Tectonic rotations in extensional regimes and their paleomagnetic consequences for oceanic basalts

Abstract

Paleomagnetic samples of many Deep‐Sea Drilling Project (DSDP) basalts have shallower inclinations than would be inferred from the paleolatitude at which the basalts were extruded at mid‐ocean ridges. Rather than arising from peculiarities of the geomagnetic field, the anomalous magnetic directions may result from tectonic rotations along the listric normal faults which often characterize extensional tectonic regimes. Several lines of evidence support this conclusion: observations of the Basin and Range Province, an extensional tectonic regime occurring in continental crust; observations of the Troodos complex, Cyprus, an oceanic extensional tectonic regime now exposed on land; and analysis of possible fault patterns associated with the magma chambers postulated to occur at some mid‐oceanic ridges. The tectonic rotations resulting from movement along the listric normal faults occur about a horizontal axis generally perpendicular to the extension direction and typically amount to 30° to 50°, although rotations as large as 70° to 90° are not uncommon. The paleomagnetic consequences of these rotations can be calculated for DSDP basalts if the original strike and latitude of a ridge are known. At four DSDP sites for which adequate data are available, namely sites 319, 332, 410, and 417, rotations ranging from 50° to 70° are required to explain the paleomagnetic results. If the original strike and location of the ridge are not known, as is the case for most of the older parts of the Pacific Ocean, the paleomagnetic data and the inferred rotation determine a relationship between possible ridge strikes and locations. This information, combined with a similar but independent relationship derived from the marine magnetic anomalies, puts severe constraints on the original strike and location of the ridge.