Intraplate seismicity, reactivation of preexisting zones of weakness, alkaline magmatism, and other tectonism postdating continental fragmentation

Abstract

The distribution of intraplate earthquakes and of igneous rocks postdating continental rifting is summarized and placed into a plate tectonic framework for the following continental areas: eastern and central North America, Africa, Australia, Brazil, Greenland, Antarctica, Norway, Spitsbergen, India, and the margins of the Red Sea and Gulf of Aden. In continents, intraplate earthquakes tend to be concentrated along preexisting zones of weakness within areas affected by the youngest major orogenesis that predates the opening of the present oceans. Many preexisting zones of weakness (including fault zones, suture zones, failed rifts, and other tectonic boundaries), particularly those near continental margins, were reactivated during the early stages of continental separation. In contrast, intraplate shocks rarely occur within the older oceanic lithosphere or within the interiors of ancient cratonic blocks of the continents. In several continental areas, rocks and tectonic features postdating the opening of present‐day oceans, including carbonatites, kimberlites, other alkalic rocks, mafic dikes, and ring dikes, as well as some of the largest intraplate shocks, seem to be located preferentially along old zones of weakness near the ends of major oceanic transform faults that were active in the early opening of adjacent oceans. In several places, alkaline magmatism and earthquakes extend several hundred kilometers inland from the ends of oceanic transform faults (but not necessarily with the same strike as the transform fault). Major preexisting zones of weakness that are oriented subparallel to the directions of relative continental separation appear to control the locations of transform faults that develop in a new ocean. In some instances, alkaline magmatism persisted along reactivated features of this type for as long as 100 m.y. after the initial stages of continental fragmentation. Most kimberlites in South Africa seem to have been emplaced along preexisting zones of weakness that were reactivated during the early opening of the South Atlantic. The type of intraplate magmatism appears to be related to the thickness of the lithosphere. Unlike oceanic transform faults where large horizontal movements have occurred, reactivated zones of weakness in continents usually appear to have been the sites of only relatively small displacement. Seismic activity and alkaline magmatism may be controlled by deep fractures that penetrated the entire lithosphere to tap asthenospheric sources of magma. Seismic activity along these zones seems to occur in response to the present‐day stress regime, which is not necessarily the same as that which was active during the emplacement of the alkaline rocks. Other intraplate shocks are concentrated along old zones of weakness that are subparallel to continental margins. Such shocks are found in the Appalachians, northeastern and northern Greenland, Norway, Great Britain, Spitsbergen, northern Canada, and Australia. These zones of weakness were also reactivated during continental separation...

Lithosphere, asthenosphere, and perisphere

“…There is evidence that lithosphere splits or pulls apart [Sykes, 1978;Bailey, 1992], rather than stretching, thinning, eroding, or delaminating. The rheology of the overlying crust and the underlying TBL is quite different, and these elements may stretch and thin.…”

Intraplate seismicity in the western Bohemian Massif (central Europe): A possible correlation with a paleoplate junction

“…1b)? First, there is a preexisting zone of weakness (cf., in other regions, e.g., Sykes, 1978;Talwani and Rajendran, 1991) located on the crossing of two major active faults -the KHF and the MLF. The KHF dominates in morphology separating the uplifted Krušné hory Mts.…”

Spatial variation of shear‐wave splitting across an active fault and its implication for stress accumulation mechanism of inland earthquakes: The Atotsugawa fault case

“…[2] Stress accumulation models of inland faults have been proposed by several studies [e.g., Sykes, 1978;Campbell, 1978]. Iio and Kobayashi [2002] classified these models into two contrastive groups.…”