白垩纪中期地球磁场与全球地质现象

摘要

白垩纪超静磁带(CNS)是指白垩纪中期约40 Ma内(Aptian-Santonian, 121～83 Ma)地球磁场没有出现倒转的异常现象.这一现象引起地球学界的极大关注,原因在于它与白垩纪中期(124～90 Ma)其他一系列事件在时间上非常吻合,包括洋底扩张速率的快速增加和洋底高原、海山链、大型火成岩省等生产速率的快速增加.过去20多年许多研究强调所有这些现象与下地幔对流的联系.近期研究的一个重要的进展在于把地球磁场的变化(反转频率和古强度)与许多重要的地质和地球物理过程结合起来,如周期性地幔对流、地幔柱活动、全球热流、真极移、大型火成岩省的产生和生物群集绝灭.文中回顾了白垩纪地球磁场研究以及与白垩纪中期所发生的全球性的地质事件之间的可能存在的关联,并讨论在这一重要研究领域内未来研究的发展方向.%The mid-Cretaceous(124～90 Ma)was a period of great unrest in geologic history. In the mid-Cretaceous the breakup of the supercontinent Gondwana into separate continents was still underway and the Tethys Ocean was closing. During this time several mountain ranges were formed, such as Californias Sierra Nevada, the Rocky mountains in the western USA, the Andes mountains of western South America, and the European Alps. The Gondwana breakup caused South America and Australia-Antarctic-New Zealand to begin advancing into Pacific from east and west, respectively, and slab penetration events began below these margins. Rates of continental drift(i.e. sea-floor spreading rate)were about three times as great as they are now. During the mid-Cretaceous, large scale plutonism occurred in eastern Asia, western Antarctic, New Zealand, the southern Andes, and western North America. This is best documented in western North America where more than 50% of the exposed batholiths are dated between 115 and 85 Ma. The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Sea levels were up to 250 m higher than present day levels. Geological evidence also points towards an unusually high rate of volcanic activity in the mid-Cretaceous. Large volumes of magmatic material flowed to Earths surface from the mantle, and not just from the typical seafloor spreading. The "large igneous provinces" or LIPs made a very important contribution to the lithosphere during the mid-Cretaceous. Several LIPs such as the Ontong Java Plateau(120 Ma), the main portion of the Kerguelen Plateau(115～80 Ma), the Madagascar LIP(84～91Ma), the High Arctic LIP(85～95 Ma), and the Caribbean LIP(88～91 Ma)were all formed during this time. Carbon dioxide emissions released by the Cretaceous LIPs may have triggered two to seven oceanic anoxic events(OAEs) and led to a greenhouse effect that raised the Earths surface temperature by roughly 10 degrees Celsius, reduced temperature gradients from the Equator to the poles, and resulted in general absence of polar ice caps and oceans much warmer than todays. The mid-Cretaceous was also a time of rapid radiation and turnover in the marine plankton, benthic foraminifera, and terrestrial plants. This evolutionary activity transformed many of these groups on an ocean-wide basis. The reorganization of the global biosphere during the mid-Cretaceous paralleled the marked changes in the ocean-climate system, suggesting a causal relationship between biotic evolution and environments. Other geologic anomalies associated with the mid-Cretaceous thermal activity include the extremely high development of the Earths diamond bearing kimberlites in mid-East and South Africa and markedly reduced(less radiogenic)seawater strontium isotope ratio(87Sr/86Sr). A key piece in the mid-Cretaceous geologic phenomena is the Cretaceous Normal Superchron. This is a period in which Earths magnetic field was uncharacteristically steady that did not switch from normal to reversed polarity for about 40 million years(Aptian to Santonian, 121～83 Ma).It is remarkable that the onset time of the Cretaceous Normal Superchron coincides closely with the mid-Cretaceous pulse of volcanism. Some researchers argue that, during mid-Cretaceous when magnetic reversal frequency dropped to zero, true polar wander rates were also higher, and remagnetization of rock formations was more widespread in many regions of the world. These remarkable geological and geophysical signals in the mid-Cretaceous have excited great interest in the geoscience community, leading many to suggest a connection between all of these phenomena and deep-mantle convection. In the past two decades, a number of attempts have addressed the connections between all these phenomena and deep-mantle convection. Significant progress has been made to tie changes in the geomagnetic field(reversal frequency and paleointensity)to many important geological and geophysical processes, such as episodic mantle convection, mantle plume activity, global heat flow, true polar wander, production of large igneous provinces, and mass extinctions. This paper presents an overview of the Earths magnetic field in mid-Cretaceous and its possible connections with these mid-Cretaceous global geologic events. The present debates on the magnetic field of the Earth in mid-Cretaceous, especially about its intensity during the Cretaceous Normal Superchron, are discussed in detail. This paper also discusses directions of future research in this important area, revealing how much work still remains to be done.