Structure of accretionary orogens

One of the most fully investigated belts of accreted terranes is the Cordillera of western North America (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, 1989). Some also experienced lateral translations along strike-slip faults. This latter process of dispersal, where accreted terranes become detached and are redistributed along the margin, is still occurring today as active strike-slip faults dismember and transport terranes within Canada, the USA, and México.

Two composite cross-sections across the Canadian Cordillera (Fig. 10.33a) illustrate the large-scale tectonic structure of a major accretionary orogen. The sections were constructed by combining deep seismic reflection and refraction data from the Lithoprobe Slave-Northern Cordillera Lithospheric Evolution (SNORCLE) and Southern Cordillera transects, with geologic information and the results of other geophysical surveys (Clowes et al., 1995, 2005; Hammer & Clowes, 2007). Figure 10.33b shows a part of the Northern Cordillera, where subduction has ceased and the western side of the orogen is marked by a zone of active strike-slip faulting. Figure 10.33c shows a part of the Southern Cordillera, where subduction is still occurring. These transects elucidate the youngest part of a four billion year history of subduction, arc-continent collision, and terrane accretion along the western margin of North America (Clowes et al., 2005) (see also Section 11.4.3).

Following the amalgamation of the Canadian Shield during the Proterozoic (Section 11.4.3), a number of

Figure 10.32 Generalized map of suspect terranes in western North America. Stippled ornament, North American cratonic basement; barbed line, eastern limit of Cordilleran Mesozoic-Cenozoic deformation; solid ornament, Wrangellia; diagonal ornament, Cache Creek terrane (redrawn from Coney et al., 1980, with permission from Nature 288,329-33. Copyright © 1980 Macmillan Publishers Ltd).

Principal terranes

Alaska and Western Canada

NS North Slope

Kv Kagvik

En Endicott

R Ruby

Sp Seaward Peninsula

I Innoko

NF Nixon Fork

PM Pingston and McKinley

YT Yukon - Tanana

Cl Chuiitna

P Peninsular

W Wrangellia

Cg Chugach and Prince William

TA Tracy Arm

T Taku

Ax Alexander

G Goodnews

Ch Cache Creek

St Stikine

BR Bridge River

E Eastern assemblages

Washington, Oregon and California

Ca Northern Cascades

SJ San Juan

O Olympic

S Siletzia

BL Blue Mountains

Trp Western Triassic and Paleozoic of Klamath Mountains

KL Klamath Mountains

Fh Foothills Belt

F Franciscan and Great Valley

C Calaveras

Si Northern Sierra

SG San Gabriel

Mo Mohave

Sa Salinia

Or Orocopia

Nevada

S Sonomia

RM Roberts Mountains

GL Golconda

Mexico

Baja Vizcaino

Figure 10.32 Generalized map of suspect terranes in western North America. Stippled ornament, North American cratonic basement; barbed line, eastern limit of Cordilleran Mesozoic-Cenozoic deformation; solid ornament, Wrangellia; diagonal ornament, Cache Creek terrane (redrawn from Coney et al., 1980, with permission from Nature 288,329-33. Copyright © 1980 Macmillan Publishers Ltd).

rifting events between 1.74 Ga and the Middle Devonian created thick passive margin sequences that were deposited on top of Proterozoic crust of the North American craton (Thorkelson et al., 2001). These sequences occupy the central part of SNORCLE Line 2B/21 (Fig. 10.33a,b). During the Middle Jurassic, a composite terrane, called the Intermontane Superterrane, began to accrete onto the continental margin. The collision shortened the passive margin sequences and translated them eastward, resulting in a major foreland fold and thrust belt that now forms most of the eastern Cordillera. West of the foreland, the Omineca belt consists of

Accretionary Orogens

DE FUCA-125 Sea,,le PLATE SC-FF

Strike-slip fault ^^ Subduction

Outer Terranes

|c[ Chugach Crescent I^O^ Olympic

Pacific Rim I I Yakutat Insular Terranes Alexander

Wrangellia

Plutonios and

Undivided

Metamorphios

Displaoed Continental Margin

□ Foreland fold and thrust

DE FUCA-125 Sea,,le PLATE SC-FF

Cassiar

I NS I Nisling

A A Thrust front Seismic profiles

Intermontane Terranes

I st I Stikinia I SM I Slide Mountain I cc I Cache Creek I QN I Quesnellia I BR I Bridge River |ca-mt| Cadwallader-Methow I e I Easton and Melange I ha I Harrison-Nooksack

Preicratonic Terranes

E^ Kocjtenay i i Yukon-Tanana

I MO | Monshee

North American Ancestral Basement

^ Proterozoic Q Archean

PACIFIC

Pacific Plate

INSULAR

W'langellia | aX

INTERMONTANE

Pacific Ocean

Queen Charlotte Coast Shear

Fairweather Fault Zone

Queen Charlotte Prince

¿p<TiIslands Hecate Strait Rupert i OMNCA .

FORELAND

M'-onh American Miogeocline | NahannTI

Bowser Basin

Stiknie Arch

Dease Lake

Kechika Fault

Tintina

Fault Watson

PACIFIC

Pacific Plate

INSULAR

W'langellia | aX

INTERMONTANE

Pacific Ocean

Queen Charlotte Coast Shear

Fairweather Fault Zone

Queen Charlotte Prince

¿p<TiIslands Hecate Strait Rupert i OMNCA .

FORELAND

M'-onh American Miogeocline | NahannTI

Bowser Basin

Stiknie Arch

Dease Lake

Kechika Fault

Tintina

Fault Watson

Stikinia

Fort Nelson -0 20 40 60 80 100

600 700

Distance (km)

SNORCLE 2A/22

1100 1200

SNORCLE 2B/21

1000

1300

Queen Charlotte Traverse

ACCREIE

600 700

Distance (km)

SNORCLE 2A/22

1100 1200

SNORCLE 2B/21

Fort Nelson -0 20 40 60 80 100

Stikinia

1000

1300

Queen Charlotte Traverse

ACCREIE

Figure 10.33 (a) Tectonic terrane map and interpreted cross-sections of the (b) northern and (c) southern Canadian Cordillera based on lithospheric velocities, seismic reflection data (solid black lines), and geologic information (images provided by R. Clowes and P. Hammer and modified from Hammer and Clowes, 2007, with permission from the Geological Society of America). Data sources: Queen Charlotte Traverse - Spence & Asudeh (1993), Dehler & Clowes (1988); Accrete - Morozov et al. (1998,2001), Hammer et al. (2000); SNORCLE - Hammer & Clowes (2004), Cook et al. (2004), Welford et al. (2001); Offshore surveys - Rohr et al. (1988), Hasselgren & Clowes (1995), Drew & Clowes (1990); SHIPS - Ramachandran et al. (2006); Southern Cordillera Transect - Clowes et al. (1987), Hyndman et al. (1990), Clowes et al. (1995), Varsek et al. (1993), Cook et al. (1992); Alberta Basement - Chandra & Cumming (1972), Lemieux et al. (2000). QCT, Queen Charlotte Transform; WCF, West Coast Fault.

Figure 10.33 (a) Tectonic terrane map and interpreted cross-sections of the (b) northern and (c) southern Canadian Cordillera based on lithospheric velocities, seismic reflection data (solid black lines), and geologic information (images provided by R. Clowes and P. Hammer and modified from Hammer and Clowes, 2007, with permission from the Geological Society of America). Data sources: Queen Charlotte Traverse - Spence & Asudeh (1993), Dehler & Clowes (1988); Accrete - Morozov et al. (1998,2001), Hammer et al. (2000); SNORCLE - Hammer & Clowes (2004), Cook et al. (2004), Welford et al. (2001); Offshore surveys - Rohr et al. (1988), Hasselgren & Clowes (1995), Drew & Clowes (1990); SHIPS - Ramachandran et al. (2006); Southern Cordillera Transect - Clowes et al. (1987), Hyndman et al. (1990), Clowes et al. (1995), Varsek et al. (1993), Cook et al. (1992); Alberta Basement - Chandra & Cumming (1972), Lemieux et al. (2000). QCT, Queen Charlotte Transform; WCF, West Coast Fault.

highly deformed and metamorphosed rocks of Middle Jurassic age that represent the suture zone created by the Intermontane-North American collision. A key result of the Northern Cordillera transect (Fig. 10.33b) is that most of the accreted terranes are relatively thin flakes of crust with a vertical extent of less than 10 km. Crustal thickness is unusually low and almost uniform across the entire Cordillera, ranging between 33 and 36 km (Clowes et al., 2005). Lithospheric thickness also is unusually thin and gradually thickens to the east beneath the Precambrian shield. These observations demonstrate that many accreted terranes lack the thick mantle roots that characterize most continental cratons (Section 11.3.1) (Plate 11.1 between pp. 244 and 245).

During the Late Cretaceous, the North American Cordillera again grew westward as another composite terrane, called the Insular Superterrane, accreted to the margin. This composite assemblage was exotic to North America and consisted mostly of two island arc terranes: Alexander and Wrangellia. The latter terrane, which is named after the Wrangell Mountains in Alaska (Jones et al., 1977), is particularly well studied and comprises upper Paleozoic island arc rocks overlain by thick, subaerial lava flows, and capped by a Triassic carbonate sequence. This distinctive geology has allowed investigators to identify several fragments of the terrane that are now scattered along some 2500 km of the Cordillera, occupying a latitudinal spread of almost 24° (Fig. 10.32). However, some paleomagnetic data suggest that the original spread was 4°, implying that a large amount of post-accretion fragmentation and dispersal has occurred. The paleolatitude of the fragments is centered on 10° N or S (the hemisphere is unknown due to uncertainties in the polarity of the Earth's magnetic field during the Triassic) and is in accord with a tropical environment suggested by their geology. It thus appears that Wrangellia may have originated in the western Pacific in Triassic times near the present position of New Guinea. Following its formation, it appears to have traversed the Pacific as a complete entity and accreted to North America where it was subsequently fragmented and translated to its present locations by strike-slip faulting.

The arrival of the Insular Superterrane deformed the interior of the North American continent and formed a major part of the Coast belt. Prior to and during the amalgamation, subduction beneath the margin formed the Coast Plutonic Complex (Hutchison, 1982; Crawford et al., 1999; Andronicos et al., 2003) during a major period of crustal growth by magma addition. The ACCRETE marine seismic transect across the Coast belt (Hollister & Andronicos, 1997) (Fig. 10.33b) shows the presence of layered, high velocity intrusions and a thinner than average (32 km) continental crust (Morozov et al., 1998; Hammer et al., 2000). At its western margin, the lithospheric-scale Coast Shear Zone (Klepeis et al., 1998) separates rocks of the Coast Plutonic Complex from those of the Alexander terrane, which lies on continental crust only 25 km thick. The Queen Charlotte Fault, which forms the boundary between the Pacific and North American plates, coincides with the western boundary of the orogen. This transform, and the Denali Fault in the western Yukon and southeast Alaska, represent the only major active strike-slip faults in the transect.

Strike-slip displacements also accommodated some relative motion between the accreted terranes and North America in the Canadian Cordillera. One of the most prominent of these zones is the Tintina Fault, which now forms the boundary between the accreted terranes to the west and ancestral North America to the east. This fault is a major lithospheric-scale structure that may record several hundred kilometers of dextral displacement since the Paleocene (Clowes et al., 2005). Other major strike-slip displacements are more speculative. For example, some paleomagnetic data suggest that, from 90 Ma to 50 Ma, many of the terranes in southeast Alaska and British Columbia were displaced several thousand kilometers northward parallel to the margin from a latitude near present day Baja, California (Umhoefer, 1987; Irving et al., 1996). This interpretation is known as the Baja-British Columbia or Baja-BC hypothesis. However, the great magnitude of the postulated displacements has been disputed mainly because the faults along which the terranes may have moved great distances have not been found (e.g. Cowan et al., 1997). Many correlations of stratigraphy and structures across faults suggest that the displacements are much less than those indicated by some paleomagnetic data. Numerous attempts to resolve these conflicting observations have been proposed, including: (i) tests of the paleomagnetic data using other types of data (Housen & Beck, 1999; Keppie & Dostal, 2001); (ii) determining reasons why strike-slip faults that record large translations are unlikely to be preserved (Umhoefer, 2000); and (iii) evaluating alternative correlations of units across terranes (Johnston, 2001). It seems probable that only through an interdisciplinary approach that combines paleomagnetic data, plate motions, paleontologic data, and geologic evidence will the history of large-magnitude terrane translation in western North America be resolved.

In general, these observations from the Canadian Cordillera and elsewhere suggest that ancient accretion-ary orogens are characterized by the following (Clowes et al., 2005):

1 An extremely heterogeneous seismic velocity structure in the crust, produced by both thin-skinned and thick-skinned (Section 10.3.4) deformation, with the majority of terranes consisting of thin (<10 km thick) crustal flakes and lacking the thick mantle roots that characterize most continental cratons. There are exceptions to this 'thin flake' pattern, such as the Stikinia terrane (Fig. 10.33b), which exhibits a full crustal extent. Thick-skinned belts commonly display crustal-scale tectonic wedges characterized by a complex pattern of indentations and interfingering.

2 Observed crustal thicknesses are unusually low (33-36 km) compared to global averages (Section 2.4.3) except for averages in zones of continental extension.

3 The Moho remains mostly flat regardless of the age of crustal accretion or the age at which the last major tectonic deformation occurred. Lateral changes in crustal thickness tend to be gradual, with abrupt variations occurring at major terrane boundaries.

4 The dispersal of terranes by strike-slip faulting is an important process that occurs in most orogens. Subtle variations in seismic velocity and/or crustal thickness typically occur across these faults.

The structure of the Southern Cordillera (Fig. 10.33c), where subduction is occurring, provides some additional information on the mechanisms that result in many of the characteristics of the northern transect at the lithospheric scale. This southern part of the margin shows shortening and crustal thickening in the forearc region and an active volcanic arc within the Coast belt. The mantle lithosphere shows evidence of hydrothermal alteration (serpentinization) in the upper mantle wedge beneath the arc and substantial thinning for several hundred kilometers toward the interior of the continent. This thinning of the lithosphere in the backarc region is similar to that observed in other ocean-continent convergent margins (e.g. Fig. 10.8) and appears to reflect processes closely associated with subduction. These processes could include thinning by delamination or tectonic erosion driven by convective flow in the mantle (Section 10.2.5).

At scales smaller than that of the transects shown in Fig. 10.33b and c, the structure of ancient accre-tionary orogens provides a record of the processes involved in terrane accretion, including subduction and the formation of crustal-scale wedges. For example, seismic reflection data collected across the Appalachian orogen in Newfoundland provide an image (Fig. 10.34) of an Ordovician-Devonian collisional zone that resulted when several exotic terranes accreted onto the margin of Laurentia (Hall et al., 1998; van der Velden et al., 2004). Prior to the collision, thick sequences of sedimentary rock were deposited on a passive continental margin located outboard of the craton. These sequences record the stretching, thinning and eventual rupture (Sections 7.2, 7.7) of Pro-terozoic continental lithosphere as the Iapetus Ocean opened during the Late Proterozoic and Early Cambrian. This rifting event was followed by a series of terrane collisions and accretionary cycles that formed the Paleozoic orogenies of the Appalachian Mountains (Section 11.5.4). Many of the accreted terranes, such as Meguma and Avalonia, were microcontinents and composite terranes rifted from northwestern Gond-wana during the Early Ordovician (Section 11.5.5, Fig. 11.24a).

The seismic reflection data (Fig. 10.34b) show prominent reflectivity at deep levels of the Appalachian crust that taper westward and merge with a well-defined Moho (van der Velden et al., 2004). The shape and character of these reflections suggest that they mark the location of an old Ordovician-Devonian subduction zone. A similar feature occurs beneath the Canadian Shield (Fig. 11.15b), suggesting that the preservation of ancient subduction channels may be relatively common. Above and to the east of the paleosubduction zone are a series of dipping thrust faults and tectonic wedges composed of interlayered slices of the amalgamated terranes. Some reflections are truncated by a near vertical strike-slip fault that cuts through the entire crust.

These and other relationships observed in the Appalachians and the northern Canadian Cordillera show that ancient accretionary orogens tend to preserve the large-scale tectonics structures and lithologic contrasts associated with terrane accretion and dispersal. By contrast, active orogens such as the Andes, the Himalayan-Tibetan orogen, and the southern Canadian Cordillera produce seismic reflection profiles whose

Accretionary Orogens

suture

Accretionary Orogens

Laurentia

Gondwana

suture

Laurentia

□ Passive margin sequences and Taconic allochthons

Notre Dame arc and obducted oceanic rocks of lapetus

Gondwana

Clastic sedimentary rocks

Proterozoic basement

Microcontinental blocks and arc terranes

| Avalonia

Figure 10.34 (a) Tectonic provinces of Newfoundland in eastern Canada and plot (b), and interpretation (c), of seismic reflection data (after van der Velden et al., 2004, with permission from the Geological Society of America). Migrated data are plotted with no vertical exaggeration and an assumed average velocity of 6 km s~'.

suture deep structure exhibits the effects of processes related to the release, trapping, and consumption of fluids above a subduction zone (Figs 10.6a, 10.20b).

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