Page 353 - Global Tectonics

336 CHAPTER 10

(a) preserve isotopic evidence of multiple phases of
igneous and metamorphic growth. Comparisons of the
A Successor basins or age spectra from detrital zircon populations collected
B overlap sequences from sedimentary and metasedimentary rocks are espe-
cially useful for determining provenance history (e.g.
Sections 3.3, 11.1). Analyses of the composition and
(b) petrologic evolution of xenoliths carried to the surface
from great depths provide another important means
A Provenance linking of probing the deep roots of terranes to determine
B their age, sources, and tectonic evolution (e.g. Section
11.3.3).

(c)
10.6.2 Structure of
A Pluton stitching
B accretionary orogens

Figure 10.31 Geologic relationships that help One of the most fully investigated belts of accreted
establish the timing of terrane amalgamation and terranes is the Cordillera of western North America
accretion (redrawn from Jones et al., 1983).
(Fig. 10.32). The distribution of terranes in this region
forms a zone some 500 km wide that makes up about
30% of the continent (Coney et al., 1980). Most of the
terranes in the Cordillera accreted onto the margin of
ancestral North America during Mesozoic times (Coney,
terrane include the Intermontane and Insular
1989). Some also experienced lateral translations along
Superterranes of the Canadian Cordillera
strike-slip faults. This latter process of dispersal, where
(Fig. 10.33a) and Avalonia (Figs 10.34;
accreted terranes become detached and are redistrib-
11.24b).
uted along the margin, is still occurring today as active
The chronological sequence of terrane accretion strike-slip faults dismember and transport terranes
onto a continent can be determined from geologic within Canada, the USA, and México.
events that postdate accretion and link adjacent terranes Two composite cross-sections across the Canadian
(Fig. 10.31). These include the deposition of sediments Cordillera (Fig. 10.33a) illustrate the large-scale tectonic
across terranes boundaries (Fig. 10.31a), the appearance structure of a major accretionary orogen. The sections
of sediments derived from an adjacent terrane (Fig. were constructed by combining deep seismic refl ection
10.31b), and the “stitching” together of terranes by plu- and refraction data from the Lithoprobe Slave-North-
tonic activity (Fig. 10.31c). ern Cordillera Lithospheric Evolution (SNORCLE) and

Following the identification of the terranes that Southern Cordillera transects, with geologic informa-
comprise an orogen, a variety of analytical tools may tion and the results of other geophysical surveys (Clowes
be applied to determine whether they are exotic or et al., 1995, 2005; Hammer & Clowes, 2007). Figure
native to the adjacent cratons. In addition to paleomag- 10.33b shows a part of the Northern Cordillera, where
netic, structural, and paleontologic studies; these subduction has ceased and the western side of the
include the application of isotope geochemistry and orogen is marked by a zone of active strike-slip faulting.
geochronology to determine the thermal evolution, Figure 10.33c shows a part of the Southern Cordillera,
provenance history, and crustal sources of the terranes. where subduction is still occurring. These transects elu-
The most commonly used provenance techniques cidate the youngest part of a four billion year history of
include the dating and geochemical characterization of subduction, arc–continent collision, and terrane accre-
zircon using U, Pb, and Hf isotope compositions (e.g. tion along the western margin of North America
Gehrels, 2002; Hervé et al., 2003; Griffi n et al., 2004). (Clowes et al., 2005) (see also Section 11.4.3).
Zircon is a highly refractory mineral that commonly Following the amalgamation of the Canadian Shield
occurs in granitoids and sedimentary rock and may during the Proterozoic (Section 11.4.3), a number of