Speakers

A trans-disciplinary and community-driven database to unravel subduction zone initiation

I will provide an overview over subduction zone initiation (SZI) and its study involving geologic evidence, plate reconstructions, seismic tomography, and geodynamic modelling. Moreover, I will introduce our brand-new trans-disciplinary, community-driven SZI database and timely online platform, how to use it, and what we already learned from it.

Melt focusing beneath mid-ocean ridges: implications for the lithosphere asthenosphere boundary

At mid-ocean ridges, oceanic crust is emplaced in a narrow neo-volcanic region on the seafloor, whereas basaltic melt that forms this oceanic crust is generated in a wide region beneath as suggested by a few geophysical surveys. The combined observations suggest that melt generated in a wide region at depths has to be transported horizontally to a small region at the surface. We present results from a suite of two-phase models applied to the mid-ocean ridges, varying half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere asthenosphere boundary (LAB). Three distinct melt focusing mechanisms are recognized in these models: 1) melting pressure focusing, 2) decompaction layers and 3) ridge suction, of which the first two play dominant roles in focusing melt. All three of these mechanisms exist in the fundamental two phase flow formulation but the manifestation depends largely on the choice of rheological model. The models show that regardless of spreading rates, the amount of melt and melt transport patterns are sensitive to changes in intrinsic permeability, K0. Geophysical observations place the LAB at a steeper incline as compared to the gentler profile suggested by modeling efforts. The decompaction melt-rich layer roughly follows and itself can define the lithosphere-asthenosphere boundary (LAB), which without the melt layer, would be along the temperature dependent rheological and freezing boundaries. Melting pressure focusing is the only focusing mechanism that can focus melt before reaching the typical model thermal LAB. The lack of the decompaction layers in the geophysical observations hint at the possibility that melting pressure focusing could be significant, which could provide constraints for mantle rheology, permeability and the lithosphere-asthenosphere boundary.

Seismic imaging of the crust and upper mantle beneath New England: Past tectonic process and present-day dynamics

The eastern margin of North America has been affected by a range of fundamental tectonic processes in the geologic past. Major events include the Paleozoic Appalachian orogeny, which culminated in the formation of the supercontinent Pangea, and the breakup of Pangea during the Mesozoic. The southern New England Appalachians exhibit a particularly rich set of geologic and tectonic structures that reflect multiple episodes of subduction and terrane accretion, as well as subsequent continental breakup. It remains poorly known, however, to what extent structures at depth in the crust and lithospheric mantle reflect these processes, and how they relate to the geological architecture at the surface. Furthermore, the origin of a prominent low-velocity anomaly in the upper mantle beneath New England, known as the Northern Appalachian Anomaly, and its possible links to volcanism that post-dates the breakup of Pangea, remain poorly understood. In this talk I will discuss new results on imaging the crust and upper mantle beneath New England, including results from the recent SEISConn experiment and the ongoing NEST deployment.

Sequencing seismograms: A panoptic view of scattering in the core-mantle boundary region.

Scattering of seismic waves can reveal subsurface structures but usually in piecemeal way focused on specific target areas. We used a manifold learning algorithm called "the Sequencer" to simultaneously analyze thousands of seismograms of waves diffracting along the core-mantle boundary and obtain a panoptic view of scattering across the Pacific region. In nearly half of the diffracting waveforms, we detected seismic waves scattered by three-dimensional structures near the core-mantle boundary. The prevalence of these scattered arrivals shows that the region hosts pervasive lateral heterogeneity. Our analysis revealed loud signals due to a plume root beneath Hawaii and a previously unrecognized ultralow-velocity zone beneath the Marquesas Islands. These observations illustrate how approaches flexible enough to detect robust patterns with little to no user supervision can reveal distinctive insights into the deep Earth.

The Deep Water Cycle: Mantle Mixing and the Surface Ocean

Water plays a vital role in planetary evolution. The deep water cycle is often examined using parametrised models as they are computationally inexpensive, allowing for a broad exploration of the parameter space. However, these models also assume that mixing is instantaneous and the mantle is a homogenous reservoir. When mixing is implemented in the deep water cycle of parameterised models, it acts to trap water in the mantle interior. Periods of net degassing can also be induced, with consequences for the surface reservoir mass and sea level. Geological evidence suggests that the Earth’s surface has been sub-aerial since ~3.5 Ga. Therefore, adopting this exposure as a constraint, we can conclude that no more than half an ocean’s worth of water from the surface has been introduced to the mantle.

Dynamics, evolution and seismic visibility of melting zones in the lowermost mantle: Implications for the origin of ultra-low velocity zones

Work with: Robert Myhill, Rene Gassmöller, Sanne Cottaar

Seismic observations of the lowermost mantle show several thin patches of extremely low seismic velocities just above the core-mantle boundary. But the nature and origin of these ultra-low velocity zones (ULVZs) remain uncertain. One hypothesis is that ULVZs contain partial melt constrained to a thin layer above the core-mantle boundary.

We have developed coupled geodynamic and thermodynamic models that test this hypothesis by simulating the dynamics and evolution of zones of partial melt near the core-mantle boundary. Our models make predictions about the shape, stability and melt distribution within these regions, and the corresponding effect on seismic velocities. The results suggest that mantle melting alone can not explain the observed velocity reductions, because the generated melt would likely be too dense to remain suspended within the partially molten regions.

Deep Slab Seismicity Limited by Rate of Slab Deformation in the Transition Zone

Deep earthquakes within subducting lithosphere (slabs) have remained enigmatic because they have many similarities to shallow earthquakes, but frictional failure of rocks is strongly inhibited at high pressure. Proposed mechanisms for deep earthquakes include dehydration embrittlement, shear instability, or transformation faulting. Previous attempts to explain the global depth distribution of deep earthquakes, in terms of the thermal conditions at which possible triggering mechanisms are viable, fail to explain the variability in seismicity within and between slabs. In addition to thermal constraints, the proposed failure mechanisms for deep earthquakes require that sufficient strain accumulates in the slab at a high enough level of stress.

I will show simulations of subduction with non-linear rheology and compositionally-dependent phase transitions, which exhibit strongly variable strain-rate magnitude in space and time similar to observed seismicity versus depth profiles. High strain-rates occur in bending regions of the slab and migrate as the slab buckles and folds at the base of the transition zone. However, in between these strongly-deforming regions the strain rate is low due to the strong temperature-dependence of viscosity and high yield strength of the slab. I argue that, in addition to thermal constraints on deep earthquake mechanisms, variations in strain-rate determine the spatially-variable distribution of deep earthquakes and explains why there are large gaps in seismicity (low strain-rate), variable peaks in seismicity (high strain-rate bending regions) and, an abrupt cessation of seismicity below 660 km.

India-Eurasia convergence history revisited

During the Cretaceous, the Indian plate moved towards Eurasia at the fastest rates ever recorded. The details of this journey are preserved in the Indian Ocean seafloor, which document two distinct pulses of fast motion, separated by a noticeable slowdown. The nature of this rapid acceleration, followed by a rapid slowdown and then succeeded by a second speedup, is puzzling to explain. Using an extensive observation dataset and numerical models of subduction, we show that the arrival of the Reunion mantle plume started a sequence of events that can explain this history of plate motion. The forces applied by the plume initiate an intra-oceanic subduction zone, which eventually adds enough additional force to drive the plates at the anomalously fast speeds. The two-stage closure of a double subduction system, including accretion of an island arc at 50 million years ago, may help reconcile geological evidence for a protracted India-Eurasia collision.

Uniform versus Variable Tectonic Coupling on 3D Subduction Dynamics of South-central Alaska

Subduction interface viscosity and variability are important parameters for understanding the 3D dynamics of a subduction system. These properties, however, are difficult to quantify. In south-central Alaska, geodetic modeling and rupture zones from the 1964 great earthquake suggest that the flat slab is highly coupled to the upper plate with coupling decreasing southwestward. Here, we present a suite of 3D numerical models that test uniform tectonic coupling, and variable tectonic coupling that increases locally within the region of flat subduction. The model with a low-viscosity Denali fault shear zone (10¹⁷ Pa-s) and a variably coupled subduction interface (10²⁰ Pa-s to 10²¹ Pa-s) is found to best fit post-seismic corrected GPS velocities. Observed and model-predicted rotational motion of the lithosphere between the trench and Denali fault, analysis of Euler poles, and a measure of plateness suggest that southern Alaska can be viewed as a microplate, the Wrangell microplate. This implies that the long-term 3D tectonics of south-central Alaska is dominated by three lithospheric plates: the Pacific plate, the North American plate, and the Wrangell microplate. Models show that motion of the Wrangell microplate is governed by both the geometry and viscosity of the plate boundary and Denali fault shear zones. Thus, the models suggest there may be connectivity between the motion of the intracontinental fault and motion along the subduction interface via the intervening microplate. With the Wrangell microplate being the upper plate to megathrust earthquakes in Alaska, the geodynamic models suggest that in subduction zones with obliquity, intracontinental shear zones may play an important role in hazard assessment.

A Geophysical Investigation of the Puzzle within the Continental Lithosphere

The lithosphere is the stiff outer shell of our planet, underlain by a soft and weak layer, the asthenosphere, which flows more readily. These are concepts that are fundamental to plate tectonics. The puzzling observation of internal layering within the cratonic lithosphere and its possible explanations (i.e., partial melt, anisotropy, chemical stratification, or short-term rheological weakening) are still vigorously debated. In our group, we construct high-resolution images of global lithospheric structure, which when interpreted with complementary experimental and geophysical constraints (e.g., conductivity), may help resolve this puzzle, with important implications for the geological evolution of continents and oceans.

In this talk, I describe how seismic results interpreted alongside other geophysical constraints e.g., conductivity and mineral physics is valuable for testing for successful models. This is possible in the Southeastern US, where passive-source seismic and magnetotelluric experiments have been conducted. I compare new seismic results with magnetotelluric observations on the conductivity structure of the lithosphere in this region. I also provide overlapping constraints on fine-scale lithospheric layering using a high-resolution Ps receiver function study. Taken together, these complementary geophysical constraints support a dominant role of sub-solidus grain-boundary sliding as a preferred mechanism for lithospheric layering in the south-eastern US.

Xenon isotopes and mantle heterogeneity

The noble gas isotopic composition of the mantle reflects an integrated history of volatile delivery and loss during accretion, long-term deep Earth volatile transport and radiogenic production. The short-lived, extinct I-Xe and Pu-Xe systems are sensitive to degassing that occurred during the Hadean, and preserve signatures of ancient mantle heterogeneity. Long-lived radioactive nuclides (U, Th, K) generate noble gas signatures that reflect long-term degassing. Injection and incorporation of compositionally distinct atmospheric noble gases (“regassing”) also affects the mantle isotopic composition. Thus, a full set of noble gas isotopes measured in mantle-derived samples provides constraints on volatile transport processes on a broad range of timescales. Here I discuss high precision Xe isotope systematics among mantle-derived samples. While variations in radiogenic and fissiogenic noble gas isotopes reflect differential degassing of the plume and MORB mantle sources, these two reservoirs are not simply related by different extents of degassing from a common ancient source. Rather, the signatures of long-term differential degassing are instead superimposed on ancient heterogeneities and atmospheric volatile regassing.

Evolving viscous anisotropy in the asthenosphere

Asthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated with laboratory measurements of olivine aggregates. This model characterizes the time-dependent coupling between large-scale formation of lattice-preferred orientation (i.e., texture) and changes in asthenospheric viscosity for a series of simple deformation paths that represent upper-mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a-axes of olivine into the plate-motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. Our models indicate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a-axes. This increase should slow plate motions and new texture development in this perpendicular direction, and could impede changes to the plate motion direction for 10s of Myrs. However, the same well-developed asthenospheric texture may foster subduction initiation perpendicular to the plate motion and deformations related to transform faults, as shearing on vertical planes seems to be favored across a sub-lithospheric olivine texture. These end-member cases examining shear-deformation in the presence of a well-formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes.

Orphaning Regimes: The missing link between flattened and penetrating slab morphologies

The mantle transition zone and the uppermost lower mantle host a plethora of slab morphologies from flattened, to penetrative, to broken or remnant slabs. The remnant slabs at these depths are generally interpreted as vertically sinking fragments resulting from slab break-off at shallow depths (<300km), and are associated with subduction cessation following tectonic regime shifts. However, slabs can also orphan (break-off) directly at 660 km depth.

Slab orphaning describes a new phenomenological behaviour where the slab tip breaks off at the top of the lower mantle and is subsequently abandoned by its parent. Upon orphaning, subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. Similar to other deep slab morphologies, orphaning can be the end-result for a wide range of physical parameters governing slab dynamics. Slab orphaning persists across wide variations in slab dip, slab yield stress/strength, Clapeyron slope values and overriding plate nature. The diversity in orphan slab sizes and orphaning periods is tied to the orphaning regime space which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes. Orphaning provides a simple dynamic link between the well-known deflective and penetrative modes. It is one possible way for slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments in the process. Orphan slabs are the intermediary between these two extensively studied slab morphologies and highlight the first order effect of the mantle radial viscosity profile on deep slab morphology.

The Formation of Hot Thermal Anomalies in Cold Subduction‐Influenced Regions of Earth's Lowermost Mantle

Link to paper

The Earth's lowermost mantle is characterized by two large low shear velocity provinces (LLSVPs). The regions outside the LLSVPs have been suggested to be strongly influenced by subducted slabs and, therefore, much colder than the LLSVPs. However, localized low‐velocity seismic anomalies have been detected in the subduction‐influenced regions, whose origin remains unclear. Here, three‐dimensional geodynamic calculations are performed, and they show that linear, ridge‐like hot thermal anomalies, or thermal ridges, form in the relatively cold, downwelling regions of the lowermost mantle. Like the formation of Richter rolls due to sub-lithosphere small‐scale convection (SSC), the thermal ridges form as a result of SSC from the basal thermal boundary layer and they extend in directions parallel to the surrounding mantle flow. The formation of thermal ridges in subduction regions of the lowermost mantle is very sensitive to the thermal structures of the subducted materials, and thermal heterogeneities brought to the bottom of the mantle by subducting slabs greatly promote the formation of thermal ridges. The formation of thermal ridges is also facilitated by the increase of core‐mantle boundary heat flux and vigor of lowermost mantle convection. The thermal ridges may explain the low‐velocity seismic anomalies outside of the LLSVPs in the lowermost mantle. The results suggest that the relatively cold, subduction‐influenced regions of the Earth's lowermost mantle may contain localized hot anomalies.

Flow in the Deep Earth: Observations and Modeling of Lowermost Mantle Seismic Anisotropy

Plate tectonics represents the surface expression of Earth’s convecting mantle resulting from the continued cooling of the planet. Deep processes near the core-mantle boundary can have direct consequences on the Earth’s surface (e.g., hotspot volcanism). In order to understand these processes, we need to discern their driving forces in the deep Earth. One way to go about understanding this connection is to characterize the flow pattern of the entire convecting mantle, including how material is transported from the surface to the core-mantle boundary and vice versa. In this seminar, I explore the links between seismic observations and mineral physics experiments and theory to better understand processes in the deep Earth, with an emphasis on the lowermost mantle (the D” region: 200-300 km thick region directly above the core mantle boundary). A robust seismic observation from the D” region is seismic anisotropy, a phenomenon where seismic waves propagate with different speeds in different directions. A variety of mechanisms have been proposed as explanations for seismic anisotropy observed at the base of the mantle, including crystallographic preferred orientation of various minerals (bridgmanite, post-perovskite, and ferropericlase) and shape preferred orientation of elastically distinct materials such as partial melt. However, based on recent modeling of anisotropy, we have constrained the most likely mechanism to be post-perovskite or ferropericlase (or some mixture of the two). Ray theory is commonly used to distinguish these different proposed models of anisotropy and is appropriate within certain limits, but not all implications of such an approach have been explored. Ray theory’s validity depends on the period of waves, the scale of heterogeneities, the length of its propagation path, and the superposition of multiple arrivals, making interpreting seismic anisotropy observations more difficult. We illustrate some preliminary work on the assumptions made in shear wave splitting in the deep mantle, by conducting numerical simulations via 3D global wave propagation solver SPECFEM3D_GLOBE, a numerical solver that simulates global wave propagation.

Mapping mantle heterogeneities through redox state

To further our understanding of the communication between the deep Earth and its surface and atmosphere, we explore how redox state affects mantle mineralogy, density and seismic velocities and in turn what this could mean for convection and thermochemical evolution of the planet. Mantle ferric iron Fe³⁺ content and its distribution are largely unconstrained and are influenced by the disproportionation of Fe²⁺ through the formation of Earth’s most abundant mineral bridgmanite, and the presence of other minor elements such as aluminum and water. Here we show that redox-induced density contrast (~1-2%) affects mantle convection and may potentially cause the oxidation of the upper mantle. Our geodynamic simulations suggest that such a density contrast causes a rapid ascent and accumulation of oxidized material in the upper mantle, with descent of the denser, reduced material to the core–mantle boundary. The resulting heterogeneous redox conditions in Earth’s interior may have contributed to the large low-shear velocity provinces in the lower mantle and the rise of oxygen in Earth’s atmosphere. Using a combination of laser-heated diamond anvil cell experiments, Monte Carlo simulations, geodynamic modeling, and seismic forward modeling calculations, our work shows the importance of ferric iron content and its effects on overall rock mineralogy in the lower mantle. Therefore, geophysical anomalies in the mantle may reflect not only differences in bulk composition or temperature, but also mantle oxidation state.

Geomorphic and stratigraphic signatures of mantle-flow-induced dynamic subsidence

Earth’s interior viscous flows induced by density variations exert radial stresses to the base of lithosphere, generating vertical displacements to the surface. This dynamic component of surface topography (i.e. dynamic topography) affects landscape evolution, sediment routing systems and stratal architectures over millions of years. However, its contributions remain less quantified. We use source-to-sink landscape evolution models to investigate surface responses to dynamic topography, particularly dynamic subsidence related with downwelling flows within the underlying mantle. Our results show that downward tilting of the surface leads to large-scale continental flooding and sediment redistributions. Reorganizations of river flow patterns in response to surface subsidence contribute to varying sediment supply to continental margin, and therefore affect stratal stacking patterns along the margin. The sedimentation across continental interior could provide important constraints on the evolving pattern of the downwelling flows in the mantle. Finally, we show that correlating offshore depositional unconformities with hinterland erosional anomalies has the potential to constrain the amplitude and wavelength of dynamic topography.

Subduction, underplating, and return flow recorded in the Cycladic Blueschist Unit exposed on Syros Island, Greece

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20-70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit (CBU) preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution associated with Eocene subduction. Despite decades of research on Syros, the pressure-temperature-deformation history (P-T-D), and timing of subduction and exhumation, are matters of ongoing discussion. Here we show that the CBU on Syros comprises three coherent tectonic slices, and each one underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and return flow are distinguished by stretching lineations and ductile fold axis orientations: top-to-the-S (prograde-to-peak subduction), top-to-the-NE (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole chemical zonations record cooling during decompression, indicating return flow along the top of a cold subducting slab. New multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology demonstrate that three nappes record distinct stages of peak subduction (53 Ma,~50 Ma (?), and 47 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50-40 Ma and ~43-20 Ma, respectively, and also young with structural depth. The datasets support a revised tectonic framework for the CBU, involving subduction of structurally distinct nappes and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5-5 mm/yr, and accommodated ~80% of the total exhumation of this HP/LT complex.

The multiple personalities of the lithosphere : insights from geodynamical models

The lithosphere is perceived all at once as a cold, rigid, undeformed and/or translating block. However, thermo-mechanical models of plate and mantle dynamics emphasize that temperature, viscosity and strain rate vary gradually with depth with no sharp discontinuities. A dynamical lithosphere-asthenosphere boundary (LAB) can be defined as the base of a “constant-velocity” plate (i.e. the material translating at constant horizontal velocity). This constant-velocity plate is not fully rigid, but deforms at its base. The thermal structure has a major control on this dynamical LAB, which deepens with increasing plate age. The constant-velocity plate thickens as plate velocity increases (larger mantle drag), with the transient dynamical LAB adjusting to flow field evolution. The concept of a constant-velocity plate can be extended to a constant-velocity slab, which also deforms at its borders and drags the surrounding mantle. This framework is relevant to address the issue of mass transport within the Earth's mantle.

The ups and downs of NW Africa margin hinterlands

The evolution of rifted margins is usually described as tectonically active during rifting, dominated by lithospheric stretching and a variable degree of magmatism, and tectonically passive following continental breakup, when the thermal cooling of the lithosphere becomes the prevailing process.

In the last two decades, the widespread use of low temperature thermochronology techniques revealed unexpected cooling and heating events along the hinterland of many rifted margins. These events, which are interpreted as km-scale exhumation and burial of the crust, respectively, took place tens of million years after continental breakup and challenge our understanding of the processes that control the post-rift evolution of rifted margins.

Here, we examine the post-rift history of vertical movements along the Atlantic margins of NW Africa. We use low temperature thermochronology to quantify their amplitude and magnitude and discuss the processes that might be driving them.

High-resolution seismic mapping of the Hawiian ULVZ

Ultra low velocity zones (ULVZs) represent extreme but relatively small-scale seismic features within landscape of the core-mantle boundary (CMB). Open questions remain over what they are formed of (partial melt or compositionally distinct material?), where they are found (potentially associated with large-low velocity provinces LLVPs or the base of plumes?) and what role they play within wider-scale convective processes within the Earth?s mantle. The first step in unravelling the mysteries of these extreme lower-mantle structures is high-quality seismic constraints.

I will present results of a recent seismic analysis of the Hawaiian ULVZ, making using of high-resolution core-reflected shear waves (ScS phases) recording at the US and Alaskan Transportable Arrays. This dataset allows us to map out the detailed 3D shape of the Hawaiian ULVZ for the first time. Results suggest the Hawaiian ULVZ is not an isolated patch of material, but instead represents a regional layer of varying topography, shaped by surrounding mantle structures. The location and variable shape of the ULVZ are more indicative of a compositional distinct formative material rather than partial melt.

Compositional heterogeneities in the mid-mantle revealed by seismic discontinuities and reflectors

We investigate seismic discontinuities and reflectors in the mid-mantle by analyzing SS precursors recorded at global seismic stations. Our results confirm the global existence of the 520-km discontinuity. Although its depth variations are generally correlated with temperature in the mid-mantle, they cannot be fully explained by the Clapeyron slope of the wadsleyite-ringwoodite phase transition, suggesting both thermal and compositional heterogeneities in the mantle transition zone. A second discontinuity at ~560-km depth, previously interpreted as splitting of the 520-km discontinuity, is most commonly detected in cold subduction zones and hot mantle regions. The depth separation between the 520- and 560-km discontinuities varies from ~80 km in cold regions to ~40 km in hot areas. Because the only known transition in major minerals at this depth in the mantle transition zone is the formation of Ca-pv, the existence of the 560-km discontinuity may imply localized high calcium concentrations in the mid-mantle, possibly related to the recycling of oceanic crust.

We also discover a megameter-scale seismic reflector at about 810-km depth west of the Sea of Okhotsk. It can only be explained as a portion of thickened oceanic crust subducted to the uppermost lower mantle. We propose that this thick crust represents the oceanic plateau, or at least its major part, that was created by massive decompression melting during the initiation stage of the Hawaiian hotspot and then subducted about 20 Ma ago. Combined with plate reconstruction models, our discovery provides temporal and spatial clues on the early history of the Hawaiian plume.

LLSVPs viewed by tomography and fluid mechanics : bundles of mantle thermochemical hot instabilities rather than thick stagnant "piles"

The large-low shear velocity provinces (LLSVPs) present at the base of the Earth's mantle beneath the Pacific and Africa cover about 25% of the core-mantle boundary. At long- wavelength, they appear as compact, uniform structures, and have often been interpreted as hot but denser thermochemical piles. In contrast, based on the higher resolution SEMUCB-WM1 tomographic model and fluid mechanics constraints, we show that LLSVPs are in fact bundles of hot thermochemical instabilities.

Each LLSVPs contains roots of a number of well-separated, low velocity conduits that extend vertically throughout most of the lower mantle. However, their thicker size, uneven amplitude and sometimes contorted shapes do not belong to the classical purely thermal plumes. Instead, they can be explained by the presence of compositional heterogeneities in the deep lower mantle, which strongly influence the development, shape, and time- dependence of hot instabilities there. Comparing tomographic images, completed by gravity informations and surface volcanism evolution wherever possible, with an experimental data base of thermochemical instabilities shapes and evolution, we can begin to infer the magnitude of the compositional density anomaly relative to the density anomaly of thermal origin in the deep mantle. Moreover, the overall shape of the LLSVPs is probably controlled by the distribution of subducted slabs, and due to their thermochemical nature, the position of both LLSVPs and individual upwelling dynamics should be time dependent.

Controls of Stratigraphic Architecture on Along Strike Cooling Age Patterns: an Example from the Himalaya

Abstract:

Cooling related to moving rocks over an ~8.5 km vertically thick ramp in the Main Himalayan thrust (MHT) produced 0.8 – 2 Ma zircon (U-Th)/He (ZHe) and apatite fission track (AFT) ages from the Main Central thrust to 40 km south in central Nepal. The ramp size is controlled by thicker Proterozoic stratigraphy. ZHe ages in NW India and eastern Bhutan over a similar 40 km wide swath are 1.4 – 12 Ma. In addition, the ZHe ages are consistently 2 – 5 million years older than the AFT ages. I argue that the MHT ramps are smaller because of thinner Proterozoic stratigraphy, with two vertically thick ramps of 2.5 km each in eastern Bhutan and one vertically thick ramp of ~5 km in NW India. Thus, the original stratigraphy and associated weaker décollement horizons control location and size of active MHT ramps, resulting vertical uplift, and along strike pattern of cooling ages.

Significance Statement:

The original stratigraphic architecture of the Greater Indian margin controls the vertical thickness of the Main Himalayan thrust ramps. Central Nepal has the Paleoproterozoic Kuncha Formation, cropping out nowhere else in the Himalaya, which makes the lower Lesser Himalayan stratigraphy thicker and stronger, producing a ~8.5 km vertically thick Main Himalayan thrust ramp. This very large ramp produced very young ZHe ages. This strong rock is not present in eastern Bhutan and NW India; thus, the ramps are smaller corresponding to the thinner original stratigraphic architecture. Smaller ramps produce older ZHe ages. Thus, the young low temperature thermochronologic cooling ages collected from Greater Himalayan rocks in the foothills of the highest peaks of the Himalaya are a function of the vertical thickness of the Main Himalayan thrust ramps at depth. The continuity of these ramps, their vertical thickness and their location focuses exhumation in central Nepal and elevates the subsurface thermal field (see Ghoshal et al., 2020) and facilitates young ZHe ages. Researchers who work in mountain belts will be interested because it shows how the original stratigraphic architecture influences low temperature thermochronologic ages.

New constraints on seismic structures in sparsely instrumented regions of Africa

The continent of Africa lacks a high-resolution seismic model due to the paucity of broadband seismic stations and relatively low seismicity. However, due to several small-scale deployments in recent years, improved images of the crust and upper mantle structure at different parts of the continent have been developed. To date, most of the existing broadband seismic stations are deployed in Southern and Eastern Africa. In contrast, only a handful of stations exists in the Central, Northern, and Western parts leaving the structures in large part of the continent poorly constrained seismologically. The current situation stems from the fact that most deployments are targeted at understanding specific geological structures (e.g., cratons and mineralization in Southern Africa) or answer specific scientific questions (e.g., continental rift initiation in Eastern Africa) of interest to the network managers. Therefore, our current understanding of seismic structure in other parts of Africa mostly came from large-scale global or regional tomography studies, essentially using teleseismic earthquake data. However, the resulting models usually lack enough resolution to reveal the lateral and depth variations of seismic structure in these regions, thereby limiting our capability to answer specific scientific questions of interest (e.g., the tectonic evolution of the Saharan Metacraton).

In the first part of my presentation, I will generally talk about our effort to develop a new uniform 3D seismic model in Africa and surrounding regions with good constraints to shallow and relatively deep structures for a more comprehensive understanding of the African tectonics. In the end, I will focus on new constraints on seismic structures in sparsely instrumented regions, especially within the Saharan Metacraton in North-Central Africa.

Complex seismo-tectonics of northeast India

Abstract:

The complexity in seismotectonics of North Eastern India is well accepted. Typically, the region is wedged among particularly three distinct regional dynamics - collision boundary to the north comprising Arunachal Himalaya, the syntaxial bend towards north-eastern most corner and Indo-Myanmar subduction zone towards east. In addition, Holocene upliftment of Shillong-Mikir plateau is another critical dynamic of the region. All these tectonic domains are the source and contribute to the intense seismicity pattern of the region. As a result, the region produced two great earthquakes of 1897 and 1950 (M>8.0) and twenty numbers of large earthquakes. Seismicity of the region is mostly constrained by shallower to intermediate depth earthquakes except Indo-Myanmar region where deeper depth earthquakes occur. Characteristics of these earthquakes are vivid and inferences on mechanism of these earthquakes and subsequent stress regime are a critical parameter broadly evolved from regional dynamics. Consequently these are sometimes functions of depth and space mostly independent of magnitude. In fact the complexity can be lucidly explained if in-depth study on the stress regime in terms of space , time and depth are estimated and interpreted. Although a broad understanding of the stress regime in NE-India is established, however much intense investigation in this domain is required so that the differences in stress pattern may carry information about the ongoing status of geodynamics. It is really praiseworthy to note that several researchers have contributed using different tools to substantiate the tectonic model out of extensive field cum experimental work, needless to say a proper tectonic model is still awaited where the only hindrance is complexity of geodynamics. Understanding seismic behavior of the complex domain is really frustrating since there is no expression of surface rupture questioning the epicenter of these great earthquakes. Slip rate, co-seismic uplift mismatching indicate perhaps the existence of blind fault in potential source zones which poses big threat on the contrary to clearly ascertain seismic cycle scenario of the region. The topic covers mainly the seismicity pattern, its characteristics, inferences on stress embedded with some field evidence which can be the prime inputs to the seismic hazard assessment of the region. So what is next, a comprehensive and focused multi-institutional study is the imminent need to predict a valid seismotectonic model of the northeastern region of India which will be the key driver to seismic hazard assessment.

Bio-data:

Dr. Saurabh Baruah is working as the Chief Scientist at CSIR-NEIST, Assam. He has decades of research experience in the field of seismology. Under his supervision dozens of PhD thesis have been supervised. Dr. Baruah is the pioneer of the foundation of the Multi Parametric Geophysical Observatory (MPGO) Tezpur for earthquake precursor studies in North East India context. He was also a winter member of the 18th Indian scientific expedition to Antarctica.