Geology Reference
In-Depth Information
Fig. 1.7
Seismic reflection-refraction cross-section along the eastern US Atlantic margin (Modified from Talwani and
Abreu
2000
. Numbers represent
P
-waves velocity in km s
1
.
SDR
D
Seaward dipping reflectors)
anomalous composition or volatile content (e.g.,
Anderson and Natland
2005
). In this instance,
the acceleration that accompanies the transition
from the rifting stage to drifting may induce
a “spurt” of
active
upwelling, with anomalous
production of melt and formation of compressive
structures along the continental margins. For ex-
ample, the break-up of the supercontinent Pan-
gaea at
200 Ma was accompanied by an event
of extensive volcanism, which gave rise to the
Central Atlantic Magmatic Province
(CAMP).
This event was also responsible for the forma-
tion of a thick initial oceanic crust along the
eastern margin of North America (Talwani and
Abreu
2000
). At the same time, the extensional
structures associated with the rift basins were
inverted as a consequence of horizontal compres-
sion (Schlische et al.
2002
).
A similar mechanism is probably incipient in
the present-day northern Red Sea (Ligi et al.
2011
), thereby the magmatism of the Afar re-
gion could be only the “epicenter” of a future
larger magmatic pulse. Figure
1.7
shows a com-
bined seismic reflection-refraction cross-section
along the Atlantic margin of the United States
(Talwani and Abreu
2000
), which illustrates the
main features of volcanic margins. We note the
presence of a more than 20 km thick atypical
oceanic crust, whose upper part has higher ve-
locities with respect to both the continental crust
and the extrusive layers (2A, 2B) of the normal
oceanic crust. The considerable thickness of such
initial oceanic crust is a direct consequence of
the anomalously high potential temperature of the
asthenosphere beneath these rifts (100-200
ı
C
above normal
T
p
, White and McKenzie
1989
). A
distinctive feature of this kind of OCT is the pres-
ence of
seaward dipping reflectors
(SDR), which
are surfaces of discontinuity within the extrusive
region, having a characteristic dip towards the
ocean (Fig.
1.7
). Volcanic OCTs belong to the
world's
Large Igneous Provinces
(LIPs, Coffin
and Eldholm
1994
). They are also the magmatic
expression of catastrophic events that have deter-
mined the continental break-up and huge volcan-
ism. Well-known examples are the East Coast of
the US from Georgia to Connecticut, the western
Indian margin, the conjugate margins of South
America and the South African craton, Greenland
and Eurasia, Eritrea and Yemen.
At the opposite of the quite common volcanic
rifts, there exist only a few examples of
non-
volcanic passive margins
, which are character-
ized by low magma supply and ultraslow velocity
of the separating plates. An abrupt decrease in the
production of melt can be observed when the full
spreading rates are below
20 mm year
1
(White
et al.
2001
). At these very slow spreading rates,
the upwelling of asthenosphere is not adiabatic,
so that conductive cooling prevails and melting
is inhibited. This favors the growth of new litho-
spheric mantle beneath the rift, according to a
thermodynamic process that will be clarified in
