Article

Thermal and tectonic structure of Escanaba Trough: new heat-flow measurements and seismic-reflection profiles

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Abstract

A layer of turbidites roughly 600 m thick buries basement across the full width of the rift valley in the study area. The sediment section is disrupted by numerous normal faults and volcanic intrusions. Displacements on individual faults have been accumulating at rates of up to 15 mm/yr. The valley floor appears to be subsiding in most areas, at rates of up to 20 mm/yr. The sedimentation rate during the late Pleistocene is calculated to be in excess of 25 mm/yr. Heat flow in the valley is regionally low and relatively constant, averaging about 0.2 W/m2, as are temperatures predicted for the sediment-basement interface. Some variability is associated with disrupted zones in the inner rift. The highest heat-flow values are near the centers of intrusive volcanism. -from Authors

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... Turbiditic sediment enters the trough at the southern end and is channeled northward by the axial valley walls (Vallier et al., 1973;Normark et al., 1993). Sedimentation was relatively rapid (up to 25 mm yr -1) during low stands of sea level in the Pleistocene, and the entire sediment fill of the trough probably was deposited within the last 100,000 years (Normark et al., 1993;Davis and Becker, 1993). Holocene sedimentation is dominantly hemipelagic and sedimentation rates are much slower, approximately 1 mm yr -1 (Karlin and Lyle, 1986; Karlin and Zierenberg, 1993 , 1970). ...
... Volcanic rocks are exposed at each of the three strongly disturbed areas that have been investigated by submersible . The strongly disturbed zones are also areas of high heat flow (Abbott et al., 1986; Davis and Becker, 1993). ...
... There is, however, a possible addition of recharge fluid in local circulation cells which are zoned around sites of hydrothermal upflow. Heat flow in the Escanaba trough is quite uniform away from the zones of hydrothermal discharge (Davis and Becker, 1993), but the density of the measurements may not be adequate to define local anomalies related to individual hydrothermal circulation cells. ...
Article
The Escanaba trough is a sediment-filled axial valley in the slow-spreading (2.3 cm/yr) southern part of Gorda Ridge. The hemipelagic and turbiditic sediment fill is 300 to > 1,200 m thick and was rapidly deposited during Pleistocene low stands of sea level. Local areas of excess magmatism, relative to the rate of extension, form igneous centers a few kilometers in diameter that are spaced at intervals of approximately 15 km along the spreading axis. Sediment cover is thinner over these igneous centers and the sedimentary sequence is disrupted by igneous intrusions and faulting. The coexistence of tectonic extension with rapid sediment deposition favors the formation of sheeted sills rather than basalt flows that form the uppermost oceanic crust at sediment-free spreading centers. Circular sediment hills as much as 1,200 m in diameter and 120 m high are interpreted as uplifted fault blocks above laccolithic intrusions emplaced above the igneous centers. Massive sulfide deposits that formed on the peripheries of these hills have surface exposures of greater than 100 m in at least one direction, but the full dimensions of sulfide mineralization are poorly known. The sulfide deposits are composed predominantly of pyrrhotite with less abundant isocubanite, chalcopyrite, sphalerite, arsenopyrite, and marcasite. Barite-rich and polymetallic massive sulfide occur locally and have higher contents of Zn, Pb, Ag, As, Sb, and Sn than does pyrrhotite-rich massive sulfide. Polymetallic massive sulfide has low Au contents, but barite- rich and pyrrhotite-rich massive sulfide samples are enriched in gold relative to most sediment-hosted massive sulfide deposits, averaging more than 1 g/t Au. Massive sulfide from the Escanaba trough is enriched in group IV, V, and VI elements relative to deposits formed on sediment-free spreading centers due to interaction of hydrothermal fluid with sediment. Further evidence of hydrothermal fluid-sediment interaction is provided by the alkali-rich nature of hydrothermal fluid sampled from 220°C vents, the presence of sulfide samples containing thermogenic hydrocarbon derived from terrigenous organic matter in the sediment, and radiogenic Pb isotope ratios of massive sulfide. Sulfide sulfur is derived from basaltic rocks and from seawater sulfate that is reduced by high-temperature reaction with iron silicates or sedimentary organic matter. Sediment is extensively altered to clinochlore by Mg metasomatism in localized mixing zones where seawater is drawn into the upper part of hydrothermal discharge zones. Shallow subsurface deposition of sulfide is interpreted to be an important process in the formation of the deposits. A geologic model of the hydrothermal circulation proposes that the heat to drive the hydrothermal circulation system is provided both by a regionally extensive sheeted sill complex and by local laccolithic intrusions. Reaction of heated seawater with basaltic rocks controls the initial composition of the hydrothermal fluid, but interaction of the hydrothermal fluid with sediment in the upflow zone alters the fluid chemistry and results in enrichment of the sulfide deposits in group IV, V, and VI metals. The geologic setting in an oceanic rift environment, associated lithologies such as mixed flyschlike sediment and tholeiitic basalt, and the composition (Fe sulfide-dominant, Cu-Zn deposits with Cu, Zn >> Pb) of the Escanaba trough deposits generally are analogous to ancient massive sulfide deposits that are classified as Besshi type. The difference in tectonic settings among the modern sediment- hosted deposits which formed in open-ocean spreading centers and rifted continental margins and the contrast in morphology and composition compared with many ancient sediment- hosted deposits imply that Besshi-type deposits form within a multitude of ocean rift environments.
... Volcanogenic massive sulfide deposits and associated hydrothermal vents were found at several localities on and within the sediment fill near the axis of the Escanaba Trough Morton et al. 1994a;Zierenberg et al. 1994). To understand the interaction of the hydrothermal fluids with the host sedimentary sequence, the lateral extent, sedimentologic processes, and thermal structure of the axial valley fill were investigated (e.g., Davis and Becker 1994;Morton and Fox 1994;Normark et al. 1994). The areas of volcanogenic massive sulfides are generally associated with hills formed by local uplifts of the turbidite fill. ...
... Unit C and the upper part of unit D were deposited at 14.8 m/k.yr., and the lower part of unit D was deposited at 7.6 m/k.yr. Measured sedimentation rates are comparable to previously estimated rates (Normark et al. 1994;Davis and Becker 1994) and confirm extraordinarily fast deposition at Escanaba Trough. ...
... The basis of the correlation among the sediment log, MST logs, and 4.5-kHz profile is discussed in the text. The low-resolution seismicreflection profile segment on the right side of the figure is extracted from line 3 of figure 3.3 in Davis and Becker (1994); the profile is in the vicinity of Hole 1037B. mon occurrence on flat areas, slopes, and blocks separated by apparent faults (fig. ...
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Stratigraphy boundary surfaces and internal anatomy of unconformity-bounded units may be deciphered by using properly conceived petrography that identifies compositional, spatial, and temporal characteristics of sand grains. Discussion of two case studies of synorogenic deep marine successions indicates the potential of this method. These results emphasize the potential for use of sand petrography in sequence stratigraphy with particular reference to (1) basinal successions, where superposed sequences are conformable, (2) field mapping of structurally complex or poorly exposed areas where sequence-stratigraphic relationships are hidden, and (3) stratigraphic analysis of subsurface bodies. -from Authors
... Volcanogenic massive sulfide deposits and associated hydrothermal vents were found at several localities on and within the sediment fill near the axis of the Escanaba Trough Morton et al. 1994a;Zierenberg et al. 1994). To understand the interaction of the hydrothermal fluids with the host sedimentary sequence, the lateral extent, sedimentologic processes, and thermal structure of the axial valley fill were investigated (e.g., Davis and Becker 1994;Morton and Fox 1994;Normark et al. 1994). The areas of volcanogenic massive sulfides are generally associated with hills formed by local uplifts of the turbidite fill. ...
... Unit C and the upper part of unit D were deposited at 14.8 m/k.yr., and the lower part of unit D was deposited at 7.6 m/k.yr. Measured sedimentation rates are comparable to previously estimated rates (Normark et al. 1994;Davis and Becker 1994) and confirm extraordinarily fast deposition at Escanaba Trough. ...
... The basis of the correlation among the sediment log, MST logs, and 4.5-kHz profile is discussed in the text. The low-resolution seismicreflection profile segment on the right side of the figure is extracted from line 3 of figure 3.3 in Davis and Becker (1994); the profile is in the vicinity of Hole 1037B. mon occurrence on flat areas, slopes, and blocks separated by apparent faults (fig. ...
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Escanaba Trough is the southernmost segment of the Gorda Ridge and is filled by sandy turbidites locally exceeding 500 m in thickness. New results from Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) 14C dates and revised petrographic evaluation of the sediment provenance, combined with high-resolution seismic-reflection profiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining-upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and Deep Sea Drilling Program (DSDP) Site 35. Six AMS 14C ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates exceeded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition of one approximately 60-m interval of sand. Petrography of the sand beds is consistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High-resolution acoustic stratigraphy shows that the turbidites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100-km-long pathway from the Columbia River to the west flank of the Gorda Ridge. The age of the upper fining-upward unit of sandy turbidites appears to correspond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or jökulhlaups, from the glacial lakes probably continued flowing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.
... Hightemperature hydrothermal discharge and sulfide deposits have been observed and sampled at all three locations (e.g., Lonsdale and Becker, 1985; Davis, Goodfellow, et al., 1987; Koski et al., 1988). Numerous detailed geophysical studies have been carried out that define the structural and in particular the thermal setting in which these deposits have formed (e.g., Guaymas Basin: Lawver et al., 1975; Lawver and Williams, 1979; Williams et al., 1979; Lonsdale and Becker, 1985; Becker and Fisher, 1991; Fisher and Becker, in press; Middle Valley: Davis and Lister, 1977a; Davis, Goodfellow, et al., 1987; Escanaba Trough: Morton et al, 1987; Abbott et al., 1986; Davis and Becker, 1992). The results have shown that reconnaissance surveys can easily detect the large variability and the isolated highs in heat flow caused by hydrothermal activity, but that much more detailed surveys, with spacings between measurements on the order of a few hundred meters or less, are required to characterize properly the nature of heat flow variability so that clear inferences can be drawn about the associated hydrothermal processes. ...
... The coincidence of the surface deformation and the bright reflection beneath strongly suggests that the deformation is the simple and direct consequence of the high-level intrusion that is imaged seismically . Similar structures occur in Escanaba Trough (Davis and Becker, 1992; Dellinger and Holmes, 1992), although they differ in detail. In many of those examples, the sediment is domed and uplifted by an amount considerably greater than that seen here (up to 150 m), and while intrasedimentary intrusions (bright reflectors) are common there also, there is no direct correlation between the surface deformation and the disposition of the reflectors. ...
Article
Recently acquired single- and multichannel seismic reflection profiles and over 500 heat flow measurements augment bathymetric and side-scan acoustic data to provide new constraints on the tectonic setting and thermal structure of the sediment-filled rift, Middle Valley. Over most of the length of the Juan de Fuca Ridge, spreading takes place along high-standing volcanic ridges, which are broken only by relatively small axial rift grabens. Near the northern end of the ridge the volcanic supply is diminished, resulting in the formation of deep rift valleys at the spreading axis. The deepest of these, Middle Valley, has been buried syntectonically by Pleistocene turbidite sediment. Heat flow in this valley varies inversely with sediment thickness, suggesting that the sediment forms a hydrologic seal over permeable igneous crust, where efficient hydrothermal circulation maintains relatively uniform temperatures. This simple model is investigated by comparing directly the thermal regime at depth and the seismic structure of the valley. -from Authors
... If it is assumed that the temperature gradient measured in the upper 60 m of hole 1038I can be linearly extrapolated to ϳ135-161 mbsf, it suggests a temperature of ϳ340°C for this depth (Fig. 5). For comparison, the extrapolated temperature gradient measured in Hole 1037B, drilled outside of the area of hydrothermal venting, indicated significantly lower temperatures at depth, reaching just ϳ87°C at the bottom of the hole (546 mbsf), which is interpreted to have penetrated ϳ50 m into igneous basement, in agreement with the estimates of Davis and Becker (1994) and Davis and Villinger (1992). ...
... This larger sill, the top of which may have been intersected at a depth of 403 mbsf at the bottom of Hole 1038I (Zierenberg and Miller, 2000) may be the heat source responsible for the hydrothermal deposits at Central Hill. Thermal and mechanical modeling (Davis and Becker, 1994;Denlinger and Holmes, 1994) shows that intrusion of a sill equivalent to the volume of the uplifted hill is capable of sustaining high temperature hydrothermal circulation sufficient to generate the amount of massive sulfide deposited at this site. ...
Article
The composition and temperature of vent fluids sampled from the active hydrothermal system in Escanaba Trough, Gorda Ridge in 2000 and 2002 remain unchanged from the only time this field was previously sampled, in 1988. ODP Leg 169 drilled nine bore holes at this site in 1996, some within meters of the vents, yet this disturbance has not impacted the measured compositions or temperatures of the fluids exiting at the seafloor. The fluids have maximum measured temperatures of 218°C and contain ∼20% more chloride than local ambient seawater. Our interpretation is that the fluid compositions are generated by supercritical phase separation of seawater, with much of the water-rock reaction occurring within the ∼400m thick sedimentary section that overlies the basalt at this site. The ODP drilling results provide information on the mineralogy and composition of materials below the seafloor, as well as direct constraints not typically available on the physical conditions occurring below the seafloor hydrothermal system. Calculations utilizing geochemical modeling software suggest the fluids are close to saturation with a suite of minerals found subsurface, suggesting equilibrium between the fluids and substrate. These results provide an explanation for why the fluids have remained chemically stable for 14 yrs. The pore water data from drilling suggest that the hydrology and chemistry of the hydrothermal system are much more complex within the sediment cover than would be expected from the surface manifestations of the hydrothermal system. While the pore waters have chloride contents both greater and less than the local seawater, only fluids with higher chloride contents vent at the seafloor. Our calculations suggest that at the current conditions the “brines” (fluids with chlorinity greater than seawater) are actually less dense than the “vapors” (fluids with chlorinity less than seawater). These density relationships may provide an explanation for why the “brines” are now venting preferentially to the “vapors,” a situation opposite to what is usually observed or inferred.
... Deformation, dewatering, resedimentation and low-grade metamorphism of the enclosing clastic facies are inferred to have accompanied emplacement of the intrusions (cf. Einsele et al., 1980;Delaney, 1982;Kokelaar, 1982;Duffield et al., 1986;Hanson, 1991;McPhie, 1993;Davis and Becker, 1994;Brooks, 1995). Quenching of the cryptodomes generated abundant glassy, perlitic, fractured coherent and breccia facies, and slumping at the margins of some partly extrusive cryptodomes introduced small volumes of glassy detritus into flanking environments. ...
... Updoming of sediment above the intrusions probably generated seafloor topography (cf. Yamamoto et al., 1991;Davis and Villinger, 1992;Goodfellow and Franklin, 1993;Davis and Becker, 1994;Denlinger and Holmes, 1994) that could have been important in trapping hydrothermal fluids exhaled onto the seafloor (e.g. Rio Tinto, Boulter 1993a,b). ...
Article
The Highway–Reward massive sulphide deposit is hosted by a silicic volcanic succession in the Cambro-Ordovician Seventy Mile Range Group, northeastern Australia. Three principal lithofacies associations have been identified in the host succession: the volcanogenic sedimentary facies association, the primary volcanic facies association and the resedimented syn-eruptive facies association. The volcanogenic sedimentary facies association comprises volcanic and non-volcanic siltstone and sandstone turbidites that indicate submarine settings below storm wave base. Lithofacies of the primary volcanic facies association include coherent rhyolite, rhyodacite and dacite, and associated non-stratified breccia facies (autoclastic breccia and peperite). The resedimented volcaniclastic facies association contains clasts that were initially formed and deposited by volcanic processes, but then redeposited by mass-flow processes. Resedimentation was more or less syn-eruptive so that the deposits are essentially monomictic and clast shapes are unmodified. This facies association includes monomictic rhyolitic to dacitic breccia (resedimented autoclastic facies), siltstone-matrix rhyolitic to dacitic breccia (resedimented intrusive hyaloclastite or resedimented peperite) and graded lithic-crystal-pumice breccia and sandstone (pumiceous and crystal-rich turbidites). The graded lithic-crystal-pumice breccia and sandstone facies is the submarine record of a volcanic centre(s) that is not preserved or is located outside the study area. Pumice, shards, and crystals are pyroclasts that reflect the importance of explosive magmatic and/or phreatomagmatic eruptions and suggest that the source vents were in shallow water or subaerial settings.
... The mound in Middle Valley occurs over igneous crust less than 100 ka old and is just 5 km from the southernmost tip of the NNF (see Figures 3b, 6a, and 7 in Davis & Villinger, 1992). Multiple mounds were imaged along the spreading axis of Escanaba Trough (Davis & Becker, 1994); they were often surrounded by high-temperature hydrothermal deposits (Denlinger & Holmes, 1994). ...
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Geophysical and geochemical data indicate there is abundant fluid expulsion in the Nootka fault zone (NFZ) between the Juan de Fuca and Explorer plates and the Nootka continental slope. Here we combine observations from >20 years of investigations to demonstrate the nature of fluid‐flow along the NFZ, which is the seismically most active region off Vancouver Island. Seismicity reaching down to the upper mantle is linked to near‐seafloor manifestation of fluid flow through a network of faults. Along the two main fault traces, seismic reflection data imaged bright spots 100–300 m below seafloor that lie above changes in basement topography. The bright spots are conformable to sediment layering, show opposite‐to‐seafloor reflection polarity, and are associated with frequency reduction and velocity push‐down indicating the presence of gas in the sediments. Two seafloor mounds ~15 km seaward of the Nootka slope are underlain by deep, nonconformable high‐amplitude reflective zones. Measurements in the water column above one mound revealed a plume of warm water, and bottom‐video observations imaged hydrothermal vent system biota. Pore fluids from a core at this mound contain predominately microbial methane (C1) with a high proportion of ethane (C2) yielding C1/C2 ratios <500 indicating a possible slight contribution from a deep source. We infer the reflective zones beneath the two mounds are basaltic intrusions that create hydrothermal circulation within the overlying sediments. Across the Nootka continental slope, gas hydrate‐related bottom‐simulating reflectors are widespread and occur at depths indicating heat flow values of 80–90 mW/m².
... Unfortunately, the poor recovery of densely fractured materials from the ocean crust has significantly reduced the feasibility of structural studies in active seafloor deposits. Therefore, the study of the structural evolution of sulfide mounds on the seafloor has largely been limited to information obtained through seismic analyses (Macdonald and Luyendyk, 1977;Karson and Rona, 1990;Davis and Villinger, 1992;Davis and Becker, 1994). One viable means by which the structural framework of massive sulfide deposits can be constrained has largely been ignored in drilled core. ...
... Estimating 300 ka as a maximum possible age of the present rift valley, Normark et al. (1994) showed that the turbidite sedimentation rates could be as much as two orders of magnitude faster than measured regional Holocene rates of ~0.1 m/k.y. (depending upon the total amount of turbidite beds in the fill; see Normark et al., 1994;Davis and Becker, 1994;Karlin and Lyle, 1986). The estimated rates, as much as 25 m/k.y., would be as rapid as those for actively growing levees on the Amazon fan (Flood et al., 1995), startlingly fast for the oceanic setting and the great distance from continental sources (Fig. 1). ...
Article
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New results from Ocean Drilling Program Site 1037 and U.S. Geological Survey high-resolution seismic-reflection profiles confirm the great thickness, fast deposition rate, distant source, and convolute path of turbidites that fill the Escanaba Trough, the rift valley of the southernmost segment of the Gorda Ridge. Accelerator mass spectrometry 14C measurements provide the first direct dating of the Escanaba Trough turbidites, demonstrating an average deposition rate faster than 10 m/k.y. between 32 and 11 ka and as fast as 15 m/k.y. during the oxygen isotope stage 2 lowstand. In the upper 60 m of sediment, the petrology of turbidite sand beds, which are as much as 12 m thick, show that the dominant source for the turbidites is from the Columbia River, which is more than 800 km to the north, rather than from the much closer rivers of northern California. New high-resolution seismic-reflection profiles show that, except for areas of very recent volcanism, the entire Escanaba Trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at Site 1037B. The ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest Quaternary jokulhlaups from glacial Lake Missoula. The age and source characteristics suggest that these megaturbidite beds in Escanaba Trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the Lake Missoula floods entered the sea.
... In these cases, regionally uniform temperatures appear to be maintained at the interface between the igneous basement and the sediment section. Such conditions have been inferred to exist from co-loc ited seismic reflection and heat flow observations at sites up to 3.5Ma in age on the eastern flank of the Juan de Fuca Ridge (Davis ef al. 1989; 1992) and within a sedirnented rift valley at the northern end of the Juan de Fuca Ridge (Davis & Lister 1977; Davis & Villinger 1992). In the former case, estimated sediment-basement interface temperatures fall ...
Article
In a hydrothermally active ocean basin, vigorous hydrothermal circulation in highly permeable basement rocks maintains a nearly constant temperature at the base of the overlying accumulating sediment section. To investigate the thermal effects of sedimentation in such geological settings, we have developed a simple one-dimensional finite element model and applied it to cases in the northeast Pacific. The model accounts for differential motion of fluids and sediment grains during compaction, and can be used with any porosity-depth function. Results demonstrate clearly that the constant basal temperature of an accumulating sediment section, maintained by convective heat transfer in the basement, causes the section to remain thermally near steady state for even very high rates of accumulation, particularly when compared to conditions estimated for a section where heat is transported in the basement by conduction. A 10-kyr period of thermal recovery due to the highly diminished sediment supply during the post-Pleistocene further reduces the thermal effects of sedimentation by a significant amount. Only in rare cases where rates of accumulation exceed 10 mm yr−1 and sediment thicknesses exceed 1 km are the sea-floor heat flow and temperatures at depth diminished significantly. An example is found in Middle Valley of the Juan de Fuca Ridge, in a part of which over 2 km of sediment has accumulated in the past 200 kyr. Even in this extreme case, the heat flow is estimated to be lower than that of the steady state by about only 15 per cent. While rates of accumulation are also high in other parts of Middle Valley and in many other hydrothermally active areas, such as Guayamas Basin, Escanaba Trough and the eastern flank of the Juan de Fuca Ridge, these rates and the accumulated sediment thicknesses are found to be insufficient to cause appreciable thermal anomalies.
... These included studies over the ridge crest itself, where sediments fill an axial rift valley (Davis and Lister, 1977), as well as on the ridge flank (Davis et al., 1980). Later investigations included a regional transect across Cascadia Basin south of 47ûN (Moran and Lister, 1987) and detailed measurement arrays in Escanaba Trough, a partially sediment-filled rift valley of the southern Gorda Ridge (Abbott, et al., 1986;Davis and Becker, 1994) and in the Middle Valley sedimented rift of the northern Juan de Fuca Ridge (Davis and Villinger, 1992;Fisher et al., 1996). Attempts have been made recently to extend heat-flow measurements to very thinly sedimented areas using a short probe deployed by a submersible vehicle (Johnson et al., 1993). ...
Article
Using a complex of analytical methods, clay minerals were studied in Pleistocene sediments from Hole ODP 1038B, 120.50 m deep, drilled on the northwestern edge of the Central Hill, located in the Escanaba Trough (Gorda Ridge) near a hydrothermal source with a temperature of 108°C, as well as in Pleistocene background terrigenous sediments from reference Hole ODP 1037B, drilled in the Escanaba Trough, 5 km south of Central Hill. The association of terrigenous clay minerals in sediments from Hole 1037B consists of mixed-layer smectite-illites, smectite, chlorite, illite, and kaolinite. In sediments from Hole 1038B in the interval from the bottom surface to a depth of 5–7 m, clay minerals are terrigenous. In the rest of the sedimentary section, clay minerals are represented by newly formed biotite, chlorite, and dioctahedral smectite. Their formation occurred under the conditions that arose during the intrusion of basaltic melt into the Escanaba trough with the formation of a laccolith and the subsequent rapid cooling of its flank; the intrusion was accompanied by the ascent of high-temperature hydrothermal fluid in the central discharge channel, interacting with the adjacent sediments. As a result, at the high-temperature stage of this interaction, finely dispersed biotite was formed in the sediments due to the original terrigenous clay minerals, K-feldspar and amphiboles. Then, at the rapid cooling of the hydrothermal fluid to a temperature presumably 270–330°C, partial replacement of biotite by chlorite. With further rapid cooling of the hydrothermal fluid to a temperature of 200°C and below and its mixing with sea water seeping into the sediments of the Central Hill, smectite was formed.
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Heat flow measurements collected throughout the Auka and JaichMaa Ja'ag' hydrothermal vent fields in the central graben of the Southern Pescadero Basin, southern Gulf of California, indicate upflow of hydrothermal fluids associated with rifting dissipate heat in excess of 10 W/m² around faults that have a few kilometers in length. Paradoxically, longer faults do not show signs of venting. Heat flow anomalies slowly decay to background values of ∼2 W/m² at distances of ∼1 km from these faults following an inverse square‐root distance law. We develop a near‐fault model of heat transport in steady state for the Auka vent field based on the fundamental Green's function solution of the heat equation. The model includes the effects of circulation in fracture networks, and the lateral seepage of geothermal brines to surrounding hemipelagic sediments. We use an optimal fitting method to estimate the reservoir depth, permeability, and circulation rate. Independently derived constraints for the model, indicate the heat source is at a depth of ∼5.7 km; from the model, permeability and flow rates in the fracture system are ∼10⁻¹⁴ m² and 10⁻⁶ m/s, respectively, and ∼10⁻¹⁶ m² and 10⁻⁸ m/s in the basin aquitards, respectively. Model results point to the importance of fault scaling laws in controlling sediment‐hosted vent fields and slow circulation throughout low permeability sediments in controlling the brine's chemistry. Although the fault model seems appropriate and straightforward for the Pescadero vents, it does seem to be the exception to the other known sediment‐hosted vent fields in the Pacific.
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Escanaba Trough, which forms the southernmost part of the axial valley of the actively spreading Gorda Ridge, is filled with several hundred meters of sediment of presumed late Quaternary age. Surficial sediment samples from gravity cores, deeper samples (as much as 390 m) from Site 35 of the Deep Sea Drilling Program (Leg 5), and the acoustic character of the sediment fill observed on seismic-reflection profiles indicate that much of the sediment fill is of turbidite origin. Gross composition and heavy- mineral analyses of sand samples show that two distinct petrofacies comprise the sediment fill. The lower part of the fill was derived primarily from the Klamath River source of northern California while the younger fill, including the surficial sand beds, are from the Columbia River drainage much farther north. The Escanaba Trough sediment provides an opportunity to evaluate concepts for paleogeographic and paleotectonic reconstructions that are based on facies analysis and compositional and textural data for the volcanic components because both intrabasinal and extrabasinal sources are present as well as coeval (neovolcanic) and non coeval (paleovolcanic) sourcre This study of a modern basin shows, that although the sediment sources could be identified, it was useful to have some knowledge of the sediment pathway(s), the effects of diagenesis, and the possible effects of sediment sorting as a result of long transport distances from the source area for some components. Application of these same techniques to ancient deposits without benefit of the additional parameters will face limitations.
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The size, chemical composition, energy flux, and fluid composition of the TAG hydrothermal sulfide deposit at the Mid-Atlantic Ridge and the size, chemical composition and reaction zone characteristics of the Skouriotissa volcanogenic massive sulfide deposit of the Troodos ophiolite in Cyprus are used to examine the energy requirements and chemical balances associated with the generation of a large volcanogenic massive sulfide deposit. We conclude that formation of large sulfide deposits from oceanic hydrothermal systems is a geologically rapid process and occurs on timescales of hundreds of years, with episodes of activity as short as a few tens of years separated by thousands of years of inactivity. About 2×1019J of energy supplied at high temperature is required to form a deposit the size of the TAG mound. Metals (up to 4 times the mass of any element present in the sulfide deposit) are leached out of relatively small reaction zones most likely at the base of the sheeted dikes. Chemical balance can be struck for all elements except sulfur with a reaction zone 1-2 km3 in volume from which a small proportion of iron and large proportions of copper, zinc, and manganese are removed. A sulfur balance requires that a significant fraction of sulfur be derived from reduction of seawater sulfate, as suggested by stable isotope analyses. We argue that the principal source of energy that drives hydrothermal circulation is latent heat of crystallization of magma close to the top of the plutonic section. Furthermore, we speculate that activity of the TAG hydrothermal system is related to periods of more rapid magma supply from the mantle at magma supply rates similar to those observed in volcanoes in Hawaii and Iceland.
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Escanaba Trough, the southernmost segment of the Gorda Ridge, is filled by as much as 500 m of late Quaternary turbidite and hemipelagic sediment. Coring at Deep Sea Drilling Project Site 35 and Ocean Drilling Program (ODP) Sites 1037 and 1038 together with 4.5-kHz deep-tow and 3.5-kHz surface-ship seismic reflection profiles enable a distinct pattern of reflections to be mapped throughout Escanaba Trough in the upper part of this sediment fill. The uppermost 80 m of turbidite sediment, which includes at least 11 turbidity current events, were deposited in 3200 m. The turbidity currents were trapped upon entering Escanaba Trough, resulting in all of the sediment in suspension in the flows being deposited. The thickness of the turbidite layers reflects both the flow thickness and the vertical grain concentration within the flow that deposited the layer. Variations in the turbidite thickness with respect to water depth can be used to estimate the degree of relative vertical movement within the floor of Escanaba Trough. In the area of hydrothermal activity near ODP Site 1038, uplift of as much as 140 m has occurred over the past 8 kyr.
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One of the major unresolved issues of the Late Pleistocene catastrophic-flood events in the northwestern United States (e.g., from glacial Lake Missoula) has been what happened when the flood discharge reached the ocean. This study compiles available 3.5-kHz high-resolution and airgun seismic reflection data, long-range sidescan sonar images, and sediment core data to define the distribution of flood sediment in deepwater areas of the Pacific Ocean. Upon reaching the ocean at the mouth of the Columbia River near the present-day upper continental slope, sediment from the catastrophic floods continued flowing downslope as hyperpycnally generated turbidity currents. The turbidity currents resulting from the Lake Missoula and other latest Pleistocene floods followed the Cascadia Channel into and through the Blanco Fracture Zone and then flowed west to the Tufts Abyssal Plain. A small part of the flood sediment, which was stripped off the main flow at a bend in the Cascadia Channel at its exit point from the Blanco Fracture Zone, continued flowing more than 400 km to the south and reached the Escanaba Trough, a rift valley of the southern Gorda Ridge. Understanding the development of the pathway for the Late Pleistocene flood sediment reaching Escanaba Trough provides insight for understanding the extent of catastrophic flood deposits on the Pacific plate.
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