摘要:

Two prominent features mark the passage of oceanic lithosphere over a hotspot. The first is the initiation of oceanic volcanism leading to a chain of islands or seamounts. The second is the generation of a ∼1‐km‐high, ∼1000‐km‐wide bathymetric swell around the volcanic island chain. Here we show that recent estimates for the volume of hotspot volcanism and the size of the swell suggest a shared origin: swell relief is created by the density reduction created by melting beneath the hotspot. This results in a seafloor age dependence to swell size and volcanism along the Hawaiian chain: beneath younger, thinner lithosphere the hotspot undergoes more decompression melting, resulting in both a larger swell volume and greater island building. For rapidly moving plates the swell root residue from hotspot melting is dragged away from the hotspot by the overriding lithosphere; its buoyancy induces further spreading and thinning of swell root material, producing, for example, the characteristic bow‐shaped form of the 0–5 Ma section of the Hawaiian swell. This post emplacement spreading and thinning of the swell root may be the reason for the ∼5 m.y. duration of late stage melting and volcanism along the Hawaiian hotspot chain. The ∼5 m.y. timescale for spreading of the swell root implies a characteristic viscosity of the depleted swell root of ∼ 1–3×1020Pa s, which is less fluid than underlying, less melted asthenosphere. Melt extraction at the hotspot is our preferred mechanism for the increase in viscosity of the swell root relative to

ISSN:0148-0227    

DOI:10.1029/94JB02887

年代:1995

数据来源: WILEY

摘要:

Multibeam bathymetry and gravity coverage of the East Pacific Rise (EPR) between 18° and 21°30′S is used to investigate the relation between melt supply and tectonic segmentation at ultrafast spreading rates. The long‐wavelength features in the residual anomaly show a good correlation with those in the bathymetry. The highest residual anomaly values occur over the broad discordant zone of the 20°40′S overlapping spreading center (OSC), for seafloor ages of 0 Ma to at least 1.5 Ma. We interpret the deepening of the bathymetry and the increase of the residual anomaly toward that discordant zone as due to a decrease of 500 plusmn;200 m of the crustal thickness. Hence the 20°40′S OSC has been associated with a reduced magmatic budget for at least the past 1.5 m.y. and represents a persistent segmentation of the EPR. This is consistent with models in which mantle upwelling, even at the fastest spreading centers, is enhanced between large discontinuities rather than evenly distributed along axis. However, this decrease of the crustal thickness toward an axial discontinuity is several times smaller than that typically documented for slow spreading ridges, which suggests that mantle upwelling is less focused at fast spreading ridges, or that along‐axis transport of crustal material is more efficient, or both. Across the study area, the residual anomaly decreases toward the NW by 15–20 mGal. This regional gradient can be modeled with lateral temperature variations in the upper mantle of up to 60°C, increasing toward the NW. This interpretation is consistent with the numerous seamounts present to the NW and the robust magmatic budget of the ridge between 17° and 18°S, and it could also explain why the ridge segments defined by the smaller OSCs between 18° and 19°S propagate very rapidly away from the robust area. Similar patterns of ridge propagation away from the shallowest section of a ridge have been documented near the Galapagos and Easter Island hot spots. Hence these shorter ridge segments may not be associated with significant individual melt sources. Rather, they may represent a superficial segmentation due to the interaction between the EPR and a mantle heterogeneity located

ISSN:0148-0227    

DOI:10.1029/95JB00243

年代:1995

数据来源: WILEY

摘要:

We present the results of a numerical simulation study of the thermo‐convective circulation in a sedimentary pile, in the distal part of the Bengal Fan. The occurrence of faults and the variability of surface heat flow data led previous workers to suggest hydrothermal circulation in this area. According to the purpose of Ocean Drilling Program survey Leg 116, data were collected which are adequate to constrain a model for the comprehension and the quantification of the relationship between ocean basin heat flow and pore fluid circulation. In this paper the sedimentary pile is modeled as an anisotropic and inclined porous layer interrupted by faults with a periodic distribution. An important observation the model tries to reproduce is the shift of approximately 2–3 km between the surface expression of the heat flow maximum and the fault zone near site 718. The influence of permeability anisotropy and of permeability within those fault zones is analyzed. When the maximum vertical permeability in the fault zones is 5 times greater than the vertical permeability of the porous block, the circulation consists of longitudinal rolls (i.e., rolls with their axis oriented downslope), with more vigorous convection in fault zones. When the maximum permeability of the fault zone is 10 or more times greater than the vertical permeability of the porous layer, convection is primarily restricted to the fault plane. In all cases maximum heat flow values are located directly above faults. This paper shows that the best agreement between those data suggests a 1 m yr−1forced convection and a permeability of the sediments of 0.5 darcy (5 × 10−13m2) and 5 mdarcy respectively, in the horizontal and vertical de

ISSN:0148-0227    

DOI:10.1029/95JB00072

年代:1995

数据来源: WILEY

摘要:

We investigate the sedimentary and volcanic structure of the Tuamotu Plateau with multichannel seismic, seismic refraction, and gravity data along a ship track crossing the plateau near 15°S. The volcanic basement of the central portion of the plateau is capped with a 1 to 2‐km‐thick sediment layer composed of two compositional sequences. The uppermost sequence, with semblance‐derivedPwave velocities of 1.6–1.9 km/s and thicknesses of 0.2–0.9 km, is composed of pelagic sediments. The underlying sequence, with velocities 2.5–3.5 km/s and thicknesses of 0.5–1.5 km, is composed of limestone and volcaniclastic sediments. Sonobuoy refraction data show the upper 1 km of the volcanic basement to have velocities 4.5–5.5 km/s. The gravity data indicate that the platform is compensated by an elastic lithosphere with effective thickness 5±5 km and that the volcanic thickness is 9–10 km thicker than normal oceanic crust with a volume of 2.0–2.6×l06km3. The inferred eruption rates of 0.1–0.13 km3/yr are comparable to those of the Hawaiian and Marquesas island chains but substantially less than those of many oceanic plateaus. Radiometric and paleontological ages for the plateau and geomagnetic dates of the surrounding seafloor indicate that the northwestern portion of the plateau formed –600 km off the axis of the paleo‐Pacific‐Farallon spreading center, on lithosphere of age ∼10–20 Ma. Linear volcanic ridges and scarps bounding deep sediment‐filled basins, however, are similar to features of oceanic plateaus which formed at or near accretionary plate boundaries. We suggest that these volcanic ridges and the gross plateau like morphology were formed by magma that was channelled along the lithospheric discontinuities left behind by a southward propagating rift segment of the nearby spreading center. We attribute the formation of the northwestern portion of the Tuamotu Plateau to the passage of two hotspots during times 50–30 Ma as they migrated beneath the Pacific plate but r

ISSN:0148-0227    

DOI:10.1029/95JB00071

年代:1995

数据来源: WILEY

摘要:

We present a conceptual model of fluid circulation in a ridge flank hydrothermal system, the Mariana Mounds. The model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep‐sea research vesselAlvinin small, meter‐sized mounds situated on a local topographic high. These mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow. We have interpreted the chemical data with time‐dependent transport‐reaction models to estimate pore water velocities. In the mounds themselves pore water velocities reach several meters per year to kilometers per year. Within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year. A larger area of low heat flow surrounds these heat flow and topographic highs, with upwelling pore water velocities less than 2 cm/yr. In some nearby cores, downwelling of bottom seawater is evident but at speeds less than 2 cm/yr. Downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement. We conclude that the principal source of seawater recharge to basement is where basement outcrops exist, most likely a scarp about 2–4 km to the east and southeast of the s

ISSN:0148-0227    

DOI:10.1029/95JB00047

年代:1995

数据来源: WILEY

摘要:

A new ∼117 Ma paleomagnetic pole has been defined from the study of volcanic and plutonic rocks from the eastern portion Marie Byrd Land (MBL). The new pole (185.6°E/56.8°S,A95= 8.7°) implies that the eastern portion of MBL was an integral part of Weddellia, which included the ancestral Antarctic Peninsula, Thurston Island, and Ellsworth‐Whitmore Mountains blocks of West Antarctica. This pole is generally similar to a ∼125 Ma pole from Thurston Island. Both poles call for major clockwise rotation and poleward motion of eastern MBL and Thurston Island between the Early Cretaceous (125–117 Ma) and the mid‐Cretaceous (110–100 Ma). We propose that in the Early Cretaceous, eastern MBL and the Eastern Province of New Zealand were part of a continuous active Pacific margin of Gondwana, connecting with the Antarctic Peninsula, and distinct from western MBL, the Western Province of New Zealand, and North Victoria Land. These western terranes are thought to have accreted to Gondwana in the Devonian. Eastern MBL and the Eastern Province of New Zealand amalgamated with western MBL and the Western Province of New Zealand by the mid‐Cretaceous. Major Early Cretaceous motions of the Weddellia blocks postdate the estimated initiation of seafloor spreading in the Weddell Sea and therefore may be the result of plate reorganization during the Creta

ISSN:0148-0227    

DOI:10.1029/95JB00042

年代:1995

数据来源: WILEY

摘要:

Geophysical and tectonic evidence suggests that the principal vertical axis bending of the Bolivian Orocline is a late Cenozoic event related to subduction and Andean uplift. To further test this hypothesis, paleomagnetic data are presented from Miocene basins within the Altiplano and Cordillera Oriental of Bolivia, located in the presumed southern limb of the orocline. To factor out potential VGP data, samples were collected from sedimentary sections spanning at least 0.5 m.y. New paleomagnetic data are presented for two sites, i.e., Gerdas (21°S latitude, 16–15 Ma) and Quehua (20°S latitude, 13–7 Ma). In conjunction with previously published paleomagnetic results from middle to late Tertiary rocks from Bolivia and Peru spanning some 16° of latitude (6–22°S), a consistent pattern of oroclinal development is as follows: significant counterclockwise rotation (about 15–20°) in the northern limb, negligible rotation in the axis (at the latitude of the Arica Deflection), and significant clockwise rotation (about 15–20°) in the southern limb. Although these paleomagnetic data could represent independent, mesoscale (>102to<104km2) block rotations, the preferred interpretation is that they confirm the hypothesis that a significant portion (about 30–40°) of the oroclinal bending occurred during the late Cenozoic. Using the unit mean paleomagnetic declinations in the well‐dated sections, minimum rates of oroclinal bending during the late Cenozoic are calculated to range from 0.6°/m.y. to 1.7°/m.y. The remaining 15–20° of observed curvature of the orocline probably resulted from an earlier phase of Andean deformation or may represent the sha

ISSN:0148-0227    

DOI:10.1029/95JB00149

年代:1995

数据来源: WILEY

摘要:

Tertiary volcanic and sedimentary rocks were collected at 50 sites of both the central Ryukyu arc and the southern Ryukyu arc for a paleomagnetic and geochronological study, in an attempt to understand the opening of the Okinawa Trough back arc basin. Stable primary components of remanent magnetization were isolated through both thermal and alternating field demagnetization experiments from 12 sites of three geological units of the central Ryukyu arc and from 12 sites of two geological units of the southern Ryukyu arc. The paleomagnetic results were compared with the Tertiary apparent pole wander path of Eurasia in order to investigate the tectonic movement of the Ryukyu arc with respect to the Asian continent. The comparison and the radiometric ages indicate that the southern Ryukyu arc rotated clockwise through 25° between 10 Ma and 6 Ma. In contrast, the central Ryukyu arc experienced little rotational motion since 17 Ma. We propose the following two‐phase opening model for the Okinawa Trough opening. The first stage took place between 10 Ma and 6 Ma. The southern part of the trough opened by means of a “wedge” mode to cause the clockwise rotation of the southern Ryukyu arc. In the central part the “parallel” opening occurred, and the Ryukyu arc drifted without any rotation. The second phase started recently in the whole Okinawa Trough. It is observed as the present activities in

ISSN:0148-0227    

DOI:10.1029/95JB00034

年代:1995

数据来源: WILEY

摘要:

Five geodetic arrays (10 to 40 km aperture) located along the San Andreas fault have been surveyed frequently (several times in most years) over the 1980–1991 interval to detect possible fluctuations in the deformation rate. In each survey of an array the distances between the same four to seven pairs of geodetic monuments were measured. The distances measured (with corresponding standard deviation) range from 8.4 km (3.4 mm) to 38 km (8.2 mm). The data are displayed as plots of measured distance as a function of time. Linear fits in such plots furnish a satisfactory representation of the data. That is, the scatter of the data about the linear fits is within the range expected for the estimated standard deviations in measurement. However, there are coherent low‐amplitude (within the observational error) fluctuations apparent in some of the measured distances. Those fluctuations need not be tectonic effects but rather may be either random patterns or artifacts introduced by systematic drift in instrument calibration or wander of the geodetic monuments. We conclude that the measurements are consistent with steady deformation of the arrays over the 1980–1991 interval, and we find no convincing evidence in the data for fluctuations in the rate of deform

ISSN:0148-0227    

DOI:10.1029/95JB00611

年代:1995

数据来源: WILEY

摘要:

By comparing UT1 determinations from the International Radio Interferometric Surveying intensive program of quasi‐daily, single‐baseline, 1‐hour very long baseline interferometry (VLBI) measurement sessions with coincident, multibaseline 24‐hour VLBI determinations, we estimate the overall accuracy of the intensive UT1 results, averaged over 5.4 years of data, to be ∼52 μs, depending somewhat on the amount of error attributed to the multibaseline results. The largest error contributions are due to interpolated values for polar motionXandycoordinates (<38 μs), short‐term variability in atmospheric propagation (∼48 μs during summers but much less at other times), and extended brightness structures in the radio sources (∼22 μs). Because the atmospheric effect is most pronounced during summers, the UT1 accuracy of the intensives is ∼47 μs most of the year but degrades to ∼67 μs during the t

ISSN:0148-0227    

DOI:10.1029/95JB00151

年代:1995

数据来源: WILEY