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OROGENIC BELTS 339

highly deformed and metamorphosed rocks of Middle the Coast belt (Hollister & Andronicos, 1997) (Fig.
Jurassic age that represent the suture zone created by 10.33b) shows the presence of layered, high velocity
the Intermontane–North American collision. A key intrusions and a thinner than average (32 km) continen-
result of the Northern Cordillera transect (Fig. 10.33b) tal crust (Morozov et al., 1998; Hammer et al., 2000). At
is that most of the accreted terranes are relatively thin its western margin, the lithospheric-scale Coast Shear
fl akes of crust with a vertical extent of less than 10 km. Zone (Klepeis et al., 1998) separates rocks of the Coast
Crustal thickness is unusually low and almost uniform Plutonic Complex from those of the Alexander terrane,
across the entire Cordillera, ranging between 33 and which lies on continental crust only 25 km thick. The
36 km (Clowes et al., 2005). Lithospheric thickness also Queen Charlotte Fault, which forms the boundary

is unusually thin and gradually thickens to the east between the Pacific and North American plates, coin-
beneath the Precambrian shield. These observations cides with the western boundary of the orogen. This
demonstrate that many accreted terranes lack the thick transform, and the Denali Fault in the western Yukon
mantle roots that characterize most continental cratons and southeast Alaska, represent the only major active
(Section 11.3.1) (Plate 11.1 between pp. 244 and 245). strike-slip faults in the transect.
During the Late Cretaceous, the North American Strike-slip displacements also accommodated some
Cordillera again grew westward as another composite relative motion between the accreted terranes and
terrane, called the Insular Superterrane, accreted to the North America in the Canadian Cordillera. One of the
margin. This composite assemblage was exotic to North most prominent of these zones is the Tintina Fault,
America and consisted mostly of two island arc ter- which now forms the boundary between the accreted
ranes: Alexander and Wrangellia. The latter terrane, terranes to the west and ancestral North America to the
which is named after the Wrangell Mountains in Alaska east. This fault is a major lithospheric-scale structure
(Jones et al., 1977), is particularly well studied and com- that may record several hundred kilometers of dextral
prises upper Paleozoic island arc rocks overlain by thick, displacement since the Paleocene (Clowes et al., 2005).

subaerial lava flows, and capped by a Triassic carbonate Other major strike-slip displacements are more specula-
sequence. This distinctive geology has allowed investi- tive. For example, some paleomagnetic data suggest
gators to identify several fragments of the terrane that, from 90 Ma to 50 Ma, many of the terranes in
that are now scattered along some 2500 km of the southeast Alaska and British Columbia were displaced
Cordillera, occupying a latitudinal spread of almost 24° several thousand kilometers northward parallel to the
(Fig. 10.32). However, some paleomagnetic data suggest margin from a latitude near present day Baja, California
that the original spread was 4°, implying that a large (Umhoefer, 1987; Irving et al., 1996). This interpretation
amount of post-accretion fragmentation and dispersal is known as the Baja–British Columbia or Baja–BC
has occurred. The paleolatitude of the fragments is cen- hypothesis. However, the great magnitude of the postu-
tered on 10° N or S (the hemisphere is unknown due lated displacements has been disputed mainly because
to uncertainties in the polarity of the Earth’s magnetic the faults along which the terranes may have moved
field during the Triassic) and is in accord with a tropical great distances have not been found (e.g. Cowan et al.,

environment suggested by their geology. It thus appears 1997). Many correlations of stratigraphy and structures
that Wrangellia may have originated in the western across faults suggest that the displacements are much
Pacific in Triassic times near the present position of less than those indicated by some paleomagnetic data.

New Guinea. Following its formation, it appears to have Numerous attempts to resolve these confl icting obser-
traversed the Pacific as a complete entity and accreted vations have been proposed, including: (i) tests of the

to North America where it was subsequently frag- paleomagnetic data using other types of data (Housen
mented and translated to its present locations by strike- & Beck, 1999; Keppie & Dostal, 2001); (ii) determining
slip faulting. reasons why strike-slip faults that record large transla-
The arrival of the Insular Superterrane deformed tions are unlikely to be preserved (Umhoefer, 2000); and
the interior of the North American continent and (iii) evaluating alternative correlations of units across
formed a major part of the Coast belt. Prior to and terranes (Johnston, 2001). It seems probable that only
during the amalgamation, subduction beneath the through an interdisciplinary approach that combines
margin formed the Coast Plutonic Complex (Hutchi- paleomagnetic data, plate motions, paleontologic data,
son, 1982; Crawford et al., 1999; Andronicos et al., 2003) and geologic evidence will the history of large-magni-
during a major period of crustal growth by magma tude terrane translation in western North America be
addition. The ACCRETE marine seismic transect across resolved.