PALEOMAGNETIC EVIDENCE OF VERTICAL AXIS BLOCK ROTATIONS FROM THE MESOZOIC OF NORTHERN CHILE

We present paleomagnetic results for three mesozoic formations from no rthern Chile: La Ternera Formation (upper Triassic), Quebrada Monardes Formation (Upper Jurassic); Cerrillos Formation (Upper Cretaceous). R esults from the Cerrillos are divided into eastern (Cuesta El Gao (CEG )) and western (Elisa De Bordo (EBD)) localities. Most specimens from La Ternera volcanic and sedimentary rocks are magnetically stable, as shown by alternating field and thermal demagnetization. More complicat ed but still reliable results were obtained from Quebrada Monardes red beds. Normal and reverse polarities are present in both units; means of both populations are antiparallel at 95% confidence. The Quebrada M onardes Formation also yields positive conglomerate and fold tests. Pa leomagnetic poles for La Ternera and Quebrada Monardes are 60.9S, 218. 3E (A95, 7.8-degrees), and 66.9S, 191.6E (A95, 12.7-degrees), respecti vely. Comparison with appropriate reference poles shows that this regi on of Chile has undergone about 25-degrees of clockwise rotation, with negligible latitudinal transport. Cerrillos CEG results are less reli able and possibly complicated by remagnetization during emplacement of early Tertiary intrusives. Most Cerrillos EDB specimens are stable, b ut marked increase in scatter upon unfolding suggests remagnetization. Results for the Cerrillos CEG locality, which is contiguous to the sa mpling area of the La Ternera and Quebrada Monardes formations, show o nly about half the rotation of those two units, suggesting that rotati on commenced after deposition of the Quebrada Monardes rocks in the La te Jurassic and was approximately half complete by the time Cerrillos CEG rocks acquired their magnetization. Cerrillos EDB results come fro m an area approximately 40 km to the west; these show roughly 45-degre es of clockwise rotation. Dispersion is very low between EDB sites, su ggesting that secular variation may not be completely averaged. Nevert heless, the great difference in direction between the two Cerrillos lo calities suggests that they lie in different structural blocks.