Nonvolcanic margins

The occurrence of nonvolcanic margins (Fig. 7.34a) shows that extreme thinning and stretching of the crust is not necessarily accompanied by large-scale volcanism and melting. Nonvolcanic margins lack the large volume of extrusive and intrusive material that characterizes their volcanic counterparts. Instead, the crust that characterizes this type of margin may include highly faulted and extended continental lithosphere, oceanic lithosphere formed by very slow sea floor spreading, or continental crust intruded by magmatic bodies (Sayers et al., 2001). In addition, these margins may contain areas up to 100 km wide that are composed of exhumed, serpentinized upper mantle (Fig. 7.34b,c) (Pickup et al., 1996; Whitmarsh et al., 2001). Dipping reflectors in seismic profiles also occur within nonvolcanic margins. However, unlike in volcanic varieties, these reflectors may be preferentially tilted continentward and do not represent sequences of volcanic rock (Pickup et al., 1996). Some of these continentward-dipping reflectors represent detachment faults (Section 7.3) that formed during rifting (Boillot & Froitzheum, 2001).

Two end-member types of nonvolcanic margins have been identified on the basis of relationships preserved in the North Atlantic region (Louden & Chian, 1999). The first case is derived from the southern Iberia Abyssal Plain, Galicia Bank, and the west Greenland margins. In these margins rifting of the continent

Distance (km)
100-150km

Figure 7.34 (a) Map of the North Atlantic showing location of selected nonvolcanic margins. MAR, Mid-Atlantic Ridge. (b) Velocity model of the West Iberia margin and the Iberia Abyssal Plain (image provided by T. Minshull and modified from Minshull, 2002 with permission from Royal Society of London). Data are from Dean et al. (2000). The dashed lines mark the approximate edges of the ocean-continent transition zone. Velocities in km s~'. (c,d) Two end-member types of nonvolcanic margin (images provided by K. Louden and modified from Louden & Chian, 1999, with permission from the Royal Society of London). PR, peridotite ridge; S, reflections interpreted to represent a detachment fault or shear zone; M, Moho reflections.

Figure 7.34 (a) Map of the North Atlantic showing location of selected nonvolcanic margins. MAR, Mid-Atlantic Ridge. (b) Velocity model of the West Iberia margin and the Iberia Abyssal Plain (image provided by T. Minshull and modified from Minshull, 2002 with permission from Royal Society of London). Data are from Dean et al. (2000). The dashed lines mark the approximate edges of the ocean-continent transition zone. Velocities in km s~'. (c,d) Two end-member types of nonvolcanic margin (images provided by K. Louden and modified from Louden & Chian, 1999, with permission from the Royal Society of London). PR, peridotite ridge; S, reflections interpreted to represent a detachment fault or shear zone; M, Moho reflections.

produced a zone of extremely thin continental crust. This thin crust is characterized by tilted fault blocks that are underlain by a prominent subhorizontal reflector (S) that probably represents a serpentinized shear zone at the crust-mantle boundary (Fig. 7.34c) (Reston et al., 1996). The reflector occurs seaward of stretched continental basement and above a high velocity lower layer of serpentinized mantle. Below the reflector seismic velocities increase gradually with depth and approach normal mantle velocities at depths of15-20 km. Seaward of the thinned continental crust and landward of the first oceanic crust, a transitional region is characterized by low basement velocities, little reflectivity, and a lower layer of serpentinized mantle showing velocities (Vp > 7.0 km s-1) that are similar to high velocity lower crust. Farther seaward, the basement is characterized by a complex series of peridotite ridges (PR), which contain sea floor spreading magnetic anomalies that approximately parallel the strike of the oceanic spreading center. Although this zone is composed mostly of serpentinized mantle, it may also contain minor intrusions. Thus, basement at these margins consists of faulted continental blocks, a smooth transitional region, and elevated highs. Moho reflections (M) are absent within the ocean-continent transition zone. Instead, this region displays landward and seaward dipping reflectors that extend to depths of 15-20 km.

In the second type of nonvolcanic margin (Fig. 7.34d), based primarily on the Labrador example, only one or two tilted fault blocks of upper continental crust are observed and the S-type horizontal reflection is absent. A zone of thinned mid-lower continental crust occurs beneath a thick sedimentary basin. A transitional region occurs farther seaward in a manner similar to the section shown in Fig. 7.34c. However, dipping reflections within the upper mantle are less prevalent. For Labrador, the region of extended lower continental crust is very wide with a thick sedimentary basin, while for Flemish Cap and the Newfoundland basin, the width of extended lower continental crust is narrow or absent. Moho reflections (M) indicate very thin (~5 km) oceanic crust.

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