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Most of the world's explosive volcanoes are located in volcanic arcs, formed by fluid-fluxed melting of upper mantle rocks. The fluids that facilitate melting are released from subducted tectonic plates as they sink into the mantle. Yet, we have sparse knowledge of the migration pathways of melts through the upper mantle (i.e., between the surface of the subducted plate and arc volcanoes). We are also uncertain of the time required for this migration to occur. Here, we show evidence of two earthquake swarms that occur in the upper mantle beneath the Mariana and Izu-Bonin arc systems. The best-resolved swarm occurs beneath the Mariana arc, where the earthquakes define a sub-vertical pipe-like structure with a diameter of ∼50 km and occurs between depths of ∼10-250 km. To test the robustness of depth locations, we used a fully non-linear grid search algorithm, double-difference relocation, as well as an analysis of pP-P arrival times and depth sensitive phases. In addition, we calculated centroid moment tensor solutions using a 3D Earth model to understand the mechanism of failure within the swarms. These data demonstrate that the sub-vertical earthquake swarm occurs within the upper mantle between the subducted slab and the overriding volcanic arc, with seismicity concentrated within discrete day- to month-long swarms of activity over a single two-year period. The Izu-Bonin example shares a similar sub-vertical pipe-like geometry with seismic activity bracketed within a two-year period. We infer that these rare earthquake swarms record the ascent of hydrous melt and/or fluid, from dehydration of the subducting plate. This implies that hydrous minerals within subducted slabs continue to dehydrate to depths of at least 200-250 km. Also, the short duration of earthquake swarms implies that fluids/melts can be rapidly transported through the sub-arc mantle at rates in the order of km/hr. This is consistent with rapid melt ascent rates inferred from geospeedometry and exp

Subducting slabs commonly acquire complex geometries from the migration of subduction trenches, slab-mantle interaction, slab tearing, and collision of slabs at depth. Although it is possible to construct three-dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models, it is often not possible to rigorously test their accuracy. Here we present a methodology for performing such a test, by “reverse-engineering” the presubduction configuration of a slab of oceanic lithosphere from interpretations of its present-day morphology. We illustrate our approach for the Ryukyu and Shikoku slabs, northwest Philippine Sea, having simulated them as viscoelastic sheets that we unfolded and “floated” to the surface. The net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable (minimal net strain) and which parts of the mesh required additional tears and/or zones of localized ductile extension to have enabled the slab to deform during subduction. In the instance of the Ryukyu and Shikoku slabs, the Palau-Kyushu and Gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears. These subducted ridges are connected by a trench-parallel tear that represented the former contact between the Huatung and West Philippine Basins. The fossil spreading center of the Shikoku Basin formed a separate slab window upon subduction along the Nankai Trough. The methodology presented herein is a powerful tool to evaluate complex slab morphologies, infer the locations of slab tears, and therefore reconstruct intricate configurations of subducted oceanic lithosphere.

The locations of major gold and copper deposits on the island of New Guinea are considered by many to be controlled by a series of transfer faults that strike N-S to NE-SW, perpendicular to the long axis of the island. The premise is that these faults dilate perpendicular to the regional stress field, forming conduits for metalliferous gases and fluids to drop out of solution. However, the data on which this idea was first proposed were often not presented or, when the data were presented, were of poor quality or low resolution. We therefore present a review of the existing structural interpretations and compare these with several recently published geophysical data sets to determine if the mineralization controlling transfer faults could be observed. These data were used to produce a new lineament map of New Guinea. A comparison of the lineaments with the location of major gold and copper deposits indicates there is a link between the arc-normal structures and mineralization. However, it is only those deposits that are less than 4.5 million years old that could be associated with these structures. Gravity and seismic tomography data indicate that some of these structures could penetrate deep levels of the lithosphere, providing some support to the earlier idea that the arc-normal structures act as conduits for the younger mineral deposits of New Guinea. The gravity data can also be used to infer the location of igneous intrusions at depth, which could have brought metal-bearing fluids and gases closer to the Earth's surface. These regions might be of interest for future exploration campaigns, particularly those areas that are crosscut by deep, vertical faults. However, new exploration models are needed to explain the location of the deposits that are older than 5 Ma. 2014 Author(s).

We would like to congratulate Sen and Collins (2012) for producing a detailed study of the structural history of the Ladakh Batholith according to new LA-ICPMS U/Pb analyses of zircon and AMS measurements. This work provides some significant findings to the evolution of this part of the Himalaya. However, we wanted to draw the author's attention to the omission of a discussion of other geochronological results from the region (White et al. 2012). This is because the results are particularly important for understanding the India-Asia collision. We also raise concerns about some of the conclusions that were drawn from AMS measurement in regard to postcrystallization deformation of the Ladakh Batholith granitoids.

In recent years several tectonic reconstructions have been presented for Australia-Antarctica break-up, with each putting the Australian plate in a different location with respect to Antarctica. These differences reflect the different datasets and techniques employed to create a particular reconstruction. Here we show that some of the more recent reconstructions proposed for Australia-Antarctica break-up are inconsistent with both our current knowledge of margin evolution as well as the inferred match in basement terranes on the two opposing conjugate margins. We also show how these incorrect reconstructions influence the fit of the Indian plate against Antarctica if its movement is tied to the Australian plate. Such errors can have a major influence on the tectonic models of other parts of the world. In this case, we show how the position of the Australia plate can predetermine the extent of Greater India, which is (rightly or wrongly) used by many as a constraint in determining the timing of India-Asia, or India-Island Arc collisions during the closure of Tethys. We also discuss the timing of Australia-Antarctica break-up, and which linear magnetic features are a product of symmetric sea-floor spreading versus those linear magnetic features that result from rifting of a margin. The 46. Ma to 84. Ma rotational poles previously proposed for Australia-Antarctica break-up, and confined to transitional crust and the continent-ocean transition zone, more likely formed during earlier stages of rifting rather than during symmetric sea-floor spreading of oceanic crust. So rotation poles that have been derived from magnetic anomalies in such regions cannot be used as input in a plate reconstruction. A new reconstruction of the Australia-Antarctica margin is therefore proposed that remains faithful to the best available geological and geophysical data. 2013 International Association for Gondwana Research.

We review the relative motion of India and Asia for the last 100 million years and present a revised reconstruction for the India-Antarctica-Africa-North America-Eurasia plate circuit based on published motion histories. Deformation of these continental masses during this time introduces uncertainties, as does error in oceanic isochron age and location. Neglecting these factors, the data ipso facto allow the inference that the motion of India relative to Eurasia was distinctly episodic. Although motion is likely to have varied more smoothly than these results would allow, the geological record also suggests a sequence of distinct episodes, at about the same times. Hence we suggest that no single event should be regarded as the collision of India with Asia. The deceleration of the Indian plate commencing at ~65. Ma is matched by an equally significant prior acceleration and this aspect must be taken into account in geodynamic scenarios proposed to explain the collision of India with Asia.

The three-dimensional (3D) geometry of subducting lithospheric slabs has four-dimensional (4D) implications, i.e. in three dimensions plus time. To understand the implications of present 3D geometry, we need to consider what has gone before, what is happening now, and what will happen in the future. We illustrate this point by examining the 4D evolution of the New Hebrides slab, concluding that the Australian lithosphere tore as it began to subduct, and is still ripping in the present day. Southward motion of a north-dipping flap has been enabled as the result of westward propagation of an active rip, accompanied by southward foundering of newly created transform segments. Foundering was progressive but the rate appears to have been episodic. Additional transforms formed in the upper-plate in consequence. Subduction transform foundering is reflected by steps in the height of the subducted slab, and this needs to be taken into account so as not to significantly overestimate the area of subducted material. The principles illustrated are of general interest because foundering of subduction transforms may be a common occurrence in curved subduction zones worldwide. 2012 Copyright Taylor and Francis Group, LLC.

The switch from I- to S-type magmatism in the Trans-Himalayan Batholith is one of the criteria used to determine when India and Eurasia collided. We present new 238U/ 206Pb SHRIMP data from the Karakorum Batholith, NW India. Our results suggest that an I-type hornblende-biotite granodiorite was emplaced at 31.4 ± 0.4 Ma and crosscut by S-type granite dykes at 18.0 ± 0.4 Ma. We interpret these data to indicate that volatile-fluxed melting of a mantle wedge above an active subduction zone and/or decompression melting of sub-arc asthenosphere was possible until at least 31 Ma. This interpretation uses the same rationale as previous workers have adopted to propose that the India-Eurasia collision occurred between 57 Ma and 47 Ma. A review of published data provides further evidence of &50 Ma magmatism and Cu-Au-Mo porphyry deposit mineralisation north of the Himalayan range. These rocks would be interpreted to have formed in an arc setting in other parts of the world. However, most workers consider that &50 Ma magmatism and porphyry deposit formation in the Himalaya is unrelated to subduction of oceanic lithosphere and are instead considered to form owing to 'post-collisional' processes. We argue that these interpretations have been based on an established model where the India-Eurasia collision is deemed to occur at ca 50 Ma, instead of using the data to question and refine existing tectonic models. 2012 Copyright Taylor and Francis Group, LLC.

The Indus Suture Zone is defined as the plate boundary between India and Eurasia. Here we document geochronological data that suggest that Indian rocks outcrop to the north of this suture zone. The inherited age spectrum of zircons from mylonitic gneiss collected in the southern part of the Karakorum Batholith is similar to those obtained from the Himalayan Terrane, the Pamir and is apparently Gondwanan in its affinity. These data are taken to indicate that the Karakorum Terrane was once a component of Gondwana, or at least derived from the erosion of Gondwanan material. Several continental ribbons (including the Karakorum Terrane) were rifted from the northern margin of Gondwana and accreted to Eurasia prior to India-Eurasia collision. Many therefore consider the Karakorum Terrane is the southern margin of Eurasia. However, we do not know if rifting led to the creation of a new microplate(s) or simply attenuated crust between Gondwana and these continental ribbons. Thus there is a problem using inherited and detrital age data to distinguish what is "Indian" and what is "Eurasian" crust. These findings have implications for other detrital/inherited zircon studies where these data are used to draw inferences about the tectonic history of various terranes around the world.

We present U/Pb SHRIMP dates of zircons from 7 key samples of the Ladakh Batholith to reconcile the many, often conflicting ages that have been proposed according to various geochronological techniques. Various crosscutting magmatic phases were dated to determine the age of granitoid emplacement. Our results indicate that granitoid emplacement occurred between 66Ma and 46Ma. The final phase of magmatism is characterized by the emplacement of crosscutting fine-grained granite (leucogranite), and pegmatite near the village of Chumatang. These dykes are defined as I-type granitoids according to their mineralogy. This indicates that I-type magmatism in the Ladakh Batholith continued until at least ~46Ma. This implies either that India had not accreted to Eurasia by 46Ma, or that one of the widely used geological criteria to define the timing of India-Asia collision is incorrect. In addition, SHRIMP analyses of metamorphic zircon rims yield ages between 20 and 10Ma. These ages may relate to the exhumation of the batholith or to metamorphic activity after exhumation. Evidence of metamorphic zircon growth between 20 and 10Ma may provide an explanation for many of the similar ages obtained from geochronometers that are affected or reset by temperatures lower than those expected for zircon (e.g. K/Ar and 40Ar/39Ar dating of mica). 2011 Elsevier B.V.