Page 215 - Global Tectonics

CONTINENTAL RIFTS AND RIFTED MARGINS 201

(a) (b)
Stage 1: Continental breakup and mantle exhumation
Orphan Basin
West Newfoundland Incipient spreading center East
48 N Flemish Line 1 5 km Margin PR Galicia
Margin
Pass Basin Flemish
Cap ? “S” detachment
Line 3 5 km
Line 2 Serpentinized upper mantle
46 N Grand Model section Stage 2: Magma-starved seafloor spreading
Banks
50 W 30 W 10 W M0 PR
44 N MAR 50 N “S”
IAM-11
Survey area 40 N
IAM-9 M0 Stage 3: Mantle exhumation
50 W 45 W
“Z”

Stage 4: Magmatic event (submarine flood volcanism)

“Z”

Stage 5: Late extension, normal seafloor spreading
I
“Z”
Serpentinized upper mantle
Unaltered upper mantle

Figure 7.36 (a) Location of seismic surveys of the Flemish Cap and (b) five stage model of nonvolcanic margins (after
Hopper et al., 2004, with permission from the Geological Society of America). MO in (a) is magnetic anomaly. Random-
dash pattern, continental crust; v pattern, oceanic crust; light gray shading, serpentinized upper mantle; dark gray

shading, unaltered upper mantle; thick lines, strong reflections; dashed lines, inferred crust–mantle boundary; dotted
lines, oceanic layers; PR, peridotite ridge; S, reflections interpreted to represent a detachment fault; I, unusual very

reflective oceanic crust; Z, reflections interpreted to represent a detachment fault buried by deep marine flood basalts.

reduction in magma supply led to about 20 km of exten- Z). The intrusion of gabbroic material may have accom-
sion that was accommodated mostly by detachment panied this volcanism. This magmatic activity marked

faulting. The detachment faulting led to the exhuma- the beginning of sea floor spreading that formed normal
tion of the mantle and formed an oceanic core complex (6 km) thickness ocean crust (stage 5).

that is similar to those found in slow-spreading environ- This example shows that, to a first order, the transi-
ments at ridge–transform intersections (Section 6.7). tion from rift to oceanic crust at nonvolcanic margins
Voluminous but localized magmatism during stage 4 is fundamentally asymmetric and involves a period
resulted in a 1.5-km-thick layer of deep marine fl ood of magmatic starvation that leads to the exhuma-
basalts that buried the detachment surface (refl ection tion of the mantle. This type of margin may typify