LLSVPs与全球热点空间叠置性再分析

Reanalysis of the spatial correlations between LLSVPs and the global hotspots

地震层析成像显示在非洲和太平洋之下的核幔边界处(~2800km)存在两个大型低剪切波速省(LLSVPs),分别被命名为Tuzo和Jason,指示温度或成分的异常。前人通过研究发现,大火成岩省喷发的古位置以及现今活动热点位置多位于LLSVPs边界±10°附近。由此,基于空间相关性,将LLSVPs狭窄的边界视为地幔热柱生成区,广泛用于解释地幔柱与热点的成因。但是,将一个深度2800km的地球物理异常与地表热点建立联系,仍存在诸多不确定性。而且LLSVPs作为全球尺度的异常区,在地表的映射范围巨大,不可避免地与大量热点存在天然的空间叠置性,这种空间关联的程度以及是否具有成因联系仍然未知。本文考虑随机状态下热点落入LLSVPs范围的概率,对全球热点与LLSVPs边界重新进行空间分析,判断两者之间的空间叠置性。统计结果显示,全球热点与LLSVPs边界的空间相关性不及预期,Tuzo和Jason之间存在较大差异,Jason与热点缺乏空间位置相关性,而Tuzo与热点空间相关性较强。热点的分布与成因可能并不完全受控于LLSVPs。研究结果对普遍认为的LLSVPs与热点分布之间的强相关性提出质疑,在具体的热点和地幔柱成因研究中,不能盲目认为热点与LLSVPs有关,而需要更加审慎的关注地质事实本身。

Seismic tomographic images show that there are two large low shear-wave velocity provinces(LLSVPs)beneath Africa and Pacific Ocean at the Earth's core-mantle boundary,named Tuzo and Jason,respectively.It implies the anomalies in temperature or composition of the deep mantle.Previous studies have found that the reconstructed eruption sites of large igneous provinces and current active hotspots are mostly located near the LLSVPs margins with a tolerance of±10°.Based on spatial correlations the narrow margins of LLSVPs are regarded as the mantle plume generation zone and is widely used to explain the genesis of mantle plumes and hotspots.However,there are still many uncertainties in connecting the surface hotspots to a deep geophysical anomaly 2800km beneath.Moreover,LLSVPs,as a global-scale anomalies,are huge in extent if projected on the Earth's surface,and will inevitably be overlapping with a large number of hotspots.Whether the two components are genetically connected remains unknown.Here,we consider the random probability of a hotspot falling into the LLSVPs ranges,and reanalyze the spatial correlation between LLSVPs'margins and global hotpots to determine the confidence level of the potential correlation.The statistical results show that the spatial correlation between global hotspots and LLSVPs'margins is not significant as proposed.There are great differences between Tuzo and Jason in their relationships to the hotspots.The Tuzo is considered well correlated with hotspots,while the Jason don't.Thus,the distribution and genesis of hotspots may not be fully controlled by LLSVPs.The result challenges the generally accepted strong correlation between LLSVPs and hotspots.We should not take it for granted that hot spots are related to LLSVPs,but should be more careful in study of the genesis of hot spots and mantle plumes,with more attention to the geological facts and observations.