South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.

Extension within the southern Mariana margin occurs both normal to and parallel to the trench. Trench‐normal extension takes place along focused and broad backarc spreading axes forming crust that is passively accreted to the rigid Philippine Sea plate flank to the northwest. To the southeast, trench‐parallel extension has split apart the Eocene‐Miocene forearc terrain accreting new crust diffusely over a 150–200 km wide zone forming a pervasive volcano‐tectonic fabric oriented at high angles to the trench and the backarc spreading center. Earthquake seismicity indicates active extension over this forearc region and basement samples date young although waning volcanic activity. Such diffuse formation of new oceanic crust and lithosphere is unusual; in most oceanic settings extension rapidly focuses to narrow plate boundary zones—a defining feature of plate tectonics. Diffuse crustal accretion has been inferred to occur during subduction zone infancy, however. We hypothesize that in a near‐trench extensional setting, the continual addition of water from the subducting slab creates a weak overriding hydrous lithosphere that deforms broadly. This process counteracts mantle dehydration and strengthening proposed to occur at mid‐ocean ridges that may help to focus deformation and melt delivery to narrow plate boundary zones. The observations from the southern Mariana margin suggest that where lithosphere is weakened by high water content narrow seafloor spreading centers cannot form. These conditions likely prevail during subduction zone infancy, explaining the diffuse contemporaneous volcanism inferred in this setting.

Back‐arc spreading center characteristics reflect interactions between plate‐driven mantle advection and melting and slab‐driven hydrous melting and buoyant upwelling in the mantle wedge. At the Fonualei Rift and Spreading Center (FRSC) in the Lau Basin, spreading rates decrease from slow to ultraslow, providing an opportunity to examine crustal accretion as the plate‐driven component is minimized. A new Lau Basin kinematic analysis predicts FRSC spreading rates of ~32–8 mm/yr southward, much slower than previous estimates of ~95–47 mm/yr. Here we examine FRSC morphology and geophysical characteristics as it approaches the Tofua arc volcanic front southward and spreading rates decrease, minimizing the plate‐driven component of mantle advection and maximizing buoyant hydrous flux melting. Axial morphology changes abruptly from a deep, flat, faulted axis ~100 km away from the arc to a volcanic ridge that shoals and increases in relief southward. Within ~50 km of the arc at the south end, the volcanic ridge is abruptly replaced by isolated volcanic cones bisected by volcanic rift zones and surrounded by anomalously deep seafloor. These morphologic changes likely reflect along‐axis focusing of mantle upwelling and melting similar to that seen at ultraslow mid‐ocean ridges, causing the change in morphology from a segmented ridge to spaced axial cones. We propose that as opening rates slow and the ridge approaches the arc, more of the inherently three‐dimensional pattern of hydrous flux melting and buoyant upwelling in the mantle wedge is expressed volcanically. With faster opening, two‐dimensional plate‐driven mantle advection dominates melt production, favoring ridges over point‐source features.

We present a revised three-plate kinematic model for the Lau Basin, focusing on new interpretations of the geologic characteristics of the Niuafo'ou microplate boundaries. It builds upon the three-plate model by Zellmer and Taylor (2001) while resolving some geologic discrepancies in that model. Along the southern branch of the Mangatolu Triple Junction and the Fonualei Rift and Spreading Center, which together form the Niuafo'ou-Tonga plate boundary, our model predicts slow to ultraslow opening rates (similar to 32-8mm/yr southward), consistent with axial morphology and magnetic profile modeling. Our interpretation of the seafloor magnetization pattern and flank morphology along the Central Lau Spreading Center (CLSC) indicates that opening rates along the Australia-Niuafo'ou boundary are higher than predicted in the previous model, thus compensating for slower rates along the Niuafo'ou-Tonga boundary. Lastly, we show geologic evidence that Peggy Ridge (PR), interpreted previously as an active transform fault and therefore a strong azimuthal constraint on Niuafo'ou plate motion, is inactive and that the active plate boundary is the adjacent Lau Extensional Transform Zone, extending from the CLSC to its intersection with the Northwest Lau Spreading Center. The removal of the PR azimuthal constraint allows us to determine an Euler pole for Australia-Niuafo'ou opening that satisfies three-plate closure criterion and develop a geologically consistent model of Lau Basin kinematics. While the final model focuses on constraining current plate velocities assuming rigid plate Eulerian rotations, the geologic analyses presented herein also provide insight into long-term basin evolution, which appears to involve significant nonrigid plate kinematics.

Back-arc spreading centers increasingly depart from mid-ocean ridge (MOR) characteristics with proximity to the arc volcanic front. The close association of these departures with slab-derived materials in erupted lavas suggests that subduction-related chemical effects are their primary cause. The Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR) are type examples of this process. Together they constitute a first-order spreading center in the Lau back-arc basin that progressively converges on the Tofua arc volcanic front from north to south. Here we use ship multibeam and deep-towed side-scan sonar data to examine variations in axial morphology and volcanism at the second-and third-order segment scale along these ridges and develop a model for the processes that control them. Closest to the arc, VFR, and the southern segment of the ELSC shoal toward second-order segment ends, in contrast to MORs. Northward and beyond similar to 70 km from the arc, the axis becomes abruptly deeper and flatter and no longer shoals toward second-order segment ends. At VFR, along-axis topographic highs correlate with the location of arc volcanoes along slab flow lines. These correlations are weaker along the southernmost ELSC segment and absent along ELSC segments farther north. The observations show a modulation of back-arc segmentation with arc proximity that rapidly diminishes with distance. They support a model of the mantle wedge with a strongly hydrous domain within similar to 70 km of the arc within which the arc and ridge interact and a much less hydrous domain farther from the arc without evident arc-ridge interactions.

Graduate seminars typically involve reviewing key papers, ranging from seminal contributions by pioneers to the latest cutting‐edge results published in journals such as Nature, Science, and Geophysical Research Letters. Ideally, guest lecturers are invited to add specific expertise and stimulate discussions. However, the increasing costs for travel make this an expensive proposition. Recent improvements in free videoconferencing over the Internet make it possible to invite distant experts to participate in seminars without involving expensive and time‐consuming travel. Tools to record videoconferences allow anyone to combine narrations with slide presentations and prepare merged video files for distribution over the Internet (i.e., podcasts). This brief report discusses such an experiment in a graduate seminar on advanced plate tectonics in the Department of Geology and Geophysics at University of Hawai'i at Manoa's (UHM) School of Ocean and Earth Science and Technology (SOEST). The podcasts are freely available from iTunes (UHM; SOEST; Public Course Lectures) and from http://www.soest.hawaii.edu/pwessel/podcasts.