基于约束三维重力反演的南极大陆地壳密度结构研究

地壳密度是表征其物质组成以及结构特征的一个基本参数,是深入理解构造演化、地球动力学等问题的重要基础.本文利用最新发布的布格重力异常数据,以现有的三维S波速度模型、地表热流以及岩石学数据作为约束条件,采用引入深度加权函数的三维重力反演算法,获取了南极大陆三维精细密度结构模型.研究结果显示,南极大陆地壳尺度范围内密度异常在-0.25～0.20g·cm^-3之间.大致以南极横断山脉为界,受中生代与新生代多期拉张活动的影响,西南极上地壳密度异常以低值为主,与东南极克拉通的高密度异常呈现明显对比关系.通过对比发现,反演的密度结构与岩石圈强度信息存在较好的相关性.进一步分析西南极裂谷系统的密度结构与岩石圈强度,认为受新生代拉张活动和岩浆活动影响的局部地区除外,热沉降作用很可能使得地壳趋于冷却.此外,西南极的低密度特征延伸到南极横断山脉,结合现有的研究结果认为可能需要多种机制来解释南极横断山脉的隆升过程.

Crustal density structure of the Antarctic continent from constrained 3-D gravity inversion

Crustal density is a fundamental physical parameter that helps to reveal its composition and structure, and hence it is also significantly related to the tectonic evolution and geodynamics. Based on the latest complete Bouguer gravity anomalies and by the constrains of 3-D shear velocity model, surface heat flow and petrology data, the 3-D gravity inversion method, incorporating deep weighting function, has been used to obtain the refined density structure over the Antarctic continent. Our results show that the density anomalies changes from -0.25 g·cm^-3 to 0.20 g·cm^-3. Due to the multi-phase extensional tectonics in Mesozoic and Cenozoic, respectively, the low density anomalies dominates in the West Antarctica, whereas the East Antarctica is characterized by high values of density anomalies. By comparing with the variations of effective elastic thickness, the inverted density structure correlates well with the lithospheric strength. According to the mechanical strength and inverted density anomalies in the West Antarctic Rift System (WARS), our analysis find that except for the small-scale area affected by the Cenozoic extension and magmatic activity, the crustal thermal structure in the most part of WARS tends to be normal under the effect of thermal diffusion. Finally, the low density anomalies features in West Antarctica extend to beneath the Transantarcitc Mountains (TAMs), however, we hypothesize that a single rift mechanism seems not be used to explain the entire TAMs range.