Page 212 - Global Tectonics

198 CHAPTER 7

the formation of intracontinental rifts (Section 7.6).
produced a zone of extremely thin continental crust. Thermal and crustal buoyancy forces, lithospheric

This thin crust is characterized by tilted fault blocks that flexure, rheological contrasts, and magmatism all may
are underlain by a prominent subhorizontal refl ector (S) affect margin behavior during continental break-up,
that probably represents a serpentinized shear zone at although the relative magnitudes and interactions
the crust–mantle boundary (Fig. 7.34c) (Reston et al., among these factors differ from those of the pre-break-
1996). The reflector occurs seaward of stretched conti- up rifting stage. Two sets of processes that are especially

nental basement and above a high velocity lower layer important during the transition from rifting to sea fl oor
of serpentinized mantle. Below the refl ector seismic spreading include: (i) post-rift subsidence and stretching;
velocities increase gradually with depth and approach and (ii) detachment faulting, mantle exhumation, and
normal mantle velocities at depths of 15–20 km. Seaward ocean crust formation at nonvolcanic margins.
of the thinned continental crust and landward of the
fi rst oceanic crust, a transitional region is characterized
by low basement velocities, little reflectivity, and a lower Post-rift subsidence and stretching

layer of serpentinized mantle showing velocities (V p >
−1
7.0 km s ) that are similar to high velocity lower crust. As continental rifting progresses to sea fl oor spreading,
Farther seaward, the basement is characterized by a the margins of the rift isostatically subside below sea
complex series of peridotite ridges (PR), which contain level and eventually become tectonically inactive. This
sea floor spreading magnetic anomalies that approxi- subsidence is governed in part by the mechanical effects

mately parallel the strike of the oceanic spreading of lithospheric stretching (Section 7.6.2) and by a
center. Although this zone is composed mostly of ser- gradual relaxation of the thermal anomaly associated
pentinized mantle, it may also contain minor intrusions. with rifting. Theoretical considerations that incorpo-
Thus, basement at these margins consists of faulted rate these two effects for the case of uniform stretching
continental blocks, a smooth transitional region, and predict that subsidence initially will be rapid as the
elevated highs. Moho reflections (M) are absent within crust is tectonically thinned and eventually slow as the

the ocean–continent transition zone. Instead, this effects of cooling dominate (McKenzie, 1978). However,
region displays landward and seaward dipping refl ectors the amount of subsidence also is influenced by the

that extend to depths of 15–20 km. flexural response of the lithosphere to loads generated
In the second type of nonvolcanic margin (Fig. by sedimentation and volcanism and by changes in
7.34d), based primarily on the Labrador example, only density as magmas intrude and melts crystallize and
one or two tilted fault blocks of upper continental crust cool (Section 7.6.7). Subsidence models that include the
are observed and the S-type horizontal refl ection is effects of magmatism and loading predict signifi cant
absent. A zone of thinned mid-lower continental crust departures from the theoretical thermal subsidence
occurs beneath a thick sedimentary basin. A transitional curves.
region occurs farther seaward in a manner similar to the The amount of subsidence that occurs at rifted
section shown in Fig. 7.34c. However, dipping refl ec- margins is related to the magnitude of the stretching
tions within the upper mantle are less prevalent. For factor (β). There are several different ways of estimating
Labrador, the region of extended lower continental the value of this parameter, depending on the scale of
crust is very wide with a thick sedimentary basin, while observation (Davis & Kusznir, 2004). For the brittle
for Flemish Cap and the Newfoundland basin, the width upper crust, the amount of extension typically is derived
of extended lower continental crust is narrow or absent. from summations of the offsets on faults imaged in

Moho reflections (M) indicate very thin (∼5 km) oceanic seismic refl ection profiles that are oriented parallel to

crust. fault dips. Estimates of the combined upper crustal
extension and lower crustal stretching are obtained
from variations in crustal thickness measured using
7.7.3 The evolution of wide-angle seismic surveys, gravity studies, and seismic
reflection data. This latter approach relies on the

rifted margins assumption that the variations are a consequence of
crustal extension and thinning. At the scale of the entire
The evolution of rifted continental margins is governed lithosphere, stretching factors are obtained through
by many of the same forces and processes that affect considerations of the flexural isostatic response to