New and reprocessed seismic reflection data on the Alaskan and Canadian Arctic margins of the Canada Basin, together with geologic constraints from exploration wells and outcrops, reveal structural and stratigraphic relationships in pre-Mississippian rocks that constrain models of Canada Basin opening. Lithostratigraphic age and acoustic character indicate that the Devonian and older passive-margin to foreland-basin succession in the Canadian M’Clure Strait is also found on the central Alaska margin. This succession also displays similar structural geometry and relief as well as deformational age on both margins. Moreover, Middle Devonian to Early Mississippian tectonic vergence—north directed on the central Alaska margin and east directed in the Canadian M’Clure Strait—indicates a common direction of tectonic transport if the two margins were conjugate. All of these observations demonstrate that pre-Mississippian rocks of the Alaskan and Canadian Arctic margins share a common tectonic history of uplift and exhumation and that the two margins were conjugates prior to opening of the Canada Basin.
The origin and plate tectonic reconstruction of the Arctic Ocean Basin remains controversial, in part because of limited data from this remote and inaccessible ice-covered region of Earth. One of the contentious topics is the opening of the Canada Basin, part of which lies between high-standing continental crust of Canada, Alaska, and the Chukchi Borderland (Fig. 1A; Grantz et al., 2011). A popular model considers the Alaskan and Canadian Arctic margins to be conjugates separated by a Bay of Biscay–type (Vissers and Meijer, 2012) counterclockwise opening of the Canada Basin about a pole of rotation in the Mackenzie Delta region (Fig. 1A; Grantz et al., 2011). Geological and geophysical evidence pertinent to this model has been presented by numerous authors (e.g., Embry, 1990; Miller et al., 2010; Grantz et al., 2011; Strauss et al., 2013; Gottlieb et al., 2014; Chian et al., 2016).
Significantly, the conjugate relationship between the Alaskan and Canadian Arctic margins and the rotational-opening model have been challenged recently by Lane et al. (2016). Those authors cite the absence of evidence for Late Devonian tectonism in Arctic Alaska. Access to new and reprocessed seismic reflection data on the Alaskan and Canadian submerged margins of the Canada Basin (Fig. 1A) provides an unprecedented opportunity to interpret the crustal-scale structure and deep stratigraphy that support a conjugate relationship between the two margins.
PERTINENT DATA AND METHODS
The geology of the Canada Basin and its submerged margins is inferred mainly from geophysical data. Regional potential field grids, in combination with sonobuoy velocity and seismic reflection data, have been used to infer a relict spreading center in the Canada Basin and the crustal character across the Alaskan and Canadian margins (e.g., Grantz et al., 2011; Chian et al., 2016).
High-resolution seismic reflection data are limited mainly to shelf areas. Public-domain seismic reflection and refraction data that constrain shelf to deep-basin geology are sparse and of limited depth resolution (Grantz et al., 1990; Jackson et al., 1990). Newer seismic reflection and refraction data provide enhanced imaging (Chian et al., 2016), but are also sparse in coverage. Thus, current understanding of Canada Basin tectonics is based mainly on integration of onshore and nearshore geology with sparse data from the offshore.
We use two sets of two-dimensional seismic reflection data, integrated with well and outcrop data, to interpret stratigraphy and structure across the potentially conjugate Alaskan and Canadian margins. The first is a grid of A.D. 2006–2012, long-offset data on the Canada and Alaska margins. The second is a grid on the Alaska margin collected in the 1980s to 1990s, some of which has been reprocessed recently to enhance deeper imaging.
Pre-Mississippian rocks, the focus of this paper, are well known from outcrop and subsurface studies in the Canadian Arctic Islands. There, Cambrian to Lower Devonian strata were deposited on a north-facing (present coordinates) passive margin (Franklinian strata of Trettin et al. ) of an ocean basin that subsequently was consumed by terrane accretion. As multiple terranes were accreted, or as individual terranes were accreted in multiple phases, a clastic wedge was shed southwestward and progressively deformed and partly cannibalized during Devonian to Early Mississippian time in a series of events broadly known as the Ellesmerian orogeny (Trettin et al., 1991; Embry, 1991; Harrison and Brent, 2005; Anfinson et al., 2012; Gottlieb et al., 2014).
In Arctic Alaska, pre-Mississippian rocks are known from outcrops in the Brooks Range and well penetrations in a narrow area along the Arctic coast (Fig. 1). These rocks were broadly deformed by the pre–Middle Devonian Romanzof orogeny (Moore et al., 1994; Lane, 2007; Strauss et al., 2013), and clear evidence of subsequent pre-Mississippian deformation is lacking (Lane et al., 2016). Certain pre-Mississippian rocks in Arctic Alaska have been suggested to represent part of the Franklinian margin and Devonian clastic wedge, based on seismic reflection character, and separated from Arctic Canada during opening of the Canada Basin (Embry, 1990; Sherwood, 1994).
Deep structure related to the Devonian to Early Mississippian deformation can be seen in Figure 2A. Interpretations are constrained by published surface and subsurface studies and sparse well control on adjacent islands (Embry, 1991; Trettin et al., 1991; Harrison, 1995; Harrison and Brent, 2005). The shallow M’Clure Strait and Eglinton Basins contain mainly Mesozoic strata that display no contractional structures (Fig. 2A). These two basins are separated by a submarine exposure of Devonian strata on the Mecham high, a 30-km-wide antiform (Fig. 2A) that extends from Prince Patrick Island to Banks Island (Harrison and Brent, 2005).
Beneath a sub-Mesozoic unconformity, we recognize a well-stratified succession with relatively high amplitude and laterally continuous reflections alternating with acoustically transparent strata at fairly regular intervals. This succession is at least 3 s (two-way traveltime, TWT) (>4 km) thick at maximum (Fig. 2A). We interpret these rocks as the Middle to Upper Devonian clastic wedge (Embry, 1991) in the Hardinge synclinorium (Harrison and Brent, 2005). Here, this sediment was shed southwestward into the foreland basin from an emergent Middle to Late Devonian fold-and-thrust belt (Embry, 1991; Anfinson et al., 2012).
A deeper unit grades downward from a moderately stratified succession into acoustically transparent rocks punctuated by widely spaced, laterally continuous, low- to moderate-amplitude reflections. We interpret this unit as Lower Devonian and older strata of the Franklinian margin in the Banks Island belt, an east-vergent fold-and-thrust belt that extends from southwestern Prince Patrick Island to western Banks Island (Harrison and Brent, 2005).
This seismic line in M’Clure Strait displays deep-seated contractional structures erosionally truncated beneath Mesozoic strata (Fig. 2A). Contractional deformation was accommodated by imbricated, east-vergent thrust faults and related folding. A hinterland detachment climbs eastward from ∼11 s TWT (20–25 km) to 7–8 s TWT (10–15 km) in the foreland beneath the Hardinge synclinorium. Thrust faults are interpreted to propagate forward beneath the Hardinge synclinorium, where they accommodate a series of upright, open folds in the upper Franklinian margin and Devonian clastic wedge. Thrust faults do not ramp upward into the Hardinge synclinorium.
Our interpretation of the structure of pre-Mississippian rocks in this seismic line, namely an east-vergent, hinterland to foreland, eastward shallowing of structural levels and progressive decrease in contraction, is similar to that of Harrison and Brent (2005). These structures, which clearly postdate the Middle to Upper Devonian clastic wedge in the Hardinge synclinorium, likely formed during the latest Devonian to Early Mississippian (Harrison and Brent, 2005).
Alaska—Central North Slope and Beaufort Shelf
A composite seismic line (Fig. 2B) from the Brooks Range foothills to the Beaufort shelf edge illustrates the stratigraphy and structure of the Alaska Arctic margin to moderate depths in the crust. This line crosses the Prudhoe Bay oil field on the Barrow arch, a regional uplift commonly considered part of a Jurassic to Hauterivian rift shoulder related to opening of the Canada Basin (Hubbard et al., 1987; Houseknecht and Bird, 2011). Generalized stratigraphy (Fig. 2B) is based on direct ties to numerous wells and regional seismic mapping.
Pre-Mississippian rocks of the North Slope of Alaska are mainly acoustically transparent (Fig. 2B). In some areas, laterally continuous reflections of variable dip direction and magnitude define a stratigraphy that can be correlated across tens of kilometers. However, near the coast and continuing as far as 50 km offshore, stratification dips 30° to 45° to the north. This seismic facies has been penetrated by ∼100 wells along the Barrow arch (Fig. 1B), where lithofacies are mainly argillite and chert with dips that exceed 60° in many cores (Dumoulin, 1999, 2001). These rocks are Neoproterozoic to Silurian in age based on a cosmopolitan fauna that is non-diagnostic regarding affinity to coeval shelf facies bearing both Siberian and Laurentian fauna in the Alaska Brooks Range and only Laurentian fauna in the Canadian Arctic Islands (Dumoulin et al., 2000; Dumoulin, 1999, 2001). In addition, a core from one well (Fig. 1B, well T) contains Lower to Middle Devonian nonmarine strata, and two nearby wells likely penetrated coeval strata (Dumoulin, 2001). We interpret these Devonian strata to be concordant with the older rocks and that both were deformed by a tectonic event that postdated deposition of the Lower to Middle Devonian rocks.
Significantly, the dipping fabric in pre-Mississippian rocks is overlain in angular unconformity by Lower to Middle Mississippian strata in extensional grabens (Wicks et al., 1991). This angular unconformity, which is confirmed by seismic observations and well penetrations across the region, demonstrates that contractional deformation of pre-Mississippian rocks is bracketed between Middle Devonian and Early Mississippian time and that the region was subjected to extension shortly thereafter.
The Cambrian to Silurian argillite and chert (Fig. 1B) along the Barrow arch likely were deposited in a deep-water basin, perhaps located between the Canadian Franklinian margin (Trettin et al., 1991) and the Arctic Alaska margin prior to accretion of the Arctic Alaska terrane to Laurentia (Dumoulin et al., 2000). We interpret the seismic geometries and steep dips in cores to indicate that these rocks are deformed by contraction and significantly more shortened, structurally elevated, and erosionally truncated onshore and nearshore (inferred Paleozoic orogenic hinterland) compared to offshore (inferred Paleozoic foreland, see below). We infer that this Paleozoic structural domain is analogous to the Banks Island belt in the Canadian Arctic Islands. And, we suggest that the local occurrence of Lower to Middle Devonian nonmarine strata on the south flank of the Barrow arch is analogous to the faulted syncline interpreted as part of the Devonian clastic wedge on the back of the large M’Clure Strait structure (Fig. 2A).
Seismic data from the outer Beaufort shelf north of Prudhoe Bay, including the northern Dinkum graben and Dinkum plateau (Fig. 1B; Hubbard et al., 1987; Grantz et al., 1990), image pre-Mississippian rocks of significantly different acoustic character. There, we interpret the top of pre-Mississippian rocks by the presence of an angular unconformity at the base of Mississippian or younger strata (Fig. 3).
The seismic geometries below the pre-Mississippian unconformity display discrete domains of parallel concordant strata (Fig. 3, label 1) bound by low-angle thrust faults (2) that sole into a deeper detachment below the base of the seismic record. Back-limb (3) and front-limb (4) cutoff geometries are consistent with imbricate fault-bend folding (Shaw et al., 2005). These relationships document the presence of a north-vergent fold-and-thrust belt. Furthermore, the reflective character and relief of the pre-Mississippian unconformity suggests preservation of an irregular paleotopography that clearly influenced accommodation in the Dinkum graben, as indicated by onlap of graben-filling strata. The seismic facies described above has not been penetrated by drilling on the Alaska margin, and we interpret it as part of, or analogous to, the Devonian clastic wedge in the Canadian Arctic Islands (Embry, 1991). This interpretation is based on its structural relationship to Silurian and older strata on the Barrow arch and its seismic similarities with strata in the Hardinge synclinorium (Fig. 2). Alternatively, this reflective succession could represent older Devonian strata sourced from the Romanzof orogeny.
DISCUSSION AND SUMMARY
Our interpretation of significant northward (present coordinates) tectonic vergence bracketed between the Middle Devonian and Early Mississippian represents the first evidence for such young Paleozoic tectonism in Arctic Alaska, other than local and relatively minor north-vergent Mississippian thrusting documented by Mauch (1989). Other accounts of significant tectonism in pre-Mississippian rocks involve Early Devonian deformation associated with the Romanzof orogeny (Moore et al., 1994; Lane, 2007).
Northward tectonic vergence on the Alaska Arctic margin during the Middle Devonian to Early Mississippian matches coeval eastward tectonic vergence in the Canadian M’Clure Strait (Banks Island belt of Harrison and Brent ), if the two margins were conjugate prior to opening of the Canada Basin. Moreover, the similarity of lithostratigraphic age and acoustic character and structural relief, geometry, and vergence demonstrates that pre-Mississippian rocks of the two areas share a common tectonic history that included significant uplift and exhumation. Thus, our results provide new evidence that prior to opening of the Canada Basin the Alaskan and Canadian Arctic margins were conjugates along a Late Devonian–Early Mississippian orogenic belt. A counterclockwise rotation of Arctic Alaska during opening of the Canada Basin would broadly be supported by the observations here, however a translational component to the opening cannot be ruled out. Further analysis of the lateral extent of this Late Devonian–Early Mississippian orogenic belt along the Arctic margin is warranted to constrain the full geometry of the belt.
We appreciate constructive reviews by U.S. Geological Survey colleagues Deborah Hutchinson and Thomas Moore and by Geology reviewers Ashton Embry, Elizabeth Miller, and Justin Strauss.