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.

Palaeomagnetic insights into the evolution of Neotethyan oceanic crust in the eastern Mediterranean

“…The wider western flank is distinctly different, with north-south-trending dykes dipping at low angles of 25-45 ~ to the east. The lowangle orientations of these dykes were attributed by Verosub & Moores (1981) to listric normal faulting associated with the Kakopetria detachment, above a magma chamber at the active spreading axis. In this model, plate separation was at least partly accommodated by tectonic thinning of the upper crust during periods of amagmatic stretching.…”

“…Spreading took place either by steady-state processes (Allerton & Vine 1987), or by formation of discrete, ephemeral, sea-floor grabens (Varga & Moores 1985). The best documented and most distinctive graben runs through the Solea area to the north of the plutonic complex, above the 'Kakopetria Detachment' fault (Verosub & Moores 1981). This graben is interpreted as a fossil spreading axis (the 'Solea axis'; see Fig.…”

Tectonic setting of faulted Tertiary strata associated with the Catalina core complex in southern Arizona

“…The dozens of Cordilleran metamorphic core complexes distributed along some 2,500 km of the intermountain belt from southern Canada to northern Mexico (Coney, 1980b) are worthy of careful study because they constitute perhaps the most dramatic record of intraplate extension exposed to view anywhere on land. As close analogies can be drawn between the geometry of basement structures beneath rifted continental margins (LePichon and Sibuet, 1981) and the structural configuration of typical core complexes (Lister and others, 1986), geologic relationships similar to those observed in exposed core complexes may exist in the substratum beneath many sedimentary basins (Verosub and Moores, 1981).…”

Comment on “Tectonic rotations in extensional regimes and their paleomagnetic consequences for ocean basalts” by Kenneth L. Verosub and Eldridge M. Moores

“…Only a few deep penetration sites are thought to provide an adequate sampling of the earth's magnetic field during formation of the oceanic crust and thus data relevant to tectonic rotations [Harrison and Watkins, 1977;Hall and Robinson, 1979;Verosub and Moores, 1981]. Consequently, it is not entirely clear from such few determinations how pervasive and systematic are the directional deviations.…”

“…Consequently, it is not entirely clear from such few determinations how pervasive and systematic are the directional deviations. A major piece of evidence pertaining to this question not fully addressed by Verosub and Moores [1981] is the shape or skewness of seafloor spreading anomalies. A marine magnetic anomaly represents the aggregate effect of the magnetization of the entire oceanic crust over an area of several tens of square kilometers.…”