Vlad Constantin Manea

Chilean flat slab subduction controlled by overriding plate thickness and trench rollback

How flat slab geometries are generated has been long debated. It has been suggested that trenchward motion of thick cratons in some areas of South America and Cenozoic North America progressively closed the asthenospheric wedge and induced flat subduction. Here we develop time-dependent numerical experiments to explore how trenchward motion of thick cratons may result in flat subduction. We find that as the craton approaches the trench and the wedge closes, two opposite phenomena control slab geometry: the suction between ocean and continent increases, favoring slab flattening, while the mantle confined within the closing wedge dynamically pushes the slab backward and steepens it. When the slab retreats, as in the Peru and Chile flat slabs, the wedge closure rate and dynamic push are small and suction forces generate, in some cases, flat subduction. We model the past 30 m.y. of subduction in the Chilean flat slab area and demonstrate that trenchward motion of thick lithosphere, 200–300 km, currently ∼700–800 km away from the Peru-Chile Trench, reproduces a slab geometry that fits the stress pattern, seismicity distribution, and temporal and spatial evolution of deformation and volcanism in the region. We also suggest that varying trench kinematics may explain some differing slab geometries along South America. When the trench is stationary or advances, the mantle flow within the closing wedge strongly pushes the slab backward and steepens it, possibly explaining the absence of flat subduction in the Bolivian orocline.

Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico

The aseismic slow slip event of 2001–2002 in Guerrero, Mexico, with an equivalent magnitude MW ~ 7.5, is the largest silent earthquake (SQ) among many recently recorded by GPS in different subduction zones (i.e. Japan, Alaska, Cascadia, New Zealand). The sub-horizontal and shallow plate interface in Central Mexico is responsible for specific conditions for the ~100 km long extended transient zone where the SQs develop from ~80 to ~190 km inland from the trench. This wide transient zone and relatively large slow slips of 10 to 20 cm displacements on the subduction fault result in noticeable surface displacements of 5–6 cm during the SQs. Continuous GPS stations allow one to trace the propagation of SQs, and to estimate their arrival time, duration and geometric attenuation. These propagation parameters must be accounted in order to locate source of slow slips events and to understand the triggering effect that they have on large subduction earthquakes. We use long-baseline tiltmeter data to define new time limits (onset and duration) for the SQs and continuous records from 8 GPS stations to determine the propagation of the 2001–2002 SQ in Central Mexico. Data from the CAYA and IGUA GPS stations, separated by ~170 km and located along the profile perpendicular to the trench, are used to determine that the surface deformation from the 2001–2002 SQ started almost instantaneously. It propagated parallel to the coast at ~2 km/day with an exponential attenuation of the horizontal surface displacement and a linear decrease of its duration with distance. Campaign data obtained yearly from 2001 to 2005 at the Oaxaca GPS network have been modeled according to a propagation of the 2001–2002 SQ step-like displacement anomaly. This modeling shows that the SQ ceased gradually in the central part of the Oaxaca segment of the subduction zone (west of Puerto Angel, PUAN) and then it apparently triggered another SQ in SE Oaxaca (between PUAN and Salina Cruz, SACR). The estimated horizontal velocities for inter-event epochs at each GPS site are used to assess an average interplate coupling in the Central Oaxaca subduction zone.

Mantle temperature control on composition of arc magmas along the Central Kamchatka Depression

Abundant volcanism in the Central Kamchatka Depression (CKD) adjacent to the Kamchatka–Aleutian Arc junction occurs where the Pacific slab edge is subducting beneath Kamchatka. Here we summarize published data on CKD rocks and demonstrate a systematic south-to-north change of their compositions from moderately fractionated basalt-andesite tholeiitic series to highly fractionated basalt-rhyolite calc-alkaline series including high-magnesian andesites near the slab edge. Localized slab melting at the slab edge cannot explain these regional geochemical variations. Instead, we propose that the thermal state of the mantle wedge can be the key factor governing the composition of CKD magmas. We integrate the results from petrology and numeric modeling to demonstrate the northward decrease of the mantle wedge temperatures beneath CKD volcanoes, which correlates with decreasing slab dip, length of mantle columns, and magma flux. We envision two petrogenetic models, which relate the composition of erupted magmas to the subduction parameters beneath the CKD. The first model suggests that mantle temperature governs melt-peridotite equilibria and favors generation of andesitic primary melts in cold mantle regions above the shallowly subducting Pacific slab edge. Alternatively, mantle temperature may control magmatic productivity along the CKD, which decreases sharply toward the slab edge and thus allows more extensive magma fractionation deeper in the crust and mixing of highly evolved and mantle-derived magmas to generate Si-rich “primitive” magmas. These results point to a possible casual link between deep mantle and shallow crustal magmatic processes. Similar effects of mantle temperature on the composition and productivity of arc magmatism are expected elsewhere, particularly in volcanic regions associated with significant slab dip variation along the arc.

Thermal models, magma transport, and velocity anomaly estimation beneath southern Kamchatka

A finite-element method is applied to model the thermal structure of the subducted Pacific plate and overlying mantle wedge beneath the southern part of the Kamchatka peninsula. A numerical scheme solves a system of 2D Navier-Stokes equations and a 2D steady-state heat transfer equation. A model with isoviscous mantle exposed very low temperatures (~800 °C) in the mantle wedge, which cannot account for magma generation below the volcanic belt. Instead, a model with strong temperature-dependent viscosity shows a rise in the temperature in the wedge. At a temperature of more than 1300 °C beneath the active volcanic chain, melting of wedge peridotite becomes possible. Although the subducting slab below the Kamchatka peninsula is rather old (ca. 70 Ma), some frictional heating (m = 0.034) along the interface between the subducting oceanic slab and the overlying Kamchatka peninsula lithosphere would be enough to melt subducted sediments. Dehydration (>5 wt% H2O release) occurs in the subducting slab because of metamorphic changes. As a consequence, hydration of the mantle wedge peridotite might produce melt, which may rise to the base of the continental crust as diapirlike blobs. Considering that melting processes in the subducting plate generate most of the volcanic material, we developed a dynamic model that simulates the migration of partially melted buoyant material in the form of blobs in the viscous mantle wedge flow. Blobs with diameters of 0.4–10.0 km rise to the base of the continental lithosphere within 0.002–10 m.y. depending on blob diameter and surrounding viscosity. The thermal structure obtained in the model with temperature-dependent viscosity is used to estimate seismic compressional wave (P-wave) velocity anomalies (referenced to the Preliminary Reference Earth Model) associated with subduction beneath Kamchatka. A low-velocity zone (~–7% velocity anomaly) is obtained beneath the volcanic belt, and a high-velocity anomaly (~4%) is obtained for the cold subducted lithosphere. These results agree with seismic tomography results from P-wave arrivals.

Subduction of fracture zones controls mantle melting and geochemical signature above slabs.

For some volcanic arcs, the geochemistry of volcanic rocks erupting above subducted oceanic fracture zones is consistent with higher than normal fluid inputs to arc magma sources. Here we use enrichment of boron (B/Zr) in volcanic arc lavas as a proxy to evaluate relative along-strike inputs of slab-derived fluids in the Aleutian, Andean, Cascades and Trans-Mexican arcs. Significant B/Zr spikes coincide with subduction of prominent fracture zones in the relatively cool Aleutian and Andean subduction zones where fracture zone subduction locally enhances fluid introduction beneath volcanic arcs. Geodynamic models of subduction have not previously considered how fracture zones may influence the melt and fluid distribution above slabs. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations of subduction, we show that enhanced production of slab-derived fluids and mantle wedge melts concentrate in areas where fracture zones are subducted, resulting in significant along-arc variability in magma source compositions and processes.

Three‐dimensional numerical modeling of thermal regime and slab dehydration beneath Kanto and Tohoku, Japan

Although the thermal regime of the interface between two overlapping subducting plates, such as those beneath Kanto, Japan, is thought to play an important role in affecting the distribution of interplate and intraslab earthquakes, the estimation of the thermal regime remains challenging to date. We constructed a three-dimensional (3-D) thermal convection model to simulate the subduction of the Pacific plate along the Japan Trench and Izu-Bonin Trench, including the subduction of the Philippine Sea beneath Kanto and investigated the slab thermal regime and slab water contents in this complex tectonic setting. Based on the subduction parameters tested in generic models with two flat oceanic plates, a faster or thicker plate subducting in a more trench-normal direction produces a colder slab thermal regime. The interplate temperature of the cold anomaly beneath offshore Kanto was approximately 300°C colder than that beneath offshore Tohoku at a same depth of 40 km and approximately 600°C colder at a depth of 70 km. The convergence between the two subducting plates produces an asymmetric thermal structure in the slab contact zone beneath Kanto, which is characterized by clustered seismicity in the colder southwestern half. The thermo-dehydration state of the mid-ocean ridge basalt near the upper surface of the subducted Pacific plate controls the interplate seismicity beneath the Kanto-Tohoku region according to the spatial concurrence of the thermo-dehydration and seismicity along the megathrust fault zone of the subducted Pacific plate.

Thermal Models Beneath Kamchatka and the Pacific Plate Rejuvenation from a Mantle Plume Impact

The Northwest Pacific area, comprising the Kamchatka peninsula, is a distinctive area where a series of on going geodynamical processes like: plate rejuvenation from a mantle plume impact, slab detachment, slab edge melting and exotic volcanism, take place. With the help of finite element modeling we infer the thermal structure across Kamchatka in a series of 2D profiles normal to the trench. We chose the location at these profiles based on seismicity, geochemical variation and offshore heat flow measurements. Assuming that the transition from brittle to ductile behavior inside the subducting slab corresponds to the 650°C isotherm, our thermal models predict a good fit with maximum depth of seismicity (∼500 km) for southern Kamchatka only if the exothermic olivine-spinel phase transition is introduced. In the central part of Kamchatka, a good fit is obtained if the hot mantle plume, located just beneath Meiji Guyot seamount, thermally rejuvenates the subducting Pacific plate. Further to the north, the seismicity shallows more (200-100 km) and slab rejuvenation alone cannot provide a thermal structure with a good fit with seismically active subducting slab. A good explanation for such shallow seismicity might be the slab detachment due to cessation of subduction just north of Kamchatka-Aleutians junction. The thermal structure beneath the northernmost active volcano in Kamchatka, Scheveluch, which exhibits a strong adakitic signature, shows that slab edge exposure to the hotter asthenosphere creates the favorable conditions for oceanic crust melting at ∼70 km depth, just beneath Scheveluch. Our numerical models show that plate rejuvenation from a mantle plume, slab edge exposure to hot upper mantle and probably slab detachment play an essential role in subduction slabs thermal structure, seismicity down-dip extension and geochemical variations of lavas in Kamchatka.

ON THE GEOCHRONOLOGICAL METHOD VERSUS FLOW SIMULATION SOFTWARE APPLICATION FOR LAHAR RISK MAPPING: A CASE STUDY OF POPOCATÉPETL VOLCANO, MEXICO

Abstract. Lahars are hazardous events that can cause serious damage to people who live close to volcanic areas; several were registered at different times in the last century, such as at Mt St Helens (USA) in 1980, Nevado del Ruiz (Colombia) in 1985 and Mt Pinatubo (Philippines) in 1990. Risk maps are currently used by decision‐makers to help them plan to mitigate the hazard‐risk of lahars. Risk maps are acquired based on a series of tenets that take into account the distribution and chronology of past lahar deposits, and basically two approaches have been used: (1) The use of Flow Simulation Software (FSS), which simulates flows along channels in a Digital Elevation Model and (2) The Geochronological Method (GM), in which the mapping is based on the evaluation of lahar magnitude and frequency. This study addresses the production of a lahar risk map using the two approaches (FSS and GM) for a study area located at Popocatépetl volcano – Central Mexico. Santiago Xalitzintla, a town located on the northern flank of Popocatépetl volcano, where volcanic activity in recent centuries has triggered numerous lahars that have endangered local inhabitants, has been used for the case study. Results from FSS did not provide satisfactory findings because they were not consistent with lahar sediment observations made during fieldwork. By contrast, the GM produced results consistent with these observations, and therefore we use them to assess the hazard and produce the risk map for the study area.

Seismogenesis of dual subduction beneath Kanto, central Japan controlled by fluid release

Dual subduction represents an unusual case of subduction where one oceanic plate subducts on top of another, creating a highly complex tectonic setting. Because of the complex interaction between the two subducted plates, the origin of seismicity in such region is still not fully understood. Here we investigate the thermal structure of dual subduction beneath Kanto, central Japan formed as a consequence of a unique case of triple trench junction. Using high-resolution three-dimensional thermo-mechanical models tailored for the specific dual subduction settings beneath Kanto, we show that, compared with single-plate subduction systems, subduction of double slabs produces a strong variation of mantle flow, thermal and fluid release pattern that strongly controls the regional seismicity distribution. Here the deepening of seismicity in the Pacific slab located under the Philippine Sea slab is explained by delaying at greater depths (~150 km depth) of the eclogitization front in this region. On the other hand, the shallower seismicity observed in the Philippine Sea slab is related to a young and warm plate subduction and probably to the presence of a hot mantle flow traveling underneath the slab and then moving upward on top of the slab.

Deep embrittlement and complete rupture of the lithosphere during the M w 8.2 Tehuantepec earthquake

Subduction zones, where two tectonic plates converge, are generally dominated by large thrust earthquakes. Nonetheless, normal faulting from extensional stresses can occur as well. Rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending. Such earthquakes are limited to regions of the subducting slab cooler than 650 °C and can be highly tsunamigenic, producing tsunamis similar in amplitude to those observed during large megathrust events. Here, we show from analyses of regional geophysical observations that normal faulting during the moment magnitude Mw 8.2 Tehuantepec earthquake ruptured the entire Cocos slab beneath the megathrust region. We find that the faulting reactivated a bend-fault fabric and ruptured to a depth well below the predicted brittle–ductile transition for the Cocos slab, including regions where temperature is expected to exceed 1,000 °C. Our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension in the subducted slab, possibly due to fluid infiltration. We conclude that lithosphere can sustain brittle behaviour and fail in an earthquake at greater temperatures and ages than previously considered.Geophysical observations of the 2017 Tehuantepec earthquake suggest that oceanic lithosphere can sustain brittle behaviour and rupture in an earthquake at greater depths than previously assumed.