R-3 Repository :: Browsing by Author "Morgan, Julia K."

Browsing by Author "Morgan, Julia K."

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3D seismic characterization of the Peridotite Ridge in the Deep Galicia Margin
(2018-04-19) Linkevich, Gary D.; Morgan, Julia K.
In the Galicia magma-poor rifted margin, hyperextended continental crust is separated from oceanic crust by a wide transitional zone of exhumed mantle, which contains a series of margin-parallel basement ridges. The most landward of these, the Peridotite Ridge (PR), has been described for decades as largely devoid of coherent internal structure. Using new 3D seismic data, this study describes a variety of features inside and on the PR, which show it was heavily influenced by mass wasting processes. Its eastern flank contains a series of overlapping lenticular slump blocks, separated from the PR’s core by a large, landward-dipping normal fault. The western flank’s surface contains arcuate fault scarps, and its upper layer is underlined by a potential serpentinization front. Both flanks have large, chaotic landslides at their base. Normal faulting in the post-rift strata overlying the PR correlate with these mass-wasting features, suggesting a likely causal relationship.
Compactive deformation of incoming calcareous pelagic sediments, northern Hikurangi subduction margin, New Zealand: Implications for subduction processes
(Elsevier, 2023) Wang, Maomao; Barnes, Philip M.; Morgan, Julia K.; Bell, Rebecca E.; Moore, Gregory F.; Wang, Ming; Fagereng, Ake; Savage, Heather; Gamboa, Davide; Harris, Robert N.; Henrys, Stuart; Mountjoy, Joshu; Tréhu, Anne M.; Saffer, Demian; Wallace, Laura; Petronotis, Katerina
Calcareous rocks are commonly found in subduction zones, but few studies have investigated the consolidation and compactive deformation of these rocks prior to subduction, and their potential effects on subduction and accretionary processes are thus poorly understood. Using drilling data obtained during International Ocean Discovery Program (IODP) Expeditions 372 and 375 combined with 2D and 3D seismic reflection data, the structure, growth history, and slip rates of normal faults identified in the incoming pelagic sedimentary sequences of the Hikurangi Margin were investigated. A seismic coherence depth slice and vertical profiles show that these faults exhibit polygonal structure that has rarely been documented at subduction margins. The polygonal faults are closely spaced and layer-bound within sequences dominated by pelagic carbonate and calcareous mudstone of Paleocene-Pliocene age. Kinematic modeling and 2D displacement analysis reveal that fault throws decrease toward the upper and lower tipline. In detail, two groups of throw profiles are defined by locations of displacement maxima, possibly reflecting lateral variations in physical properties. The polygonal fault system (PFS) likely formed by syneresis processes that involve diagenetically induced shear failure and volumetric contraction of the pelagic unit associated with fluid escape. Fault growth sequences reveal multiple, weakly correlated intervals of contemporaneous seafloor deformation and sedimentation and allow estimates of fault slip rates. We find evidence for a significant increase in typical slip rates from 0.5-3 m/Ma during pelagic sedimentation to >20 m/Ma following the onset of terrigenous sedimentation. These observations suggest that rapid loading of the pelagic sediments by the trench-wedge facies was associated with renewed and faster growth of the PFS. The PFS will eventually be transported into the base of the accretionary wedge, enhancing geometric roughness and heterogeneity of materials along the megathrust, and providing inherited zones of weakness. Selective fault reactivation may facilitate deformation and episodic vertical fluid migration in the lower wedge associated with microearthquakes, tremor, and slow slip events.
Deformation styles of allochthonous salt sheets during differential loading conditions: Insights from discrete element models
(2009) Maxwell, Scott Allen; Morgan, Julia K.
The Discrete Element Method (DEM) was used to model the advance of allochthonous salt sheets through differential loading. The effects of basal slope angle, initial salt thickness, sediment thickness, loading time, progradation rate, and the mechanical strength of the overburden were tested to determine their influence on the development of stratigraphic and structural relationships in the emerging salt and sediment structures. These simulations show that the advance of salt is driven by gravitational instability and sediment loading. Salt advance is greatest with high basal slopes, thick salt, weak sediments, and high sediment progradation rates. The rate of salt advance determines the angle of a subsalt sediment ramp, which influences the final geometry of the system. High angle ramps form with slow salt front advance rates and tend to form counterregional sediment geometries, while low angle ramps form with rapid salt front advance and tend to produce roho geometries.
Discrete simulations of density-driven volcanic deformation: Applications to Martian caldera complexes
(2010) Zivney, Lindsay Laurel; Morgan, Julia K.
We have carried out 2-D numerical simulations using the discrete element method (DEM) to investigate density-driven deformation in Martian volcanic edifices and how it affects the development of caldera complexes. These simulations demonstrate that the presence of a dense and weak cumulate body within a volcanic edifice strongly influences the volcano morphology and enhances volcanic spreading. The settling of a cumulate body generates distinctive structural and morphological features characteristic of Olympus Mons and Arsia Mons, including low flank slopes and pronounced summit calderas. We show that gravitational spreading of a cumulate body can play a primary role in the long-term development of calderas. We conclude that a cumulate body that is both shallow and wide could generate a single large depression similar to the Arsia-type caldera, while our simulations of a narrow cumulate body are capable of generating summit subsidence that is similar in dimension to the Olympus Mons caldera.
Effects of cohesion on the structural and mechanical evolution of fold and thrust belts and contractional wedges: Discrete element simulations
(Wiley, 2015) Morgan, Julia K.
Particle-based numerical simulations of cohesive contractional wedges can yield important perspectives on the formation and evolution of fold and thrust belts, offering particular insights into the mechanical evolution of the systems. Results of several discrete element method simulations are presented here, demonstrating the stress and strain evolution of systems with different initial cohesive strengths. Particle assemblages consolidated under gravity, and bonded to impart cohesion, are pushed from the left at a constant velocity above a weak, unbonded décollement surface. Internal thrusting causes horizontal shortening and vertical thickening, forming wedge geometries. The mean wedge taper is similar for all simulations, consistent with their similar residual and basal sliding friction values. In all examples presented here, both forethrusts and back thrusts occur, but forethrusts accommodate most of the shortening. Fault spacing and offset increase with increasing cohesion. Significant tectonic volume strain also occurs, with the greatest incremental volume strain occurring just outboard of the deformation front. This diffuse shortening serves to strengthen the unfaulted domain in front of the deformed wedge, preconditioning these materials for brittle (dilative) failure. The reach of this volumetric strain and extent of décollement slip increase with cohesive strength, defining the extent of stress transmission. Stress paths for elements tracked through the simulations demonstrate systematic variations in shear stress in response to episodes of both décollement slip and thrust fault activity, providing a direct explanation for stress fluctuations during convergence.
Effects of Stress on Failure Behavior of Shallow, Marine Muds from the Northern Gulf of Mexico
(2014-06-18) Zhao, Xin; Dugan, Brandon; Morgan, Julia K.; Sawyer, Dale S.
Direct simple shear (DSS) experiments on mud samples from 4.3-13.4 meters below sea floor (mbsf) document that stress impacts soil strength and pore pressure genesis during failure. As burial depth increases from 7.3 to 13.4 mbsf, cohesion decreases from 12.3 to 6.5 kPa and internal friction angle increases from 18° to 21°. For the same depth increase, peak shear strength increases from 30 to 63 kPa. For a specimen from 11.75 mbsf, an increase in maximum consolidation stress from 45 to 179 kPa results in an increase in the shear-induced pore pressure from 29 to 150 kPa. The normalized shear strength at peak shear, however, decreases from 0.37 to 0.25 over this consolidation range. Our results indicate that compaction induces a positive feedback on pore pressure genesis. This feedback suggests an increase in failure potential during burial at shallow depth. To further understand the physical controls on this behavior, we complete DSS experiments on resedimented samples to erase stress history and sediment fabric. For the resedimented samples, cohesion is 3.2 kPa and internal friction angle is 24°. An increase in maximum consolidation stress from 40 to 254 kPa results in an increase in the peak shear strength from 14 to 91 kPa and an increase in the shear-induced pore pressure from 22 to 203 kPa; however, the normalized shear strength at peak shear decreases from 0.32 to 0.28. Resedimented samples show similar strength and failure behavior to intact samples. By constraining pore pressure and strength response to initial stress state and fabric, we are beginning to gain better insight on slope failure dynamics. Thus, this study may provide constraints on submarine landslide risks by investigating impact of stress and sedimentary fabric on soil strength and pore pressure genesis during shear failure.
Improved 3-D Models of Seismic Velocity and Density for the Island of Hawaii: Implications for Volcano-tectonics
(2008) Park, Jaewoo; Morgan, Julia K.; Zelt, Colin A.
Improved 3-D models of P-wave velocity and density are presented for better understanding of volcano-tectonic processes around the Island of Hawaii. The summit and upper rift zones of Kilauea are underlain by high-velocity and positive-density anomalies, indicative of magma intrusives dominated by dikes and melt-rich olivine cumulates. Seismicity is clustered at the seaward edge of this body, indicating that the cumulate body pushes the flank outward above a frictional decollement. The intrusive rocks along Kilauea's and Mauna Loa's rift zones are not continuous along their lengths, suggesting that eruptions along the lower rift zones could be fed vertically from the mantle, rather than downrift from the summit reservoirs. Mauna Loa's southeast flank is underlain by an anomalously large volume of intrusive materials that lacks the distinctive positive density anomaly observed above active rift zones. Therefore, this cumulate body is probably now cold and solidified, representing an ancient rift zone. Similar to Kilauea, earthquakes are concentrated along the boundary of this body, but here accommodate seaward motion of the adjacent flank rather than the cumulate body. Mauna Loa also appears to have a buried northwest rift zone, overlying the older flanks of Hualalai and Mauna Kea. Both Hualalai and Mauna Kea show south trending high-velocity and density features, also indicative of buried rift zones. High- and low-velocity anomalies beneath Loihi seamount are interpreted to indicate the presence of intrusive cumulates within the volcanic edifice and oceanic crust, and partial melt within the upper mantle, respectively. Low velocities beneath the Hilina and Kao'iki fault zones are attributed to thick piles of volcaniclastic sediments deposited on the submarine flanks. In contrast, the submarine outer bench of Kilauea is marked by anomalously high-velocity materials, possibly evidence for a buried seamount that may impede outward spreading of the flank today.
Mauna Loa's submarine western flank: Evidence for deep volcanic spreading and hydrothermal alteration
(2005) Borchers, Deanna Caroline; Morgan, Julia K.
Observations of submersible dives across Mauna Loa's submarine western flank suggest the flank has been influenced by a number of interacting processes, including large-scale sector collapses, deep volcanic spreading, and hydrothermal alteration. Subaerially derived pillow lavas observed draping the upper flank are in strong contrast to the volcaniclastic debris discovered at the toe of the flank. The distribution of rock types, deformation structures, and alteration phases within the volcaniclastic rocks suggest the mid-slope bench is constructed of a series of thrust sheets formed by seaward sliding of the western flank. Volcaniclastic sediments within the bench have been locally altered by circulation of hot hydrothermal fluids along the faults. The remnant landslide scar, from which much of the clastic debris within the bench derived, has largely been concealed by a flood of new lava flows, many of which crossed the shoreline and were deposited on the upper flank as pillow basalts.
Microstructural Evolution of Porosity and Stress During the Formation of Brittle Shear Fractures: A Discrete Element Model Study
(Wiley, 2018) LongJohn, Tamunoisoala; Morgan, Julia K.; Dugan, Brandon
Brittle fracturing in rocks is a progressive process involving changes in stress, strain, and porosity. Changes in these properties occur heterogeneously within a rock and are manifest at the grain scale, which is difficult to observe directly in the laboratory or the field. This study uses the discrete element method to show that fractures correspond to zones of generally lower stresses, higher porosity, and highly localized dilation and distortional strain. Using the discrete element method, we conducted numerical biaxial experiments at different confining pressures to probe the internal conditions of a low cohesive sandy sediment numerical analog. When compression begins, the stresses within the sandy sediment are relatively homogeneous with anastomosing stress chains. At yield stress, when the confining pressure is relatively low, multiple dilational bands start to open. At peak stress, high‐magnitude stress chains localize adjacent to the developing shear band and distortion is evident. Postpeak stress, through‐going shear fractures are fully developed. High stresses are transmitted across the fracture where porosity is low through a network of particles in contact. With increasing confining pressure, distortion is favored over dilation during deformation. Also, the number of particles that define the width of a stress chain across a shear fracture, and the steepness of the fracture, increases. Our results can be applied to understanding stress conditions of field samples, and in constraining rock property changes during reservoir modeling of fractured reservoirs.
Microstructural Evolution of Stress and Porosity During the Formation of Brittle Shear Fractures: A Discrete Element Model Study
(2017-08-11) LongJohn, Tamunoisoala; Dugan, Brandon; Morgan, Julia K.
Brittle fracturing in rocks is a progressive process involving changes in stress, strain, and porosity. Changes in these properties occur heterogeneously within a rock, and are manifest at the grain scale, which is difficult to observe directly in the laboratory or the field. This study uses the discrete element method (DEM) to show that fractures correspond to zones of generally lower stresses, higher porosity, and highly localized dilation and distortional strain. Using the DEM, we conducted numerical biaxial experiments at different confining pressures to probe the internal conditions of a sandstone numerical analog. When compression begins, the stresses within the sandstone are relatively homogeneous with anastomosing stress chains. At yield stress, when the confining pressure is relatively low, multiple dilational bands start to open. At peak stress, high magnitude stress chains localize adjacent to the developing shear band and distortion is evident. Post peak stress, through-going shear fractures are fully developed. High stresses are transmitted across the fracture where porosity is low through a network of particles in contact. With increasing confining pressure, distortion is favored over dilation during deformation, Also, the number of particles that define the width of a stress chain across a shear fracture, and the steepness of the fracture increases. Our results can be applied to understanding stress conditions of field samples, and in constraining rock property changes during reservoir modeling of fractured reservoirs.
Numerical Modeling of the Formation and Evolution of Basement-Involved Structures in Wyoming
(2014-01-21) Zhang, Jie; Morgan, Julia K.; Anderson, John B.; Akin, John Edward.; Dugan, Brandon; Sawyer, Dale S.
The Wyoming foreland is composed of basement-involved structures and intermontane basins formed during the Laramide Orogeny. Based on their sizes, structures in this area can be categorized into primary uplifts and secondary folds. Tectonic models suggest the primary uplifts form by sliding the crustal slabs along a deep-seated, large-scale regional detachment in the lower crust, and rotating the basement wedges along listric primary faults. The secondary folds are located close to and trend sub-parallel to the adjacent primary structures, suggesting a causative or correlative relationship between the two, although this connection has not been firmly established through field and seismic investigations. I carry out numerical simulations using both the finite element method (FEM) and discrete element method (DEM) to explore the structural evolution of these secondary basement-involved structures. The first study investigates the Laramide-age Sheep Mountain anticline, located in the eastern Bighorn Basin of Wyoming, using comparative FEM and DEM simulations. The kinematic and mechanical results of the two simulations are similar, thus verifying the methodological comparison. Differences in the geometric details, however, provide important perspectives on the capabilities of the two methods. The mechanical properties defined through this comparative study are then employed in DEM simulations that investigate the relationships between primary and secondary structures during the displacement of large crustal slabs along primary thrust faults. My results show that the displacements and geometries of the primary faults have great impact on the distributions and throw values of the secondary faults. For shallow primary faults with limited regional shortening, the numbers and the displacements of secondary faults are evenly distributed across the basin, with no preference in dip direction. For steep primary faults with significant regional shortening, conjugate faults form early and subsequently cluster into groups. I also explore the influences of initial sedimentary thickness, sedimentary mechanical stratigraphy, and syn-tectonic sedimentation on the distribution of secondary faults. Thicker Pre-Laramide deposits allow more secondary faults to form early during deformation, absorbing the horizontal shortening within the sedimentary layer. The presence of weak shale layers in the sedimentary section allows numerous small faults to form, and limits the depth of all the faults. Syn-tectonic sedimentation reduces the number of secondary faults that form in the basinal area, and displacements along those faults are very small. In this case, most of the deformation is accommodated by the faults located above the ramp take-off location, at the edge of the syn-tectonic deposits.
Overpressure and Earthquake Initiated Slope Failure in the Ursa Region, Northern Gulf of Mexico
(2010) Stigall, Justin Lee; Dugan, Brandon; Morgan, Julia K.; Anderson, John B.
We use two-dimensional fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our model predictions match measured overpressures from Integrated Ocean Drilling Project Expedition 308 Site U1324 above 200 mbsf, but overpredicts deeper overpressures by 0.4-1.1 MPa. Slope stability models predict a slope failure at 61 ka on the eastern end of the Ursa region. This predicted failure corresponds to a mass transport deposit (MTD) that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second MTD at ~27 ka. We predict that a magnitude 5 earthquake within 140 km of the Ursa region would initiate this failure. We conclude that overpressure could drive submarine slope failures and horizontal acceleration from earthquakes can further facilitate this process.
Sediment microstructures and deformation at the toe of the Nankai accretionary prism: Effects of accretion and diagenesis, and evolution of the decollement
(2004) Sunderland, Elizabeth Blanche; Morgan, Julia K.
Sediments incorporated into accretionary prisms experience significant changes in physical properties, grain fabric, and diagenesis. The record of such changes is preserved in sediment microstructures. Scanning electron microscopy (SEM) analyses were carried out on sediments collected from within and seaward of the Nankai accretionary prism to document microstructural changes with depth, lithology, and discrete deformation during the earliest stages of tectonic deformation. Clay mineral preferred orientations (CPOs) increased with depth due to burial, inclined CPOs were evident adjacent to discrete deformation structures and within the accretionary prism, reflecting the influence of sub-horizontal tectonic stress with accretion. Mineralogical and microstructural contrasts across the decollement and the seaward proto-decollement suggest strength differences that may help to localize the fault at this location.
Seismic imaging of the Upper Mantle structure and dynamics beneath the Southern Caribbean plate boundary and Venezuela
(2013-11-01) Masy, Jeniffer; Niu, Fenglin; Levander, Alan R.; Alexander, David; Morgan, Julia K.
The Caribbean-South America plate boundary has a complicated tectonic history that has been matter of debate and the focus of many studies for decades, yet many questions remain unanswered. The aim of this work, developed within the framework of the BOLIVAR (USA) and GEODINOS (Venezuela) projects, is to use different seismological techniques to study the lithospheric structure under the southern Caribbean and Venezuela, in order to understand some aspects of the present structure and its tectonic evolution. A shear wave splitting analysis in northwestern Venezuela revealed three areas with different deformation mechanisms: (1) Islands and coastal regions have large splitting times (~2-3 s) and a fast polarization direction parallel to the direction of the relative plate motion of the Caribbean plate respect to South America, which can be explained by a strong eastward flow confined at the CAR-SA plate boundary. (2) The stable South America plate showed weak seismic anisotropy with an origin likely in the asthenosphere. (3) Large splitting times and a ~NE-SW fast direction are observed at stations deployed along the Mérida Andes range, suggesting that the subcontinental mantle is also deformed beneath the range. It is likely the lithospheric mantle played a major, if not dominant, role in the formation of the Mérida Andes. The upper mantle structure of the area was obtained by combining three types of seismic data: Ps and Sp receiver functions and Rayleigh wave tomography. Results reveal the presence of the Moho of the subducting Caribbean Plate beneath the northwestern part of the Maracaibo Block. Tomographic images indicate that the subducting Atlantic slab appears to be attached to the continental South American lithosphere, pulling it down and removing the continental lithospheric mantle beneath the Serrania del Interior. A lithospheric thickness map was also obtained. The lithosphere asthenosphere boundary shows significant variations and seems to correlate well with major tectonic provinces in the region. Finally ambient noise cross-correlations between station pairs yields to Empirical Greens Function as waveform data input for the adjoint tomography based on spectral element methods. The adjoint tomography utilizes a more accurate full wave finite-frequency theory compared to the previous ray theory, and will iteratively refine the initial smooth 3D model to achieve more detailed high-resolution images of the upper most mantle structure of eastern Venezuela. Low velocity anomalies correspond to the major sedimentary basins and high velocity anomalies correspond to the stable craton.
Study of fault gouge influences on mechanical and frictional behavior of granular shear zones using the distinct element method
(2006) Guo, Yonggui; Morgan, Julia K.
Studies of fault gouge and its role in shear zone deformation are the key to understanding the mechanics of earthquakes and fault zone evolution. With the advantage of exploring the micromechanical process of gouge deformation in "real time", the combination of the Distinct Element Method (DEM) and linear elastic contact bonds provides an opportunity to deform complex, heterogeneous granular assemblages that approximate natural shear zones in a more realistic way, and to study gouge deformation processes that are responsible for unstable sliding of fault zones. Granular assemblages of multiple shaped grains were sheared over a range of normal stresses, sigman, in order to examine the influences of sigman gouge grain shape, grain comminution, and associated dynamic changes in grain characteristics on the frictional behavior of granular shear zones. The results show an inverse power law relationship between sigman and maximum sliding friction, where both its coefficient and exponent are dependent on gouge angularity. Enhanced grain rolling alone does not explain the low frictional strengths of simulated granular assemblages. Shear zone strength is dependent on the competition between strength reduction by fracturing and strength variation by changes in grain characteristics that are related to the partitioning of different deformation mechanisms. DEM experiments were also conducted to simulate the growth of fault gouge zones, for the purposes of studying the processes of gouge zone evolution, and its dependence on sigman and uniaxial compressive strength, sigmaucs. The simulated fault gouge zones exhibit two distinct stages of evolution, i.e., fast growth and slow growth, distinguished by a switch in deformation mechanism from dominantly wear of the fault blocks to dominantly shearing of existing fault gouge. During the fast growth stage, the rates of gouge thickening and bond breakage decrease exponentially and are proportional to sigman and inversely proportional to sigmaucs the rates become relatively constant and the dependency reverses during the slow growth stage. Gouge properties show complex correlations and dependences on shear displacement, sigman and sigma ucs, demonstrating the important effects of depth, mechanical properties of fault rocks, and gouge properties on the evolution and stability of natural faults.
Tectonic and gravitational fold and thrust belts: Insights from discrete element simulations and Galicia ocean-continent transition zone: New seismic reflection constraints
(2014-07-21) Dean, Sarah; Morgan, Julia K.; Dugan, Brandon; Sawyer, Dale; Anderson, John; Akin, John E.
The evolution and formation of tectonically and gravitationally driven fold and thrust belts were investigated with 2D numerical simulations using the discrete element method (DEM). In the tectonic study, the occurrence of triangle zones at the front of thrust belts was investigated, specifically how mechanical stratigraphy affects their formation. Simulations with homogenous stratigraphy deformed predominantly along forethrusts. Adding a weak upper unit caused more forethrusts and popup structures in the upper unit relative to the lower unit. Thicker décollement surfaces and strong upper and lower units enhanced decoupling and formed triangle zones. Results compared favorably with triangle zones in Alberta. Simulations of gravitationally driven thrust belts consisted of an updip extensional zone, syntectonically loaded with sediments, which is connected at depth to a downdip contractional zone. The whole system overlies a mobile shale unit. Our simulations show more diffuse décollements connecting the normal faults in the extensional zone with toe thrusts in the contractional zone, then are interpreted on seismic profiles. We also look at the distributions of stress and strain within out simulations, relating the distributions of σ1 to the vergence of thrust faults. We compare out models to the Niger Delta type locale for shale tectonics. The West Iberia continental margin is a type locale for magma-poor rifting, and has been instrumental in changing the classical view of the ocean-continent transition (OCT) from a discrete boundary juxtaposing continental and oceanic crust, into a more complicated zone of varying width that can include exhumed mantle. This study examines two new seismic lines in the Galicia Bank area extending west of the Peridotite Ridge, showing high resolution images of five new ridges. These ridges could be hyperextended continental crust, exhumed continental mantle, or rough ultra-slow spreading oceanic crust. There are no tilted fault blocks with pre-syn rift stratigraphy that would indicate continental crust. There are also no faults indicating mid-ocean spreading with seismic layer stratigraphy indicating normal oceanic crust. Therefore, it is likely the western ridges are also made of serpentinized mantle, with a wide OCT similar to the Southern Iberia Abyssal Plain.
The role of protothrusts in frontal accretion and accommodation of plate convergence, Hikurangi subduction margin, New Zealand
(The Geological Society of America, 2018) Barnes, Philip M.; Ghisetti, Francesca C.; Ellis, Susan; Morgan, Julia K.
Protothrusts mark the onset of deformation at the toe of large subduction accretionary wedges. They are recognized in seismic reflection sections as small-displacement (tens of meters) faults seaward of the primary frontal thrust fault. Although assumed to reflect incipient accretionary deformation and to mark the location of future thrusts, few studies discuss their displacement properties, evolution, and kinematic role during frontal accretion and propagation of the subduction décollement. We analyze high-quality geophysical and bathymetric images of the spectacular 25-km-wide Hikurangi margin protothrust zone (PTZ), the structure of which varies along strike north and south of the colliding Bennett Knoll seamount. We provide a quantitative data set on protothrust scaling relationships and fractal fault population characteristics. Our analyses lead us to speculate on the importance of stratigraphic heterogeneity in structural development, and highlight the role of protothrust arrays in the formation of the frontal thrust. We document a migrating wave of protothrust activity in association with forward advancement of the décollement and deformation front. Shortening east of the present frontal thrust, calculated from displacements on seismically imaged faults and from subseismic faulting derived from power law relationships, reveal the significant role of the PTZ in accommodating shortening. There is possibly as much as ∼7.4 km and ∼4.0 km of shortening accommodated by the PTZ south and north, respectively, of Bennett Knoll seamount. As much as ∼90% of the total shortening may be accommodated at subseismic scale. These data indicate that the active PTZ, together with older accreted PTZs, may accommodate ∼10%–50% of the total margin-normal convergence rate at the Hikurangi margin.
The S-reflector detachment from the Galicia margin, offshore Spain: New insights on fault zone thicknesses, fault surface morphology, and mantle serpentinization associated with a large-scale detachment fault
(2019-04-15) Schuba, Cemile Nur; Morgan, Julia K.; Lee, Cin-Ty
The Galicia magma-poor margin is considered an archetypal location to explore detachment faulting in a passive rift margin setting. A new 3-D seismic reflection volume acquired over the Galicia continent-ocean transition zone provides an unprecedented view of the prominent S-reflector detachment fault from the Deep Galicia Margin. This new dataset shows that the interface of the S-reflector detachment fault has heterogenous seismic reflection amplitudes. The seismic reflection amplitudes are analyzed and discussed relative to fault surface morphology and variations in the fault rock layer above and upper mantle layer below the S-reflector. The fault surface map of the S-reflector shows coherent corrugations parallel to the expected paleo-extension directions with an average azimuth of 107o. Locally, the corrugations are continuous beneath multiple fault blocks, spanning crustal fault intersections with the S-reflector, which suggest that during the final stages of the detachment did not demonstrate sequential crustal faulting. A second somewhat discontinuous layer above the detachment fault, here named the S-interval, interpreted to be as zone of fault rock above the S-reflector. Overall, the S-interval increases in thickness by tens of meters to the northwest, in the direction of hanging wall transport. There are localized thick accumulations of fault rock near overlying fault intersections, suggesting either non-uniform fault rock production, or redistribution of fault rock during slip. An artificial neural network has been utilized to estimate the P-wave velocities of the upper mantle, 100 ms (~400 m) below the S-reflector detachment. This approach estimates seismic velocity along a target interface, based on geological and geophysical inputs from both the 3-D seismic reflection survey and and additional 2-D wide-angle seismic profile. This non-linear regression technique results in estimated velocities 100 ms above and below the targeted interface, the S-reflector. An ensemble of similarly trained neural networks is used to estimate previously unknown target velocities where the S-reflector is the cust-mantle boundary within the 3-D dataset, where the S-reflector is the crust-mantle boundary. These networks reached minimum achievable error of 2% on the test data, and the low standard deviation (<300 m/s) between different networks demonstrate that the input features were sufficient to capture variations in the velocity above and below the targeted S-reflector. The upper mantle seismic velocities are used to estimate distribution and the degree of serpentinization in the upper mantle and its distribution beneath the S-reflector detachment fault. The degree of serpentinization is heterogeneous, varying between 0 and 90% across the domain. The distribution pattern shows relationships with both the current intersections of crust-cutting faults soles and to paleo-temperatures estimated for ~112 Ma. High degrees of serpentinization are only observed in areas where overburden thickness at the end of rifting was sufficient to provide optimal serpentinization temperatures. The alignment of crust-cutting fault intersections and serpentinized areas suggest the crustal faults were water conduits that hydrated the mantle peridotite. The correspondence between the fault intersections and present-day serpentinite anomalies indicates that the most of the serpentinization postdated slip on the S-reflector detachment. Volumetric expansion caused by the serpentinization process may also lead to undulations on the fault detachment fault surface. High localized reliefs (0.9 km and 1.4 km) on the S-reflector coincide with highly serpentinized (80-90%) areas in the 3-D seismic reflection survey. These observations have important implications for understanding how detachment faults form and evolve during and after rifting. 3-D seismic reflection imaging and amplitude analysis have enabled unique insights into fault slip history, fault rock production and redistribution, and causal mechanisms behind mantle serpentinization in a passive rift margin environment.
Tomographic study of mantle structure beneath eastern Asia
(2014-04-24) Huang, Tao; Niu, Fenglin; Levander, Alan R.; Morgan, Julia K.
A high-resolution 3D P-wave velocity model of the mantle beneath eastern Asia was obtained through performing seismic travel-time tomography by using recently emerging new datasets. An improved relocation method that accounts for correlated travel-time errors among similar ray paths was applied to reduce location bias caused by unmodeled heterogeneities. The model demonstrates more distinct structures beneath eastern Asia in comparison with the one obtained by applying the standard linear relocation method. This model GAP_C1 clearly shows that vast areas of low-velocity structures dominate the upper mantle above the transition zone. The late Cenozoic volcanisms are mostly distributed above these slow anomalies, implying their deep mantle origin. In the mantle transition zone, high-velocity structures with complex configurations become more prominent. These fast anomalies are most likely subducted oceanic lithospheres stagnant in the transition zone. Furthermore, their extent could be attributed to the coexistence of slabs being subducted at different stages. However, comparing the transition zone structures in model GAP_C1 to the plate reconstructions, slabs subducted at the same stage are separated beneath the Changbai volcano and the South China Fold Belt, creating slab gaps. A model was proposed to explain this phenomenon. The slab gaps are related with the slab fragmentation that happens at the slab-slab junction when the slab with different dipping directions flattens against the top of the lower mantle.

Browsing by Author "Morgan, Julia K."

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