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Lithospheric density structure of the southern Central Andes constrained by 3D data-integrative gravity modelling
International Journal of Earth Sciences ( IF 2.3 ) Pub Date : 2020-12-21 , DOI: 10.1007/s00531-020-01962-1
Constanza Rodriguez Piceda , Magdalena Scheck Wenderoth , Maria Laura Gomez Dacal , Judith Bott , Claudia Beatriz Prezzi , Manfred R. Strecker

The southern Central Andes (SCA) (between 27° S and 40° S) is bordered to the west by the convergent margin between the continental South American Plate and the oceanic Nazca Plate. The subduction angle along this margin is variable, as is the deformation of the upper plate. Between 33° S and 35° S, the subduction angle of the Nazca plate increases from sub-horizontal (< 5°) in the north to relatively steep (~ 30°) in the south. The SCA contain inherited lithological and structural heterogeneities within the crust that have been reactivated and overprinted since the onset of subduction and associated Cenozoic deformation within the Andean orogen. The distribution of the deformation within the SCA has often been attributed to the variations in the subduction angle and the reactivation of these inherited heterogeneities. However, the possible influence that the thickness and composition of the continental crust have had on both short-term and long-term deformation of the SCA is yet to be thoroughly investigated. For our investigations, we have derived density distributions and thicknesses for various layers that make up the lithosphere and evaluated their relationships with tectonic events that occurred over the history of the Andean orogeny and, in particular, investigated the short- and long-term nature of the present-day deformation processes. We established a 3D model of lithosphere beneath the orogen and its foreland (29° S–39° S) that is consistent with currently available geological and geophysical data, including the gravity data. The modelled crustal configuration and density distribution reveal spatial relationships with different tectonic domains: the crystalline crust in the orogen (the magmatic arc and the main orogenic wedge) is thicker (~ 55 km) and less dense (~ 2900 kg/m3) than in the forearc (~ 35 km, ~ 2975 kg/m3) and foreland (~ 30 km, ~ 3000 kg/m3). Crustal thickening in the orogen probably occurred as a result of stacking of low-density domains, while density and thickness variations beneath the forearc and foreland most likely reflect differences in the tectonic evolution of each area following crustal accretion. No clear spatial relationship exists between the density distribution within the lithosphere and previously proposed boundaries of crustal terranes accreted during the early Paleozoic. Areas with ongoing deformation show a spatial correlation with those areas that have the highest topographic gradients and where there are abrupt changes in the average crustal-density contrast. This suggests that the short-term deformation within the interior of the Andean orogen and its foreland is fundamentally influenced by the crustal composition and the relative thickness of different crustal layers. A thicker, denser, and potentially stronger lithosphere beneath the northern part of the SCA foreland is interpreted to have favoured a strong coupling between the Nazca and South American plates, facilitating the development of a sub-horizontal slab.



中文翻译:

3D数据综合重力模型约束南部安第斯山脉中部岩石圈密度结构

南部中部安第斯山脉(SCA)(介于27°S和40°S之间)与南美西部大陆板块与大洋纳斯卡板块之间的会聚边缘接壤。沿着该边缘的俯冲角是可变的,上板的变形也是如此。在33°S和35°S之间,纳斯卡板块的俯冲角从北部的亚水平(<5°)到南部的相对陡峭(〜30°)。SCA包含地壳内继承的岩性和结构异质性,自从俯冲和安第斯造山带内部的新生代变形开始以来,这种异质性已被重新激活和套印。SCA内的变形分布通常归因于俯冲角的变化和这些遗传异质性的重新激活。然而,大陆壳的厚度和成分对SCA短期和长期变形的可能影响尚待深入研究。在我们的调查中,我们得出了组成岩石圈的各层的密度分布和厚度,并评估了它们与安第斯造山运动历史上发生的构造事件的关系,特别是研究了该岩层的短期和长期性质。当前的变形过程。我们在造山带及其前陆(29°S–39°S)下建立了岩石圈的3D模型,该模型与当前可用的地质和地球物理数据(包括重力数据)相一致。建模的地壳构造和密度分布揭示了不同构造域的空间关系:3)比在弧前(〜35公里〜2975公斤/米3)和前陆(〜30公里,〜3000千克/米3)。造山带中的地壳增厚可能是由于低密度区域的堆积而造成的,而前臂和前陆下方的密度和厚度变化最有可能反映了地壳增生后每个区域构造演化的差异。在岩石圈内的密度分布与早古生代期间积聚的地壳边界之间没有明确的空间关系。正在进行变形的区域与那些具有最高地形梯度且平均地壳密度对比度突然变化的区域显示出空间相关性。这表明,安第斯造山带及其前陆内部的短期变形从根本上受到地壳成分和不同地壳层相对厚度的影响。SCA前陆北部下方较厚,更密,甚至可能更坚固的岩石圈被认为有利于纳斯卡和南美板块之间的强耦合,从而促进了亚水平平板的发育。

更新日期:2020-12-22
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