Magnetic Anomaly Map of Shatsky Rise and Its Implications for Oceanic Plateau Formation

摘要

Shatsky Rise oceanic plateau was emplaced during a period of frequent geomagnetic polarity reversals, allowing reconstruction of its tectonic evolution using magnetic anomalies. Prior studies mainly focused on identifying magnetic isochrons and encountered difficulties in tracing magnetic lineations over high relief. We complied a large magnetic data set over Shatsky Rise and its environs, using 5.5 x 10(6) data points from 96 geophysical cruises spanning 54 years. The long-time span and heterogeneity of component data sets made data merger a challenge. Contributions of internal and external fields, and spurious readings were removed during processing. A "backbone" method, using recent GPS-navigated data as a foundation, was developed to improve the coherency of the data set. The singular characteristic of the new magnetic anomaly map is that linear magnetic anomalies are ubiquitous. In nearly every place where data are dense enough to delineate anomaly trends, the plateau and surrounding crust are characterized by linear anomalies. Discordant anomalies in some areas imply complex tectonics related to triple junction migration and ridge reorientation. Tamu Massif apparently formed along a segment of Pacific-Farallon spreading ridge that rotated by 90 degrees as a triple junction migrated through the edifice. Ori Massif appears to have formed on the Pacific-Izanagi ridge between triple junctions. Shirshov Massif contains discordant lineations that may indicate a microplate. The pervasive occurrence of linear magnetic anomalies within Shatsky Rise implies that these volcanic edifices must have formed by spreading analogous to mid-ocean ridges that formed anomalously thick crust.