Abstract: Inversion and the development of the Murdoch Block, Silver Pit Basin, Southern North Sea

HOOPER, ROBERT J., MARK BOYD, CAROL GUNN, PETE HILL, and PHIL SALVADOR

The Silver Pit Basin, one of a series of NE-SW trending basins in the UK southern North Sea, experienced a long and protracted deformational history beginning in the late Paleozoic and culminating in contraction during the Tertiary Alpine events. Of particular importance is the influence on later deformation provided by structures developed during the Caledonian and Variscan orogenic cycles. The subsequent late Paleozoic and Mesozoic tectonic history of the area has been, in large part, controlled by the reactivation of these fundamental elements.

The most prominent sub-salt structural feature in the Silver Pit Basin is the Murdoch Block, a large "horst-like" feature that is host to two Carboniferous gas fields, Murdoch and Caister. The Murdoch Block was previously interpreted as a "pop-up" structure set in an area cross-cut by wrench-grabens. Our evaluation, which is based on the interpretation of a semi-regional 3D seismic survey (~ 1600 km²), provides an alternative explanation. The Murdoch block is not a simple "pop-up" structure; the sides of the block have been formed at different times by different deformational events. The NE-side of the block is defined by an inversion related contractional fault that extends for many tens of kilometers across the Silver Pit Basin. In the area of the Murdoch field, this fault formed during the late Cretaceous/early Tertiary contractional events. In other places, however, the Murdoch boundary fault links together segments of Carboniferous and Mesozoic extensional faults. The fault is thus a complex composite of reactivated extensional faults and newly formed contractional faults. The NW side of the block is defined by an inverted late Carboniferous extensional fault. The SW sideboard is a series of late Carboniferous NW-SE trending graben, that belong to a system of similar trending graben found throughout the area. The main Murdoch block, while having the appearance of a "pop-up", is actually the high point on a much wider area affected by inversion.

The more detailed interpretation possible with 3D seismic data provides no evidence for large-scale strike-slip motion within the study area; all the main structures can be created by oblique extension and subsequent oblique inversion. Minor strike-slip movement probably occurs along some faults simply as a kinematic necessity within the extensional and inversional deformation.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria