Non-volcanic passive margins

NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling.

NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.

NVPM are the result of rifting when a continent breaks up to form an ocean, producing transitional crust without volcanism.

Seismic reflection lines across passive margins show many structural features common to both VPM and NVPM, such as faulting and crustal thinning, with the primary contra-indicator for volcanism being the presence of continent-ward dipping reflectors.

In some cases, an extremely thick igneous underplating of a VPM will display similar P-wave velocity (7.2-7.8 km/s, but with a lower gradient).

The oceanic crust density is then further enhanced with gabbros and basalts and additionally contributes to the regional gravity trend.

Isostatic compensation and gravity anomalies result from balance between mass excess of the extra mantle beneath the thinned lithosphere and the overlying low-density crust.

Positive gravity anomalies result from the relatively low flexural strength of the lithosphere during the beginning of rifting.

As the passive margin matures, the crust and uppermost mantle become colder and stronger, so that the compensating deflection in the base of the lithosphere is broader than the actual rift.

Continued extension of the lithosphere will eventually lead to decompression melting of the mantle and the formation of a mid-ocean ridge.

In a simple shear model, a basin is stretched asymmetrically by a large scale detachment fault extending from the upper crust to the lower lithosphere and even asthenosphere.

[3] During the Late Jurassic-Early Cretaceous, tectonic extensional forces created a shallow angle east-dipping detachment fault.

Mantle material composed of peridotites is serpentinized by circulating seawater after it rises close enough to the upper crust due to its low density and isostatic forces.