科研之友
微信
新浪微博
Facebook
分享链接
摘要
Neoproterozoic orogenesis in the southern sector of the Southern Brasilia Belt was the result of subduction and terminal collision between the Socorro-Guaxupe Arc and Paranapanema Block to the west (present coordinates), and the passive margin along the southern portion of the western side of the Sao Francisco Craton to the east. We report the results of a thermochronologic study of the uppermost passive margin derived high-pressure granulite facies nappes of the Andrelandia Nappe Complex to better constrain the timing of exhumation of these nappes. Based on Ar-40/Ar-39 biotite plateau ages, the Tres Pontas-Varginha Nappe at the top of the Andrelandia Nappe Complex was the first nappe to be exhumed through similar to 300 degrees C during the interval 591 to 567 Ma; this was followed by exhumation of the underlying Carmo da Cachoeira Nappe through similar to 300 degrees C at circa 540 Ma. Initially, the Tres Pontas-Varginha Nappe cooled slowly at a rate of <1 degrees C Ma(-1) from similar to 800 degrees C at circa 648 Ma to similar to 750 degrees C at circa 590 Ma, but then the rate of cooling increased dramatically to >20 degrees C Ma(-1). This increase is attributed to tectonically-driven exhumation during orogenic collapse triggered by regional extension as a far-field effect of eastward subduction of the passive margin on the eastern side of the Sao Francisco Craton during accretion of a magmatic arc terrane associated with the formation of the Ribeira Belt to the east. The average rate of cooling in the underlying Carmo da Cachoeira Nappe was 4 to 5 degrees C Ma(-1). This slower rate reflects collapse of the nappe stack, which is likely to have led to faster exhumation of nappes higher in the stack. Exhumation of the Andrelandia Nappe through similar to 300 degrees C circa 50 Ma after exhumation in the correlative Carmo da Cachoeira Nappe demonstrates that the two nappes were exhumed separately. The delayed exhumation of the Andrelandia Nappe may be due to reworking during orogenesis associated with formation of the Ribeira Belt. The Andrelandia Nappe records cooling from the bottom up; this was likely a consequence of final emplacement on the colder Sao Francisco Craton.
- 出版日期2010-12
