Bedrock fission‐track analysis, high‐resolution petrography and heavy mineral analyses of sediments are used to investigate the relationships between erosion and tectonics in the Western Alps. Along the Aosta Valley cross‐section, exhumation rates based on fission‐track data are higher in the fault‐bounded western block than in the eastern block (0.4–1.5 vs. 0.1–0.3 mm yr−1). Erosion rates based on the analysis of bed‐load in the Dora Baltea drainage display the same pattern and have similar magnitudes in the relative sub‐basins (0.4–0.7 vs. 0.04–0.08 mm yr−1). Results highlight that climate, relief and lithology are not the controlling factors of erosion in the Western Alps. The main driving force behind erosion is instead tectonics that causes the differential upward motion of crustal blocks. 相似文献
Several volcanic ash layers were identified in cores collected in the Bannock Basin (Eastern Mediterranean) during cruises BAN-84, BAN-86 and BAN-88 of the R.V. Bannock. Lithological, microscopic, mineralogical and chemical analyses together with stratigraphic position help in identifying them as tephra layers Y-1, Y-5, X-2 and W-1 of Keller et al. (GSA Bull. 89, 1978).
Tephra layer Y-1 is stiff and usually deformed, and is black to very dark brown in colour. It is mainly composed of highly vesicular micropumice with dark brown and colourless limpid glass and it has a benmoritic chemical composition, which is typical of Mt. Etna material.
Tephra layer Y-5 is composed of soft, limpid vitric shards and subordinate micropumice ranging in composition from alkaliphonolitic trachyte to latite and may be correlated with the Campanian Ignimbrite eruption which occurred about 35,000 years ago in the Phlegrean Fields (the Campanian volcanic area in Italy).
Tephra layer X-2 is olive-grey in colour, and is composed of micropumice and glass shards with a chemical composition ranging from alkali-phonolitic trachyte to latite; its source is probably the Campanian volcanic area.
Tephra layer W-1 is dark grey and made up of micropumice and glass shards with a chemical composition ranging from tephrite-phonolite to alkali-phonolitic trachyte; its source is probably the Roman volcanic area in Italy.
The volcanic layers have been identified in cores of the basin sill, in the central bulge of the basin and on the basin flanks leading up to the “cobblestone topography” of the Mediterranean Ridge; they could not, however, be identified in any core raised from beneath the anoxic hypersaline brines.
Important volcanological results are: (a) an extension of the areal distribution of Y-1, X-2 and W-1, (b) correlation of Y-1 with the Biancavilla Ignimbrite of Mt. Etna dated in previous works at about 14,000 yrs B.P., and (c) determination of the bimodal chemical composition of Y-5 showing that the latter has a composition in accordance with that of the Campanian Ignimbrite in the Phlegrean Fields. 相似文献
Sediment in coastal Namibia to southern Angola is supplied dominantly from the Orange River with minor additional fluvial input and negligible modifications by chemical processes, which makes this a great test case for investigating physical controls on sand texture and composition. This study monitored textural, mineralogical and geochemical variability in beach and aeolian‐dune sands along a ca 1750 km stretch of the Atlantic coast of southern Africa by using an integrated set of techniques, including image analysis, laser granulometry, optical microscopy, Raman spectroscopy and bulk‐sediment geochemistry. These results contrast with previous reports that feldspars and volcanic detritus break down during transport, that sand grains are rounded rapidly in shallow‐marine environments, and that quartzose sands may be produced by physical processes. Mechanical wear is unable to modify the relative abundance of detrital components, including pyroxene and mafic volcanic rock fragments traditionally believed to be destroyed rapidly. The sole exceptions are poorly lithified or cleaved sedimentary/metasedimentary rock fragments, readily lost at the transition to the marine environment, and slow‐settling flaky micas, winnowed and deposited offshore. Coastal sediments tend to be depleted in relatively mobile amphibole, preferentially entrained offshore or re‐deposited in sheltered beaches, while less mobile garnet is retained onshore. No detrital mineral displays a significant increase in grain roundness after 300 to 350 km of longshore transport in high‐energy littoral environments from the Orange mouth to south of the Namib Erg, but all minerals get rapidly rounded after passing into the dunefield. Pyroxene and opaques get rounded faster than harder quartz and garnet, but sand mineralogy remains unchanged. Excepting strong transient selective‐entrainment effects, physical processes are unable to modify sand composition significantly. Selective mechanical breakdown can be largely neglected in quantitative provenance analysis of sand and sandstone even in the case of ultra‐long‐distance transport in high‐energy environments dominated by strong persistent winds and waves. 相似文献
This article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition, which cause systematic distortion in quantitative provenance analysis (“environmental bias”). Traditional procedures aimed at eliminating textural noise find limited success. The Gazzi–Dickinson method cannot remove hydrodynamic-related modal variability. Multiple size-window strategies are time-consuming. Narrow or moving size-window strategies represent misleading or impractical short cuts, being less convenient options than simply analysing each sample in bulk. New concepts introduced here unravel the superposed causes of compositional variability in modern sediments. Intrasample modal variability, fundamentally explained by settling-equivalence relationships, can be accurately modelled mathematically. Intersample modal variability, principally resulting from selective entrainment, can be assessed and removed by a simple principle. In absence of provenance changes and environmental bias, the weighted average density of terrigenous grains (SRD index) should be equal, for each sample and each grain-size class of each sample, to the weighted average density of source rocks. By correcting relative abundances of detrital minerals in proportion to their densities, we can restore the appropriate SRD index for any provenance and subprovenance type in each sample or grain-size class. Modal variability is effectively reduced by this procedure, which can be applied confidently to modern sediments deposited by tractive currents in any environment. Good results are obtained even for placer sands and finest classes where heavy-mineral concentration is strongest. Such “SRD correction” also successfully compensates for biased narrow-window modes, thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology. After compensating for settling-equivalence and selective-entrainment effects, residual size-dependent compositional variability may be provenance-related. Minor in Ganga–Brahmaputra sediments, provenance-related effects are spectacularly displayed in the Nile basin, where volcaniclastic silt mixes with basement-derived quartzofeldspathic sand and wind-blown Saharan quartz. 相似文献
This paper aims to aid understanding of the complicated interplay between construction and destruction of volcanoes, with an emphasis on the role of substrate tectonic heritage in controlling magma conduit geometry, lateral collapse, landslides, and preferential erosion pathways. The influence of basement structure on the development of six composite volcanoes located in different geodynamic/geological environments is described: Stromboli (Italy), in an island arc extensional tectonic setting, Ollagüe (Bolivia–Chile) in a cordilleran extensional setting, Kizimen (Russia) in a transtensional setting, Pinatubo (Philippines) in a transcurrent setting, Planchon (Chile) in a compressional cordilleran setting, and Mt. Etna (Italy) in a complex tectonic boundary setting. Analogue and numerical modelling results are used to enhance understanding of processes exemplified by these volcanic centres. We provide a comprehensive overview of this topic by considering a great deal of relevant, recently published studies and combine these with the presentation of new results, in order to contribute to the discussion on substrate tectonics and its control on volcano evolution. The results show that magma conduits in volcanic rift zones can be geometrically controlled by the regional tectonic stress field. Rift zones produce a lateral magma push that controls the direction of lateral collapse and can also trigger collapse. Once lateral collapse occurs, the resulting debuttressing produces a reorganization of the shallow-level magma migration pathways towards the collapse depression. Subsequent landslides and erosion tend to localize along rift zones. If a zone of weakness underlies a volcano, long-term creep can occur, deforming a large sector of the cone. This deformation can trigger landslides that propagate along the destabilized flank axis. In the absence of a rift zone, normal and transcurrent faults propagating from the substrate through the volcano can induce flank instability in directions respectively perpendicular and oblique to fault strike. This destabilization can evolve to lateral collapse with triggering mechanisms such as seismic activity or magmatic intrusion. 相似文献
Understanding the relationships between sedimentation, tectonics and magmatism is crucial to defining the evolution of orogens and convergent plate boundaries. Here, we consider the lithostratigraphy, clastic provenance, syndepositional deformation and volcanism of the Almagro‐El Toro basin of NW Argentina (24°30′ S, 65°50′ W), which experienced eruptive and depositional episodes between 14.3 and 6.4 Ma. Our aims were to elucidate the spatial and temporal record of the onset and style of the shortening and exhumation of the Eastern Cordillera in the frame of the Miocene evolution of the Central Andes foreland basin. The volcano‐sedimentary sequence of the Almagro‐El Toro basin consists of lower red floodplain sandstones and siltstones, medial non‐volcanogenic conglomerates with localised volcanic centres and upper volcanogenic coarse conglomerates and breccia. Coarse, gravity flow‐dominated (debris‐flow and sheet‐flow) alluvial fan systems developed proximal to the source area in the upper and medial sequence. Growing frontal and intrabasinal structures suggest that the Almagro‐El Toro portion of the foreland basin accumulated on top of the eastward‐propagating active thrust front of the Eastern Cordillera. Synorogenic deposits indicate that the shortening of the foreland deposits was occurring by 11.1 Ma, but conglomerates derived from the erosion of western sources suggest that the uplift and erosion of this portion of the Eastern Cordillera has occurred since ca.12.5 Ma. An unroofing reconstruction suggests that 6.5 km of rocks were exhumed. A tectono‐sedimentary model of an episodically evolving thick‐skinned foreland basin is proposed. In this frame, the NW‐trending, transtensive Calama–Olacapato–El Toro (COT) structures interacted with the orogen, influencing the deposition and deformation of synorogenic conglomerates, the location of volcanic centres and the differential tilt and exhumation of the foreland. 相似文献
The Eastern Cordillera (Central Andes, 24°S) consists of a basement-involved thrust system, resulting from Miocene–Quaternary eastward migrating compression, separating the Puna plateau from the Santa Barbara System foreland. The inferred Tertiary strains arising from shortening in the Eastern Cordillera and Santa Barbara System are similar, higher than in the Puna. Slip data collected on the major N–S trending faults of Eastern Cordillera show a westward progression from dip-slip (contraction) to dextral and sinistral motions. This, consistently with established tectonic models, may result from partitioning due to the oblique Mio-Quaternary underthrusting of the Brazilian Shield north of 24°S. This strain partitioning has three main implications. (1) As the dextral and sinistral shear in the Eastern Cordillera are 62% and 29% of the compressive strain respectively, the Eastern Cordillera results more strained than Santa Barbara System foreland, contrary to previous estimates. (2) The partitioning in the Eastern Cordillera may find its counterpart in that to the west of the Central Andes, giving a possible structural symmetry to the Central Andes. (3) The easternmost N–S strike-slip structures in the Eastern Cordillera coincide with the easternmost Mio-Pliocene magmatic centres in the Central Andes, at 24°S. Provided that, further to the east, the crust is partially molten, the absence of magmatic centres may be explained by the presence of pure compressive structures in this portion of the Eastern Cordillera. 相似文献