Page 214 - Global Tectonics

200 CHAPTER 7

Observations of the southeast Greenland volcanic Magma accretion, mantle exhumation,

margin support the idea that the flow of low- and detachment faulting
density mantle during the transition to sea fl oor

spreading strongly influences subsidence and stretch-
ing patterns. Hopper et al. (2003) found distinctive The transition from rifting to sea fl oor spreading at
changes in the morphology of basaltic layers in the nonvolcanic margins is marked by the exhumation of

crust that indicate significant vertical motions of the large sections of upper mantle. Seismic refl ection data
ridge system. At the start of spreading, the system collected from the Flemish Cap off the Newfoundland
was close to sea level for at least 1 Myr when margin provide insight into the mechanisms that lead
spreading was subaerial. Later subsidence dropped to this exhumation and how they relate to the forma-
the ridge to shallow water and then deeper water tion of ocean crust.
ranging between 900 and 1500 m depth. This history The Flemish Cap is an approximately circular shaped
appears to reflect the dynamic support of the ridge block of 30-km-thick continental crust that formed

system by upwelling of hot mantle material during during Mesozoic rifting between Newfoundland and
the initiation of spreading. Exhaustion of this the Galicia Bank margin near Iberia (Fig. 7.36a). The
thermal anomaly then led to loss of dynamic two conjugate margins show a pronounced break-up
support and rapid subsidence of the ridge system asymmetry. Seismic images from the Galicia Bank show
over a 2 Ma period. In addition, nearly double the a transition zone composed of mechanically unroofed
volume of dikes and volcanic material occurred on continental mantle (Fig. 7.36b) and a strong regional

the Greenland side of the margin compared to the west-dipping S-type reflection (Fig. 7.36b, stages 1 & 2)
conjugate Hatton Bank margin located south of (Section 7.7.2). The transition zone is several tens of
Iceland on the other side of the North Atlantic kilometers wide off the Galicia Bank and widens to
ocean. These observations indicate that interactions 130 km to the south off southern Iberia. The S-refl ec-
between hot asthenosphere and the lithosphere con- tion is interpreted to represent a detachment fault
tinue to influence the tectonic development of rifted between the lower crust and mantle that underlies a

margins during the final stages of continental break- series of fault-bounded blocks. By contrast, the New-

up when sea floor spreading centers are estab- foundland margin lacks a transition zone and shows no

lished. evidence of any S-type reflections or detachment faults

The flow of low-density melt-depleted astheno- (Hopper et al., 2004). Instead, this latter margin shows
sphere out from under a rift also may help explain an abrupt boundary between very thin continental
the lack of magmatic activity observed at rifted non- crust and a zone of anomalously thin (3 to 4 km thick),
volcanic margins. The absence of large volumes of highly tectonized oceanic crust (Fig. 7.36b, stages 3, 4,
magma could be linked to the effects of prior and 5). Seaward of this boundary the oceanic crust thins
melting episodes, convective cooling of hot astheno- even further to <1.3 km and exhibits unusual very refl ec-
sphere, and/or the rate of mantle upwelling (Buck, tive layering (I).

2004). As sublithospheric mantle wells up beneath a The five stage model of Hopper et al. (2004) explains
rift it melts and cools. This process could result in these structural differences and the evolution of the
shallow mantle convection due to the presence of conjugate margins. In Fig. 7.36b, the top panel shows
cool, dense melt-depleted material overlying hotter, a reconstruction of the two margins emphasizing

less dense mantle. Cooling also restricts further their asymmetry at final break-up when the continental
melting by bringing the mantle below its solidus crust was thinned to a thickness of only a few kilome-
temperature (Section 7.4.2). If some of this previ- ters (stage 1). During break-up, displacement within
ously cooled, melt-depleted asthenosphere is pulled an extensional detachment fault (labeled S in Fig. 7.36b)
up under the active part of the rift during the tran- unroofed a peridotite ridge (PR) above a zone of weak
sition to sea floor spreading, its presence would sup- serpentinized upper mantle. Break-up west of the

press further melting, especially if the rate of rifting ridge isolated it on the Galicia Bank margin when,
or sea floor spreading is slow. The slow rates may during stage 2, mantle melts reached the surface and

not allow the deep, undepleted asthenosphere to sea floor spreading was established. Limited magma-

reach the shallow depths that generate large amounts tism produced the thinner than normal (3–4 km),
of melting. highly tectonized ocean crust. During stage 3, a