THE CRETACEOUS-TERTIARY DEFORMATION OF THE LHASA BLOCK AND ITS IMPLICATIONS FOR CRUSTAL THICKENING IN TIBET
THE CRETACEOUS-TERTIARY DEFORMATION OF THE LHASA BLOCK AND ITS IMPLICATIONS FOR CRUSTAL THICKENING IN TIBET
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DOI：
10.1029/tc005i001p00001
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发表时间：
1986-02-01
期刊：
影响因子：
4.2
通讯作者：
SEARLE, M
中科院分区：
文献类型：
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作者：
ENGLAND, P;SEARLE, M
印度板块北缘和桑坡缝合带的第三纪强烈变形与拉萨地块内部的类似变形不相匹配，但地块南缘在整个晚白垩世都是大规模侵入活动的场所(冈底斯或穿越什马拉雅岩基)，因此可能比周围弱。这对任何用地壳弥漫增厚来解释西藏变形的方法都是一个问题。然而，有证据表明，拉萨地块在白垩纪末期无论在高程上还是在岩浆类型上都是安第斯边缘，我们认为与地壳抬升有关的浮力足以抑制拉萨地块南部的挤压变形。通过将大陆岩石圈视为含有均衡补偿高差的粘性薄片，对这一观点进行了定量检验。当受到代表印度和亚洲碰撞的边界条件时，该板块通过在与青藏高原大小相当的区域内的扩散变形来适应会聚；在没有海拔对比的情况下，该变形在该区域上由大约100%的增厚应变组成。拉萨地块碰撞前高差为3000m时，其碰撞后增厚约为40%，而不是100%；较小的高差将导致较大的增厚，反之亦然。这些计算结果对藏北地区的地壳增厚模式有一定的意义：由于现有证据表明，南亚中生代早期构造活动的地形表达在印亚碰撞前已被抹去，因此我们预计，在第三纪期间，有广泛且相对均匀的地壳缩短从拉萨地块向北迁移。
The strong Tertiary deformation of the northern margin of the Indian plate and of the Tsangpo suture zone is not matched by comparable deformation within the Lhasa block, yet the southern margin of the block was the site of large‐scale intrusive activity (the Gangdese, or Transhimalayan, batholith) throughout the late Cretaceous and was, therefore, presumably weaker than its surroundings. This poses a problem for any approach that interprets the deformation of Tibet in terms of diffuse crustal thickening. However, there is evidence that the Lhasa block was, in elevation as well as in magma type, an Andean margin by the end of the Cretaceous, and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern Lhasa block. This suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast. When subjected to boundary conditions representing the collision between India and Asia, the sheet accommodates convergence by diffuse deformation over a region comparable in size with the Tibetan plateau; in the absence of an elevation contrast this deformation consists of approximately 100% thickening strain over the region. A precollision elevation contrast of 3000 m in the Lhasa block would have resulted in its experiencing roughly 40%, rather than 100%, postcollisional thickening; smaller elevation contrasts would have resulted in greater thickening, and vice versa. These calculations have implications for the mode of crustal thickening in northern Tibet: As the available evidence suggests that the topographic expression of the earlier Mesozoic tectonic activity in southern Asia was erased before the India‐Asia collision, we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the Lhasa block during the Tertiary.