zone is the widest accretionary prism in the world (~400km), generated by convergence between the Arabian and Eurasian tectonic plates. It represents a global end-member, with a 7km thick incoming sediment section. Accretionary prisms have traditionally been thought to be aseismic due to the presence of unconsolidated sediment and elevated basal pore pressures. The seismogenic potential of the Makran subduction zone is unclear, despite a Mw 8.1 earthquake in 1945 that may have been located on the plate boundary beneath the prism. In this study, a series of imbricate landward dipping (seaward verging) thrust faults have been interpreted across the submarine prism (outer 70 km) using over 6000km of industry multichannel seismic data and bathymetric data. A strong BSR (bottom simulating reflector) is present throughout the prism (excluding the far east). An unreflective décollement is interpreted from the geometry of the prism thrusts. Two major sedimentary units are identified in the input section, the lower of which contains the extension of the unreflective décollement surface. Between 60%-100% of the input section is currently being accreted. The geometry of piggy-back basin stratigraphy shows that the majority of thrusts, including those over 50km from the trench, are recently active. Landward thrusts show evidence for reactivation after periods of quiescence. Negative polarity fault plane reflectors are common in the frontal thrusts and in the eastern prism, where they may be related to increased fault activity and fluid expulsion, and are rarer in older landward thrusts. Significant NE-SW trending basement structures (The Murray Ridge and Little Murray Ridge) on the Arabian plate intersect the deformation front and affect sediment input to the subduction zone. Prism taper and structure are apparently primarily controlled by sediment supply and the secondary influence of subducting basement ridges. The thick, likely distal, sediment section in the west produces a prism with a simple imbricate structure. As basement depth is reduced over the Little Murray Ridge, the accretionary prism structure (fault spacing and deformation front position) changes. In the east, proximity to the Murray Ridge and triple junction is expressed through a reduction in prism width and reduced fault activity. The resulting prism structure and morphology can ultimately be used to assess likely sediment properties and hence seismic potential at the plate boundary.
ABSTRACT The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of... more ABSTRACT The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of any subduction zone. These sediments have formed a wide accretionary prism (˜400 km). Seismicity in the Makran is generally low; however the margin experienced an Mw 8.1 earthquake in 1945 which generated a significant regional tsunami. Seismic reflection data and swath bathymetry data from offshore Pakistan are used to analyze the structure and fault activity of the outer accretionary prism. The outer prism has a simple structure of seaward verging imbricate thrust faults, many continuous for over 100 km along strike. Fault activity is analyzed using basin stratigraphy and fault geometry, revealing a frontal continuously active zone, a central intermittently active zone, and a landward inactive zone. Over 75% of the faults in the seaward ˜70 km of the prism show evidence for recent activity. The décollement occurs within the lower sediment section, but steps onto the top-basement surface in regions of elevated basement topography. Fault spacing (6 km) and taper (4.5°) are comparable to other margins such as S. Hikurangi, Cascadia and Nankai, suggesting that high sediment input is not leading to an unusual prism structure. The décollement is unreflective, which is unexpected considering other prism characteristics predicting a weak surface, and may suggest a potentially stronger décollement than previously predicted. This study provides a significant advance in our understanding of the structure of an end-member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential.
Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8... more Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8.6 and M w 8.2 strike-slip intraplate earthquakes on 11 April 2012, while clearly occurring in the equatorial Indian Ocean diffuse plate boundary zone, are a case in point, with disagreement on the nature of the focal mechanisms and the faults that ruptured. We use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern Wharton Basin to demonstrate pervasive brittle deformation between the Ninet-yeast Ridge and the Sunda subduction zone. In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. However, left-lateral strike-slip reactivation of existing fracture zones started earlier, in the Paleocene to early Eocene, and compartmentalizes the Wharton Basin. Modeled rupture during the 11 April 2012 intraplate earthquakes is consistent with the location of two reactivated, closely spaced, approximately north-south–trending fracture zones. However, we find no evidence for WNW-ESE–trending faults in the shallow crust, which is at variance with most of the earthquake fault models.
Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor... more Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmen-tation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the M w = 8.7, A.D. 2005 Simeulue-Nias earthquake , and the northern termination of a major 1797 earthquake that was partly filled by an M w = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.
The rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a ... more The rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.
ABSTRACT In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and p... more ABSTRACT In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution 1–3. V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume 4–7. Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle 8. Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years—providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5–10 • C. From 35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25–30 • C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core–mantle boundary. Spatial and temporal patterns of convective circulation beneath lithospheric plates cause regional elevation changes at the Earth's surface that have important—but poorly understood— implications for the development of dynamic topography on geologic timescales. As the Rayleigh number of convecting mantle is 10 6 –10 8 , this circulation is expected to be transient, varying on timescales of 1–100 Myr and on length scales of hundreds to thousands of kilometres 3,9. A global network of mid-ocean ridges provides a useful means of estimating the temperature of underlying asthenospheric mantle 10,11. At spreading mid-ocean ridges, accretion of oceanic crust is sensitive to small temperature fluctuations that change the thickness of newly formed crust by kilometres 2. In the North Atlantic Ocean, the Reykjanes Ridge crosses the Icelandic plume, a hot convective upwelling with a radius of at least 1,200 km (ref. 12). Within the region influenced by this plume, average thickness of oceanic crust increases from 7 to 14 km and the seabed is anomalously shallow by up to 2 km. Both observations are consistent with an average temperature anomaly of 150 • C (ref. 2). Several different timescales of transient behaviour are sampled by the mid-ocean ridge's interaction with this plume. On the shortest timescale, the most obvious and best-known features are diachronous V-shaped ridges (VSRs) that are visible on either side of the ridge axis where sedimentary cover is minimal (Fig. 1). These VSRs probably reflect minor changes in the thickness and composition of oceanic crust and are generated when hotter than average parcels of plume material travel radially away from the plume's conduit 13,14. On a much longer timescale, there is a transition from smooth crust without fracture zones, accreted over hotter asthenosphere, to rough crust with fracture zones, accreted over colder asthenosphere (Fig. 1). This observation suggests that the plume's planform has changed through time. Today, the plume's thermal (as opposed to chemical) influence extends to the intersection between the mid-ocean ridge and the Bight Fracture Zone at 57 • N and 33 • W (refs 2,5,15). Despite their importance in providing otherwise inaccessible insights into convective processes, the structure and extent of these VSRs are poorly known and their origin is still debated 16,17. It is especially unclear how many VSRs exist and how far back in time their history can be traced. To address these general issues, we acquired two regional (>1,200 km) seismic reflection profiles that traverse the oceanic basin south of Iceland (Fig. 1). Crucially, these profiles provide conjugate images of the Iceland and Irminger basins, because each one of them is oriented parallel to plate-spreading flowlines 18. Acquisition and processing details are provided in the Methods. We have two significant findings. First, we have mapped the sediment– basement interface, which demonstrates that VSR activity can be continuously traced back to 55 Myr ago (Ma). Second, this activity has been used to build a detailed chronology of asthenospheric potential temperature, T p. This continuous record provides a reference frame for analysing relationships between plume activity and other geologic observations. Profile 2 resolves the detailed structure of the Iceland and Irminger basins (Fig. 2a). Away from a prominent mid-ocean ridge, the top of oceanic crust is clearly imaged beneath layered sediments, which thicken in either direction. A sediment–basement interface can easily be traced, despite being cut by minor faults (Fig. 2b). The sedimentary pile is dominated by contourite drift deposits that record the history of deep-water…
ABSTRACT Understanding the formation and evolution of relatively uniform ocean crust formed at fa... more ABSTRACT Understanding the formation and evolution of relatively uniform ocean crust formed at fast spreading rates provides an important benchmark to understand more complex ocean crust formed at slow spreading rates. In this study, the thermal structure of the lower oceanic crust has been investigated at Hess Deep and ODP Hole 1256D using the calcium in olivine geospeedometer. Diffusion rates of calcium in olivine vary significantly with crystallographic axes. In previous studies, the diffusion coefficient for the fastest crystallographic axis has been used and the calculated cooling rates represent the fastest possible cooling, potentially overestimating the cooling rate by up to 50%. To improve upon this, we have used electron backscatter diffraction measurements to determine the orientation of the olivine crystals analysed for Ca diffusion profiles, allowing a profile specific diffusion coefficient to be calculated and used in the cooling rate calculations. The cooling rates through the lower crust at Hess Deep decrease by 1.5 to 2 orders of magnitude from near the dike/gabbro transition to the middle lower crust. Samples from 2 - 4 km show no variation with depth. These cooling rates are one order of magnitude faster than those published for the Wadi Abyad section of the Oman ophiolite (Coogan et al., 2002) but 2 orders of magnitude slower than those from the Wadi Tayan section in Oman (VanTongeren et al., 2008). Cooling rates from gabbros within and below the dike/gabbro boundary at Hole 1256D, Hess Deep and the Oman ophiolite are similar, reflecting the proximity to the upper crustal hydrothermal system that must influence the thermal structure at this key crustal transition.
Eos, Transactions American Geophysical Union, 2010
A series of linked marine and land studies have recently targeted the Sumatra subduction zone, fo... more A series of linked marine and land studies have recently targeted the Sumatra subduction zone, focusing on the 2004 and 2005 plate boundary earthquake ruptures in Indonesia. A collaborative research effort by scientists from the United Kingdom (UK Sumatra ...
Andesitic magma erupted at island arc strato-volcanoes is stored in the shallow crust prior to er... more Andesitic magma erupted at island arc strato-volcanoes is stored in the shallow crust prior to eruption. Constraining the size, location and characteristics of magma reservoirs is key to forecasting the likelihood and characteristics of future eruptions. The island of Montserrat, in the Lesser Antilles, has been the subject of an active-source seismic tomography experiment (the SEA-CALIPSO experiment) with the main aim of studying the magmatic system of the active Soufriere Hills Volcano (SHV). We present the results of the three-dimensional travel-time inversion of the dataset. We are able to image the main features of the volcanic system of the island including SHV and the two older and extinct volcanoes. The shallow structure is dominated by the presence of high-velocity cores beneath the three volcanic centres. Beneath SHV at depth between 5 and 8 km we observe a negative velocity anomaly (NVA) roughly 4 km across. Checkerboard tests show that a pattern with wavelength similar t...
As part of U.K./Indonesian/U.S. project CRUST (Collaborative Research into Understanding Sumatran... more As part of U.K./Indonesian/U.S. project CRUST (Collaborative Research into Understanding Sumatran Tectonics), the National Science Foundation funded one week of multichannel seismic acquisition during July 2008. Using the German research vessel Sonne, we acquired 25 reflection profiles using an array of 12 G- guns totaling 5420 cu. in recorded on a 192 channel, 2.4 km hydrophone streamer. These profiles, totaling ~1250 km in length, image the southern portion of the rupture area of the December 26, 2004 Mw 9.3 earthquake, in the vicinity of the epicenter. Nineteen profiles image the subducting ~70 myr old Indian Plate crust as it enters the Sunda Trench and underthrusts the resultant accretionary prism. A clear negative-polarity décollement lies close to the top of the oceanic plate, and numerous faults cut the extremely thick (>5.5 km) incoming sediments. These sediments originated as part of the Bengal Fan and may arrive at the Trench having already undergone significant dewatering and consolidation. The accretionary prism consists largely of landward vergent thrusts for the outer ~40-45 km, followed by a marginal plateau up to 70 km wide within which vergence appears to switch from landward to seaward, and an inner prism ~50 km wide exhibiting primarily seaward vergence and allowing for lower acoustic penetration. There is no evidence for a megasplay fault similar to that observed in the Nankai Trough near the Kii Peninsula. Strata from the incoming plate are well preserved and minimally deformed in sections several kilometers thick as far inboard as the landward edge of the marginal plateau. Significant for tsunamigenesis, the suggested strength of the incoming section inherent in these observations supports models involving updip rupture propagation that appears to reach the Trench. Six profiles image the forearc basin and the structural boundary between the inner prism and the basin, which we interpret as the West Andaman Fault. This structure is near vertical and thought to be a strike-slip response of strain-partitioning in this obliquely convergent system. Several basin unconformities are present, as well as debris flows that together may yield insight into margin history and past seismic events.
The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosp... more The Lesser Antilles arc is only one of two subduction zones where slow‐spreading Atlantic lithosphere is consumed. Slow‐spreading may result in the Atlantic lithosphere being more pervasively and heterogeneously hydrated than fast‐spreading Pacific lithosphere, thus affecting the flux of fluids into the deep mantle. Understanding the distribution of seismicity can help unravel the effect of fluids on geodynamic and seismogenic processes. However, a detailed view of local seismicity across the whole Lesser Antilles subduction zone is lacking. Using a temporary ocean‐bottom seismic network we invert for hypocenters and 1D velocity model. A systematic search yields a 27 km thick crust, reflecting average arc and back‐arc structures. We find abundant intraslab seismicity beneath Martinique and Dominica, which may relate to the subducted Marathon and/or Mercurius Fracture Zones. Pervasive seismicity in the cold mantle wedge corner and thrust seismicity deep on the subducting plate interf...
Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreadi... more Deep water cycle studies have largely focused on subduction of lithosphere formed at fast spreading ridges. However, oceanic plates are more likely to become hydrated as spreading rate decreases.
ABSTRACT Fluids in subduction zones can influence seismogenic behaviour and prism morphology. The... more ABSTRACT Fluids in subduction zones can influence seismogenic behaviour and prism morphology. The Eastern Makran subduction zone, offshore Pakistan, has a very thick incoming sediment section of up to 7.5 km, providing a large potential fluid source to the accretionary prism. A hydrate-related bottom simulating reflector (BSR), zones of high amplitude reflectivity, seafloor seep sites and reflective thrust faults are present across the accretionary prism, indicating the presence of fluids and suggesting active fluid migration. High amplitude free gas zones and seep sites are primarily associated with anticlinal hinge traps, and fluids here appear to be sourced from shallow biogenic sources and migrate to the seafloor along minor normal faults. There are no observed seep sites associated with the surface expression of the wedge thrust faults, potentially due to burial of the surface trace by failure of the steep thrust ridge slopes. Thrust fault reflectivity is restricted to the upper 3 km of sediment and the deeper décollement is non-reflective. We interpret that fluids and overpressure are not common in the deeper stratigraphic section. Thermal modelling of sediments at the deformation front suggests that the deeper sediment section is relatively dewatered and not currently contributing to fluid expulsion in the Makran accretionary prism.
ABSTRACT The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and rece... more ABSTRACT The Makran subduction zone experienced a tsunamigenic Mw 8.1 earthquake in 1945 and recent, smaller earthquakes also suggest seismicity on the megathrust; however, its historical record is limited and hazard potential enigmatic. We have developed a 2-D thermal model of the subduction zone. The results are twofold: (1) The thick sediment cover on the incoming plate leads to high (~150°) plate boundary temperatures at the deformation front making the megathrust potentially seismogenic to a shallow depth, and (2) the shallow dip of the subducting plate leads to a wide potential seismogenic zone (up to ~350 km). Combining these results with along strike rupture scenarios indicates that Mw8.7-9.2 earthquakes are possible in the seaward Makran subduction zone. These results have important earthquake and tsunami hazard implications, particularly for the adjacent coastlines of Pakistan, Iran, Oman, and India, as the Makran has not been previously considered a likely candidate for a Mw > 9 earthquake.
ABSTRACT The Makran subduction zone is the widest accretionary prism in the world (~400km), gener... more ABSTRACT The Makran subduction zone is the widest accretionary prism in the world (~400km), generated by convergence between the Arabian and Eurasian tectonic plates. It represents a global end-member, with a 7km thick incoming sediment section. Accretionary prisms have traditionally been thought to be aseismic due to the presence of unconsolidated sediment and elevated basal pore pressures. The seismogenic potential of the Makran subduction zone is unclear, despite a Mw 8.1 earthquake in 1945 that may have been located on the plate boundary beneath the prism. In this study, a series of imbricate landward dipping (seaward verging) thrust faults have been interpreted across the submarine prism (outer 70 km) using over 6000km of industry multichannel seismic data and bathymetric data. A strong BSR (bottom simulating reflector) is present throughout the prism (excluding the far east). An unreflective décollement is interpreted from the geometry of the prism thrusts. Two major sedimentary units are identified in the input section, the lower of which contains the extension of the unreflective décollement surface. Between 60%-100% of the input section is currently being accreted. The geometry of piggy-back basin stratigraphy shows that the majority of thrusts, including those over 50km from the trench, are recently active. Landward thrusts show evidence for reactivation after periods of quiescence. Negative polarity fault plane reflectors are common in the frontal thrusts and in the eastern prism, where they may be related to increased fault activity and fluid expulsion, and are rarer in older landward thrusts. Significant NE-SW trending basement structures (The Murray Ridge and Little Murray Ridge) on the Arabian plate intersect the deformation front and affect sediment input to the subduction zone. Prism taper and structure are apparently primarily controlled by sediment supply and the secondary influence of subducting basement ridges. The thick, likely distal, sediment section in the west produces a prism with a simple imbricate structure. As basement depth is reduced over the Little Murray Ridge, the accretionary prism structure (fault spacing and deformation front position) changes. In the east, proximity to the Murray Ridge and triple junction is expressed through a reduction in prism width and reduced fault activity. The resulting prism structure and morphology can ultimately be used to assess likely sediment properties and hence seismic potential at the plate boundary.
ABSTRACT The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of... more ABSTRACT The Makran Subduction Zone has the highest incoming sediment thickness (up to 7.5 km) of any subduction zone. These sediments have formed a wide accretionary prism (˜400 km). Seismicity in the Makran is generally low; however the margin experienced an Mw 8.1 earthquake in 1945 which generated a significant regional tsunami. Seismic reflection data and swath bathymetry data from offshore Pakistan are used to analyze the structure and fault activity of the outer accretionary prism. The outer prism has a simple structure of seaward verging imbricate thrust faults, many continuous for over 100 km along strike. Fault activity is analyzed using basin stratigraphy and fault geometry, revealing a frontal continuously active zone, a central intermittently active zone, and a landward inactive zone. Over 75% of the faults in the seaward ˜70 km of the prism show evidence for recent activity. The décollement occurs within the lower sediment section, but steps onto the top-basement surface in regions of elevated basement topography. Fault spacing (6 km) and taper (4.5°) are comparable to other margins such as S. Hikurangi, Cascadia and Nankai, suggesting that high sediment input is not leading to an unusual prism structure. The décollement is unreflective, which is unexpected considering other prism characteristics predicting a weak surface, and may suggest a potentially stronger décollement than previously predicted. This study provides a significant advance in our understanding of the structure of an end-member convergent margin and demonstrates that systematic analyses of accretionary prism structure can help to elucidate subduction zone dynamics with ultimate relevance to seismogenic potential.
Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8... more Large-magnitude intraplate earthquakes within the ocean basins are not well understood. The M w 8.6 and M w 8.2 strike-slip intraplate earthquakes on 11 April 2012, while clearly occurring in the equatorial Indian Ocean diffuse plate boundary zone, are a case in point, with disagreement on the nature of the focal mechanisms and the faults that ruptured. We use bathymetric and seismic reflection data from the rupture area of the earthquakes in the northern Wharton Basin to demonstrate pervasive brittle deformation between the Ninet-yeast Ridge and the Sunda subduction zone. In addition to evidence of recent strike-slip deformation along approximately north-south–trending fossil fracture zones, we identify a new type of deformation structure in the Indian Ocean: conjugate Riedel shears limited to the sediment section and oriented oblique to the north-south fracture zones. The Riedel shears developed in the Miocene, at a similar time to the onset of diffuse deformation in the central Indian Ocean. However, left-lateral strike-slip reactivation of existing fracture zones started earlier, in the Paleocene to early Eocene, and compartmentalizes the Wharton Basin. Modeled rupture during the 11 April 2012 intraplate earthquakes is consistent with the location of two reactivated, closely spaced, approximately north-south–trending fracture zones. However, we find no evidence for WNW-ESE–trending faults in the shallow crust, which is at variance with most of the earthquake fault models.
Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor... more Earthquakes in subduction zones rupture the plate boundary fault in discrete segments. One factor that may control this segmen-tation is topography on the downgoing plate, although it is controversial whether this is by weakening or strengthening of the fault. We use multichannel seismic and gravity data to map the top of the downgoing oceanic crust offshore central Sumatra, Indonesia. Our survey spans a complex segment boundary zone between the southern termination of the M w = 8.7, A.D. 2005 Simeulue-Nias earthquake , and the northern termination of a major 1797 earthquake that was partly filled by an M w = 7.7 event in 1935. We identify an isolated 3 km basement high at the northern edge of this zone, close to the 2005 slip termination. The high probably originated at the Wharton fossil ridge, and is almost aseismic in both local and global data sets, suggesting that while the region around it may be weakened by fracturing and fluids, the basement high locally strengthens the plate boundary, stopping rupture propagation.
The rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a ... more The rise of atmospheric carbon dioxide (CO2) principally due to the burning of fossil fuels is a key driver of anthropogenic climate change. Mitigation strategies include improved efficiency, using renewable energy, and capture and long-term sequestration of CO2. Most sequestration research considers CO2 injection into deep saline aquifers or depleted hydrocarbon reservoirs. Unconventional suggestions include CO2 storage in the porous volcanic lavas of uppermost oceanic crust. Here we test the feasibility of injecting CO2 into deep-sea basalts and identify sites where CO2 should be both physically and gravitationally trapped. We use global databases to estimate pressure and temperature, hence density of CO2 and seawater at the sediment-basement interface. At previously suggested sites on the Juan de Fuca Plate and in the eastern equatorial Pacific Ocean, CO2 is gravitationally unstable. However, we identify five sediment-covered regions where CO2 is denser than seawater, each sufficient for several centuries of anthropogenic CO2 emissions.
ABSTRACT In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and p... more ABSTRACT In the North Atlantic Ocean, a mid-ocean ridge bisects the Icelandic mantle plume, and provides a window into its temporal evolution 1–3. V-shaped ridges of thick oceanic crust observed south of Iceland are thought to record pulses of upwelling within the plume 4–7. Specifically, excess crust is thought to form during the quasi-periodic generation of hot solitary waves triggered by thermal instabilities in the mantle 8. Here we use seismic reflection data to show that V-shaped ridges have formed over the past 55 million years—providing the longest record of plume periodicity of its kind. We find evidence for minor, but systematic, asymmetric formation of crust, due to migration of the mid-ocean ridge with respect to the underlying plume. We also find changes in periodicity: from 55 to 35 million years ago, the V-shaped ridges form every 3 million years or so and reflect small fluctuations in plume temperature of about 5–10 • C. From 35 million years ago, the periodicity changes to about 8 million years and reflects changes in mantle temperature of 25–30 • C. We suggest that this change in periodicity is probably caused by perturbations in the thermal state at the plume source, either at the mantle-transition zone or core–mantle boundary. Spatial and temporal patterns of convective circulation beneath lithospheric plates cause regional elevation changes at the Earth's surface that have important—but poorly understood— implications for the development of dynamic topography on geologic timescales. As the Rayleigh number of convecting mantle is 10 6 –10 8 , this circulation is expected to be transient, varying on timescales of 1–100 Myr and on length scales of hundreds to thousands of kilometres 3,9. A global network of mid-ocean ridges provides a useful means of estimating the temperature of underlying asthenospheric mantle 10,11. At spreading mid-ocean ridges, accretion of oceanic crust is sensitive to small temperature fluctuations that change the thickness of newly formed crust by kilometres 2. In the North Atlantic Ocean, the Reykjanes Ridge crosses the Icelandic plume, a hot convective upwelling with a radius of at least 1,200 km (ref. 12). Within the region influenced by this plume, average thickness of oceanic crust increases from 7 to 14 km and the seabed is anomalously shallow by up to 2 km. Both observations are consistent with an average temperature anomaly of 150 • C (ref. 2). Several different timescales of transient behaviour are sampled by the mid-ocean ridge's interaction with this plume. On the shortest timescale, the most obvious and best-known features are diachronous V-shaped ridges (VSRs) that are visible on either side of the ridge axis where sedimentary cover is minimal (Fig. 1). These VSRs probably reflect minor changes in the thickness and composition of oceanic crust and are generated when hotter than average parcels of plume material travel radially away from the plume's conduit 13,14. On a much longer timescale, there is a transition from smooth crust without fracture zones, accreted over hotter asthenosphere, to rough crust with fracture zones, accreted over colder asthenosphere (Fig. 1). This observation suggests that the plume's planform has changed through time. Today, the plume's thermal (as opposed to chemical) influence extends to the intersection between the mid-ocean ridge and the Bight Fracture Zone at 57 • N and 33 • W (refs 2,5,15). Despite their importance in providing otherwise inaccessible insights into convective processes, the structure and extent of these VSRs are poorly known and their origin is still debated 16,17. It is especially unclear how many VSRs exist and how far back in time their history can be traced. To address these general issues, we acquired two regional (>1,200 km) seismic reflection profiles that traverse the oceanic basin south of Iceland (Fig. 1). Crucially, these profiles provide conjugate images of the Iceland and Irminger basins, because each one of them is oriented parallel to plate-spreading flowlines 18. Acquisition and processing details are provided in the Methods. We have two significant findings. First, we have mapped the sediment– basement interface, which demonstrates that VSR activity can be continuously traced back to 55 Myr ago (Ma). Second, this activity has been used to build a detailed chronology of asthenospheric potential temperature, T p. This continuous record provides a reference frame for analysing relationships between plume activity and other geologic observations. Profile 2 resolves the detailed structure of the Iceland and Irminger basins (Fig. 2a). Away from a prominent mid-ocean ridge, the top of oceanic crust is clearly imaged beneath layered sediments, which thicken in either direction. A sediment–basement interface can easily be traced, despite being cut by minor faults (Fig. 2b). The sedimentary pile is dominated by contourite drift deposits that record the history of deep-water…
ABSTRACT Understanding the formation and evolution of relatively uniform ocean crust formed at fa... more ABSTRACT Understanding the formation and evolution of relatively uniform ocean crust formed at fast spreading rates provides an important benchmark to understand more complex ocean crust formed at slow spreading rates. In this study, the thermal structure of the lower oceanic crust has been investigated at Hess Deep and ODP Hole 1256D using the calcium in olivine geospeedometer. Diffusion rates of calcium in olivine vary significantly with crystallographic axes. In previous studies, the diffusion coefficient for the fastest crystallographic axis has been used and the calculated cooling rates represent the fastest possible cooling, potentially overestimating the cooling rate by up to 50%. To improve upon this, we have used electron backscatter diffraction measurements to determine the orientation of the olivine crystals analysed for Ca diffusion profiles, allowing a profile specific diffusion coefficient to be calculated and used in the cooling rate calculations. The cooling rates through the lower crust at Hess Deep decrease by 1.5 to 2 orders of magnitude from near the dike/gabbro transition to the middle lower crust. Samples from 2 - 4 km show no variation with depth. These cooling rates are one order of magnitude faster than those published for the Wadi Abyad section of the Oman ophiolite (Coogan et al., 2002) but 2 orders of magnitude slower than those from the Wadi Tayan section in Oman (VanTongeren et al., 2008). Cooling rates from gabbros within and below the dike/gabbro boundary at Hole 1256D, Hess Deep and the Oman ophiolite are similar, reflecting the proximity to the upper crustal hydrothermal system that must influence the thermal structure at this key crustal transition.
Eos, Transactions American Geophysical Union, 2010
A series of linked marine and land studies have recently targeted the Sumatra subduction zone, fo... more A series of linked marine and land studies have recently targeted the Sumatra subduction zone, focusing on the 2004 and 2005 plate boundary earthquake ruptures in Indonesia. A collaborative research effort by scientists from the United Kingdom (UK Sumatra ...
Andesitic magma erupted at island arc strato-volcanoes is stored in the shallow crust prior to er... more Andesitic magma erupted at island arc strato-volcanoes is stored in the shallow crust prior to eruption. Constraining the size, location and characteristics of magma reservoirs is key to forecasting the likelihood and characteristics of future eruptions. The island of Montserrat, in the Lesser Antilles, has been the subject of an active-source seismic tomography experiment (the SEA-CALIPSO experiment) with the main aim of studying the magmatic system of the active Soufriere Hills Volcano (SHV). We present the results of the three-dimensional travel-time inversion of the dataset. We are able to image the main features of the volcanic system of the island including SHV and the two older and extinct volcanoes. The shallow structure is dominated by the presence of high-velocity cores beneath the three volcanic centres. Beneath SHV at depth between 5 and 8 km we observe a negative velocity anomaly (NVA) roughly 4 km across. Checkerboard tests show that a pattern with wavelength similar t...
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