Conjugate strike-slip faulting across a subduction front driven by incipient seamount subduction

Geology ◽  
2020 ◽  
Vol 48 (5) ◽  
pp. 493-498
Author(s):  
Sam R. Davidson ◽  
Philip M. Barnes ◽  
Jarg R. Pettinga ◽  
Andrew Nicol ◽  
Joshu J. Mountjoy ◽  
...  
Keyword(s):  
New Zealand ◽  
Seismic Reflection ◽  
Fluid Pressure ◽  
Strike Slip ◽  
Accretionary Wedge ◽  
Pore Fluid Pressure ◽  
Plate Convergence ◽  
Swath Bathymetry ◽  
Seamount Subduction ◽  
Overlying Strata

Abstract The initial stages of seamount subduction and associated deformation in an overriding accretionary wedge is rarely documented. Initial subduction of Bennett Knoll seamount and faulting of the overlying strata along the Hikurangi subduction margin, New Zealand, are here studied using multibeam swath bathymetry, subbottom profiles, and regional seismic reflection lines. Our results provide new insights into the earliest stages of seamount collision at sediment-rich margins. Differential shortening along the subduction front induced by seamount subduction is initially accommodated in the accretionary wedge by conjugate strike-slip faults that straddle the buried flanks of the seamount and offset the frontal thrusts by as much as 5 km. The geometries of the strike-slip faults are controlled by the seamount’s dimensions and aspect, the obliquity of plate convergence, pore-fluid pressure, and the thickness and rheology of the incoming sedimentary section. Strike-slip faults in such settings are ephemeral and overprinted by the formation of new structures as seamount subduction advances.

scholarly journals Focused fluid seepage related to variations in accretionary wedge structure, Hikurangi margin, New Zealand

Geology ◽  
2019 ◽  
Vol 48 (1) ◽  
pp. 56-61 ◽  
Author(s):  
Sally J. Watson ◽  
Joshu J. Mountjoy ◽  
Philip M. Barnes ◽  
Gareth J. Crutchley ◽  
Geoffroy Lamarche ◽  
...  
Keyword(s):  
Fluid Flow ◽  
New Zealand ◽  
Fluid Pressure ◽  
Data Sets ◽  
Accretionary Wedge ◽  
Fault Mechanics ◽  
Spatial Coverage ◽  
Fluid Seepage ◽  
Pressure Regime ◽  
Subduction Setting

Abstract Hydrogeological processes influence the morphology, mechanical behavior, and evolution of subduction margins. Fluid supply, release, migration, and drainage control fluid pressure and collectively govern the stress state, which varies between accretionary and nonaccretionary systems. We compiled over a decade of published and unpublished acoustic data sets and seafloor observations to analyze the distribution of focused fluid expulsion along the Hikurangi margin, New Zealand. The spatial coverage and quality of our data are exceptional for subduction margins globally. We found that focused fluid seepage is widespread and varies south to north with changes in subduction setting, including: wedge morphology, convergence rate, seafloor roughness, and sediment thickness on the incoming Pacific plate. Overall, focused seepage manifests most commonly above the deforming backstop, is common on thrust ridges, and is largely absent from the frontal wedge despite ubiquitous hydrate occurrences. Focused seepage distribution may reflect spatial differences in shallow permeability architecture, while diffusive fluid flow and seepage at scales below detection limits are also likely. From the spatial coincidence of fluids with major thrust faults that disrupt gas hydrate stability, we surmise that focused seepage distribution may also reflect deeper drainage of the forearc, with implications for pore-pressure regime, fault mechanics, and critical wedge stability and morphology. Because a range of subduction styles is represented by 800 km of along-strike variability, our results may have implications for understanding subduction fluid flow and seepage globally.

Underplating altered oceanic crust: Insights from numerical modelling

2021 ◽  
Author(s):  
Zoe Braden ◽  
Jonas B. Ruh ◽  
Whitney M. Behr
Keyword(s):  
Numerical Modelling ◽  
Oceanic Crust ◽  
Fluid Pressure ◽  
Geothermal Gradient ◽  
Pore Fluid ◽  
Surface Erosion ◽  
Accretionary Wedge ◽  
Pore Fluid Pressure ◽  
Mafic Rocks

<p>Observations of several active shallow subduction megathrusts suggest that they are localized as décollements within sedimentary sequences or at the contact between sedimentary layers and the underlying mafic oceanic crust.  Exhumed accretionary complexes from a range of subduction depths, however, preserve underplated mafic slivers, which indicate that megathrust faults can occasionally develop within the mafic oceanic crustal column. The incorporation of mafic rocks into the subduction interface shear zone has the potential to influence both long-term subduction dynamics and short-term seismic and transient slip behaviour, but the processes and conditions that favour localisation of the megathrust into deeper oceanic crustal levels are poorly understood.</p><p>In this work, we use visco-elasto-plastic numerical modelling to explore the long-term (million year) factors influencing the incorporation of mafic volcanic rocks into the subduction interface and accretionary wedge through underplating. We focus on the potential importance of oceanic seafloor alteration in facilitating oceanic crustal weakening, which is implemented through a temperature-dependent pore-fluid pressure ratio (lambda = 0.90-0.99 between 160 and 300oC). We then examine the underplating response to changes in sediment thickness, geothermal gradient, sediment fluid pressure, and surface erosion rates. Our results indicate that a thinner incoming sediment package and a lower geothermal gradient cause oceanic crustal underplating to initiate deeper beneath the backstop (overriding plate) compared to thicker incoming sediment and a higher geothermal gradient. Relative pore fluid pressure differences between sediments and altered oceanic crust control the amount of altered oceanic crust that is underplated, as well as the location of underplating beneath the backstop or accretionary wedge. When sediments on top of the altered oceanic crust have the same fluid pressure as the altered oceanic crust, no oceanic crustal underplating occurs. Modelling results are also compared to exhumed subduction complexes to examine the amount and distribution of underplated mafic rocks.</p>

scholarly journals Cenozoic tectonic and thermal history of the Nenana basin, central interior Alaska: new constraints from seismic reflection data, fracture history, and apatite fission-track analyses

2017 ◽  
Vol 54 (7) ◽  
pp. 766-784 ◽  
Author(s):  
Nilesh Dixit ◽  
Catherine Hanks ◽  
Alec Rizzo ◽  
Paul McCarthy ◽  
Bernard Coakley
Keyword(s):  
Seismic Reflection ◽  
Fission Track ◽  
Strike Slip ◽  
Apatite Fission Track ◽  
Plate Convergence ◽  
South Central ◽  
Interior Alaska ◽  
Late Paleocene ◽  
Half Graben ◽  
Nenana Basin

The Nenana basin of interior Alaska forms a segment of the diffuse plate boundary between the Bering and North American plates and is located within a complex zone of crustal-scale strike-slip deformation that accommodates compressional stresses in response to oblique plate convergence to the south. The basin is currently the focus of new oil and gas exploration. Integration of seismic reflection and well data, fracture data, and apatite fission-track analyses with regional data improves our understanding of the tectonic development of this continental strike-slip basin. The Nenana basin formed during the Late Paleocene as a 13 km wide half-graben, affected by regional intraplate magmatism and localized crustal thinning across the Minto Fault in south-central Alaska. The basin was uplifted and exhumed along this faulted margin in the Early Eocene through to Late Oligocene in response to oblique subduction along the southern Alaska margin. This event resulted in the removal of up to 1.5 km of Late Paleocene strata from the basin. Renewed rifting and subsidence during the Early Miocene widened the basin to the west resulting in deposition of Miocene non-marine clastic rocks in reactivated and newly formed extensional half-grabens. In the Middle to Late Miocene, left lateral strike-slip faulting was superimposed on this half-graben system, with rapid subsidence beginning in the Pliocene and continuing to the present day. At present, the Nenana basin is in a zone of transtensional deformation that accommodates compressional stresses in response to oblique plate convergence and allows tectonic subsidence by oblique extension along major basin-bounding strike-slip faults.

scholarly journals Crustal Structure and Lithospheric Doming: Aspects of Deformation Along an Obliquely Convergent Plate Margin, New Zealand

2021 ◽  
Author(s):  
◽  
Anna Karen Pulford
Keyword(s):  
New Zealand ◽  
Crustal Structure ◽  
Fluid Pressure ◽  
Plate Boundary ◽  
Seismic Attenuation ◽  
Southern Alps ◽  
Pore Fluid ◽  
Pore Fluid Pressure ◽  
Volcanic Region ◽  
Buoyancy Forces

<p>Lithospheric deformation along and adjacent to the Pacific-Australian Plate boundary through New Zealand has resulted in different expressions in North and South Islands. This thesis investigates some aspects of crustal and upper mantle structure in New Zealand and is divided into two distinct parts. The first examines the structure of the obliquely compressional crustal plate boundary in South Island using seismic techniques; the second focuses on the domed topography of central North Island and its relationship to mantle processes. High density active source, one and three-component, seismic data from a transect across the Southern Alps provides information on the deformation of the crust across the Australia-Pacific plate boundary of South Island. These data show 0-0.08 s ([approximately] 0.25 %) delay times between the radial and transverse directions for shear waves (Sg and SmS phases), with maximum possible delays of 140 ms and the fast direction aligned with the transverse direction (approximately parallel to the plate boundary). The transect is perpendicular to the Alpine Fault, which is slightly oblique to the fast mantle directions determined from SKS phases. The small values of crustal splitting may result from the oblique angle of the ray paths to the actual crustal structure at depth, or the complex nature of the deformation as observed at the surface, which though on a small scale can be strongly anisotropic, may not add constructively over a large region. Poisson's ratio, determined from forward modelling of both P and S phases, shows low values of 0.21 - 0.24 for the crust of South Island. A broad region of low values ([sigma]=0.15) exists at 10-20 km depth under the Southern Alps, which corresponds to a previously identified body of low Vp and high resistivity. The low [sigma] is interpreted as low pore fluid pressure and high silica composition rocks. This contrasts with previous interpretations of iii iv high pore fluid pressure at this depth. The topography of central North Island, New Zealand, describes a 250 km wide and [approximately] 500 m high dome. Exhumation estimates from mudstone porosity measurement indicate an increase in exhumation from [approximately] 500 m at the coast to 2 km in the region of the present topographic high. Combining these values gives an estimate of rock uplift of over 2.5 km for central North Island, since 4 Ma, a rate of 0.6 mm/yr. Tectonic uplift of 1.25 km indicates that [approximately] 50 % of the rock uplift occurs in response to exhumation. An independent local estimation of differential erosion in central North Island gave 300 m of exhumation since at least 500 ka, a rate of [greater than or equal to] 0.6 mm/yr. Using a digital elevation model of New Zealand the fluvial incision of the landscape was calculated and [approximately]169 m of rebound can be attributed to incision. Contouring maximum incision elucidates a region of high incision [approximately] 50 km south of the present centre of domed rock uplift. Using incision as a proxy for rock uplift, it is hypothesised that the incision signal is recent and demonstrates the southward migration of the centre of rock uplift. Rebound of sedimentary basins due to a reduction in plate coupling forces can also account for some of the observed rock uplift. Buoyancy forces required to create the pattern and magnitude of rock uplift are investigated using a 3 D loading model of the lithosphere. Strong upward forces (65 MPa) are required under the Central Volcanic Region, combined with broad uplift (36 MPa) over western North Island, to fit the observed rock uplift. Low Pn velocities under the Central Volcanic Region indicate temperatures 500 [degrees] C hotter than that of normal mantle. This temperature anomaly corresponds to 60 kg/[cubic metre] less dense than normal mantle, which is consistent with the change in density of 66 kg/[cubic metre] estimated from the loading model and aassuming the density change occurs over a 100 km depth range. The southern extent of buoyancy forces does not correspond well to regions of high seismic attenuation in the lithosphere but instead with the region of high incision.</p>

scholarly journals The contemporary force balance in a wide accretionary wedge: numerical models of the southcentral Hikurangi margin of New Zealand

2019 ◽  
Vol 219 (2) ◽  
pp. 776-795 ◽  
Author(s):  
Susan Ellis ◽  
Francesca Ghisetti ◽  
Philip M Barnes ◽  
Carolyn Boulton ◽  
Åke Fagereng ◽  
...  
Keyword(s):  
New Zealand ◽  
Laboratory Experiments ◽  
Numerical Models ◽  
Fluid Pressure ◽  
Thick Layer ◽  
Force Balance ◽  
Physical Parameters ◽  
Accretionary Wedge ◽  
Plate Interface ◽  
Seismic Section

SUMMARY The southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a >3-km-thick layer of sediments, with deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.

Main active structures in the Barbados accretionary wedge of the Lesser Antilles Subduction: implications for slip partitioning

2020 ◽  
Author(s):  
Gaëlle Bénâtre ◽  
Nathalie Feuillet ◽  
Hélène Carton ◽  
Eric Jacques ◽  
Thibaud Pichot
Keyword(s):  
Seismic Reflection ◽  
Linear Structure ◽  
Strike Slip ◽  
Lesser Antilles ◽  
Accretionary Wedge ◽  
Seismic Reflection Data ◽  
Megathrust Earthquakes ◽  
Reflection Data ◽  
Slip Partitioning ◽  
Seismic Reflection Profiles

&lt;p&gt;At the Lesser Antilles Subduction Zone (LASZ), the American plates subduct under the Caribbean plate at a slow rate of ~2 cm/yr. No major subduction megathrust earthquakes have occurred in the area since the 1839 and 1843 historical events, and the LASZ is typically considered weakly coupled. At the front of the LASZ, the Barbados accretionary wedge (BAW) is one of the largest accretionary wedges in the world. The width of the BAW decreases northward, owing to the increasing distance to the sediment source (Orinoco river) and the presence of several aseismic oceanic ridges, in particular the Tiburon ridge, that stops sediment progression. Marine geophysical studies conducted to date over the northern part of the BAW (Guadeloupe-Martinique sector) have mostly focused on resolving the geometry of the backstop. However, the structure of the wedge and the mechanical behavior of the subduction interface remain poorly known. Our study aims to describe the geometry of the BAW by a detailed morpho-tectonic analysis in order to place constraints on present and past dynamic interactions between the subducting and overriding plates.&lt;/p&gt;&lt;p&gt;New high-resolution bathymetric data (gridded at 50 meters), CHIRP data and 48-channels seismic reflection profiles were acquired over the BAW in the Guadeloupe-Martinique sector during the CASEIS cruise (10.17600/16001800) conducted in 2016 with the IFREMER vessel N/O Pourquoi Pas? We present results from the analysis of these new data, complemented by existing bathymetry and seismic reflection data acquired by several previous cruises, with an emphasis on the inner wedge domain. The data reveal a 180 km-long linear structure between 15&amp;#176;15&amp;#8217;N and 16&amp;#176;45&amp;#8217;N latitude, imaged as a positive flower structure on several CASEIS seismic reflection profiles. We interpret this structure as a strike-slip fault and name it the Seraphine fault. The identification of a horse-tail structure linked to an eastward bend of the fault trace at its northern end, as well as left-stepping &lt;em&gt;en &amp;#233;chelon&lt;/em&gt; folds west of the Seraphine fault, allow to determine the kinematics of the fault as left-lateral strike-slip. The Seraphine fault could root at the toe of the backstop (at least in its central portion). CHIRP data show evidence of folding of recent sedimentary units that are linked to the Seraphine fault, supporting the idea of recent activity. While at odds with the low obliquity of the convergence in this area, the Seraphine fault could be the expression of slip partitioning, similarly to the Bunce fault observed father north along the LASZ where obliquity is much stronger.&lt;/p&gt;

scholarly journals Crustal Structure and Lithospheric Doming: Aspects of Deformation Along an Obliquely Convergent Plate Margin, New Zealand

2021 ◽  
Author(s):  
◽  
Anna Karen Pulford
Keyword(s):  
New Zealand ◽  
Crustal Structure ◽  
Fluid Pressure ◽  
Plate Boundary ◽  
Seismic Attenuation ◽  
Southern Alps ◽  
Pore Fluid ◽  
Pore Fluid Pressure ◽  
Volcanic Region ◽  
Buoyancy Forces

<p>Lithospheric deformation along and adjacent to the Pacific-Australian Plate boundary through New Zealand has resulted in different expressions in North and South Islands. This thesis investigates some aspects of crustal and upper mantle structure in New Zealand and is divided into two distinct parts. The first examines the structure of the obliquely compressional crustal plate boundary in South Island using seismic techniques; the second focuses on the domed topography of central North Island and its relationship to mantle processes. High density active source, one and three-component, seismic data from a transect across the Southern Alps provides information on the deformation of the crust across the Australia-Pacific plate boundary of South Island. These data show 0-0.08 s ([approximately] 0.25 %) delay times between the radial and transverse directions for shear waves (Sg and SmS phases), with maximum possible delays of 140 ms and the fast direction aligned with the transverse direction (approximately parallel to the plate boundary). The transect is perpendicular to the Alpine Fault, which is slightly oblique to the fast mantle directions determined from SKS phases. The small values of crustal splitting may result from the oblique angle of the ray paths to the actual crustal structure at depth, or the complex nature of the deformation as observed at the surface, which though on a small scale can be strongly anisotropic, may not add constructively over a large region. Poisson's ratio, determined from forward modelling of both P and S phases, shows low values of 0.21 - 0.24 for the crust of South Island. A broad region of low values ([sigma]=0.15) exists at 10-20 km depth under the Southern Alps, which corresponds to a previously identified body of low Vp and high resistivity. The low [sigma] is interpreted as low pore fluid pressure and high silica composition rocks. This contrasts with previous interpretations of iii iv high pore fluid pressure at this depth. The topography of central North Island, New Zealand, describes a 250 km wide and [approximately] 500 m high dome. Exhumation estimates from mudstone porosity measurement indicate an increase in exhumation from [approximately] 500 m at the coast to 2 km in the region of the present topographic high. Combining these values gives an estimate of rock uplift of over 2.5 km for central North Island, since 4 Ma, a rate of 0.6 mm/yr. Tectonic uplift of 1.25 km indicates that [approximately] 50 % of the rock uplift occurs in response to exhumation. An independent local estimation of differential erosion in central North Island gave 300 m of exhumation since at least 500 ka, a rate of [greater than or equal to] 0.6 mm/yr. Using a digital elevation model of New Zealand the fluvial incision of the landscape was calculated and [approximately]169 m of rebound can be attributed to incision. Contouring maximum incision elucidates a region of high incision [approximately] 50 km south of the present centre of domed rock uplift. Using incision as a proxy for rock uplift, it is hypothesised that the incision signal is recent and demonstrates the southward migration of the centre of rock uplift. Rebound of sedimentary basins due to a reduction in plate coupling forces can also account for some of the observed rock uplift. Buoyancy forces required to create the pattern and magnitude of rock uplift are investigated using a 3 D loading model of the lithosphere. Strong upward forces (65 MPa) are required under the Central Volcanic Region, combined with broad uplift (36 MPa) over western North Island, to fit the observed rock uplift. Low Pn velocities under the Central Volcanic Region indicate temperatures 500 [degrees] C hotter than that of normal mantle. This temperature anomaly corresponds to 60 kg/[cubic metre] less dense than normal mantle, which is consistent with the change in density of 66 kg/[cubic metre] estimated from the loading model and aassuming the density change occurs over a 100 km depth range. The southern extent of buoyancy forces does not correspond well to regions of high seismic attenuation in the lithosphere but instead with the region of high incision.</p>

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