Toward A Miocene Origin For The Cretaceous To Present Paleomagnetic Rotations In The Peruvian Cordillera
Abstract
The Andes form the longest non-collisional mountain belt on Earth. They are the widest at the Bolivian Orocline, around 19S, where a change occurs in the strike direction of the major structures that define the chain. Thirty years of Andean paleo- magnetic research has demonstrated a fairly coherent pattern of Cretaceous to Pliocene paleomagnetic rotations that generally vary in concert with the changing structural trends. Counterclockwise rotations are found North of the orocline where structures are northwest oriented, while south of the bend, where the regional fabric strikes north- northeast, rotation sense is mainly clockwise. While trying to understand the origin of this pattern and its link with the deformational history of the chain, a controversy has emerged regarding the length scale and timing of the block rotations. Thus, to better constrain the timing and extent of the rotations, we collected more than 700 cores in widely spread Neogene basins throughout central and northern Peru. Stepwise ther- mal and alternating field demagnetization demonstrate positive fold and reversals tests. Site mean directions show rotations ranging between -26 and +7. In the Subandean zone no rotation was recorded; however Cordilleran basins exhibit a complex pattern of rotations. Interestingly, where rotations were found in Cretaceous rocks, the same amount of rotation was registered in nearby Miocene rocks. Moreover, in one case, where Cretaceous rocks recorded no rotation, none was observed either in Miocene rocks. This suggests that, despite several episodes of deformation between the Creta- ceous to Present, only the latest one has provoked vertical axis block rotations. This appears true as well from preliminary results from sediments we recently collected near the coast at about 16S. These results corroborate with our previous work in the Ayacucho basin which demonstrated the occurrence of a fast vertical axis rotation at ca. 8 Ma that affected the western Peruvian Cordillera between 9 and 15S.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.2957R