Page 207 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 193

long-term (tens of millions of years) subsidence induced transition zone between the shelf edge and the Lofoten
by cooling. basin is 50–150 km wide, includes an abrupt lateral gra-
Two other problems of rift evolution that also might dient in crustal thinning, and is covered by layers of
be resolved by incorporating the effects of magmatism volcanic material that display shallow seaward dipping

and/or flow of the asthenosphere include the extra sub- reflectors (Fig. 7.32a). The 50–150 km width of this

sidence observed at some rifted margins and the lack of zone is typical of many rifted margins, although in
magma that characterize nonvolcanic margins (Buck, some cases where there is extreme thinning the zone
2004). These effects are discussed in the context of the may be several hundred kilometers wide. Crustal relief
evolution of rifted continental margins in Section 7.7.3. in this region is related to faulted blocks that delineate
uplifted highs. In the Lofoten example, the continent–
ocean boundary occurs landward of magnetic anomaly
24B (53–56 Ma) and normal ocean crust occurs seaward
7.7 RIFTED of magnetic anomaly 23 (Fig. 7.31b). Crustal thinning
is indicated by variations in Moho depth. The Moho
CONTINENTAL reaches a maximum depth of 26 km beneath the conti-
nental shelf and 11–12 km beneath the Lofoten basin.
MARGINS Along profi le A–A′ a region of 12–16 km thick crust
within the ocean–continent transition zone coincides
with a body in the lower crust characterized by a high
−1
lower crustal velocity (7.2 km s ) (Fig. 7.32a,c). This
7.7.1 Volcanic margins body thins to the north along the margin, where it
eventually disappears, and thickens to the south, where

Rifted volcanic margins are defined by the occurrence at one point it has a thickness of 9 km (Fig. 7.31c).
−1
of the following three components: Large Igneous Oceanic layers display velocities of 4.5–5.2 km s , sedi-
−1
Provinces (Section 7.4.1) composed of thick fl ood ments show velocities of ≤2.45 km s . These seismic
basalts and silicic volcanic sequences, high velocity (V p velocities combined with gravity models (Fig. 7.32b)
−1
> 7 km s ) lower crust in the continent–ocean transition provide information on the nature of the material
zone, and thick sequences of volcanic and sedimen- within the margin (Fig. 7.32c).
tary strata that give rise to seaward-dipping reflectors on In most volcanic margins the wedges of seaward-

seismic refl ection profi les (Mutter et al., 1982). The dipping reflectors occur above or seaward of the high

majority of rifted continental margins appear to be vol- velocity lower crust in the continent–ocean transition
canic, with some notable exceptions represented by the zone. Direct sampling of these sequences indicates that
margins of the Goban Spur, western Iberia, eastern they are composed of a mixture of volcanic fl ows, vol-
China, South Australia, and the Newfoundland Basin– caniclastic deposits, and nonvolcanic sedimentary rock
Labrador Sea. Relationships evident in the Red Sea and that include both subaerial and submarine types of
southern Greenland suggest that a continuum probably deposits. Planke et al. (2000) identified six units that are

exists between volcanic and nonvolcanic margins. commonly associated with these features (Fig. 7.33): (i)
The high velocity lower crust at volcanic margins an outer wedge of seaward-dipping reflectors; (ii) an

occurs between stretched continental crust and normal outer high; (iii) an inner wedge of seaward-dipping
thickness oceanic crust (Figs 7.31, 7.32). Although these reflectors; (iv) landward flows; (v) lava deltas; and (vi)

layers have never been sampled directly, the high P n inner flows. The wedge-like shape of the refl ector pack-
wave velocities suggest that they are composed of thick ages is interpreted to reflect the infi lling of rapidly sub-

accumulations of gabbro that intruded the lower crust siding basement rock. The outer reflectors tend to be
during continental rifting. The intrusion of this material smaller and weaker than the inner variety. The outer
helps to dissipate the thermal anomaly in the mantle high is a mounded, commonly flat-topped feature that

that is associated with continental rifting. may be up to 1.5 km high and 15–20 km wide. In some
The Lofoten–Vesterålen continental margin off places this may be a volcano or a pile of erupted basalt.

Norway (Figs 7.31, 7.32) illustrates the crustal structure Landward flows are subaerially erupted fl ood basalts
of a volcanic margin that has experienced moderate that display little to no sediment layers between the
extension (Tsikalas et al., 2005). The ocean–continent flows. The inner flows are sheet-like bodies located