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1.
Daniel S. Scheirer Ken C. Macdonald Donald W. Forsyth Stephen P. Miller Dawn J. Wright Marie-Hélène Cormier Charles M. Weiland 《Marine Geophysical Researches》1996,18(1):1-12
Four large-scale bathymetric maps of the Southern East Pacific Rise and its flanks between 15° S and 19° S display many of the unique features of this superfast spreading environment including abundant seamounts (the Rano Rahi Field), axial discontinuities, discontinuity migration, and abyssal hill variation. Along with a summary of the regional geology, these maps will provide a valuable reference for other sea-going programs on-and off-axis in this area, including the Mantle ELectromagnetic and Tomography (MELT) experiment. 相似文献
2.
Gallo D. G. Kidd W. S. F. Fox P. J. Karson J. A. Macdonald K. Crane K. Choukroune P. Seguret M. Moody R. Kastens K. 《Marine Geophysical Researches》1984,6(2):159-185
During the Fall of 1979, a manned submersible program, utilizing DSRV ALVIN, was carried out at the intersection of the East Pacific Rise (EPR) with the Tamayo Transform boundary. A total of seven dives were completed in the vicinity of the EPR/Tamayo intersection depression and documented the geologic relationships that characterize the juxtaposition of these types of plate boundaries. The young volcanic terrain of the EPR axis can be traced into and across the Tamayo Transform valley but becomes buried by sedimentary talus that is being shed from sediment scarps along the unstable sediment slope that defines the north side of the intersection depression. Within 4 km of the transform boundary, the dominant trend (000°) of the fissures and faults that disrupt the rise-generated volcanics is markedly oblique to the regional direction of sea floor spreading (120°). Since no evidence was found to suggest that these structures accommodate significant amounts of strike-slip displacement, they are taken to reflect a distortion of the EPR extensional tectonic regime by a transform generated shear couple. The floor of the Tamayo Transform valley in this area is inundated by mass-wasted sediment, and the principal transform displacement zone is characterized at the surface by a narrow (<1.5 km) interval of fault scarps in sediment that trends parallel with the transform valley. Extrapolated to the west, this zone links with zones of transform deformation investigated during earlier submersible studies (CYAMEX and Pastouret, 1981). Evidence of low-level hydrothermal discharge was seen at one locality on the EPR axis and at another 8 km west of the axis at the edge of the zone of transform deformation. 相似文献
3.
John D. Bicknell Jean-Christophe Sempere Ken C. Macdonald P. J. Fox 《Marine Geophysical Researches》1987,9(1):25-45
Sea Beam and Deep-Tow were used in a tectonic investigation of the fast-spreading (151 mm yr-1) East Pacific Rise (EPR) at 19°30 S. Detailed surveys were conducted at the EPR axis and at the Brunhes/Matuyama magnetic reversal boundary, while four long traverses (the longest 96 km) surveyed the rise flanks. Faulting accounts for the vast majority of the relief. Both inward and outward facing fault scarps appear in almost equal numbers, and they form the horsts and grabens which compose the abyssal hills. This mechanism for abyssal hill formation differs from that observed at slow and intermediate spreading rates where abyssal hills are formed by back-tilted inward facing normal faults or by volcanic bow-forms. At 19°30 S, systematic back tilting of fault blocks is not observed, and volcanic constructional relief is a short wavelength signal (less than a few hundred meters) superimposed upon the dominant faulted structure (wavelength 2–8 km). Active faulting is confined to within approximately 5–8 km of the rise axis. In terms of frequency, more faulting occurs at fast spreading rates than at slow. The half extension rate due to faulting is 4.1 mm yr-1 at 19°30 S versus 1.6 mm yr-1 in the FAMOUS area on the Mid-Atlantic Ridge (MAR). Both spreading and horizontal extension are asymmetric at 19°30 S, and both are greater on the east flank of the rise axis. The fault density observed at 19°30 S is not constant, and zones with very high fault density follow zones with very little faulting. Three mechanisms are proposed which might account for these observations. In the first, faults are buried episodically by massive eruptions which flow more than 5–8 km from the spreading axis, beyond the outer boundary of the active fault zone. This is the least favored mechanism as there is no evidence that lavas which flow that far off axis are sufficiently thick to bury 50–150 m high fault scarps. In the second mechanism, the rate of faulting is reduced during major episodes of volcanism due to changes in the near axis thermal structure associated with swelling of the axial magma chamber. Thus the variation in fault spacing is caused by alternate episodes of faulting and volcanism. In the third mechanism, the rate of faulting may be constant (down to a time scale of decades), but the locus of faulting shifts relative to the axis. A master fault forms near the axis and takes up most of the strain release until the fault or fault set is transported into lithosphere which is sufficiently thick so that the faults become locked. At this point, the locus of faulting shifts to the thinnest, weakest lithosphere near the axis, and the cycle repeats. 相似文献
4.
Application of the QUILF thermobarometer to the peralkaline trachytes and pantellerites of the Eburru volcanic complex, East African Rift, Kenya 总被引:1,自引:0,他引:1
Minghua Ren Peter A. Omenda Elizabeth Y. Anthony John C. White Ray Macdonald D.K. Bailey 《Lithos》2006,91(1-4):109-124
The Quaternary Eburru volcanic complex in the south-central Kenya Rift consists of pantelleritic trachytes and pantellerites. The phenocryst assemblage in the trachytes is sanidine + fayalite + ferrohedenbergite + aenigmatite ± quartz ± ilmenite ± magnetite ± pyrrhotite ± pyrite. In the pantellerites, the assemblage is sanidine + quartz + ferrohedenbergite + fayalite + aenigmatite + ferrorichterite + pyrrhotite ± apatite, although fayalite, ferrohedenbergite and ilmenite are absent from more evolved rocks (e.g. with SiO2 > 71%). QUILF temperature calculations for the trachytes range from 709 to 793 °C and for the pantellerites 668–708 °C, the latter temperatures being among the lowest recorded for peralkaline silicic magmas. The QUILF thermobarometer demonstrates that the Eburru magmas crystallized at relatively low oxidation states (ΔFMQ + 0.5 to − 1.6) for both trachytes and pantellerites. The trachytes and pantellerites evolved along separate liquid lines of descent, the trachytes possibly deriving from a more mafic parent by fractional crystallization and the pantellerites from extreme fractionation of comenditic magmas. 相似文献
5.
Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H2O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe3+/Fe2+ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO2 were imposed (NNO + 1.5 for the 1.5 wt% H2O series, NNO + 2.3 for the 4.5 wt% H2O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H2O) and 1185°C, 16 kbar (4.5 wt% H2O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H2O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H2O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry (∼ 2 wt% H2O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with ∼ 2 wt% H2O would require a H2O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to ∼ 5.5 wt% H2O. 相似文献
6.
During the last decade, exploration and mining of modern-Tertiary heavy mineral beach and raised beach sands along the west
coast of South Africa has developed into a major industry. High resolution radiometric techniques have demonstrated their
use as a quantitative indicator of total heavy mineral concentration (THM) and also have the ability to discriminate between
sediments derived from different provenance terrains. Results indicate that it is possible to calculate the total heavy mineral
concentration from high resolution radiometric measurements, but the ability to quantify the concentration of individual mineral
fractions, such as ilmenite, requires further refinement of the method. Radiometric characteristics of the light mineral fraction
made it possible to distinguish between sediments from mixed marine-aeolian palaeoplacers and active present beach placers.
The effects of in situ alteration of the ilmenite fraction in the palaeoplacers were also reflected by subtle contrasts in
the radiometric character of the deposits. Furthermore, the uniform radiometric character of the sample population indicates
a common provenance for the heavy mineral suite and supports previous results which indicate the metamorphites of the mid-Proterozoic
Namaqua Province as the primary source terrain. The high resolution radiometric techniques discussed offer a broad spectrum
of applications in sedimentology and have the potential to aid heavy mineral exploration. With further refinement it can possibly
be used quantitatively in grade control during mining and beneficiation of heavy mineral deposits.
Received: 19 July 1996 / Accepted: 7 January 1997 相似文献
7.
8.
The sedimentary and tectonic evolution of the Amur River and North Sakhalin Basin: new evidence from seismic stratigraphy and Neogene–Recent sediment budgets 下载免费PDF全文
Uisdean Nicholson Bas van der Es Peter D. Clift Rachel Flecker David I. M. Macdonald 《Basin Research》2016,28(2):273-297
The North Sakhalin Basin in the western Sea of Okhotsk has been the main site of sedimentation from the Amur River since the Early Miocene. In this article, we present regional seismic reflection data and a Neogene–Recent sediment budget to constrain the evolution of the basin and its sedimentary fill, and consider the implications for sediment flux from the Amur River, in particular testing models of continental‐scale Neogene drainage capture. The Amur‐derived basin‐fill history can be divided into five distinct stages: the first Amur‐derived sediments (>21–16.5 Ma) were deposited during a period of transtension along the Sakhalin‐Hokkaido Shear Zone, with moderately high sediment flux to the basin (71 Mt year?1). The second stage sequence (16.5–10.4 Ma) was deposited following the cessation of transtension, and was characterised by a significant reduction in sediment flux (24 Mt year?1) and widespread retrogradation of deltaic sediments. The third (10.4–5.3 Ma) and fourth (5.3–2.5 Ma) stages were characterised by progradation of deltaic sediments and an associated increase in sediment flux (48–60 Mt year?1) to the basin. Significant uplift associated with regional transpression started during this time in southeastern Sakhalin, but the north‐eastward propagating strain did not reach the NE shelf of Sakhalin until the Pleistocene (<2.5 Ma). This uplift event, still ongoing today, resulted in recycling of older deltaic sediments from the island of Sakhalin, and contributed to a substantially increased total sediment flux to the adjacent basinal areas (165 Mt year?1). Adjusted rates to discount these local erosional products (117 Mt year?1) imply an Amur catchment‐wide increase in denudation rates during the Late Pliocene–Pleistocene; however, this was likely a result of global climatic and eustatic effects, combined with tectonic processes within the Amur catchment and possibly a smaller drainage capture event by the Sungari tributary, rather than continental‐scale drainage capture involving the entire upper Amur catchment. 相似文献
9.
Christopher Macdonald Hewitt 《Transactions in GIS》2019,23(4):823-840
In 1066, English and Norman forces engaged in a decisive battle near Hastings. The battlefield itself provides an example of the way in which physical geography can contribute to our understanding of historical events. According to historical accounts, the rolling landscape of the Hastings battle site was unquestionably an important factor in understanding the conflict. Yet surprisingly few analyses of the local terrain exist to test this proposition. In part, this is because historians of the battle have largely affirmed that the current battle site bears little or no physical resemblance to that present in 1066, and thus is unsuitable for contemporary analysis that might shed light on the outcome of the battle itself. By applying advanced geographical analysis using the RUSLE model, this study demonstrates that there is in fact considerable evidence to suggest that a limited amount of sediment was removed, hence the battlefield has in fact changed very little since the battle. This finding is significant, insofar as it opens the door to new research on the Battle of Hastings which may shed additional light on the events that occurred there and the factors that influenced the outcome of this key conflict in British history. 相似文献
10.
The WOCE-era 3-D Pacific Ocean circulation and heat budget 总被引:2,自引:0,他引:2
A.M. Macdonald S. Mecking P.E. Robbins J.M. Toole G.C. Johnson L. Talley M. Cook S.E. Wijffels 《Progress in Oceanography》2009,(4):281-325
To address questions concerning the intensity and spatial structure of the three-dimensional circulation within the Pacific Ocean and the associated advective and diffusive property flux divergences, data from approximately 3000 high-quality hydrographic stations collected on 40 zonal and meridional cruises have been merged into a physically consistent model. The majority of the stations were occupied as part of the World Ocean Circulation Experiment (WOCE), which took place in the 1990s. These data are supplemented by a few pre-WOCE surveys of similar quality, and time-averaged direct-velocity and historical hydrographic measurements about the equator.An inverse box model formalism is employed to estimate the absolute along-isopycnal velocity field, the magnitude and spatial distribution of the associated diapycnal flow and the corresponding diapycnal advective and diffusive property flux divergences. The resulting large-scale WOCE Pacific circulation can be described as two shallow overturning cells at mid- to low latitudes, one in each hemisphere, and a single deep cell which brings abyssal waters from the Southern Ocean into the Pacific where they upwell across isopycnals and are returned south as deep waters. Upwelling is seen to occur throughout most of the basin with generally larger dianeutral transport and greater mixing occurring at depth. The derived pattern of ocean heat transport divergence is compared to published results based on air–sea flux estimates. The synthesis suggests a strongly east/west oriented pattern of air–sea heat flux with heat loss to the atmosphere throughout most of the western basins, and a gain of heat throughout the tropics extending poleward through the eastern basins. The calculated meridional heat transport agrees well with previous hydrographic estimates. Consistent with many of the climatologies at a variety of latitudes as well, our meridional heat transport estimates tend toward lower values in both hemispheres. 相似文献