Plate Tectonics, Ophiolites, and Societal Significance of Geology: A Celebration of the Career of Eldridge Moores

This volume honors Eldridge Moores, one of the most accomplished geologists of his generation. The volume starts with a summary of Moores’ achievements, along with personal dedications and memories from people who knew him. Leading off the volume’s 12 chapters of original scientific contributions is Moores’ last published paper that presents an example of the Historical Contingency concept, which suggested that earlier subduction history may result in supra-subduction zone geochemical signatures for some magmas formed in non-subduction environments. Other chapters highlight the societal significance of geology, the petrogenesis of ophiolites, subduction zone processes, orogenic belt evolution, and other topics, covering the globe and intersecting with Moores’ interests and influences.

Across-arc variations in Mo isotopes and implications for subducted oceanic crust in the source of back-arc basin volcanic rocks

Molybdenum (Mo) isotope ratios provide a potential means of tracing material recycling involved in subduction zone processes. However, the geochemical behavior of Mo in subducted oceanic crust remains enigmatic. We analyzed Mo isotope ratios of arc and back-arc basin lavas from the Mariana subduction zone (western Pacific Ocean), combining newly obtained element and Sr-Nd-Pb-Li isotope data to investigate subduction zone geochemical processes involving Mo. The Mo isotope ratios (δ98/95MoNIST3134; U.S. National Institute of Standards and Technology [NIST] Mo standard) of the volcanic rocks showed clear across-arc variations, decreasing with increasing depth to the Wadati-Benioff zone. The high δ98/95Mo values in the Mariana Islands (–0.18‰ to +0.38‰) correspond to high 87Sr/86Sr, low 143Nd/144Nd, and radiogenic Pb isotope ratios, suggesting that altered upper oceanic crust played an important role in the magma source. The low δ98/95Mo values in the Central Mariana Trough (–0.65‰ to –0.17‰) with mantle-like Sr-Nd-Pb but slightly low δ7Li values provide direct evidence for the contribution of deep recycled oceanic crust to the magma source of the back-arc basin lavas. The isotopically light Mo magmas originated by partial melting of a residual subducted slab (eclogite) after high degrees of dehydration and then penetrated into the back-arc mantle. This interpretation provides a new perspective with which to investigate the deep recycling of subducted oceanic lithosphere and associated magma petrogenesis.

A Closer Look at the Creation of a Metamorphic Sole

Detailing the development of the metamorphic sole beneath the Oman–United Arab Emirates ophiolite provides insight into subduction zone processes.

Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision

The migration of arc magmatism that is a fundamental aspect of plate tectonics may reflect the complex interaction between subduction zone processes and regional tectonics. Here we report new observations on volcanic migration from northwestern Sumatra, in the westernmost Sunda arc, characterized by an oblique convergent boundary between the Indo-Australian and Eurasian plates. Our study indicates that in northwestern Sumatra, volcanism ceased at 15–10 Ma on the southern coast and reignited to form a suite of active volcanoes that erupt exclusively to the north of the trench-parallel Sumatran fault. Younger volcanic rocks from the north are markedly more enriched in K2O and other highly incompatible elements, delineating a geochemical variation over space and time similar to that in Java and reflecting an increase in the Benioff zone depth. We relate this mid-Miocene volcanic migration in northwestern Sumatra to the far-field effect of propagating extrusion tectonics driven by the India-Eurasia collision. The extrusion caused regional deformation southward through Myanmar to northwestern Sumatra and thus transformed the oblique subduction into a dextral motion–governed plate boundary. This tectonic transformation, associated with opening of the Andaman Sea, is suggested to be responsible for the volcanic migration in northwestern Sumatra.

Rapid, paced metamorphism of blueschists from laser-based Lu-Hf garnet-domain geochronology and LA-ICMPS trace element mapping

<p>Unravelling the timing and rate of subduction-zone metamorphism requires linking the composition of petrogenetic indicator minerals in blueschists and eclogites to time. Garnet is a key mineral in this regard, not in the least because it best records P-T conditions and changes therein and can be dated, using either Lu-Hf or Sm-Nd chronology. Bulk-grain garnet ages are the norm and can provide important and precise time constraints on reactions across both facies. Domain dating, i.e., dating of individual growth zones, moves beyond that. Domain dating by combining mechanical micro-milling and Sm-Nd chronology yielded important constraints on garnet-growth and fluid-release rates for blueschists (e.g., Dragovic et al., 2015). Developing this method for Lu-Hf chronology and, importantly, for "common-sized" garnet (≤1 cm) provides an important opportunity to further explore the potential of this approach.</p><p>We combined a low-loss micro-sampling technique in laser cutting with a refined Lu-Hf routine to precisely date multiple growth zones of a sub-cm-sized garnet in a blueschist. The targeted grain from a glaucophane-bearing micaschist from Syros Island, Greece, was chemically characterized by major- and trace-element mapping (EPMA, LA-ICPMS) and five zones were extracted using a laser mill. The three core and inner mantle zones are chemically comparable and identical in age within a 0.1 Myr precision (2σ). The outer two zones are chemically distinct and are resolvably younger (0.2-0.8 Myr). The timing of these two major garnet-growth episodes, together with the variations in trace-element chemistry, constrain important fluid-release reactions, such as chloritoid-breakdown. The data show that the integral history of garnet growth in subduction zones may be extremely short (<1 Myr), but may, even in that short timeframe, consist of multiple short pulses. Garnet-forming reactions clearly are localized and, thus, associated with focussed high-flux fluid flow. Beyond subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of "common-sized" garnet opens possibilities for constraining the causes and rates of garnet growth and in turn, the pace of tectonic processes in general.</p><p> </p><p><sub><em>Dragovic, B., Baxter, E.F. and Caddick, M.J., 2015. Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece. Earth and Planetary Science Letters, 413, pp.111-122.</em></sub></p>

Imaging the Upper Plate Lithosphere and Asthenosphere beneath Alaska with Sp Converted Waves

<p>To resolve the signatures of subduction zone processes in the mantle wedge, and how subduction has interacted with the upper plate, we imaged seismic velocity gradients beneath the US state of Alaska with Sp receiver function common conversion point (CCP) stacking.  Pacific plate lithosphere, and lithosphere bearing the thicker crust of the Yakutat terrane, subduct to the northwest beneath the southern margin of Alaska.   We employed data from hundreds of stations of the US NSF EarthScope Transportable Array, as well as other portable arrays and permanent networks. We calculated waveform components using a free-surface transform with improved estimates of free-surface velocities that were determined from P and SV particle motions. Sp receiver functions were calculated with time-domain deconvolution, and the CCP stack was generated with weighting functions that incorporate the properties of Sp scattering kernels. The CCP stack shows a clear interface between the North American and underthrust Yakutat crust, as well as Yakutat Moho depths of up to 60 km.  Sp phases from the negative velocity gradient at the base of the upper plate are strongest in west-central Alaska, where lithosphere-asthenosphere boundary (LAB) depths lie at 65-100 km.  In west-central Alaska, joint inversions of Sp data at single stations with Rayleigh phase velocities show comparable LAB depths as well as low asthenospheric velocities. This zone includes active magmatism and the upper plate appears to have been thinned by mantle wedge volatiles, melt, and flow.  The LAB phase deepens to the north, reaching depths of ~120 km beneath the northern Arctic Alaska terrane.  This increase in the depth of the LAB phase from the arc to the back-arc is consistent with the sculpting of the upper plate by subduction-related processes. Sp phases also delineate a prominent positive velocity gradient that represents the base of a low-velocity asthenospheric layer at depths of 100-130 km.  The positive velocity gradient is consistent with the onset of partial melting in the asthenosphere.</p><p> </p>

Synthetic Co-Existing Wadsleyite β-(Mg,Fe)2SiO4 and Ringwoodite γ-(Mg,Fe)2SiO4: an Optical Absorption Spectroscopy Study

The synthetic high-pressure α- and β-modification of (Mg1–xFex)2SiO4, wadsleyite and ringwoodite, respectively, were studied by optical absorption spectroscopy at ambient and hydrostatic high-pressure conditions. In addition, the effects of thermal annealing on the crystals were investigated. Under hydrostatic compression up to ~13 GPa and then consequent released to atmospheric pressure there were changes in the spectra and related changes in the crystal color. This is a clear indication that some Fe2+ was oxidized to Fe3+. The spectra of both ringwoodite and wadsleyite change after annealing in air at temperatures up to 300 °C. The intensities of electronic spin-allowed bands of Fe2+ decrease and the intensity of the charge-transfer electronic transition O2– → Fe3+, as given by the low-energy absorption edge in the UV region, increases. These crystal-chemical changes are shown by a weakening of the blue (ringwoodite) and green (wadsleyite) colors and a concomitant increase in yellowish tints. The effects of Fe2+ oxidation to Fe3+, upon decompression from high pressures as well as through annealing at relatively low temperatures, can cause the disintegration of both phases. Thus, both minerals have not yet been reliably identified at near surface Earth conditions after originating from deep-seated volcanism or deep subduction zone processes.

Subduction zone processes and crustal growth mechanisms at Pacific Rim convergent margins: modern and ancient analogues

Continents grow mainly through magmatism, relamination, accretionary prism development, sediment underplating, tectonic accretion of seamounts, oceanic plateaus and oceanic lithosphere, and collisions of island arcs at convergent margins. The modern Pacific–Rim subduction zone environments present a natural laboratory to examine the nature of these processes. The papers in this special issue focus on the: (1) modern and ancient accretionary margins of Japan; (2) arc–continent collision zone in the Taiwan orogenic belt; (3) accreting versus non-accreting convergent margins of the Americas; and (4) several examples of ancient convergent margins of East Asia. Subduction erosion and sediment underplating are important processes, affecting the melt evolution of arc magmas by giving them special crustal isotopic characteristics. Oblique arc–continent collisions cause strong deformation partitioning that results in orogen-parallel extension, crustal exhumation and wrench faulting in the hinterland, and thrust faulting–folding in the foreland. Trench-parallel widths of subducting slabs exert major control on slab geometries, the degree of coupling–decoupling between the lower and upper plates, and subduction velocity partitioning. An initially large width of the subducting Palaeo-Pacific Plate against East Asia caused flat subduction and resistance to slab rollback during the Triassic Period. These conditions resulted in shortening across SE China. Foundering and delamination of the flat slab during the Early Jurassic Epoch led to slab segmentation and reduced slab widths, followed by slab steepening and rollback. This pull-away tectonics induced lithospheric extension and magmatism in SE China during Late Jurassic – Cretaceous time. Melting of subducted carbonaceous sediments commonly produces networks of silicate veins in CLM that may subsequently undergo partial melting, producing ultrapotassic magmas.