Page 6 - gt1511

Lineament zone is obscure and cannot be traced beyond the area
Figure 2. Simplified map showing many of the main Cordilleran elements complexes (Foster et al., 2007) also display a sinistral separation
discussed in the text. Rotation of Colorado Plateau from Kent and Witte across the zone (Fig. 1), whereas three other Cenozoic groups of
(1993).
transcurrent. For example, it was obvious that, in addition to the Ancestral Cascades (du Bray and John, 2011), and dominantly
thrust belt, the Neogene Basin and Range structural province
occurs much farther west, north of the lineament, than its equiva- against the zone (Figs. 1 and 2). The classic Laramide basement
lent in Mexico (Fig. 1), yet I knew that it couldn’t be a younger
transcurrent fault because 18.8 Ma Peach Spring Tuff (Fig. 1)
crops out in a narrow band from Arizona to near Barstow,
California (Glazner et al., 1986) and extends unbroken over the
trace of the zone.
Besides the separation of the Basin and Range and the thrust
belt, many other features display sinistral separation across the
lineament (Fig. 1): The post-Sevier dynamic basin (as illustrated
by the 80 Ma isopachs), the Laramide belt of porphyry copper
deposits (Gilmer et al., 2003), and possibly the Oligocene ignim-
brite flare-up (Henry and John, 2013) all show sinistral separa-
tion. The oldest rocks that show obvious sinistral separation
across the zone are the Callovian salt deposits of the Gulf of
Mexico (Fig. 1).
Features restricted to the region north of the Texas Lineament
include the Colorado Plateau, the Rio Grande rift, the High Plains
province, and the Ouachita-Marathon orogen. Features largely
limited to the region south of the lineament include the Sonoran
GSA TODAY | NOVEMBER 2015 batholith, related porphyry Cu deposits, and the Pinal schist
(Fig. 1).
The Rio Grande rift disappears southward into the Mexican
Basin and Range, whereas the unbroken High Plains just west of
the 100th meridian trend southerly into the Mexican Basin and
Range (Figs. 1 and 2). Paleozoic features, such as the Ouachita-
Marathon fold-thrust belt and strata of the Permian Basin, are
truncated and do not appear south of the lineament, whereas the
much younger Late Cretaceous–Paleocene Sonoran batholith
extends northward into Arizona but remains mostly south of the
lineament and extends westward through the Transverse Ranges,
where it ends (Fig. 1). The northwesterly tip of the Texas
6
just southwest of Las Vegas, where the Cordilleran fold-thrust belt
also appears to terminate in the complexly faulted Mojave region
of eastern California (e.g., figure 14 in Burchfiel et al., 1992).
The Lewis & Clark transverse zone (Fig. 1), which King (1969)
used to separate the central from the northern Cordillera, is
similar to the Texas Lineament in that it ends at the Laramide
thrust belt, and most units north and south display sinistral sepa-
ration or are truncated against it (Figs. 1 and 2). Late Cretaceous–
Paleocene magmatic rocks in Idaho and Montana, of which the
Idaho and Boulder batholiths are examples, continue north of the
zone but display a sinistral step before continuing farther north-
ward through the High Cascades and the Coast plutonic complex
of British Columbia (Fig. 1).
Following the same trends as the plutonic rocks is the promi-
nent left step in the initial Sr isopleths of Late Cretaceous–
Paleocene rocks (Armstrong et al., 1977; Fleck and Criss, 1985).
A few plutons of a 100–80 Ma plutonic belt—interpreted farther
south as slab-failure plutons related to the 100 Ma Oregonian
event (Hildebrand and Whalen, 2014)—occur within the Idaho
batholith and to the west in the Cascades and Coast plutonic
complex north of the zone. Eocene magmatism and core
rocks—dikes and lavas of the Columbia River Basalt Group
(Reidel et al., 2013), volcanic and sedimentary rocks of the
basaltic rocks of Siletzia (Wells et al., 2014)—all abut northward
uplifts and basins (Fig. 1), characteristic of the central Cordillera,
are largely confined to the area south of the Lewis & Clark zone
and north of the Texas Lineament (Fig. 1).
Based on the absence of sinistral separation across them, rocks
of the Belt Supergroup appear to sit atop the Lewis & Clark trans-
verse zone, but the zone expresses itself in the overlying Belt rocks
with a linear band of abundant faults, folds, and intense cleavage
(Wallace et al., 1990; Sears, 1988). A conspicuous band of Late
Cretaceous sedimentary rocks located west and south of the
Boulder batholith (Fig. 1) is an order of magnitude thicker, and is
stratigraphically quite different, south of the zone than correlative
rocks to the north in the Montana disturbed belt (Wallace et al.,
1990). Jurassic-Cretaceous rocks of the Tyaughton-Methow basin
(Umhoefer et al., 2002), and those of the Upper Cretaceous
Nanaimo basin on Vancouver Island (Mustard, 1994) do not
continue south of the transverse zone (Fig. 1).
BAJA-BC RESOLVED
Several robust and repeatable paleomagnetic studies exist for
the region north of the Lewis & Clark zone. I summarize the
results of several, plus an interesting study of leaf fossils that
yielded congruent results, in Figure 3. The results are similar, but
those from older Cretaceous rocks have slightly larger amounts of
displacement relative to the craton, largely because North America
started to move southward at 90 Ma (Kent and Irving, 2010).
A breakthrough in our understanding occurred through paleo-
magnetic study of the Carmacks Group (Fig. 3), which yielded ca.
70 Ma paleopoles indicating 1950 ± 600 km northward translation
relative to cratonic North America (Enkin et al., 2006a). The
group is an amalgamation of 72–69 Ma volcanic and sedimentary