Flexural and gravity modelling of the Mérida Andes and Barinas–Apure Basin, Western Venezuela     

Luis Chacín  María I. Jcome  Carlos Izarra 《Tectonophysics》2005,405(1-4):155-167

The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction.  相似文献   

Soil liquefaction during the Arequipa Mw 8.4, June 23, 2001 earthquake, southern coastal Peru   总被引：1，自引：0，他引：1  

Franck A. Audemard M.  Juan Carlos Gmez  Hernando J. Tavera  Nuris Orihuela G. 《Engineering Geology》2005,78(3-4):237-255

The Arequipa June 23, 2001, earthquake with a moment magnitude of Mw 8.4 struck southern Peru, northern Chile and western Bolivia. This shallow (29 km deep) interplate event, occurring in the coupled zone of the Nazca subduction next to the southeast of the subducting Nazca ridge, triggered very localized but widely outspread soil liquefaction. Although sand blows and lateral spreading of river banks and road bridge abutments were observed 390 km away from the epicenter in the southeast direction (nearing the town of Tacna, close to the Chile border), liquefaction features were only observed in major river valleys and delta and coastal plains in the meizoseismal area. This was strongly controlled by the aridity along the coastal strip of Southern Peru. From the sand blow distribution along the coastal area, a first relationship of isolated sand blow diameter versus epicentral distance for a single event is ever proposed. The most significant outcome from this liquefaction field reconnaissance is that energy propagation during the main June 23, 2001, event is further supported by the distribution and size of the isolated sand blows in the meizoseismal area. The sand blows are larger to the southeast of the epicenter than its northwestern equivalents. This can be stated in other words as well. The area affected by liquefaction to the northwest is less spread out than to the southeast. Implications of these results in future paleoliquefaction investigations for earthquake magnitude and epicentral determinations are extremely important. In cases of highly asymmetrical distribution of liquefaction features such as this one, where rupture propagation tends to be mono-directional, it can be reliably determined an epicentral distance (between earthquake and liquefaction evidence) and an earthquake magnitude only if the largest sand blow is found. Therefore, magnitude estimation using this uneven liquefaction occurrence will surely lead to underrating if only the shortest side of the meizoseismal area is unluckily studied, which can eventually be the only part exhibiting liquefaction evidence, depending on the earthquake location and the distribution of liquefaction-prone environments.  相似文献   

Kamafugitic diatremes: their textures and field relationships with examples from the Goiás alkaline province, Brazil     

Tereza Cristina Junqueira-Brod  Jos Carlos Gaspar  Jos Affonso Brod  Camilla Vasconcelos Kafino 《Journal of South American Earth Sciences》2005,18(3-4):337-353

Kamafugitic rocks intruded the Precambrian basement and Phanerozoic sediments at the northeast border of the Paraná basin as part of the Late Cretaceous Goiás alkaline province (GAP). Plutonic complexes dominate the north of the province, whereas lavas and pyroclastic rocks prevail in the south. The central GAP is characterized by kamafugitic diatremes, which may crop out continuously for up to 850 m and consist of a central breccia body, surrounded and overlain by lava flows and crosscut by dykes. The breccias contain some special spheroidal juvenile fragments—namely, accretionary and armored lapilli, frozen droplets, spinning droplets, and wrapped fragments—whose textural and mineralogical aspects are described in detail. Irregularly shaped tuff pockets that occur within the breccias contain textures and structures similar to those of subaerial surge deposits and formed in confined, high gas to solid+liquid ratio domains in the conduit. Diatreme emplacement affected the country rock through thermal metamorphism, development of columnar jointing, and formation of peperite-like mixtures. There is no evidence of phreatomagmatic activity in the diatremes, and CO₂, rather than H₂O, seems to have been the major volatile component of the kamafugitic magmas. This finding implies that features such as accretionary lapilli and peperites are not exclusively associated with H₂O-dominated processes.  相似文献   

Long-term deforestation in NW Spain: linking the Holocene fire history to vegetation change and human activities     

Joeri Kaal  Yolanda Carrión Marco  Eleni Asouti  Maria Martín Seijo  Antonio Martínez Cortizas  Manuela Costa Casáis  Felipe Criado Boado 《Quaternary Science Reviews》2011,30(1-2):161-175

The Holocene fire regime is thought to have had a key role in deforestation and shrubland expansion in Galicia (NW Spain) but the contribution of past societies to vegetation burning remains poorly understood. This may be, in part, due to the fact that detailed fire records from areas in close proximity to archaeological sites are scarce. To fill this gap, we performed charcoal analysis in five colluvial soils from an archaeological area (Campo Lameiro) and compared the results to earlier studies from this area and palaeo-ecological literature from NW Spain. This analysis allowed for the reconstruction of the vegetation and fire dynamics in the area during the last ca 11 000 yrs. In the Early Holocene, Fabaceae and Betula sp. were dominant in the charcoal record. Quercus sp. started to replace these species around 10 000 cal BP, forming a deciduous forest that prevailed during the Holocene Thermal Maximum until ～5500 cal BP. Following that, several cycles of potentially fire-induced forest regression with subsequent incomplete recovery eventually led to the formation of an open landscape dominated by shrubs (Erica sp. and Fabaceae). Major episodes of forest regression were (1) ～5500–5000 cal BP, which marks the mid-Holocene cooling after the Holocene Thermal Maximum, but also the period during which agropastoral activities in NW Spain became widespread, and (2) ～2000–1500 cal BP, which corresponds roughly to the end of the Roman Warm Period and the transition from the Roman to the Germanic period. The low degree of chronological precision, which is inherent in fire history reconstructions from colluvial soils, made it impossible to distinguish climatic from human-induced fires. Nonetheless, the abundance of synanthropic pollen indicators (e.g. Plantago lanceolata and Urtica dioica) since at least ～6000 cal BP strongly suggests that humans used fire to generate and maintain pasture.  相似文献   

Application of Distributed Temperature Sensing for coupled mapping of sedimentation processes and spatio‐temporal variability of groundwater discharge in soft‐bedded streams          下载免费PDF全文

Eva Sebok  Carlos Duque  Peter Engesgaard  Eva Boegh 《水文研究》2015,29(15):3408-3422

The delineation of groundwater discharge areas based on Distributed Temperature Sensing (DTS) data of the streambed can be difficult in soft‐bedded streams where sedimentation and scouring processes constantly change the position of the fibre optic cable relative to the streambed. Deposition‐induced temperature anomalies resemble the signal of groundwater discharge while scouring will cause the cable to float in the water column and measure stream water temperatures. DTS applied in a looped layout with nine fibre optic cable rows in a 70 × 5 m section of a soft‐bedded stream made it possible to detect variability in streambed temperatures between October 2011 and January 2012. Detailed monthly streambed elevation surveys were carried out to monitor the position of the fibre optic cable relative to the streambed and to quantify the effect of sedimentation processes on streambed temperatures. Based on the simultaneous interpretation of streambed temperature and elevation data, a method is proposed to delineate potential high‐groundwater discharge areas and identify deposition‐induced temperature anomalies in soft‐bedded streams. Potential high‐discharge sites were detected using as metrics the daily minimum, maximum and mean streambed temperatures as well as the daily amplitude and standard deviation of temperatures. The identified potential high‐discharge areas were mostly located near the channel banks, also showing temporal variability because of the scouring and redistribution of streambed sediments, leading to the relocation of pool‐riffle sequences. This study also shows that sediment deposits of 0.1 m thickness already resulted in an increase in daily minimum streambed temperatures and decrease in daily amplitude and standard deviation. Scouring sites showed lower daily minimum streambed temperatures and higher daily amplitude and standard deviation compared with areas without sedimentation and scouring. As a limitation of the approach, groundwater discharge occurring at depositional and scouring areas cannot be identified by the metrics applied. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Throughfall patterns in sugarcane and riparian forest: understanding the effect of sugarcane age and land use conversion          下载免费PDF全文

Robson Willians da Costa Silva  Luiz Felippe Salemi  Rafael Pires Fernandes  Tatiana Morgan Berteli de Andrade  Jorge Marcos de Moraes  Plínio Barbosa de Camargo  Luiz Antonio Martinelli 《水文研究》2016,30(15):2579-2589

Sugarcane is an annual crop with a dynamic canopy that changes over time mainly because of genetic adaptation. There is uncertainty about the temporal trends of throughfall (TF) in this important commercial crop. In the present paper, we used troughs to measure TF in a third and fourth ratoon and subsequently in a fourth and fifth ratoon. Additional measurements were carried out in an adjacent riparian forest. There were no significant differences between cycles of sugarcane, growth phases and riparian forest. The TF results for ratoon crop and riparian forest in 2011/2012 were 76% and 79.5% of gross rainfall, respectively, while in 2012/2013, they were 79% and 78%, respectively. However, TF was remarkably lower in the riparian forest relative to ratoon from the second half of the culm formation and elongation phase (280 days after harvest) until harvest. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Dynamics of the January 2013–June 2014 explosive-effusive episode in the eruption of Volcán de Colima,México: insights from seismic and video monitoring     

Vyacheslav M. Zobin  Raúl Arámbula  Mauricio Bretón  Gabriel Reyes  Imelda Plascencia  Carlos Navarro  Armando Téllez  Arnoldo Campos  Miguel González  Zoraida León  Alejandro Martínez  Carlos Ramírez 《Bulletin of Volcanology》2015,77(4):1-13

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Improved algorithms for generalized finite element simulations of three-dimensional hydraulic fracture propagation     

Nathan Shauer  Carlos Armando Duarte 《国际地质力学数值与分析法杂志》2019,43(18):2707-2742

This paper reports improvements to algorithms for the simulation of 3-D hydraulic fracturing with the Generalized Finite Element Method (GFEM). Three optimizations are presented and analyzed. First, an improved initial guess based on solving a 3-D elastic problem with the pressure from the previous step is shown to decrease the number of Newton iterations and increase robustness. Second, an improved methodology to find the time step that leads to fracture propagation is proposed and shown to decrease significantly the number of iterations. Third, reduced computational cost is observed by properly recycling the linear part of the coupled stiffness matrix. Two representative examples are used to analyze these improvements. Additionally, a methodology to include the leak-off term is presented and verified against asymptotic analytical solutions. Conservation of mass is shown to be well satisfied in all examples.  相似文献   

Numerical modelling of desiccation cracking     

Aruna L. Amarasiri  Jayantha K. Kodikara  Susanga Costa 《国际地质力学数值与分析法杂志》2011,35(1):82-96

The ability to model and predict the formation of desiccation cracks is potentially beneficial in many applications such as clay liner design, earth dam construction, and crop science, etc. However, most studies have focused on statistical analysis of crack patterns and qualitative study of contributing factors to crack development rather than prediction. Because it is exceedingly difficult to capture the nonlinear processes during desiccation in analytical modelling, most such models handle crack formation without considering variation of material properties with time, and are unattractive to use in realistic modelling. The data obtained from laboratory experiments on clay soil desiccating in moulds were used as a basis to develop a more refined model of desiccation cracking. In this study, the properties, such as matric suction, stiffness and tensile strength of soil, and base adhesion, could be expressed approximately as functions of moisture content. The initial conditions and the development of suction due to desiccation and the varying material properties were inputted to UDEC, a distinct element code, using its internal programming language FISH. The model was able to capture some essential physical aspects of crack evolution in soil contained in moulds with varying lengths, heights, and materials of construction. Extension of this methodology is potentially beneficial not only for modelling desiccation cracking in clay, but also in other systems with evolving material properties such as concrete structures and road pavements. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Lake sedimentary processes and vegetation changes over the last 45k cal a bp in the uplands of south-eastern Amazonia     

José Tasso Felix Guimarães  Prafulla Kumar Sahoo  Mariana Maha Jana Costa De Figueiredo  Karen da Silva Lopes  Markus Gastauer  Silvio Junio Ramos  Cecilio Frois Caldeira  Pedro Walfir M. Souza-Filho  Luiza Santos Reis  Marcio Sousa Da Silva  Paulo Rogenes Pontes  Renato Oliveira Da Silva Junior  Tarcísio Magevski Rodrigues 《第四纪科学杂志》2021,36(2):255-272

This study addresses paleoclimate influences in a southern Amazonia ecotone based on multiproxy records from lakes of the Carajás region during the last 45k cal a bp. Wet and cool environmental conditions marked the initial deposition in shallow depressions with detrital sediments and high weathering rates until 40k cal a bp. Concomitantly, forest and C3 canga plants, along with cool-adapted taxa, developed; however, short drier episodes enabled expansion of C4 plants and diagenetic formation of siderite. A massive event of siderite formation occurred approximately 30k cal a bp under strong drier conditions. Afterwards, wet and cool environmental conditions returned and persisted until the Last Glacial Maximum (LGM). The LGM was marked by lake-level lowstands and subaerial exposure. The transition from the LGM to the Holocene is marked by the onset of oscillations in temperature and humidity, with an expansion of forest and canga plants. Cool taxa were present for the last time in the Carajás region ~ 9.5–9k cal a bp. After 10k cal a bp , shallow lakes became upland swamps due to natural infilling processes, but the current vegetation types and structures of the plateaus were acquired only after 3k cal a bp under wetter climatic conditions.  相似文献