期刊
JOURNAL OF SOUTH AMERICAN EARTH SCIENCES
卷 37, 期 -, 页码 60-76出版社
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jsames.2012.02.004
关键词
Brazil; Borborema Province; Borborema Plateau; Bouguer admittance; Combined surface and buried loadings; Magmatic underplating
资金
- Brazilian National Council for Scientific and Technological Development (CNPq) via Projeto Lito-borborema [501940/2003-0]
- CNPq [301568/2008-1]
In the Borborema Province (BP) - northeastern Brazil - two important Cenozoic events occurred at the surface: the Macau magmatism and the Borborema Plateau epeirogenesis. To obtain appropriated-scale geophysical data to explain the deep origins of these two events, different gravimetric/elevation databases were integrated with new surveys. Bouguer admittance estimates reveal that isostatic condition of the BP, especially in the Borborema Plateau, can be explained using elastic models to the lithosphere only if surface and buried loadings are combined. If the buried load is applied in the base of the crust, the ratio between buried and surface weights is circa 15 for a lithosphere with effective elastic thickness around 15 km and crust thickness around 33 km. From an interpretative viewpoint of the buried load, it is assumed that the lower crust under the Borborema Plateau might have an anomalous high value of density. Magmatic underplating might explain this fact as well as the observed surface magmatism and epeirogenesis. Crustal thickening of about 4 km under the Borborema Plateau and intracrustal seismic velocity discontinuity with high V-p/V-s, ratio are geophysical facts consistent with magmatic underplating. However, the surface magmatism presents low volume and mainly alkaline composition - facts that are not entirely consistent with the hypothesis of magmatic underplating. Regardless the validity of this hypothesis. Cenozoic-to-present events in BP might be somewhat associated with imbalances in lithosphere-asthenospheric mantle and/or crust-lithospheric mantle systems. The existence of free-air anomalies showing no null integral over area and of an expressive positive geoid anomaly are geophysical evidences of these imbalances. Possibly, the Borborema Plateau is still suffering epeirogenesis. Post-depositional deformation found in Barreiras Formation strata, Late Quaternay fault reactivations, and AFT thermochronology analysis suggesting the existence of a cooling stage between 20 and 0 Ma might be geologic evidences of the continued action of epeirogenesis until the present. In addition, the relatively high level of the present intraplate seismicity recorded in several regions of the BP is another unequivocal geophysical evidence that the crust of the province is still submitted to accommodation processes. (C) 2012 Elsevier Ltd. All rights reserved.
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