Passive margins

Passive margins (also known as rifted margins) mark the sites where continents have rifted apart to become separated by an ocean. Thus, passive margins consist of a seawards tapering wedge of continental crust that is dissected by faults, overlain by sedimentary basins and juxtaposed with oceanic crust. At many margins, magmatic products extruded during continental breakup occupy the outer parts of the margin. The juxtaposition with oceanic crust occurs at the so-called continent-ocean boundary. The coastal onshore parts of many passive margins are marked by mountainous escarpments of variable height. The formation of such mountains are highly debated. Our knowledge of rifted margins has increased greatly in the last few years as the higher quality of long-offset reflection seismic and other geophysical data has led to better imaging of rifted margins at depth. Due to their economical importance - they are among the world´s most prolific sources of oil and gas - passive margins are much studied and much debated among geologists. At the NGU, several research teams are dedicated to research on, and mapping of, different aspects of rifted margins.

Offshore, passive margins can be subdivided into domains that display different stages in the margin´s evolutionary history. These are the proximal domain and the necking, distal, outer and oceanic domains, respectively. The normal crustal thickness under the continents is just short of 40 km. In the proximal domain of rifted margins, the continental crystalline crust is moderately extended, and preserves a thickness between c. 40 and 20-30 km. Parts of a margin´s proximal domain may reside onshore but most of the rifted margin will reside under water as continental crust that is less than 30 km thick will subside below sea level. As crustal thinning increases seawards, the finite subsidence will also increase in that direction. Thus in the necking and distal domains, where the crust is extended by very large magnitudes, the total accommodation, thickness of sediments and/or the water depth will also be larger. In the distal parts of the mid-Norwegian margin, the sedimentary record reaches thicknesses of up to c. 15 km or more. The outer domain is located close to the line of breakup. Along a number of passive margins, this area is strongly affected by magmatic activity, with intrusions, lavas and lava deltas as evidence for active volcanism shortly before the establishment of a spreading ridge. In the oceanic domain, the crystalline crust is different from that under the continent and formed by seafloor spreading along a mid-ocean ridge. The crust in the oceanic domain normally consists of volcanic rocks and of mantle rocks exhumed in the footwalls of large extensional faults.

Studies of faults, sedimentary basins and magmatic products are important to understand the evolution of rifted margins. Normal faults are movement planes that develop in the rock mass as the crust is being extended. It is the faults that facilitate the thinning of crust in the seawards direction. However, the mechanism of faulting varies with position on the margin. Whereas the faults in the proximal domain sole out in the ductile middle crust, the faults in the necking domain cut deeper down in the crust and eventually into the mantle. In the distal margin, faults cut into the deep crust and upper mantle. The most important faults in the necking and distal domains are of very large magnitude, with displacements up to 20-30 km or more. At some rifted margins, the displacements are large enough to exhume mantle rocks to the seafloor in the footwalls of  extensional detachment faults. In such areas, the crystalline crust may become entirely removed by extension and altered mantle rocks provide the basement for overlying sediments.

The sedimentary record at rifted margins can be divided roughly into two parts, those deposited during crustal extension (that is, when faults where still active) and those deposited after active faulting, when the strongly thinned crust and mantle subsided over a large area. The first type of deposits normally reflect the geometry and magnitude of the active faults. This normally leads to rotation of strata and the formation of wedge-shaped depositional bodies that increase in thickness towards the active normal faults. In the proximal domain, arrays of so-called half-graben basins are bounded by steep normal faults with displacement of up to a few kilometers. Farther out on the margin, where the faults have larger displacements and lower dips, sediments are deposited with a different geometry. As faulting activity ceases in different parts of the margin, sediments will be deposited over larger and larger areas until most of the margin is draped by sediments.

In the outermost parts of the margin, where the crust splits and gives way to material from the mantle and from melts that rises from great depths, lava flows and sediments are deposited upon each other in large, wedge-shaped packages that dip seawards. Passive margins bear evidence for variable amounts of magmatic activity, but common to them all is that eventually, a spreading ridge forms, where new material is added to the crust in a continuous process called seafloor spreading. All the great oceans are results of this process and the spreading ridges are clearly visible on bathymetric maps as the rocks in the spreading ridge are hot and thus occupies an elevation some 2000 meters above the surrounding abyssal plain.