Paleomagnetism of the Morro Rock‐Islay Hill Complex as evidence for crustal block Rotations in central coastal California

Abstract

Paleomagnetic directions obtained from seven hypabyssal intrusive bodies of the late Oligocene Morro Rock‐Islay Hill igneous complex of central coastal California display clockwise deflections from the expected mid‐Tertiary direction. Both alternating field and thermal demagnetization experiments show these magnetization directions to be stable and essentially free from secondary overprinting. The mean paleomagnetic direction for all sites is rotated clockwise 49°. Moreover, different bodies show different amounts of clockwise rotation, with declinations ranging from 27° to 76°. These results can best be explained by the clockwise rotation of several local crustal blocks. It is suggested that these rotations are the result of right lateral shearing acting on extensionally loosened crustal fragments within a pull‐apart basin. This combines two previously suggested ideas, the ‘ball bearing’ rotation hypothesis of Beck (1976) and the pull‐apart basin process discussed by Crowell (1974), both of which appear to be important tectonic processes in the extensively sheared continental margin in California.