Mantle dynamics of the Andean Subduction Zone from continent-scale teleseismic S-wave tomography

2020 ◽  
Vol 224 (3) ◽  
pp. 1553-1571
Author(s):  
Emily E Rodríguez ◽  
Daniel Evan Portner ◽  
Susan L Beck ◽  
Marcelo P Rocha ◽  
Marcelo B Bianchi ◽  
...  
Keyword(s):  
South America ◽  
Subduction Zone ◽  
Lower Mantle ◽  
Subduction Zones ◽  
Seismic Velocity ◽  
High Amplitude ◽  
S Wave ◽  
Nazca Plate ◽  
Wave Tomography ◽  
Andean Subduction

SUMMARY The Andean Subduction Zone is one of the longest continuous subduction zones on Earth. The relative simplicity of the two-plate system has makes it an ideal natural laboratory to study the dynamics in subduction zones. We measure teleseismic S and SKS traveltime residuals at >1000 seismic stations that have been deployed across South America over the last 30 yr to produce a finite-frequency teleseismic S-wave tomography model of the mantle beneath the Andean Subduction Zone related to the Nazca Plate, spanning from ∼5°N to 45°S and from depths of ∼130 to 1200 km. Within our model, the subducted Nazca slab is imaged as a fast velocity seismic anomaly. The geometry and amplitude of the Nazca slab anomaly varies along the margin while the slab anomaly continues into the lower mantle along the entirety of the subduction margin. Beneath northern Brazil, the Nazca slab appears to stagnate at ∼1000 km depth and extend eastward subhorizontally for >2000 km. South of 25°S the slab anomaly in the lower mantle extends offshore of eastern Argentina, hence we do not image if a similar stagnation occurs. We image several distinct features surrounding the slab including two vertically oriented slow seismic velocity anomalies: one beneath the Peruvian flat slab and the other beneath the Paraná Basin of Brazil. The presence of the latter anomaly directly adjacent to the stagnant Nazca slab suggests that the plume, known as the Paraná Plume, may be a focused upwelling formed in response to slab stagnation in the lower mantle. Additionally, we image a high amplitude fast seismic velocity anomaly beneath the Chile trench at the latitude of the Sierras Pampeanas which extends from ∼400 to ∼1000 km depth. This anomaly may be the remnants of an older, detached slab, however its relationship with the Nazca–South America subduction zone remains enigmatic.

Ambient lower mantle structure and composition inferred from seismic tomography, convection models, and geochemistry.

2020 ◽  
Author(s):  
Grace E. Shephard ◽  
John Hernlund ◽  
Christine Houser ◽  
Reidar Trønnes ◽  
Fabio Crameri
Keyword(s):  
Lower Mantle ◽  
Subduction Zones ◽  
Large Scale ◽  
Seismic Velocity ◽  
Seismic Signal ◽  
High Viscosity ◽  
S Wave ◽  
Velocity Models ◽  
Reference Velocity ◽  
Seismic Anomalies

<p>The lower mantle can be grouped into high, low, and average (i.e., ambient) seismic velocity domains at each depth, based on the amplitude and polarity of wavespeed perturbations (% δlnVs, % δlnVp). Many studies focus on elucidating the thermo-chemical and structural origins of fast and slow domains, in particular. Subducted slabs are associated with fast seismic anomalies throughout the mantle, and reconstructed palaeo-positions of Cenozoic to Mesozoic subduction zones agrees with seismically imaged deep slabs. Conversely, slow wavespeed domains account for the two antipodal LLSVPs in the lowermost mantle, which are potentially long-lived features, as well as rising hot mantle above the LLSVPs and discrete mantle plumes. However, low-amplitude wavespeeds (close to the reference velocity models) are often overlooked By comparing multiple P- and S-wave tomographic models individually, and through “vote maps”, we reveal the depth-dependent characteristics and the geometry of ambient structures, and compare them to numerical convection models. The ambient velocity domains may contain early refractory and bridgmantic mantle with elevated Si/(Mg+Fe) and Mg/Fe ratios (BEAMS; bridgmanite-enriched mantle structures). They could have formed by early basal magma ocean (BMO) fractionation during a period of core-BMO exchange of SiO<sub>2</sub> (from core to BMO) and FeO (from BMO to core), or represent cumulates of BMO crystallization with bridgmanite as the liquidus phase. The high viscosity of bridgmanitic material may promote its convective aggregation and stabilise the large-scale, degree-2 convection pattern. Despite its high viscosity, bridgmanitic material, representing a primitive and refractory reservoir for primordial-like He and Ne components, might be entrained in vigorous, deep-rooted plumes. The restriction of a weak seismic signal, ascribed to iron spin-pairing in ferropericlase, to the fast and slow domains, supports the notion that the ambient lower mantle domains are bridgmanitic.</p>

Modeling guided waves to constrain the velocity structure of the oceanic crust in the subduction zone of eastern Alaska

2020 ◽  
Author(s):  
Xiaoyu Guan ◽  
Yuanze Zhou ◽  
Takashi Furumura
Keyword(s):  
Subduction Zone ◽  
Oceanic Crust ◽  
High Frequency ◽  
Subduction Zones ◽  
Guided Waves ◽  
Velocity Structure ◽  
Guided Wave ◽  
S Wave ◽  
Arrival Times ◽  
Velocity Models

<p>Fitting subduction zone guided waves with synthetics is an ideal choice for studying the velocity structure of the oceanic crust. After an earthquake occurs in subduction zones, seismic waves can be trapped in the low-velocity oceanic crust and propagated as guided waves. The arrival time and frequency characteristics of the guided waves can be used to image the velocity structure of the oceanic crust. The analysis and modeling based on guided wave observations provide a rare opportunity to understand the velocity structure of the oceanic crust and the variations in oceanic crustal materials during the subduction process.</p><p>High-frequency guided waves have been observed in the subduction zone of eastern Alaska. On several sections, observed seismograms recorded by seismic stations show low-frequency (<2Hz) onsets ahead of the main high-frequency (>2Hz) guided waves. Differences in the arrival times and dispersion characteristics of seismic phases are related to the velocity structure of the oceanic crust, and the characteristics of coda waves are related to the distribution of elongated scatters in the oceanic crust. Through fitting the observed broadband waveforms and synthetics modeled with the 2-D FDM (Finite Difference Method), we obtain the preferred oceanic crustal velocity models for several sections in the subduction zone of eastern Alaska. The preferred models can explain the seismic phase arrival times, dispersions, and coda characteristics in the observed waveforms. With the obtained P- and S- wave models of velocity structures on several sections, the material compositions they represent are deduced, and the variations of oceanic crustal materials during subducting can be understood. This provides new evidence for studying the details of the subduction process in the subduction zone of eastern Alaska.</p>

scholarly journals Relationship between the high-amplitude magnetic anomalies and serpentinized  fore-arc mantle in the Cascadia subduction zone

2021 ◽  
Author(s):  
Wen-Bin Doo
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Density Ratio ◽  
Magnetic Anomalies ◽  
High Amplitude ◽  
Cascadia Subduction Zone ◽  
Low Velocity ◽  
Temperature Isotherm ◽  
Magnetization Density ◽  
Cascadia Margin

Abstract A zone of significant high-amplitude magnetic anomalies is observed without a comparable gravity high along the Cascadia margin and is spatially correlated with the low-velocity fore-arc mantle wedge, which is understood to be serpentinized fore-arc mantle and is further considered to be the main source of the high-amplitude magnetic anomalies. To test this hypothesis, the magnetization-density ratio (MDR) is estimated along the Cascadia margin to highlight the physical characteristics of serpentinization (reduced density and increased magnetization). Interestingly, high MDR values are found only in central Oregon, where slab dehydration and fore-arc mantle serpentinization (50%-60% serpentinization) are inferred in conjunction with sparse seismicity. This result may indicate either poorly serpentinized fore-arc mantle (low degree of serpentinization) or that the fore-arc mantle is deeper than the Curie temperature isotherm for magnetite in northern and southern Cascadia. I thus propose that serpentinized fore-arc mantle may not be the major contributor to the high-amplitude magnetic anomalies in these segments. This finding means that magnetic anomaly highs and serpentinized fore-arc mantle may not be completely positively related in subduction zones. On the other hand, the MDR pattern reveals the segmentation of the Cascadia subduction zone, which is consistent with several previous observations.

scholarly journals Fluid pressurisation and earthquake propagation in the Hikurangi subduction zone

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
S. Aretusini ◽  
F. Meneghini ◽  
E. Spagnuolo ◽  
C. W. Harbord ◽  
G. Di Toro
Keyword(s):  
Shear Strength ◽  
Subduction Zone ◽  
Subduction Zones ◽  
Seismic Velocity ◽  
Fluid Pressure ◽  
Velocity Shear ◽  
Low Permeability ◽  
Earthquake Rupture ◽  
Seismic Slip ◽  
Saturated Clay

AbstractIn subduction zones, seismic slip at shallow crustal depths can lead to the generation of tsunamis. Large slip displacements during tsunamogenic earthquakes are attributed to the low coseismic shear strength of the fluid-saturated and non-lithified clay-rich fault rocks. However, because of experimental challenges in confining these materials, the physical processes responsible for the coseismic reduction in fault shear strength are poorly understood. Using a novel experimental setup, we measured pore fluid pressure during simulated seismic slip in clay-rich materials sampled from the deep oceanic drilling of the Pāpaku thrust (Hikurangi subduction zone, New Zealand). Here, we show that at seismic velocity, shear-induced dilatancy is followed by pressurisation of fluids. The thermal and mechanical pressurisation of fluids, enhanced by the low permeability of the fault, reduces the energy required to propagate earthquake rupture. We suggest that fluid-saturated clay-rich sediments, occurring at shallow depth in subduction zones, can promote earthquake rupture propagation and slip because of their low permeability and tendency to pressurise when sheared at seismic slip velocities.

scholarly journals Seismic evidence for megathrust fault-valve behavior during episodic tremor and slip

2020 ◽  
Vol 6 (4) ◽  
pp. eaay5174 ◽  
Author(s):  
Jeremy M. Gosselin ◽  
Pascal Audet ◽  
Clément Estève ◽  
Morgan McLellan ◽  
Stephen G. Mosher ◽  
...  
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Seismic Velocity ◽  
Fluid Pressure ◽  
Pore Fluid ◽  
Cascadia Subduction Zone ◽  
Low Velocity ◽  
Seismic Scattering ◽  
Fault Valve ◽  
Episodic Tremor

Fault slip behavior during episodic tremor and slow slip (ETS) events, which occur at the deep extension of subduction zone megathrust faults, is believed to be related to cyclic fluid processes that necessitate fluctuations in pore-fluid pressures. In most subduction zones, a layer of anomalously low seismic wave velocities [low-velocity layer (LVL)] is observed in the vicinity of ETS and suggests high pore-fluid pressures that weaken the megathrust. Using repeated seismic scattering observations in the Cascadia subduction zone, we observe a change in the seismic velocity associated with the LVL after ETS events, which we interpret as a response to fluctuations in pore-fluid pressure. These results provide direct evidence of megathrust fault-valve processes during ETS.

Lattice preferred orientation and seismic anisotropy of chloritoid in subduction zone

2021 ◽  
Author(s):  
Jungjin Lee ◽  
Mainak Mookherjee ◽  
Taehwan Kim ◽  
Haemyeong Jung ◽  
Reiner Klemd
Keyword(s):  
Subduction Zone ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
P Wave ◽  
Stability Field ◽  
S Wave ◽  
P Waves ◽  
Hydrous Minerals ◽  
Natural Rock

<p>Subduction zones are often characterized by the presence of strong trench-parallel seismic anisotropy and large delay times. Hydrous minerals, owing to their large elastic anisotropy and strong lattice preferred orientations (LPOs) are often invoked to explain these observations. However, the elasticity and LPO of chloritoid, which is one such hydrous phases relevant in subduction zone settings, is poorly understood. In this study, we measured the LPO of polycrystalline chloritoid in natural rock samples and obtained the LPO-induced seismic anisotropy and evaluated the thermodynamic stability field of chloritoid in subduction zones. The LPO of chloritoid aggregates displayed a strong alignment of the [001] axes subnormal to the rock foliation, with a girdle distribution of the [100] axes and the (010) poles subparallel to the foliation. New elasticity data of single-crystal chloritoid showed a strong elastic anisotropy of chloritoid with 47% for S-waves (V<sub>S</sub>) and 22% for P-waves (V<sub>P</sub>), respectively. The combination of the LPO and the elastic anisotropy of the chloritoid aggregates produced a strong S-wave anisotropy of AV<sub>S</sub> = 18% and a P-wave anisotropy of AV<sub>P</sub> = 10%. Our results indicate that the strong LPO of chloritoid along the hydrated slab-mantle interface and in subducting slabs can influence trench-parallel seismic anisotropy in subduction zones with “cold” geotherms.</p>

scholarly journals Subduction zone fluids and arc magmas conducted by lithospheric deformed regions beneath the central Andes

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
E. Contreras-Reyes ◽  
D. Díaz ◽  
J. P. Bello-González ◽  
K. Slezak ◽  
B. Potin ◽  
...  
Keyword(s):  
Electrical Resistivity ◽  
Subduction Zone ◽  
Continental Crust ◽  
Subduction Zones ◽  
Hydrothermal Systems ◽  
P Wave ◽  
Complex Permeability ◽  
Nazca Plate ◽  
Lithospheric Deformation ◽  
Permeability Structure

AbstractDehydration of the oceanic subducting slab promotes the formation of magmatic arcs, intra-slab intermediate-depth seismicity, and hydration of the overlying mantle wedge. However, the complex permeability structure of the overriding plate controls the magma and fluid migration and their accumulation at shallower depths. In this regard, mapping the inner structure of the overriding crust and mantle is crucial to understand the magmatic and hydrological processes in subduction zones. We integrate 3-D P-wave, $$V_p/V_s$$ V p / V s , and electrical resistivity tomographic models of the northern Chilean subduction zone to map the magmatic and fluids derived from the subducting oceanic Nazca plate. Results show a continental crust relatively thick (50–65 km) characterized by a lower zone of high $$V_p$$ V p values (7.2–7.6 km/s), which is interpreted as the presence of plutonic rocks. The mantle lithospheric wedge is weakly hydrated ($$V_p/V_s$$ V p / V s = 1.75–1.8) while the forearc continental crust is traversed by regions of reduced electrical resistivity values ($$< 10^2$$ < 10 2 $$\Omega m$$ Ω m ) interpreted as zones of relatively high permeability/fracturing and fluid content. These regions spatially correlate with upper plate trans-lithospheric deformation zones. Ascending melts accumulate preferentially in the back-arc, whereas hydrothermal systems form trenchward of the volcanic arc. The results highlight the complex permeability structure of the upper South American plate.

scholarly journals Seismological expression of the iron spin crossover in ferropericlase in the Earth’s lower mantle

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Grace E. Shephard ◽  
Christine Houser ◽  
John W. Hernlund ◽  
Juan J. Valencia-Cardona ◽  
Reidar G. Trønnes ◽  
...  
Keyword(s):  
Lower Mantle ◽  
Spin Crossover ◽  
Shear Velocity ◽  
Oceanic Lithosphere ◽  
P Wave ◽  
Depth Range ◽  
S Wave ◽  
Seismic Profiles ◽  
P Waves ◽  
Wave Tomography

AbstractThe two most abundant minerals in the Earth’s lower mantle are bridgmanite and ferropericlase. The bulk modulus of ferropericlase (Fp) softens as iron d-electrons transition from a high-spin to low-spin state, affecting the seismic compressional velocity but not the shear velocity. Here, we identify a seismological expression of the iron spin crossover in fast regions associated with cold Fp-rich subducted oceanic lithosphere: the relative abundance of fast velocities in P- and S-wave tomography models diverges in the ~1,400-2,000 km depth range. This is consistent with a reduced temperature sensitivity of P-waves throughout the iron spin crossover. A similar signal is also found in seismically slow regions below ~1,800 km, consistent with broadening and deepening of the crossover at higher temperatures. The corresponding inflection in P-wave velocity is not yet observed in 1-D seismic profiles, suggesting that the lower mantle is composed of non-uniformly distributed thermochemical heterogeneities which dampen the global signature of the Fp spin crossover.

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