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340 CHAPTER 10

In general, these observations from the Canadian include thinning by delamination or tectonic erosion

Cordillera and elsewhere suggest that ancient accretion- driven by convective flow in the mantle (Section
ary orogens are characterized by the following (Clowes 10.2.5).
et al., 2005): At scales smaller than that of the transects shown
in Fig. 10.33b and c, the structure of ancient accre-
1 An extremely heterogeneous seismic velocity tionary orogens provides a record of the processes
structure in the crust, produced by both thin-
involved in terrane accretion, including subduction
skinned and thick-skinned (Section 10.3.4) and the formation of crustal-scale wedges. For example,
deformation, with the majority of terranes

seismic reflection data collected across the Appalachian
consisting of thin (<10 km thick) crustal fl akes orogen in Newfoundland provide an image (Fig. 10.34)
and lacking the thick mantle roots that
of an Ordovician-Devonian collisional zone that
characterize most continental cratons. There resulted when several exotic terranes accreted onto
are exceptions to this ‘thin fl ake’ pattern, such
the margin of Laurentia (Hall et al., 1998; van der
as the Stikinia terrane (Fig. 10.33b), which Velden et al., 2004). Prior to the collision, thick
exhibits a full crustal extent. Thick-skinned belts
sequences of sedimentary rock were deposited on a
commonly display crustal-scale tectonic wedges passive continental margin located outboard of the
characterized by a complex pattern of
craton. These sequences record the stretching, thin-
indentations and interfi ngering. ning and eventual rupture (Sections 7.2, 7.7) of Pro-
2 Observed crustal thicknesses are unusually low terozoic continental lithosphere as the Iapetus Ocean
(33–36 km) compared to global averages opened during the Late Proterozoic and Early Cam-
(Section 2.4.3) except for averages in zones of brian. This rifting event was followed by a series of
continental extension. terrane collisions and accretionary cycles that formed
3 The Moho remains mostly fl at regardless of the the Paleozoic orogenies of the Appalachian Mountains
age of crustal accretion or the age at which the (Section 11.5.4). Many of the accreted terranes, such
last major tectonic deformation occurred. as Meguma and Avalonia, were microcontinents and
Lateral changes in crustal thickness tend to be composite terranes rifted from northwestern Gond-
gradual, with abrupt variations occurring at wana during the Early Ordovician (Section 11.5.5, Fig.
major terrane boundaries. 11.24a).
The seismic reflection data (Fig. 10.34b) show prom-

4 The dispersal of terranes by strike-slip faulting

inent reflectivity at deep levels of the Appalachian crust
is an important process that occurs in most
that taper westward and merge with a well-defi ned
orogens. Subtle variations in seismic velocity
Moho (van der Velden et al., 2004). The shape and char-
and/or crustal thickness typically occur across
acter of these reflections suggest that they mark the

these faults.
location of an old Ordovician–Devonian subduction
The structure of the Southern Cordillera (Fig. zone. A similar feature occurs beneath the Canadian
10.33c), where subduction is occurring, provides some Shield (Fig. 11.15b), suggesting that the preservation of
additional information on the mechanisms that result ancient subduction channels may be relatively common.
in many of the characteristics of the northern tran- Above and to the east of the paleosubduction zone are
sect at the lithospheric scale. This southern part of a series of dipping thrust faults and tectonic wedges
the margin shows shortening and crustal thickening composed of interlayered slices of the amalgamated
in the forearc region and an active volcanic arc within terranes. Some reflections are truncated by a near verti-

the Coast belt. The mantle lithosphere shows evidence cal strike-slip fault that cuts through the entire crust.
of hydrothermal alteration (serpentinization) in the These and other relationships observed in the Appa-
upper mantle wedge beneath the arc and substantial lachians and the northern Canadian Cordillera show
thinning for several hundred kilometers toward the that ancient accretionary orogens tend to preserve the
interior of the continent. This thinning of the litho- large-scale tectonics structures and lithologic contrasts
sphere in the backarc region is similar to that observed associated with terrane accretion and dispersal. By con-
in other ocean–continent convergent margins (e.g. trast, active orogens such as the Andes, the Himala-

Fig. 10.8) and appears to reflect processes closely yan–Tibetan orogen, and the southern Canadian
associated with subduction. These processes could Cordillera produce seismic refl ection profi les whose