Abstract. The input sediments of the North Sumatra subduction zone margin, drilled during IODP Expedition 362, exhibit remarkable uniformity in composition and grain size over the entire thickness of the rapidly deposited Nicobar Fan succession (sea-floor to 1500 mbsf depth), providing a unique opportunity to study the micromechanisms of compaction. Samples were prepared from dried core samples from sites (U1480 and U1481) by both Ar-ion cross-section polishing and broad-ion beam cutting, and imaged with a field-emission SEM. Shallowest samples (sea-floor to 28 mbsf) display a sharp reduction in porosity from 80 % to 52 % due to collapse of large clay-domain/matrix pores associated with rotation and realignment of clay-platelets parallel to the bedding plane. The deeper succession (28 mbsf to 1500 mbsf) exhibits less rapid reduction in porosity from 52 % to 30 % by the progressive collapse of silt-adjacent larger pores by bending and subsequent sliding/fracturing of clay particles. In addition, there is a correlated loss of porosity in the pores too small to be resolved by SEM. Clastic particles show no evidence of deformation or fracturing with increasing compaction. In the phyllosilicates, there is no evidence for pressure solution or recrystallization: thus, compaction proceeds by micromechanical processes. Increase in effective stress up to 18 MPa (~1500 mbsf) causes the development of a weakly aligned phyllosilicate fabric defined by illite clay particles and mica grains, while the roundness of interparticle pores decreases as the pores become more elongated. We propose that bending of the phyllosilicates by intracrystalline slip may be the rate-controlling mechanism. Pore size distributions show that all pores within the compactional force chain deform, irrespective of size, with increasing compactional strain. This arises because the force chain driving pore collapse is localized primarily within the volumetrically dominant and weaker clay-rich domains; pores associated with packing around isolated silt particles enter into the force chain asynchronously and do not contribute preferentially to pore loss over the depth range studied.

Natural seeps occur at the seafloor as loci of fluid flow where the flux of chemical compounds into the ocean supports unique biologic communities and provides access to proxy samples of deep subsurface processes. Cold seeps accomplish this with minimal heat flux. While individual expertize is applied to locate seeps, such knowledge is nowhere consolidated in the literature, nor are there explicit approaches for identifying specific seep types to address discrete scientific questions. Moreover, autonomous exploration for seeps lacks any clear framework for efficient seep identification and classification. To address these shortcomings, we developed a Ladder of Seeps applied within new decision-assistance algorithms (Spock) to assist in seep exploration on the Costa Rica margin during the R/V Falkor 181210 cruise in December, 2018. This Ladder of Seeps [derived from analogous astrobiology criteria proposed by Neveu et al. (2018)] was used to help guide human and computer decision processes for ROV mission planning. The Ladder of Seeps provides a methodical query structure to identify what information is required to confirm a seep either: 1) supports seafloor life under extreme conditions, 2) supports that community with active seepage (possible fluid sample), or 3) taps fluids that reflect deep, subsurface geologic processes, but the top rung may be modified to address other scientific questions. Moreover, this framework allows us to identify higher likelihood seep targets based on existing incomplete or easily acquired data, including MBES (Multi-beam echo sounder) water column data. The Ladder of Seeps framework is based on information about the instruments used to collect seep information (e.g., are seeps detectable by the instrument with little chance of false positives?) and contextual criteria about the environment in which the data are collected (e.g., temporal variability of seep flux). Finally, the assembled data are considered in light of a Last-Resort interpretation, which is only satisfied once all other plausible data interpretations are excluded by observation. When coupled with decision-making algorithms that incorporate expert opinion with data acquired during the Costa Rica experiment, the Ladder of Seeps proved useful for identifying seeps with deep-sourced fluids, as evidenced by results of geochemistry analyses performed following the expedition.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Alkalinity was determined by Gran titration with an autotitrator (Metrohm 794 basic Titrino) using 0.1 M HCl at 20 degrees C. Report includes alkalinity, correction factor (if applicable), and pH.

Descriptions of samples, generally at the section half and smear slide or thin section scale, were performed by shipboard scientists and recorded in the JRSO description software. Descriptive data for both macroscopic and microscopic examination were collected in a Microscoft Excel workbook by hole. A zip file of the entire expedition's observations is also available.

Inorganic carbon (carbonate) is determined by coulometry, which uses a photodetection cell to measure carbon dioxide evolved during sample acidification. Report includes percent inorganic carbon and calcium carbonate.

Operational rig information data were measured using a variety of sensors and compiled using the RigWatch software package. Approximately 50 channels of drilling/coring data are captured in real time during the expedition. Data are presented as ASCII files extracted from the proprietary RigWatch data files and are presented by expedition. RigWatch data in time or depth domain can be imported into graphics and analysis programs to be merged and correlated with core physical properties data to enhance assessment of poor core recovery intervals.

Paleontological data were collected using microscopes and recorded in the JRSO description software. All data for a species group (e.g., diatoms or nannofossils) were collected in a Microsoft Excel worksheet by hole. A zip file of the entire expedition's observations is also available.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

This composite report includes data from two analyses (total carbon from \Elemental analysis [CHNS]\ and inorganic carbon from \[Coulometer]\). Each row combines the CHNS and Coulometer data from measurements made on the same sample at the same time for a particular section and section offset (depth). If data do not exist for a particular expedition, the column does not appear. To identify individual samples and tests, see each separate data type (Elemental analysis and Coulometer). If the same sample was measured multiple times by any of the methods, results in the report will be combined on one line where possible. Each additional replicate result will be shown in subsequent rows and will be combined where possible. Report includes results for carbon forms: total, inorganic, calcium carbonate, and organic by difference, along with total hydrogen, nitrogen, and sulfur.

This report documents the results of X-ray diffraction analyses of 132 mud and mudstone samples collected offshore Sumatra during International Ocean Discovery Program Expedition 362. The clay-size mineral assemblage consists of smectite, illite, chlorite, kaolinite, and quartz. The relative abundance of smectite at Site U1480 decreases downsection from a mean value of 33 wt% in Unit I to a mean of 19 wt% in Unit II; illite increases from a mean of 49 wt% to a mean of 59 wt%. Smectite in Unit III increases to a mean of 73 wt%, and illite decreases to a mean of 19 wt%. Mean values are subordinate (<16 wt%) for undifferentiated chlorite + kaolinite and <7 wt% for quartz in all units. A significant compositional discrepancy occurs between Subunit IIIA at Site U1480 (mean smectite = 64 wt%) and Unit III at Site U1481 (mean smectite = 36 wt%). At Site U1480, the expandability of illite/smectite mixed-layer clays increases downsection, which is opposite to the trend expected with burial diagenesis. The maximum value is 88% within smectite-rich samples from Unit III. Values of the illite crystallinity index are between 0.42Δ°2θ and 0.76Δ°2θ, with most data straddling the generic boundary between advanced diagenesis and anchimetamorphism. Illite (060) reflections yield bo values of 8.988 to 9.000, which are indicative of low phengite contents. Smectite (060) reflections display peak apex positions of 61.998°–61.798°2θ, which are consistent with the mineral structure of montmorillonite. The detrital illite fraction contains 46%–60% 2M1 polytype, and the remainder is 1M/1Md. Introduction International Ocean Discovery Program (IODP) Expedition 362 focused on recovering the sedimentary section that will be subducted to seismogenic depths in the northern Sumatran subduction zone, which is part of the 5000 km long Sunda subduction zone system (Figure F1). Regionally, the input materials of this subduction zone are composed largely of trench-wedge and Bengal-Nicobar Fan sediments with a total thickness of 4–5 km (McNeill et al., 2017b). Beneath the fan sediments is a basal pelagic interval that overlies the basaltic crust of the subducting oceanic plate (Figure F2). Two sites (U1480 and U1481) were drilled, cored, and logged about 250 km southwest of the subduction front (Figure F1). The primary drilling objective for Site U1480 was to recover a complete section of the incoming sedimentary section and uppermost basaltic basement (McNeill et al., 2017b). Six lithologic units were identified based on major changes in grain size, bed thickness, and composition (McNeill et al., 2017a; McNeill et al., 2017b). Unit I (0–26.42 meters below seafloor [mbsf ]) is early Pleistocene to Holocene in age and composed of calcareous clay with interbeds of fine-grained sand, silty clay, and minor volcanic ash. Unit II (26.42– 1250.35 mbsf) is late Pliocene to early Miocene in age and contains thin to medium beds of sandy silt and fine-grained sand (turbidites) that transition downsection into claystone, silty claystone, and muddy sandstone. Unit III (1250.35–1327.23 mbsf ) is late Paleocene to late Miocene in age and contains claystone and tuffaceous silty claystone. Unit IV (1327.23–1349.80 mbsf ) is Late Cretaceous to late Paleocene in age and contains basalt, tuffaceous and volcaniclastic sandstone, and volcaniclastic breccia. Unit V (1349.80– 1415.35 mbsf ) is Late Cretaceous in age and contains calcareous claystone, chalk, and intercalated basalt intrusions. Unit VI (1415.35–1431.63 mbsf) is also late Cretaceous in age but composed entirely of basalt, which was interpreted to be the top of igneous oceanic crust (McNeill et al., 2017a; McNeill et al., 2017b). K. Rosenberger et al. Data report: clay mineral assemblages in hemipelagic sediments At Site U1481, the primary drilling objective was to core and log the deeper part of the stratigraphic section because it contains intervals that may become the plate boundary décollement farther landward (McNeill et al., 2017b). Two lithologic units (McNeill et al., 2017a; McNeill et al., 2017b) were correlated with Units II and III at Site U1480 on the basis of age and lithology. Unit II (1149.70– 1360.12 mbsf ) is late Miocene in age and contains claystone, silty claystone, and muddy sandstone. Unit III (1360.12–1498.72 mbsf ) is early to late Miocene in age and consists of claystone (McNeill et al., 2017a; McNeill et al., 2017b). This report summarizes the results of X-ray diffraction (XRD) analyses of clay-size fractions from 132 core samples extracted from Sites U1480 and U1481. Samples were preferentially collected from clay-rich intervals of the cores rather than interbeds of turbidites. Most sample intervals in the split cores were positioned immediately adjacent to whole-round specimens that were extracted for studies of interstitial water geochemistry and frictional-hydrogeologic properties, and clay XRD specimens were co-located in “clusters” together with discrete specimens for bulk powder XRD, carbon-carbonate, grain-size analysis, and moisture and density; additional samples were taken to document significant local lithology changes (e.g., transition from coarser grained to finer grained intervals, decrease in carbonate content, or change in color) to ensure both “representative” and end-member clay mineral compositions. Our primary objective is to document changes in proportions of smectite, illite, chlorite + kaolinite, and quartz in mud and mudstone specimens relative to depth below seafloor and lithostratigraphy. Such information can be used to infer how paleoclimate, detrital provenance, and first-order patterns of sediment dispersal may have changed over time (e.g., Biscaye, 1965; Petschick et al., 1996; Thiry, 2000; Gingele et al., 2001; Phillips et al., 2014). Such information is relevant to the objectives of Expedition 362 because large variations in the proportions of clay minerals may have significant effects on frictional properties and dehydration reactions as Figure F1. Map of eastern Indian Ocean showing locations of Sunda-Sumatra subduction zone, Bengal and Nicobar Fans, and Sites U1480 and U1481. 20° 70°E 75° 80° 85° 90° 95° 100° 105° 110° 15° 10° 5° S 0° 5° 10° 15° 20° N India Bay of Bengal

Gamma ray attenuation (GRA) data were acquired using a Cs-137 collimated source and a sodium iodide (thallium), or NaI(Tl), scintillation detector. The signal was calibrated using water and aluminum standards to provide a proxy for bulk density. This measurement was performed by a sensor mounted on either the Whole-Round Multisensor Logger (WRMSL) or the Special Task Multisensor Logger (STMSL); which track was used for a given data set is indicated in the data.

Magnetic remanence was measured on section halves (and rarely on whole-round sections) using a 2G Enterprises 760R cryogenic magnetometer, first as natural remanent magnetization (NRM) and then after demagnetization steps were performed on the samples by alternating field (AF) demagnetizer coils mounted in-line within the instrument.

P-wave velocity data were measured on undisturbed section halves using pairs of piezoelectric transducers mounted in bayonets that are inserted into soft sediment along the JRSO-defined y-axis and/or z-axis. Report includes P-wave velocity in y and/or z direction, bayonet separation, traveltime between transducers, and first arrival picks.

Operational navigation data were measured using Trimble GPS systems and saved as navigational data files including configuration, data, logs, pictures, vehicle, and waypoints. Site Fix summary data and plots are presented in Microsoft Excel. Data are presented by expedition.

ABSTRACTRadiometric dating of fault gouges has become a useful tool for regional tectonics studies and for exploring and understanding fault and earthquake processes. Methods to define the absolute age of faults achieved a solid scientific foundation almost 25 years ago when the development and application of illite age analysis for investigating sedimentary burial and thermal histories found a new potential application – defining the age of fold-and-thrust development. Since then, the methods have benefitted from further development and incorporation of the 40Ar/39Ar micro-encapsulation method and quantitative clay mineral evaluation to distinguish polytypes (Wildfire). These refinements to the methods have improved their application in fold-and-thrust terrains and have opened up applications in normal and strike-slip fault environments. Another important development is the use of absolute dating methods in retrograde clay gouges in which clays in a fault develop from igneous or metamorphic wall rocks that contain no clays. In addition, the method has also been shown to be useful at dating folds in fold-and-thrust belts. We think the method is now an established part of the geological toolkit, look forward to future fault structural and tectonic studies that incorporate fault ages and hope that researchers continue to probe and discover ways that the method can assist fault process studies, including earthquake fault studies.

L.C. McNeill, B. Dugan, K.E. Petronotis, J. Backman, S. Bourlange, F. Chemale, W. Chen, T.A. Colson, M.C.G. Frederik, G. Guèrin, M. Hamahashi, T. Henstock, B.M. House, A. Hüpers, T.N. Jeppson, S. Kachovich, A.R. Kenigsberg, M. Kuranaga, S. Kutterolf, K.L. Milliken, F.L. Mitchison, H. Mukoyoshi, N. Nair, S. Owari, K.T. Pickering, H.F.A. Pouderoux, S. Yehua, I. Song, M.E. Torres, P. Vannucchi, P.J. Vrolijk, T. Yang, and X. Zhao2

Clay smear is difficult to predict for subsurface flow applications and would benefit from an improved understanding of the processes controlling clay smear. We present water-saturated sandbox experiments with large clay smear surfaces (similar to 500cm(2)) that couple cross-fault fluid flow measurements with structural analysis of excavated clay smears. We compare measured flow data to numerical flow simulations to develop a tool to evaluate the evolving fault structure. Results show diagnostic relationships between fault structures and cross-fault flow. In experiments with one or two clay layers and a cumulative thickness of 10mm at 100mm displacement, normally consolidated clay in a structural domain of graben faulting initially yields hybrid brittle/ductile failure with early breaching of the clay layer and increased cross-fault flux. This is followed by fault backstepping, formation of clay smears, and reworking of clay fragments within the fault. Early formed holes remain open during the evolution of the faults. Fault zones are segmented by fault lenses, breached relays, and clay smears in which sand and clay mix by deformation. Experiments with two clay layers show that holes rarely form at the same position on the fault plane, producing a layered sand-clay fault rock with greater flow path tortuosity and lower permeability than in one-layer experiments. We compare our results with observations of faults in nature and discuss progress toward models with sufficient detail and understanding to allow prediction of flow across evolving faults, first in laboratory models and then in the subsurface.Plain Language Summary Fault processes are complex phenomena that defy reliable prediction. Clay that is smeared into the fault is particularly difficult to predict for subsurface flow applications and would benefit from an improved understanding of controlling processes. We present well-controlled laboratory experiments in which we couple cross-fault flow measurements with structural analysis of excavated clay smears. We further compare measured flow data to numerical flow simulations. Results show diagnostic relationships between fault structures and cross-fault flow. The normally consolidated clay initially yields in a combination of brittle and ductile deformation with early breaching of the clay layer and increased cross-fault flux. This is followed by fault backstepping, formation of clay smears, and reworking of clay fragments in the fault. Experiments with two clay layers show that holes rarely form at the same position on the fault plane, producing a layered sand-clay fault rock with greater tortuosity and lower permeability than in one-layer experiments.