Glenn A. Spinelli

Effects of fluid circulation in subducting crust on Nankai margin seismogenic zone temperatures

Vigorous fluid circulation maintained in newly subducted ocean crust significantly affects subduction zone temperatures on the Nankai margin, Japan. The shallow part of the igneous ocean crust is pervasively fractured and thus highly permeable, allowing vigorous hydro thermal circulation. This circulation has been recognized as an important control on the thermal budget and evolution of ocean crust worldwide. However, existing subduction zone thermal models either do not include hydrothermal circulation in ocean crust or assume that it abruptly stops upon subduction. Here we use a conductive proxy to incorporate the thermal effects of high Nusselt number fluid circulation in subducting crust into a subduction zone thermal model. Hydrothermal circulation reduces temperatures in the seismogenic zone of the Nankai margin plate boundary fault by ~20 °C at the updip limit of seismicity and ~100 °C at the downdip limit. With improved thermal models for subduction zones that include the effects of hydrothermal circulation in subducting crust, estimates of metamorphic reaction progress and interpretations of fault zone processes on various margins may need to be revisited.

Thermal regime of the Costa Rican convergent margin: 2. Thermal models of the shallow Middle America subduction zone offshore Costa Rica

At the Costa Rica margin along the Middle America Trench along‐strike variations in heat flow are well mapped. These variations can be understood in terms of either ventilated fluid flow, where exposed basement allows fluids to freely advect heat between the crustal aquifer and ocean, or insulated fluid flow where continuous sediment cover restricts heat advection to within the crustal aquifer. We model fluid flow within the subducting aquifer using Nusselt number approximations coupled with finite element models of subduction and explore its effect on temperatures along the subduction thrust. The sensitivity of these models to the initial thermal state of the plate and styles of fluid flow, either ventilated or insulated, is explored. Heat flow measurements on cool crust accreted at the East Pacific Rise are consistent with ventilated hydrothermal cooling that continues with subduction. These models yield much cooler temperatures than predicted from simulations initialized with conductive predictions and without hydrothermal circulation. Heat flow transects on warm crust accreted at the Cocos‐Nazca spreading center are consistent with models of insulated hydrothermal circulation that advects heat updip within the subducting crustal aquifer. Near the trench these models are warmer than conductive predictions and cooler than conductive predictions downdip of the trench. Comparisons between microseismicity and modeled isotherms suggest that the updip limit of microseismicity occurs at temperatures warmer than 100°C and that the downdip extent of microseismicity is bounded by the intersection of the subduction thrust with the base of the overriding crust.

Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone

A minor amount of opal cement inhibits consolidation of sediment approaching the Nankai Trough subduction zone at Ocean Drilling Program Sites 1173 and 1177. Secondary and backscattered electron images of sediments from Site 1173 reveal a low-density, silica phase (opal-CT) coating grain contacts. The grain-coating cement is more widespread in the upper Shikoku Basin facies than in the lower Shikoku Basin facies. Numerical models of opal-CT content display increases with depth through the cemented upper Shikoku Basin section. Once temperature increases above ∼55 °C, the rate of opal-CT dissolution outpaces precipitation, the cement can no longer support the overburden, and the open framework of the sediment begins to collapse. Cementation followed by cement failure is consistent with observed anomalies in porosity, seismic velocities, and shear rigidity. Porosity is anomalously high and nearly constant near the base of the upper Shikoku Basin facies, whereas seismic velocity increases with depth in the same interval. Across the boundary between the upper Shikoku Basin facies and the lower Shikoku Basin facies, there are step decreases in porosity from ∼60% to ∼45%, P-wave velocity from ∼1800 m/s to ∼1650 m/s, and S-wave velocity from ∼550 m/s to ∼300 m/s. Similar cementation and porosity collapse may be important in other locations where heating of hemipelagic deposits, with minor amounts of opal, is sufficient to trigger opal diagenesis.

Evolution of Continental Slope Gullies on the Northern California Margin

ABSTRACT A series of subparallel, downslope-trending gullies on the northern California continental slope is revealed on high-resolution seismic reflection profiles imaging the uppermost 50 m of sediment. The gullies are typically 0 m wide and have 1 to 3 m of relief. They extend for 10 to 15 km down the slope and merge into larger channels that feed the Trinity Canyon. In the lower half of the 50 m stratigraphic section, the gullies increase in both relief and number up section, to maxima at a surface 5 to 10 m below the last glacial maximum lowstand surface. Gully relief increased as interfluves aggraded more rapidly than thalwegs. Erosion is not evident in the gully bottoms, therefore gully growth was probably due to reduced sediment deposition within the gullies relative to that on interfluves. As the gullies increased in relief, their heads extended upslope toward the shelfbreak. At all times, a minimum of 10 km of non-gullied upper slope and shelf stretched between the heads of the gullies and the paleo-shoreline; the gullies did not connect with a subaerial drainage network at any time. Gully growth occurred when the gully heads were in relatively shallow water ( 200 m paleo-water depth) and were closest to potential sediment sources. We suggest that prior to the last glacial maximum, the Mad River, then within 10 km of the gully heads, supplied sediment to the upper slope, which fed downslope-eroding sediment flows. These flows removed sediment from nearly parallel gullies at a rate slightly slower than sediment accumulation from the Eel River, 40 km to the south. The process or processes responsible for gully growth and maintenance prior to the last glacial maximum effectively ceased following the lowstand, when sea level rose and gully heads lay in deeper water ( 300 m water depth), farther from potential sediment sources. During sea-level highstand, the Mad River is separated from the gully heads by a shelf 30 km wide and no longer feeds sediment flows down the gullies, which fill with sediment from the distal Eel River. Approximately one-half of the subsurface gullies have no expression on the seafloor, because they have completely filled with sediment following the last glacial maximum lowstand of sea level.

Heat flow along the NanTroSEIZE transect: Results from IODP Expeditions 315 and 316 offshore the Kii Peninsula, Japan

mal conductivity varies between about 1.0 W m −1 K −1 near the seafloor to 1.6 W m −1 K −1 at a depth of 1 km. Thermal conductivity generally increases with depth and correlates with variations in porosity and lithology. Thermal gradients along the transect are characterized by 48 sediment temperature measurements from 6 sites. Thermal corrections for the effects of bathymetric relief and sedimentation improve the confidence with which the heat flow values can be interpreted. Heat flow generally decreases with landward distance from the deformation front and varies from 70 mW m −2 just landward of the deformation front to 54 mW m −2 at sites characterizing the outer fore‐arc high and to 57 mW m −2 at the Kumano Basin Site. IODP heat flow measurements are significantly lower than nearby seafloor heat flow measurements. This difference is most likely due to variations in bottom water temperature that have a large effect on values of seafloor heat flow. Thus the heat flow of the Nankai accretionary prism is lower than previously thought. We present thermal models of subduction along this transect and explore the impact of the initial geotherm. Conductive plate cooling based on the age of subducting seafloor (20 Myr) under predicts the observed heat flow. We find a good fit to the data using a geotherm appropriate for 10 Myr seafloor. The extra heat is interpreted in terms of back‐arc thermal environments.

A wider seismogenic zone at Cascadia due to fluid circulation in subducting oceanic crust

In the Cascadia subduction zone, the extent of the seismogenic portion of the plate interface is poorly resolved by seismicity due to the lack of a large megathrust event during the instrumental record. Therefore, fault zone temperatures (∼150 to 350 °C) are used to estimate the limits of the seismogenic zone. Previous thermal models for the Cascadia margin estimated that 350 °C on the plate boundary occurs ∼40–70 km offshore. In contrast, models of interseismic deformation have been interpreted to indicate a seismogenic zone extending landward of the coastline to the updip edge of a region of episodic tremor and slip (ETS). We examined Cascadia subduction zone temperatures with thermal models that include the effects of fluid circulation in an ocean crust aquifer. Fluid circulation cools the subduction zone and widens the thermally defined seismogenic zone by shifting the 350 °C isotherm at the plate boundary ∼30–55 km landward relative to results from simulations without fluid flow. Our thermal models indicate a 60–80-km-wide transition zone between 350 °C on the fault and the updip edge of ETS. Under British Columbia (Canada), Washington, and Oregon (United States), ETS occurs at ∼410–550 °C. The location of the basalt-to-eclogite transition in the subducting crust provides an important constraint on the thermal models because hydrothermal circulation in the ocean crust aquifer produces only small surface heat flux anomalies on this margin with thick sediment in the trench and on the incoming plate.

Hydrothermal circulation in subducting crust reduces subduction zone temperatures

Most thermal models of subduction zones assume no advection of heat by fluid flow because slow flow through underthrusting sediment, the decollement, and wedge likely transports only a minor amount of heat. We model coupled fluid and heat transport in a subduction zone and show that hydrothermal circulation in subducting basaltic basement rocks can have a great influence on subduction zone temperatures. Fractured basaltic basement has permeability several orders of magnitude higher than a typical decollement, allowing fluid circulation to redistribute and extract heat from a subduction zone. We simulate systems with upper basaltic basement permeability ranging from 10−13 to 10−10 m2. In addition, we incorporate the effect of permeability reduction within the basaltic basement as it is subducted. The models with fluid transport show suppressed temperatures along the subducting slab relative to models with no fluid transport. With continuous sediment cover, heat is extracted from under the margin wedge to the trench. In models where faulted ocean crust exposes high-permeability basement to the ocean floor, cooling from ocean bottom water results in highly suppressed heat flow relative to conductive models. Hydrothermally cooled ocean crust also acts to slow thermally controlled diagenetic reaction progress within subducting sediment.

Hydrothermal seepage patterns above a buried basement ridge, eastern flank of the Juan de Fuca Ridge

proportion ofhemipelagic material, from 2 � 10 � 16 m 2 at30% hemipelagic to6 � 10 � 18 m 2 at 95% hemipelagic. Modeled seepage rates, derived from basement overpressures and sediment physical properties, range from 0 to 27 mm/yr. The average seepage rate over the entire 11 km of ridge within our study area is 1.3 mm/yr. On the basis of the model results, half of the total volume flux of seepage from the First Ridge is contributed from 25% of the study area, with flow rates � 1.1 mm/yr. Low seismic reflection amplitude anomalies are generally correlated with areas of high seepage rates. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1832 Hydrology: Groundwater transport; 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes; 3022 Marine Geology and Geophysics: Marine sediments—processes and transport; 3025 Marine Geology and Geophysics: Marine seismics (0935);

Effects of the legacy of axial cooling on partitioning of hydrothermal heat extraction from oceanic lithosphere

the predicted and observed heat flux across the seafloor. These methods have assumed a dynamic steady state thermal system. We show that this assumption is not warranted and leads to an incorrect partitioning of hydrothermal circulation between ridge axes and flanks. To more accurately estimate hydrothermal heat loss on ridge axes and flanks, we consider the spatial and temporal extent and vigor of axial hydrothermal circulation in calculating hydrothermal heat extraction. Axial fluid circulation perturbs the thermal state of oceanic lithosphere for ∼5 Ma after that circulation ceases, reducing the hydrothermal heat extraction on ridge flanks. We find ∼30% of hydrothermal heat extracted on axis, ∼10% extractednearaxis(fromendofaxialhydrothermalcirculationto1Ma),and ∼60%extracted from lithosphere >1 Ma.

Controls of tectonics and sediment source locations on along-strike variations in transgressive deposits on the northern California margin

Abstract We identify two surfaces in the shallow subsurface on the Eel River margin offshore northern California, a lowstand erosion surface, likely formed during the last glacial maximum, and an overlying surface likely formed during the most recent transgression of the shoreline. The lowstand erosion surface, which extends from the inner shelf to near the shelfbreak and from the Eel River to Trinidad Head (∼80 km), truncates underlying strata on the shelf. Above the surface, inferred transgressive coastal and estuarine sedimentary units separate it from the transgressive surface on the shelf. Early in the transgression, Eel River sediment was likely both transported down the Eel Canyon and dispersed on the slope, allowing transgressive coastal sediment from the smaller Mad River to accumulate in a recognizable deposit on the shelf. The location of coastal Mad River sediment accumulation was controlled by the location of the paleo-Mad River. Throughout the remainder of the transgression, dispersed sediment from the Eel River accumulated an average of 20 m of onlapping shelf deposits. The distribution and thickness of these transgressive marine units was strongly modified by northwest–southeast trending folds. Thick sediment packages accumulated over structural lows in the lowstand surface. The thinnest sediment accumulations (0–10 m) were deposited over structural highs along faults and uplifting anticlines. The Eel margin, an active margin with steep, high sediment-load streams, has developed a thick transgressive systems tract. On this margin sediment accumulates as rapidly as the processes of uplift and downwarp locally create and destroy accommodation space. Sequence stratigraphic models of tectonically active margins should account for variations in accommodation space along margins as well as across them.