Abstract The origin of orthogonally spreading ridge segments separated by oceanic transform faults, versus obliquely spreading ridge segments without transform faults (TFs), is a long‐standing enigma of plate tectonics. We address this problem using three‐dimensional (3D) geodynamic models that simulate axial magmatic intrusions along two ridge segments, initially oriented orthogonally to seafloor spreading and the intervening TF. The intrusion zones dynamically relocate to local maxima in the depth‐averaged horizontal “effective stress.” Both 3D numerical models, which simulate a viscoelastic–plastic rheology as well as 2D thin elastic plate models predict shear stress on the TF to induce an asymmetry in lithospheric tension, causing the ridge segment ends to migrate toward each other and eventually form an obliquely spreading ridge. Orthogonal spreading with a TF is promoted by weak TFs, rapid lithospheric thickening beneath the plate boundary zone, higher rates of magmatically accommodated spreading, and intrusions that are confined to a narrow zone beneath the ridge axis. These conditions are observed or expected at slow and faster spreading rates, where orthogonal spreading occurs. In contrast, oblique spreading is promoted by strong TFs, slow lithospheric thickening beneath the plate boundary, lower rates of magmatically accommodated extension, and a broader width about the ridge axis where intrusions occur. These conditions are predicted to promote the observed oblique spreading in typical slow and ultra‐slow spreading environments. At the atypical, hotspot‐influenced Reykjanes Ridge, oblique spreading is inferred to be promoted by a broad subaxial zone of magma intrusion caused by anomalously high magma supply.

Two independent studies suggest that thousands of submarine volcanoes in the Pacific Ocean and offshore Africa are likely to have resulted from two high-temperature structures in the lower mantle, whose location and morphology are influenced by plate subduction. Together, their findings imply many forms of intraplate volcanism are tied to plate tectonics through the deep mantle.

The Hawaiian Ridge is a classic example of an intraplate volcanic island chain emplaced on oceanic lithosphere. We seek to constrain both the deformation from island loading around the Hawaiian Ridge and the influence of the oceanic lithosphere, including the Molokaʻi fracture zone (MFZ), on off‐axis volcanic emplacement. To examine these processes, we conducted a marine geophysical experiment in 2018 that included the acquisition of eight multichannel seismic reflection lines and used a 6,600 in 3 tuned air gun array and an ultra‐long hydrophone streamer cable towed behind R/V Marcus G. Langseth across and around the southern Hawaiian Ridge. We image both the top of igneous oceanic crust and the Moho, and we observe significant variations in regional crustal structure and thickness variations, Moho characteristics, the locations of the buried MFZ, and the flexure of the Pacific oceanic lithosphere under the Hawaiian Ridge. We observe up to ∼4.5 km of deflection of the igneous oceanic crust in response to the volcanic load with sediment thickness increasing to ∼3–3.4 km near the ridge but not correlating with the deflection. A systematic difference in igneous oceanic crustal thickness is found north (average ∼5.2 km) and south (average ∼5.9 km), of the MFZ that also correspond to changes in Moho characteristics. The MFZ itself is associated with the largest crustal thickness variations (∼3.7–7.6 km). Magma ascent through these different crustal regions may account for some variations of magmatic flux to the surface along the Hawaiian Ridge.

Oceanic transform faults play an essential role in plate tectonics. Yet to date, there is no unifying explanation for the global trend in broad-scale transform fault topography, ranging from deep valleys to shallow topographic highs. Using three-dimensional numerical models, we find that spreading-rate dependent magmatism within the transform domain exerts a first-order control on the observed spectrum of transform fault depths. Low-rate magmatism results in deep transform valleys caused by transform-parallel tectonic stretching; intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing transform strength dependent shallow valleys; high-rate magmatism produces elevated transform zones due to local compression. Our models also address the observation that fracture zones are consistently shallower than their adjacent transform fault zones. These results suggest that plate motion change is not a necessary condition for reproducing oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries.

Abstract Oceanic transform faults are fundamental features of plate tectonics, accommodating strike-slip motion between two adjacent mid-ocean ridge segments. The continuations of these faults form tectonically inactive fracture zones, creating the longest ‘scars’ on the Earth’s surface. Yet, despite the relatively simple kinematic and thermal structures, oceanic transform faults display an enigmatic continuum of morphologies ranging from deep valleys to small ridges. Here, through three-dimensional numerical modeling of two mid-ocean ridge segments separated by a transform fault, we find that the rate of magma intrusion within the transform domain exerts a first-order control on transform topography. Low-rate magmatism results in transform-parallel tectonic stretching, generating deep transform valleys and fracture zones. Intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing shallow valleys whose depth increases with the shear strength of the fault. High-rate magmatism leads to local compression that generates fault-parallel ridges. The models not only reproduce the observed global transform valley depths but also predict the observation that fracture zones are consistently shallower than their adjacent transform valleys. These results suggest that plate motion changes are not a necessary condition for generating oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries.

Most of Hawai'i's geothermal resources are blind—their manifestations, such as hot springs and steam vents, do not appear on the ground surface because the heated water flows far below. With the exception of $K\bar{i}lauea East Rift Zone$, in most areas of Hawai'i, high lateral permeability in the first kilometer below ground surface prevents surface thermal features from developing. As a methodology for discovering these blind resources, Play Fairway Analysis (PFA) involves finding potential locations of blind hydrothermal systems and describing potential geothermal sources in rift-zone settings. Using the PFA to find Hawai'i's geothermal resources, the University of Hawai'i (UH) conducted the Hawai‘i Play Fairway Project, Hawai'i's first statewide geothermal resource assessment since 1985. Sponsored by the U.S. Department of Energy, the Hawai'i Play Fairway Project provided an updated resource assessment, a roadmap for additional exploration activities, and the identification of areas for further exploration. Benefitting from UH's core competency in earth sciences and experienced geothermal researchers, the project comprised three phases. During the first phase, the team identified, compiled, and ranked existing geologic, groundwater, and geophysical datasets relevant to subsurface heat, fluid and permeability. Using a Bayesian statistical approach, the team developed a statistical methodology to integrate these data into a resource probability map. The team evaluated the confidence in the probability value and considered development viability of areas with geothermal resources. With these analyses, the team identified 10 locations in the Hawaiian Islands for exploration activities. For the second phase, the team collected new groundwater data in 10 locations across the state and new geophysical data on $L\bar{a}na‘i, Maui$, and central Hawai'i Island and modeled topographically induced stress to better characterize subsurface permeability. Analyzing the subsurface stresses, the team evaluated the potential for fracture-induced permeability. The team inverted the MT and gravity data to produce 3D models of resistivity and density, respectively, on $L\bar{a}na‘i$, across $Haleakal\bar{a}'s$ SW rift (Maui), and surrounding Mauna Kea (Hawai‘i Island). The team developed and applied a new method for incorporating depth information about resistivity, density, and potential for fracture-induced permeability into the statistical method for computing resource probability in these three focus areas. The team incorporated the new groundwater results with the new geophysical results and the calculations of potential for fracture-induced permeability to produce updated maps of resource probability and confidence. Through combining data from the first and second phases, the team determined locations for further exploration during the third phase. For MT and gravity surveys, the team recommended $Kaua'i's$ $L\bar{i}hu'e$ $Basin$, the east rift of $Maui's$ $Haleakal\bar{a}$ volcano, and the southwest rift of Hawai'i Island's Mauna Loa volcano. The MT and gravity surveys aimed to enable improved confidence in the resource potential in these locations. For drilling deep groundwater well(s), the team recommended Southeast Mauna Kea and $L\bar{a}na's$ $P\bar{a}l\bar{a}wai$ $Basin$. During the third phase, further exploration involved drilling a groundwater well in $L\bar{a}na's$ $P\bar{a}l\bar{a}wai$ $Basin$ and performing more geophysical surveys. We deepened an existing water well proximal to our target area on $L\bar{a}na'i$ due to funding constraints that precluded us from spudding a new well that would exceed 1km depth. Drilling was preceded by a number of substantial elements including: writing an Environmental Assessment and the subsequent legal process, performance of deviation logging, lowering a camera down the well, coordinating site preparation with $P\bar{u}lama$ $L\bar{a}na'i$, shipping the UH-owned rig interisland, procuring supplies, and leading 3 community meetings on $L\bar{a}na'i$. Drilling occurred 24/7 the entire month of June 2019 over which time $L\bar{a}na'i$ $Well$ 10 was deepened from 427 m to 1057 m, with continuous core collected. We measured a roughly linear temperature gradient averaging 42°C/km and a maximum bottom hole temperature of 66°C. This gradient is more than twice the background for Hawai'i and within a range of gradients measured in this depth range for some exploration wells within KERZ. We consider these results encouraging for $L\bar{a}na'i's$ resource potential and recommend following with a slim hole within $L\bar{a}na'i's$ caldera (our target zone) to ~ 2 km. Further, the positive implications such results have for the island of O‘ahu are substantial - the shield stage of O'ahu's volcanoes ended 1-2 My earlier. However, O'ahu uses more electricity than the rest of the islands combined, and the utility recently called for 500-700MW of firm, dispatchable renewable electricity on O'ahu by 2033. In Phase 3, we also collected limited new encouraging groundwater data, and updated our thoughts on the probabilities of fluid and permeability at resource depths (PrF = 1; PrP = mostly unconstrained). Ultimately, we advocate for using our final probability of heat, and confidence in this probability, to drive the next phase of exploration. We contend further development of geothermal in Hawai‘i will enable the state to achieve its 100% renewable policy objective and Hawai'i to transition off of fossil fuels through geothermal discovery and development. The project not only produced a large amount of data and expanded the existing knowledge of Hawai'i's geothermal resources, but also produced publications, theses, presentations, core photos, datasets, media reports, television interviews, community events, and a blog. Students and new professionals benefitted from the project's hands-on research experiences and educational opportunities and earned awards and recognition.

Oceanic detachment faults represent an end-member form of seafloor creation, associated with relatively weak magmatism at slow-spreading mid-ocean ridges. We use 3-D numerical models to investigate the underlying mechanisms for why detachment faults predominantly form on the transform side (inside corner) of a ridge-transform intersection as opposed to the fracture zone side (outside corner). One hypothesis for this behavior is that the slipping, and hence weaker, transform fault allows for the detachment fault to form on the inside corner, and a stronger fracture zone prevents the detachment fault from forming on the outside corner. However, the results of our numerical models, which simulate different frictional strengths in the transform and fracture zone, do not support the first hypothesis. Instead, the model results, combined with evidence from rock physics experiments, suggest that shear-stress on transform fault generates excess lithospheric tension that promotes detachment faulting on the inside corner.

Microbial communities are found throughout the biosphere, from human guts to glaciers, from soil to activated sludge. Understanding the statistical properties of such diverse communities can pave the way to elucidate the common mechanisms ...Multiple ecological forces act together to shape the composition of microbial communities. Phyloecology approaches—which combine phylogenetic relationships between species with community ecology—have the potential to disentangle such forces but are often ...

Transform faults and non-transform offsets define the bounds of mid-ocean ridge spreading segments, but tectonic and magmatic controls on the length of segments and the morphology of intervening offsets are poorly understood. A general observation at intermediate and slow-spreading oceanic environments is that localized strike-slip motion along transform faults tends to occur on larger offsets in space or crustal age, whereas more diffuse deformation at non-transform zones occurs at shorter offsets distances. In addition, variables such as lithospheric thickness, the size and spacing of faults, and the fraction (M) of extension accommodated by magmatic accretion (rather than faulting) are known to influence the overall morphology of the ridge segment and its vicinity. We hypothesize that the decrease in the amount of magmatic extension along the ridge segment towards the discontinuity along with the ridge segment offset play a role in defining the transition between transform and non-transform offsets. In this study, we employ a 3D-numerical model to investigate how the relative amounts of fault- or magma-accommodated spreading and distance offset (D) between ridge segments control the development of transform versus non-transform offsets. Our model employs a ridge-like initial temperature structure, with magma intrusion simulated by adding a divergence to the right-hand-side of the continuity equation within a magmatic accretion zone at the ridge axis. M, the fraction of magmatically compensated spreading inside the magmatic accretion zone, can be varied along strike. By using a visco-elasto-plastic formulation the model can simulate the spontaneous formation and evolution of normal faults that accommodate part of the spreading. The temperature field is allowed to evolve and the model accounts for an increased, temperature-dependent conductivity around each ridge segment. We vary both the offset distance D separating two axes of magmatic accretion as well as the length L over which M decreases along the ridge axes towards the discontinuity. We find that increasing L leads to non-transform offsets, particularly for small offset distances D. As D increases, the occurrence of the offset zone is less prominently dominated by L. Depending on M, the style of faulting differs along the magmatic segments. While for M>0.5 we observe migrating faults creating topography similar to abyssal hills, values for M that are smaller or equal to 0.5 lead to stationary faults which are located closer to the ridge axis.

Tectonic-plate material is generally thought to be neither created nor destroyed at plate boundaries called oceanic transform faults. An analysis of sea-floor topography suggests that this assumption is incorrect. Crustal thinning and regrowth at oceanic transform faults.

Besides the large-scale wedge shape itself, the most prominent structural feature of accretionary wedges and fold-and-thrust belts is the common pattern of imbricate thrust faults. This study illuminates the fundamental mechanical processes and material properties controlling the width of the crustal blocks bounded by major thrusts using analytical solutions of stress as well as two-dimensional finite-difference models. The numerical models predict that the initial width w0 of a thrust block is set when that block first forms at the very front of the wedge. The width is found to subsequently decreases approximately in proportion to the mean horizontal strain needed for an ideally triangular-shaped Coulomb wedge with a critical taper. Block width is proportional to the thickness H of the incoming, accreting sediment. A key quantity that influences the normalized initial block width w0/H is the distance L forward of the frontal thrust needed for the net horizontal force from shear on the base of the incoming sediment to balance the net force on the frontal thrust. It is within this distance where stress in the incoming sediment is substantially elevated and thus where the new frontal thrust forms. Results show that L/H and, correspondingly, w0/H increase with increasing sediment friction angle ϕ, cohesive strength C0 and pore-fluid pressure ratio λ, and decrease with increasing basal friction angle ϕb and basal dip β. Normalized width is sensitive to ϕ and relatively insensitive to ϕb and λ. Results for submarine and subaerial wedges follow the same scaling law. The scaling law relates the observables, w0/H and β, to the material properties, ϕ, ϕb, λ, and therefore provides a theoretical relation that can be used independent of, or together with critical Coulomb wedge theory (CWT) to constrain these properties.

An ongoing challenge in studies of the oceanic upper mantle is how intraplate hotspots impact the thermal structure of the lithosphere. To address this issue at the Hawaiian hotspot, we analyze mineral compositions for a petrographically diverse suite of garnet pyroxenite xenoliths from the Salt Lake Crater (SLC) rejuvenation stage, volcanic tuff ring in Honolulu. Garnet‐clinopyroxene geobarometry and two‐pyroxene geothermometry indicate equilibrium pressures of 13–18 kbar and temperatures of 1000°C–1100°C. These pressures place the xenoliths at mid‐lithospheric depths of 45–55 km, with temperatures 200°C–300°C hotter than expected for normal 90‐Myr‐old oceanic lithosphere. Garnet and clinopyroxene occur as discrete primary grains, as well as exsolution blebs and lamellae, with lateral dimensions up to several hundred microns. Compositions within garnet and pyroxene grains are remarkably uniform and display no systematic variation with distance to grain boundaries. Together, these observations indicate that the calculated pressures and temperatures reflect the thermal state of the lithosphere under which the xenoliths last equilibrated. We attribute the elevated lithospheric temperatures under Honolulu primarily to the heating by magma as it penetrated the lithosphere during rejuvenation magmatism and the voluminous shield magmatic stage. We anticipate such magmatic heating to be common among all Hawaiian volcanoes, supporting conclusions of a recent study of earthquakes beneath Hawai‘i Island. This local lithospheric thermal anomaly may also contribute to the enigmatically weak flexural response of the lithosphere due to volcano loading along the Hawaiian hotspot chain.

This paper is the fourth in a series on a play fairway analysis of geothermal resources across the State of Hawaii. Here we describe recent exploration activities that include groundwater sampling in ten locations statewide, as well as geophysical surveys on Lana'i, across the SW rift of Haleakala Volcano (Maui), and surrounding Mauna Kea Volcano (Hawai'i Island). We derive a first-order method for computing topographic stress using Green's functions, finding that topography can induce appreciable crustal stress. We develop a new method for incorporating depth information about resistivity, density, and topographic stresses into our previously published equations to calculate resource probability and confidence. We incorporate newly collected and modeled data into our calculations to update statewide maps of probability and confidence. Lana'i Island and southeast Mauna Kea are identified as the top targets for exploratory drilling. The east rift of Haleakala Volcano, the southern-most region of Mauna Loa's SW rift (Hawai'i Island), and central Kauai are identified as targets for geophysical surveying.

B. BOSTON1*, D. J. SHILLINGTON2, R. DUNN3, A. B. WATTS4, I. GREVEMEYER5, L. GÓMEZ DE LA PEÑA5, G. ITO3, P. WESSEL3, U. TEN BRINK6, N. MILLER6 1 Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964, USA (*correspondence:brianb@ldeo.columbia.edu) 2Northern Arizona University, School of Earth and Sustainability, 624 S. Knoles Drive, Flagstaff, AZ 86011, USA 3University of Hawaii, Department of Earth Sciences, 1680 East-West Rd., Honolulu, HI 96822, USA 4University of Oxford, Department of Earth Sciences, South Parks Road, Oxford OX13AN, UK 5GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany 6Woods Hole Coastal and Marine Science Center, US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543, USA

Very few constraints exist on the structure and evolution of old oceanic crust due to a paucity of observations. Existing sparse seismic reflection data from old oceanic crust in the Pacific and Atlantic Oceans reveal diverse attributes of the oceanic crust and upper mantle, including variable crustal thickness, Moho reflectivity, and intracrustal features. To what extent does this heterogeneity reflect variations in processes during accretion at the mid-ocean ridge or modification by post-accretion magmatism? Here we present a series of long seismic reflection profiles around and across the Hawaiian-Emperor Seamount Chain, which can be used to address this question. Multichannel seismic reflection data were acquired around and across the Hawaiian Chain in 2018. In 2019, comparable seismic reflection data were acquired around and across the Emperor. Oceanic lithosphere in both study areas is Cretaceous in age. Seismic data from both cruises were collected with the 15-km-long streamer and 6600 cu in tuned airgun array of the R/V Marcus G. Langseth . We

SUMMARYWe examine the rheology and thermal structure of the oceanic lithosphere, expressed in situ by plate flexure beneath the Hawaiian Ridge, where volcanoes of variable sizes have loaded seafloor of approximately the same age, and thus where the lithosphere is expected to have had an approximately uniform age-dependent thermal structure at the time of loading. Shipboard and satellite-derived gravity, as well as multibeam bathymetry data are used in models of plate flexure with curvature-dependent flexural rigidity, the strength of which is limited, in the shallow lithosphere, by brittle failure, and in the deeper lithosphere, by low-temperature plasticity (LTP). We compute relative likelihoods and posterior probabilities for four model parameters: average crustal density ρc, friction coefficient for brittle failure ${\mu _f}$, a pre-exponential weakening factor F controlling the strength of LTP and lithospheric geotherm age t. Results show that if the lithosphere temperatures were as is expected for normal (t = ) 90-Myr-old seafloor at the time of volcano loading, the rheology must be significantly weaker than expected. Specifically, weak brittle strengths (μf ≤ 0.3) show relatively high probabilities for three of the six published LTP flow laws examined. Alternatively, moderate-to-large brittle strengths (μf ≥ 0.5) require all LTP flow laws to be substantially weakened with F = 102 to > 108 or, equivalently, activation energy reduced by 10–35 per cent. In contrast, if the lithosphere has been moderately reheated by the Hawaiian hotspot, represented by geotherms for t = 50–70 Myr, then the flow laws of Evans & Goetze, Raterron et al. and Krancj et al. require little or no weakening. Such modest thermal rejuvenation is allowed by heatflow constraints, supported by regional mantle seismic tomography imaging as well as compositions of mantle xenoliths, and reconciles previously noted discrepancies between the LTP strengths of lithosphere beneath Hawaii versus that entering the Pacific subduction zones.