排序方式： 共有18条查询结果，搜索用时 109 毫秒
1.
Archaean crustal growth through successive partial melting events in an oceanic plateau‐like setting in the Tanzania Craton 
下载免费PDF全文
下载免费PDF全文 The detrital zircon population in quartzitic conglomerates from the northern Tanzania Craton yield ages between 2640 Ma and 2790 Ma which includes most of the igneous history from this part of the craton. The igneous evolution is characterised by mafic volcanism with an oceanic plateau‐like geochemical signature at ~2800 Ma followed by diorite and tonalite–trondhjemite–granodiorite dominated magmatism between 2790 and 2700 Ma, which transitioned into more evolved high‐K magmatism between 2700 and 2620 Ma. The εHf values of the detrital zircons range from +2.4 to ?1.4 and change with time from radiogenic Hf pre‐2700 Ma (98% positive εHf) to unradiogenic Hf post‐2700 Ma (41% positive εHf). The petrological progression from mafic to felsic crust is reflected in the detrital age distribution and Hf isotopes and is consistent with juvenile mafic crust slowly maturing into more evolved felsic crust through a series of successive partial melting events in an oceanic‐plateau‐like environment. 相似文献
2.
3.
Unconsolidated mud clast breccia facies in the hominin-bearing (Homo naledi) Rising Star Cave, Cradle of Humankind, South Africa, are interpreted to have formed through a process termed sedimentary autobrecciation in this study. This process, by which most of the angular mud clast breccia deposits are thought to have formed autochthonously to para-autochthonously via a combination of erosion, desiccation, diagenesis and microbial alteration of laminated mud deposits, is thought to have taken place under relatively dry (i.e. non-flooded) conditions inside the cave. Subsequently, gravitational slumping and collapse was the dominant mechanism that produced the mud clast breccia deposits, which commonly accumulate into debris aprons. The mud clast breccia is typically associated with (micro) mammal fossils and is a common facies throughout the cave system, occurring in lithified and unlithified form. This facies has not been described from other cave localities in the Cradle of Humankind. Additionally, sedimentary autobrecciation took place during the deposition of some of the fossils within the Rising Star Cave, including the abundant Homo naledi skeletal remains found in the Dinaledi Subsystem. Reworking of the mud clast breccia deposits occurs in some chambers as they slump towards floor drains, resulting in the repositioning of fossils embedded in the breccias as evidenced by cross-cutting manganese staining lines on some Homo naledi fossil remains. The formation of the unlithified mud clast breccia deposits is a slow process, with first order formation rates estimated to be ca 8 × 10−4 mm year−1. The slow formation of the unlithified mud clast breccia facies sediments and lack of laminated mud facies within these deposits, indicates that conditions in the Dinaledi Chamber were probably stable and dry for at least the last ca 300 ka, meaning that this study excludes Homo naledi being actively transported by fluvial mechanisms during the time their remains entered the cave. 相似文献
4.
Local variations in the summertime mixing height over metropolitan St. Louis, Missouri, are examined in order to evaluate the dynamical and thermodynamical influences of an urban area. Case studies employing aircraft, lidar, pilot balloon and radiosonde data from Project METROMEX show variations in the height of a temperature inversion which caps a deep mixing layer. A dome-like lifting of the inversion structure occurs over the urban-industrial center with subsidence of the inversion layer at the perimeter of the metropolitan area. The maximum amplitude of the dome occurs shortly after mid-day, suggesting that thermal influences are primarily responsible. 相似文献
5.
T.V. Makhubela J.D. Kramers D. Scherler H. Wittmann P.H.G.M. Dirks S.R. Winkler 《地球表面变化过程与地形》2019,44(15):2968-2981
In situ cosmogenic nuclides are an important tool for quantifying landscape evolution and dating fossil-bearing deposits in the Cradle of Humankind (CoH), South Africa. This technique mainly employs cosmogenic 10-Beryllium (10Be) in river sediments to estimate denudation rates and the ratio of 26-Aluminium (26Al) to 10Be (26Al/10Be), to constrain ages of sediment burial. Here, we use 10Be and 26Al concentrations in bedrock and soil above the Rising Star Cave (the discovery site of Homo naledi) to constrain the denudation rate and the exposure history of soil on the surface. Apparent 10Be-derived denudation rates obtained from pebble- to cobble-sized clasts and coarse-sand in soil (on average 3.59 ± 0.27 m/Ma and 3.05 ± 0.25 m/Ma, respectively) are 2-3 times lower than the bedrock denudation rates (on average 9.46 ± 0.68 m/Ma). In addition, soil samples yield an average 26Al/10Be ratio (5.12 ± 0.27) that is significantly lower than the surface production ratio of 6.75, which suggests complex exposure histories. These results are consistent with prolonged surface residence of up to 1.5 Ma in vertically mixed soils that are up to 3 m thick. We conclude that the 10Be concentrations accumulated in soils during the long near-surface residence times can potentially cause underestimation of single-nuclide (10Be) catchment-wide denudation rates in the CoH. Further, burial ages of cave sediment samples that consist of an amalgamation of sand-size quartz grains could be overestimated if a pre-burial 26Al/10Be ratio calculated from the surface production is assumed. © 2019 John Wiley & Sons, Ltd. 相似文献
6.
7.
The Tia Granodiorite, a Hillgrove Suite pluton in the southern New England Fold Belt, intruded complexly deformed metasediment and metabasite belonging to the Tia Complex, which at the time of intrusion had already been affected by two deformation events at low‐T moderate‐P metamorphic conditions and two overprinting deformation events at high‐T low‐P metamorphic conditions. Emplacement took place during D5 thrusting associated with limited uplift as low‐P amphibolite facies metamorphism prevailed. Large‐scale warping during D6 was followed by a second penetrative thrusting event (D7) that caused further uplift and was initiated under lower amphibolite facies conditions. The granodiorite has been dated at ～ 300 Ma using magmatic zircon, an age which is thought to approximate the emplacement age and thus D5. Biotite grains associated with D7 uplift yield a Rb/Sr age of 264±1.3 Ma. D5 and D7 appear to have formed during one extended high‐T metamorphic event because intervening retrogression is lacking in spite of extensive hydrous fluxing, as indicated by numerous syn‐D6 quartz veins. This thermal event coincided with the opening of the extensional Permian basins. 相似文献
8.
Martin?B.?C.?BrandtEmail author Stephen?P.?Grand Andrew?A.?Nyblade Paul?H.?G.?M.?Dirks 《Pure and Applied Geophysics》2012,169(4):595-614
We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa
obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded
on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere
beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity
zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath
other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by
a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by
shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density
and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition
zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less
steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s
across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition
zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be
refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high
elevation of the Kalahari craton, and hence the southern African portion of the African Superswell. 相似文献
9.
The pattern of Au reefs at Dalny Mine and its satellite mines in the Chakari area of central Zimbabwe closely mimics an anastomosing network of narrow (< 5 m) shear zones characterised by highly strained silicified schist and graphitic shale horizons. These shears predated mineralisation and accommodated movements resulting in a duplex geometry of complexly stacked, lensoidal blocks of felsic and mafic greenstones, possibly reflecting thin-skinned processes early during the development of the Midlands Greenstone Belt. During mineralisation repeated reactivation of, and (fluid-pressure driven) dilation along these early shears resulted in the current distribution of Au reefs. The scale and orientation of the fault blocks in the early tectonic stack controls the size and spacing of the later Au reefs, explains the anomalous orientation of some reefs in the area and eliminates the need for complicated explanations involving multiple post-mineralisation-faults.Geometries like those at Dalny Mine could control a number of Au deposits in Zimbabwe, especially those that contain bifurcarting siliceous schist and black shale bands. Duplex geometries could have resulted from a number of processes including thin-skinned thrusting or strike-slip movements, although the first interpretation is preferred. The presence of imbricate stacks at Dalny implies that the stratigraphy of the thick greenstone succession may have resulted partly from tectonic processes. 相似文献
10.
Based on the Zimbabwe craton, it is suggested that, during the Archaean, full decoupling between a strong upper crust and a strong upper mantle across a weak detachment zone at the Moho allowed the independent development of crustal and mantle geometries in response to lithospheric shortening. This is an effective way to explain the field observations made in the Zimbabwe craton, which suggest a late-Archaean interplay between lateral accretionary processes through low angle thrust stacking and underplating and deep seated lineament zones with a possible mantle origin. The lineament zones play an important role in the localisation of mineral deposits such as base metals, gold, and possibly diamonds. Thickening of the mantle lithosphere occurred independently from the crust, through early Archaean melt segregation and/or lithospheric underplating. 相似文献