Slabs In Subduction Zones Research Articles

Seismic wavefield imaging of Earth’s interior across scales

Seismic full-waveform inversion (FWI) for imaging Earth’s interior was introduced in the late 1970s. Its ultimate goal is to use all of the information in a seismogram to understand the structure and dynamics of Earth, such as hydrocarbon reservoirs, the nature of hotspots and the forces behind plate motions and earthquakes. Thanks to developments in high-performance computing and advances in modern numerical methods in the past 10 years, 3D FWI has become feasible for a wide range of applications and is currently used across nine orders of magnitude in frequency and wavelength. A typical FWI workflow includes selecting seismic sources and a starting model, conducting forward simulations, calculating and evaluating the misfit, and optimizing the simulated model until the observed and modelled seismograms converge on a single model. This method has revealed Pleistocene ice scrapes beneath a gas cloud in the Valhall oil field, overthrusted Iberian crust in the western Pyrenees mountains, deep slabs in subduction zones throughout the world and the shape of the African superplume. The increased use of multi-parameter inversions, improved computational and algorithmic efficiency, and the inclusion of Bayesian statistics in the optimization process all stand to substantially improve FWI, overcoming current computational or data-quality constraints. In this Technical Review, FWI methods and applications in controlled-source and earthquake seismology are discussed, followed by a perspective on the future of FWI, which will ultimately result in increased insight into the physics and chemistry of Earth’s interior. Advances in numerical methods and high-performance computing have enabled imaging of Earth’s interior across scales using the physics of seismic-wave propagation. This Technical Review summarizes the use of full-waveform inversion in controlled-source and earthquake seismology.

Systematic shifts in subducting slab behavior with depth

When tectonic plates collide, the less buoyant plate will, in some cases, be forced beneath the other. At such subduction zones the sinking tectonic plate, known as a slab, does not follow a simple path from the surface to the deeper mantle. Instead, new research by Fukao and Obayashi suggests that subducting slabs pass through four largely distinct stages as they penetrate toward the core. To systematically catalog the stages of slab subduction, the authors analyzed roughly 10 million observations of the subsurface that were part of a tomographic study that used primary seismic waves to detect the structure of slabs in subduction zones around the Pacific.

Generation of talc in the mantle wedge and its role in subduction dynamics in central Mexico

Geophysical evidence shows the presence of low-seismic velocity material at the surface of slabs in subduction zones. In the central Mexican subduction zone this appears as a thin (∼4 km) low-velocity zone that absorbs nearly all of the strain. The P-to-S velocity ratio as a function of S wave velocity distinguishes among the various candidate hydrous (low-strength) minerals; the thin layer in the flat-slab region is most consistent with a layer showing enrichment in talc overlying normal MORB-like gabbro. Based on available thermodynamic data for equilibria for talc, its generation at the trench is nearly impossible, and hence we propose it originates from the mantle wedge during the slab flattening process coupled with trench rollback. The evolution of this low-strength zone has important implications for the dynamics of the slab-flattening process as well as the geochemistry of the mantle wedge and arc in central Mexico.

Ridge push, mantle plumes and the speed of the Indian plate

SUMMARY The buoyancy of lithospheric slabs in subduction zones is widely thought to dominate the torques driving plate tectonics. In late Cretaceous and early Paleogene times, the Indian plate moved more rapidly over the mantle than freely subducting slabs sink within it. This signal event has been attributed to arrival of the Deccan-Rmantle plume beneath the plate, but itisunknowninwhichproportionstheplumeactedtoalterthebalanceofexistingplatedriving torques and to introduce torques of its own. Our plate kinematic analysis of the Mascarene Basin yields a detailed Indian plate motion history for the period 89-60Ma. Plate speed initially increases steadily until a pronounced acceleration in the period 68-64Ma, after which it abruptly returns to values much like those beforehand. This pattern is unlike that suggested to result from the direct introduction of driving forces by the arrival of a thermal plume at the base of the plate. A simple analysis of the gravitational force related to the Indian plate's thickening away from its boundary with the African plate suggests instead that the sudden acceleration and deceleration may be related to uplift of part of that boundary during a period when it was located over the plume head. In this instance, torques related to plate accretion and subduction may have contributed in similar proportions to drive plate motion.

Origin of the Colorado Mineral Belt

The Colorado Mineral Belt (CMB) is a northeast-trending, ∼500-km-long, 25–50-km-wide belt of plutons and mining districts (Colorado, United States) that developed within an ∼1200-km-wide Late Cretaceous–Paleogene magma gap overlying subhorizontally subducted segments of the Farallon plate. Of the known volcanic gaps overlying flat slabs in subduction zones around the Pacific Basin, none contains zones of magmatism analogous to the CMB. I suggest that the primary control of the CMB was a northeast-trending segment boundary within the underlying Farallon flat slab. The boundary was dilated during warping of slab segments by the overriding thick (∼200 km) lithospheres of the Wyoming Archean craton and the continental interior craton during acceleration of Farallon–North American convergence beginning in mid-Campanian time (ca. 75 Ma). Because the primary control was not in the North American plate, the CMB cut indiscriminately across the geologic grain of Colorado, seemingly independent of the tectonic elements it crossed. A series of discontinuous shear zones of Proterozoic ancestry provided some local control at the district level but were not the primary control. Geologic contrasts north and south of the CMB reflect its relationship to a segment boundary in the Farallon plate. The dominant trends of Laramide basement-cored uplifts are northwestward north of the CMB but northward south of the CMB. Laramide sedimentary deposits of Late Cretaceous and Paleogene age (exclusive of the Sevier foredeep) are as much as 6 km thick north of the CMB versus only ≤3 km south of the CMB. The Farallon segment south of the CMB rolled back to the southwest and sank into the mantle beginning ca. 37 Ma with resultant major ignimbrite volcanism and generation of the large San Juan and Mogollon-Datil volcanic fields. Volcanism in the Rocky Mountains north of the CMB was sparse. Laramide plutons (ca. 75–43 Ma) are mainly alkaline monzonites and quartz monzonites in the northeastern CMB, but dominantly calc-alkaline granodiorites in the central CMB. Geochemical and isotopic studies indicate that CMB magmas were generated mainly in metasomatized Proterozoic intermediate to felsic lower crustal granulites and mafic rocks (± mantle). Late Eocene–Oligocene rollback magmatism superimposed on the CMB during waning of Laramide compression (ca. 43–37 Ma) resulted in world-class sulfide replacement ores in the Leadville area. Overprinting of the CMB by Rio Grande Rift extension beginning ca. 33 Ma resulted in intrusion of evolved alkali-feldspar granites and generation of major porphyry molybdenum deposits at Climax and Red Mountain.

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

It is now well established that the early continental crust was formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geologic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shallow subduction. Distinguishing between these scenarios has important implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to constrain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trondhjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG granitoids from the NAC, and their elevated garnet δ 18 O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stacking of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.

An Investigation into the Seismic Potential of the Irrawaddy Region, Northern Sunda Arc

Abstract Tearing and detachment of subducted slabs in subduction zones have been an important focus of geoscience research as they have serious seismic hazard implications. Based on evidence, including the pattern of seismicity, great earthquake ruptures, tomographic studies, back-arc volcanism, and trench morphology, we suggest that the lack of sufficient width of subducting Indian plate slab under the Irrawaddy region (between latitudes 15° N and 18° N) of the Sunda arc due to the presence of a tear in the subducting Indo-Australian slab makes the region aseismic. The lack of slab there also limits the potential for major and great earthquakes to be generated. Accordingly, the seismic and tsunami potential of this region may not be as high as it is in the Andaman and Arakan regions.

Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa

Abstract Laboratory experiments on natural, hydrous basalts at 1–4 GPa constrain the composition of “unadulterated” partial melts of eclogitized oceanic crust within downgoing lithospheric slabs in subduction zones. We complement the “slab melting” experiments with another set of experiments in which these same “adakite” melts are allowed to infiltrate and react with an overlying layer of peridotite, simulating melt:rock reaction at the slab–mantle wedge interface. In subduction zones, the effects of reaction between slab-derived, adakite melts and peridotitic mantle conceivably range from hybridization of the melt, to modal or cryptic metasomatism of the sub-arc mantle, depending upon the “effective” melt:rock ratio. In experiments at 3.8 GPa, assimilation of either fertile or depleted peridotite by slab melts at a melt:rock ratio ∼2:1 produces Mg-rich, high-silica liquids in reactions which form pyrope-rich garnet and low-Mg# orthopyroxene, and fully consume olivine. Analysis of both the pristine and hybridized slab melts for a range of trace elements indicates that, although abundances of most trace elements in the melt increase during assimilation (because melt is consumed), trace element ratios remain relatively constant. In their compositional range, the experimental liquids closely resemble adakite lavas in island-arc and continental margin settings, and adakite veins and melt inclusions in metasomatized peridotite xenoliths from the sub-arc mantle. At slightly lower melt:rock ratios (∼1:1), slab melts are fully consumed, along with peridotitic olivine, in modal metasomatic reactions that form sodic amphibole and high-Mg# orthopyroxene.

Back-arc spreading in the proterozoic — a theoretical approach

Abstract The early Proterozoic Earth probably had a hotter mantle and a lower residence time for oceanic plates at the surface than the modern one. The hotter mantle may have led to thinner oceanic plates. Young ocean floor has low negative buoyancy and therefore exerts low slab-pull forces in subduction zones. This effect would have been greater if the plates were thinner, since they would have warmed up more quickly as they descended into the mantle. Thus the negative buoyancy of slabs and the slab-pull forces were probably substantially lower in the early Proterozoic. Calculations show that this applies even if subduction rates were higher and the crust was thicker. Lower slab-pull leads to lower rates of ‘rollback’ in subduction zones and lower rates of retreat of upper plates from trenches. The limited age-range of oceanic crust and the more rapid heating of descending slabs in subduction zones probably leads to a smaller range of slab-profiles, since this range is controlled by thermal properties of the slab, and therefore to more stability in subduction zones. Thus, many of the proposed mechanisms for back-arc spreading may have been less effective, and backarc spreading may have been less common, in early Proterozoic subduction zones. This may explain the apparent absence of ophiolite complexes of that age.