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71.
Germán Bayona Camilo Montes Agustín Cardona Carlos Jaramillo Germán Ojeda Victor Valencia Carolina Ayala‐Calvo 《Basin Research》2011,23(4):403-422
The upper Campanian–Lower Eocene synorogenic sedimentary wedge of the Ranchería Basin was deposited in an intraplate basin resting on a tilted continental crustal block that was deformed by collision and subsequent subduction of the Caribbean Plate. Upper Cretaceous–Lower Eocene strata rest unconformably upon Jurassic igneous rocks of the Santa Marta Massif, with no major thrust faults separating the Santa Marta Massif from the Ranchería Basin. The upper Campanian–Lower Eocene succession includes, from base to top: foraminifera‐rich calcareous mudstone, mixed carbonate–siliciclastic strata and mudstone, coal and immature fluvial sandstone beds. Diachronous collision and eastward tilting of the plate margin (Santa Marta Massif and Central Cordillera) favoured the generation of accommodation space in a continuous intraplate basin (Ranchería, Cesar and western Maracaibo) during the Maastrichtian to Late Palaeocene. Terrigenous detritus from the distal colliding margin filled the western segments of the continuous intraplate basin (Ranchería and Cesar Basins); in the Late Paleocene, continental depositional systems migrated eastwards as far as the western Maracaibo Basin. In Early Eocene time, reactivation of former extensional structures fragmented the intraplate basin into the Ranchería‐Cesar Basins to the west, and the western Maracaibo Basin and Palmar High to the East. This scenario of continent–oceanic arc collision, crustal‐scale tilting, intraplate basin generation and fault reactivation may apply for Upper Cretaceous–Palaeogene syntectonic basins in western Colombia and Ecuador, and should be considered in other settings where arc–continent collision is followed by subduction. 相似文献
72.
Mathieu Duval Christophe Falguères Jean-Jacques Bahain Rainer Grün Qingfeng Shao Maxime Aubert Jean-Michel Dolo Jordi Agustí Bienvenido Martínez-Navarro Paul Palmqvist Isidro Toro-Moyano 《Quaternary Research》2012,77(3):482-491
The combined U-series/electron spin resonance (ESR) dating method was applied to nine teeth from two Early Pleistocene archaeological sites located in the Orce area (Guadix-Baza Basin, Southern Spain): Fuente Nueva-3 (FN-3) and Barranco León (BL). The combination of biostratigraphy and magnetostratigraphy places both sites between the Olduvai and Jaramillo subchrons (1.78–1.07 Ma).Our results highlight the difficulty of dating such old sites and point out the limits of the combined U-series/ESR dating method based on the US model. We identified several sources of uncertainties that may lead to inaccurate age estimates. Seven samples could not be dated because the dental tissues had (230Th/234U) activity ratios higher than equilibrium, indicating that uranium had probably leached from these tissues. It was however possible to calculate numerical estimates for two of the teeth, both from FN-3. One yielded a Middle Pleistocene age that seems to be strongly underestimated; the other provided an age of 1.19 ± 0.21 Ma, in agreement with data obtained from independent methods. The latter result gives encouragement that there are samples that can be used for routine dating of old sites. 相似文献
73.
The South Iceland Seismic Zone (SISZ) was loaded to failure in June 2000, resulting in two M6.6 earthquakes. The SISZ is an E–W‐trending zone with an overall sinistral movement. Numerical models indicate that, when the SISZ is loaded to failure, there are stress concentrations at its ends: tensile in the north‐east and south‐west quadrants, and compressive in the north‐west and south‐east quadrants. These model predictions fit well with observations. Geodetic measurements indicate considerable compression, uplift and associated intense seismicity in recent years in the volcanoes of Hengill and Eyjafjallajokull, located in the quadrants of compression, whereas there have been unusually frequent eruptions in the past decades in the Hekla Volcano, located in one of the quadrants of extension. The models predict that following the large June 2000 earthquakes, stress relaxation within the SISZ should lead to stopping of the intense seismicity and deformation in the volcanoes of Hengill and Eyjafjallajokull, again in agreement with observations. However, when similar episodes of deformation and seismicity start again, particularly in the Hengill Volcano, a large earthquake would be expected within several years in the SISZ. The numerical models, and the deformation and seismic data, indicate that monitoring of ‘soft’ inclusions such as volcanoes (many with magma chambers) in the vicinity of a seismic zone may serve as precursors to large earthquakes. 相似文献
74.
Reyes-Carmona Cristina Galve Jorge Pedro Moreno-Sánchez Marcos Riquelme Adrián Ruano Patricia Millares Agustín Teixidó Teresa Sarro Roberto Pérez-Peña José Vicente Barra Anna Ezquerro Pablo López-Vinielles Juan Béjar-Pizarro Marta Azañón José Miguel Monserrat Oriol Mateos Rosa María 《Landslides》2021,18(12):3781-3798
Landslides - When an active landslide is first identified in an artificial reservoir, a comprehensive study has to be quickly conducted to analyse the possible hazard that it may represent to such... 相似文献
75.
76.
77.
Jesús Carrera Xavier Sánchez-Vila Inmaculada Benet Agustín Medina Germán Galarza Jordi Guimerà 《Hydrogeology Journal》1998,6(1):178-190
Matrix diffusion has become widely recognized as an important transport mechanism. Unfortunately, accounting for matrix diffusion
complicates solute-transport simulations. This problem has led to simplified formulations, partly motivated by the solution
method. As a result, some confusion has been generated about how to properly pose the problem. One of the objectives of this
work is to find some unity among existing formulations and solution methods. In doing so, some asymptotic properties of matrix
diffusion are derived. Specifically, early-time behavior (short tests) depends only on φ
m
2
R
m
D
m
/ L
m
2, whereas late-time behavior (long tracer tests) depends only on φ
m
R
m
, and not on matrix diffusion coefficient or block size and shape. The latter is always true for mean arrival time. These
properties help in: (a) analyzing the qualitative behavior of matrix diffusion; (b) explaining one paradox of solute transport
through fractured rocks (the apparent dependence of porosity on travel time); (c) discriminating between matrix diffusion
and other problems (such as kinetic sorption or heterogeneity); and (d) describing identifiability problems and ways to overcome
them.
Received, October 1997 · Revised, November 1997 · Accepted, December 1997 相似文献
78.
The focal mechanism solutions of 83 European earthquakes withM>6, selected from a total of 140, have been used to derive the directions of the principal axes of stress along the plate boundary between Eurasia and Africa from the Azores islands to the Caucasus mountains. Along most of the region, the horizontalP-axes are at an angle of 45° to 90° with the trend of the plate boundary. HorizontalT-axes are concentrated in central Italy and northern Greece in association with normal faulting. Large strike-slip motion of right-lateral character takes place at the center of the Azores-Gibraltar fault and the North Anatolian fault. From Gibraltar to the Caucasus the boundary is complicated by the presence of secondary blocks and zones of extended deformations with earthquakes spread over wide areas. Intermediate and deep earthquakes are present at four areas with arc-like structure, namely, Gibraltar, Sicily-Calabria, Hellenic arc and Carpathians. 相似文献
79.
Agust Gudmundsson Niels Oskarsson Karl Gronvold Kristjan Saemundsson Oddur Sigurdsson Ragnar Stefansson Sigurdur R Gislason Pall Einarsson Bryndis Brandsdottir Gudrun Larsen Haukur Johannesson Thorvaldur Thordarson 《Bulletin of Volcanology》1992,54(3):238-246
The eruption that started in the Hekla volcano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km2 and has an estimated volume of 0.15 km3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02 km3 of tephra. The eruption cloud attained 11.5 km in height in only 10 min, but it became detached from the volcano a few hours later. Several fissures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially limited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m3 s–1. After this peak, the effusion rate declined rapidly to 10–20 m3 s–1, then more slowly to 1 m3 s–1, and remained at 1–12 m3 s–1 until the end of the eruption. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the eruptive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica content of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vicinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its association with a crustal weakness, all contribute to the high eruption frequency of Hekla. 相似文献
80.
Agustín Gómez-Alvarez Diana Meza-Figueroa Arturo I. Villalba-Atondo Jesús Leobardo Valenzuela-García Jorge Ramírez-Hernández Javier Almendariz-Tapia 《Environmental Geology》2009,57(7):1469-1479
The San Pedro River (SPR) is located in northern Sonora (Mexico) and southeastern Arizona (USA). SPR is a transboundary river
that develops along the Sonora (Mexico) and Arizona (USA) border, and is considered the main source of water for a variety
of users (human settlements, agriculture, livestock, and industry). The SPR originates in the historic Cananea mining area,
which hosts some of the most important copper mineralizations in Mexico. Acid mine drainage derived from mine tailings is
currently reaching a tributary of the SPR near Cananea City, resulting in the contamination of the SPR with heavy metals and
sulfates in water and sediments. This study documents the accumulation and distribution of heavy metals in surface water along
a segment of the SPR from 1993 to 2005. Total concentrations of Cd, Cu, Fe, Mn, Pb, and Zn in surface waters are above maximum
permissible levels in sampling sites near mine tailing deposits. Nevertheless, a significant decrease in the Fe and SO4
2− in surface water (SO4
2−: 7,180–460.39 mg/L; Fe: 1,600–9.51 mg/L) as well as a gradual decrease in the heavy and transition metal content were observed
during the period from 1994 to 2005. Approximately 2.3 km downstream of the mine tailings, the heavy metal content of the
water drops quickly following an increase in pH values due to the discharging of wastewater into the river. The attenuation
of the heavy metal content in surface waters is related to stream sediment precipitation (accompanied by metal coprecipitation
and sorption) and water dilution. Determining the heavy metal concentration led to the conclusion that the Cananea mining
area and the San Pedro River are ecosystems that are impacted by the mining industry and by untreated wastewater discharges
arising from the city of Cananea (Sonora, Mexico). 相似文献