Open AccessPalaeomagnetic and Rock Magnetic Studies on the Cenozoic Basalts from Western Argentina
Author(s) -
Creer K. M.,
Valencio D. A.
Publication year - 1970
Publication title -
geophysical journal of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0016-8009
DOI - 10.1111/j.1365-246x.1970.tb06037.x
Subject(s) - cenozoic , basalt , geology , paleontology , magnetic anomaly , latitude , structural basin , geodesy
Summary Results are presented of a study of 234 samples of Cenozoic basalts from the extra‐Andean region of the Provinces of Mendoza and Neuquén in the Argentine Republic, between latitudes 34° S and 40° S. These basalts have been divided into seven groups of flows on the basis of their stratigraphy, ranging in age from the Miocene (Basalts I) to historic (Basalts VII). The TRM of Basalts V, VI and VII is normally polarized and it is concluded that they belong to the Brunhes geomagnetic epoch. The TRM directions of Basalt IV flows are neither parallel nor antiparallel to the axial dipole field. These differences in direction are significant and persistent after cleaning. It is therefore suggested that Basalt IV was extruded during a magnetic transition after the Brunhes geomagnetic epoch. Some Basalt III flows are normally polarized, some are reversely polerized and some are magnetized obliquely to the axial dipole field. Hence the time of extrusion of these flows may span the Jaramillo and/or Olduvai events within the Matuyama epoch. Basalt II flows, which are stratigraphically placed in the Upper Pliocene, are all reversely magnetized while Basalt I flows, some of which are said to be Miocene, exhibit both normal and reversed polarities. It is deduced from the palaeomagnetism of these Basalts that some modifications should be made to their stratigraphy and these deductions have been substantiated by radiometric studies described in the following paper. Estimates of the palaeosecular variation have been made for the Brunhes and Matuyama epochs and compared with other similar data from other parts of the world. Ore microscope observations do not reveal any obvious correlation between polarity and state of oxidation, but a relationship between stability and the degree of exsolution is observed. Electron microprobe studies reveal a high titanium content: the unexsolved titanomagnetites have between 50 and 75 per cent ulvöspinel. It is provisionally suggested that these characteristics of these basalts vary more in time than in geographical area at a given time.