Abstract

The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments.

1. Introduction

Tectonic models for the evolution of the Arctic margin of North America (Laurentia) either support, disregard, or purport to contradict large-scale middle Paleozoic strike-slip terrane displacement [15]. Stratigraphic, paleontological, and geochronological data from Neoproterozoic to early Paleozoic strata in various allochthonous and parautochthonous terranes within the North American Cordillera suggest origins adjacent to Baltica, Siberia or the peripheries of Laurentia (Figure 1) ([1, 6] and references therein); however, the exact position and displacement history of each of these terranes is widely debated. The Northwest Passage model of Colpron and Nelson [1] argues for terrane dispersal through strike-slip displacement facilitated by the propagation of a Scotia-style subduction system between Laurentia, Siberia, and Baltica, while other models drive terrane dispersal through backarc extension and/or tectonic rotations [24, 7]. The displacement history of these terranes and their tectonic framework are critical for reconstructions of the Mesozoic-Cenozoic circum-Arctic, as they form the backdrop to the controversial opening of the Amerasian Basin of the Arctic Ocean (e.g. [810]).

A significant challenge in Neoproterozoic–early Paleozoic reconstructions of the Arctic margin of Laurentia is the current lack of detailed examination of major margin-parallel fault zones, which is amplified by often non-unique provenance data related to passive margin sedimentation along the length of the entire margin (e.g., [11] but see [12]). The Porcupine Shear Zone (PSZ) in northwestern Yukon and northeastern Alaska is potentially a major boundary separating the North Slope subterrane of the Arctic Alaska-Chukotka microplate from the autochthonous northwestern margin of Laurentia (Figure 1) [1315]. A number of researchers have postulated large-scale translation along this structure [1217], while others have argued for minimal displacement [4, 18].

Contrasting tectonic models on the significance of the PSZ reflect debate over stratigraphic correlations between the North Slope subterrane and coeval sedimentary successions of the adjacent autochthonous Yukon block and Selwyn basin (Figure 1) ([19, 20] and references therein). For example, siliciclastic-dominated Neoproterozoic–early Paleozoic strata of the British, Barn and Romanzof Mountains, Alaska and Yukon, were previously correlated by Lane [21] to coeval units exposed further south within autochthonous Laurentia. In contrast, other studies pointed out stratigraphic mismatches between these sedimentary successions [13, 20], in addition to highlighting the allochthonous nature of oceanic rocks imbricated with these strata [22] and their association with an Ordovician–Silurian overlap assemblage potentially derived from the northern Caledonides [23]. These studies point to an origin of the North Slope subterrane adjacent to the Franklinian basin of the Canadian Arctic Islands (Figure 1) with subsequent translation through strike-slip displacement along the PSZ or associated fault zones along northern Laurentia (e.g., [15]). Critically, however, the relationship between deformed strata exposed within the PSZ and adjacent rocks of the North Slope subterrane and northwestern Laurentia remain speculative [14]. Here, we present new field observations, detrital zircon U-Pb geochronological and Hf isotopic data and stable isotope geochemistry from sedimentary rocks to elucidate the stratigraphic architecture of pre-Mississippian rocks exposed along the PSZ in Alaska and Yukon. In addition, we present igneous zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Nd and Sr isotopic data from igneous rocks intruding these strata to further improve tectonic models related to the evolution of the PSZ.

2. Geological Background

2.1. Porcupine Shear Zone

The PSZ (Figure 2) was initially inferred by Grantz et al. [24] as a continuation of the right-lateral Kaltag fault of west-central Alaska [25] that was subsequently projected into Yukon [26, 27] and northeast beneath the Beaufort Sea [28]. A number of researchers proposed late Mesozoic to early Cenozoic right-lateral strike-slip translation along the PSZ during the opening of the Canada Basin (e.g., [16, 2932]), while others inferred sinistral strike-slip displacement [3234] or a complete lack of displacement [18]. Oldow et al. [34] discussed post-Jurassic translation along the PSZ, highlighting significant differences in the style of deformation and stratigraphic correlations between early Paleozoic strata north and south of this structure. These stratigraphic mismatches were documented further by Strauss et al. [13] who proposed translation of the North Slope subterrane along the PSZ and related structures from a location adjacent to the Franklinian basin of the Canadian Arctic Islands (see also [19, 20, 23, 35]). More recently, sinistral and dextral displacement along the PSZ was documented by von Gosen et al. [15], with the majority of the sinistral translation inferred to occur in the middle Paleozoic. All of these previous studies presumed deformed rocks within the PSZ belonged to autochthonous strata of the Yukon block.

2.2. Yukon Block

The autochthonous Yukon block of northwestern Laurentia (Figure 1(b)) forms a distinct lithospheric domain underlying most of northern Yukon, from the Northwest Territories to Alaskan borders [36]; this region represents a high-standing crustal block that controlled depositional patterns throughout northwestern Laurentia in the Paleozoic (e.g., [37, 38] and references therein). Its southern border is marked by the Dawson Fault, its eastern edge by the Richardson Fault Array and northern boundary by the PSZ (e.g., [14, 15, 3941]).

Proterozoic strata exposed in the Yukon block have been traditionally divided into three megasequences [42]: Paleo- to Mesoproterozoic Sequence A (ca. 1.7–1.2 Ga Wernecke Supergroup), Meso- to Neoproterozoic Sequence B (ca. 1.2–0.78 Ga Mackenzie Mountains Supergroup) and Neoproterozoic Sequence C (ca. 0.78–0.54 Ga Windermere Supergroup). These strata are variably exposed in the Tatonduk, Coal Creek, Hart River and Wernecke inliers, as well as in the Keele Range and White Uplift (Figure 1(b)) [40, 4347]. Throughout the Yukon block, these Proterozoic rocks are overlain by thick successions of upper Cambrian to Lower Devonian shallow-water carbonate and deep-water mixed carbonate and siliciclastic successions (e.g. [38, 41, 4850]). These different facies belts reflect the presence of distinct carbonate platforms (e.g., Mackenzie and Ogilvie platforms), intra-plaformal troughs (e.g., Richardson trough), and deep-water basins (Selwyn basin) (Figure 1(b)).

Autochthonous Proterozoic–Paleozoic rocks adjacent to the PSZ comprise well-studied successions in the Tatonduk inlier and more poorly understood strata of the Keele Range (also referred to as the Dave Lord Uplift) and White Uplift (Figure 1(b)). The base of the Proterozoic sedimentary succession in the Tatonduk inlier is represented by poorly understood strata of the lower Tindir Group of Young [51] or Pinguicula Group(?) of Macdonald et al. [45]. These mixed carbonate and siliciclastic deposits are unconformably overlain by carbonate strata of the Tonian Fifteenmile Group [45, 52], mafic volcanic rocks of the ca. 718 Ma Pleasant Creek Volcanics, and glacial diamictite and iron formation of the Cryogenian Rapitan Group [45, 53], all of which were previously included in the upper Tindir Group of Young [51]. The Rapitan Group is overlain by mixed carbonate and siliciclastic strata of the Cryogenian–Ediacaran Hay Creek Group and “Upper” groups [45], the latter of which are correlative with the newly defined Rackla Group [49, 54]. These Neoproterozoic units are overlain by or in structural contact with the Cambrian–Ordovician Funnel Creek Limestone, Adams Argillite, Hillard Limestone, and Jones Ridge Formation [5557], all of which are disconformably overlain by shale and limestone of the early Paleozoic Road River Group [38, 50, 55, 56].

North of the Tatonduk inlier, Proterozoic–early Paleozoic strata are exposed along the Porcupine River corridor and in the Keele Range (Figures 2 and 3). This area was initially studied in Yukon by Cairnes [58] and then mapped by Norris [27, 59] in the Old Crow and Porcupine River quadrangles. In Alaska, these strata were initially described by Kindle [60] and then investigated in detail by Brosgé et al. [61] along the Porcupine River, Brosgé and Reiser [62] in the Coleen quadrangle and by Brabb [63] in the Black River quadrangle. Rocks of the Keele Range consist of poorly studied Proterozoic argillite and quartzite, which were previously correlated with the Quartet Group (unit Hq; [27, 59]) and then more broadly included in the undivided Wernecke Supergroup (unit Hw; [64]) or quartzites equivalent to siliciclastic strata exposed along the Porcupine River (unit Pu?; [64]). Gordey and Makepeace [65] correlated these rocks with the Pinguicula or Gillespie Lake groups. In Alaska, the equivalent strata were mapped as undivided Precambrian phyllite, slate, quartzite, dolostone and limestone [63]. These Proterozoic units are inferred to be unconformably overlain by Cambrian to Devonian carbonate and siliciclastic rocks [27, 59, 6264]. Further northeast, unnamed Proterozoic rocks of the White Uplift consist of dolostone, quartzite and mafic volcanic rocks, which are unconformably overlain by early Paleozoic quartzite and carbonate [66].

Norris and Dyke [64] divided rocks along the Porcupine River corridor into brown sandstone (unit Pu1), massive gray fine-grained sandstone (unit Pu2), calcareous shale and limestone (unit Pu3) and orange weathering dolostone (unit Pu4). Unit Pu2 was correlated with the lower Tindir Group of Young [51] and the Katherine Group of the Mackenzie Mountains Supergroup; this implied units Pu3 and Pu4 were correlative with Young’s [51] black shale and dolostone units within the lower Tindir Group. The more modern stratigraphic scheme applied by Macdonald et al. [45] therefore suggest possible correlations with the Pinguicula and Fifteenmile groups; however, Norris and Dyke [64] pointed out the possibility that black shale of unit Pu3 may instead be much younger and correlate with the Road River Group or Triassic Shublik Formation. More recently, Colpron et al. [14] combined these strata into two map units, one comprised of argillite, quartzite and dolostone (Mackenzie Mountains Supergroup?) and the second of dolomitic sandstone, maroon shale, and diamictite (Rapitan/Fifteenmile groups?). Siliciclastic and carbonate rocks west of the international border in Alaska were divided into seven stratigraphically equivalent map units of the lower “Porcupine River Sequence” and assigned a Precambrian-early Paleozoic age [61, 62]. Correlations of these units with other regions of Alaska remain speculative (e.g., [67]) and no attempt has been made to correlate the “Porcupine River Sequence” across the international border.

2.3. Arctic Alaska-Chukotka Microplate

The North Slope subterrane is a part of the composite Arctic Alaska-Chukotka microplate that spans from the Yukon-Alaska border region to the Chukotka Peninsula of northeastern Russia (Figure 1) [68]. The pre-Mississippian basement of the Arctic Alaska-Chukotka microplate can be separated into two broad areas: the peri-Laurentian North Slope subterrane and the exotic southwestern subterranes ([13, 35, 69] and references therein). Across these subterrane boundaries, polydeformed Neoproterozoic and early Paleozoic rocks are intruded by Upper Devonian felsic plutons [7073] and unconformably overlain by Carboniferous Endicott and Lisburne groups [32, 68, 74]. Assembly of the Arctic Alaska-Chukotka microplate is inferred to have occurred during protracted Silurian(?)–Carboniferous tectonism near or along the North American margin ([35] and references therein).

Pre-Mississippian strata of the North Slope subterrane include a Neoproterozoic–Devonian(?) basinal siliciclastic and volcanic succession exposed in the Franklin, Romanzof, British and Barn Mountains (northeast Brooks Range basinal succession of Strauss et al. [20]) and a Neoproterozoic–Devonian carbonate succession exposed in the Shublik and Sadlerochit Mountains (northeast Brooks Range platformal succession of Strauss et al. [19]). The basinal succession is comprised of the siliciclastic-dominated Neoproterozoic–Cambrian Firth River Group, Cambrian Neruokpuk Formation and Cambrian–Ordovician Leffingwell formation [20]. These strata are disconformably overlain by texturally immature siliciclastic units of the Upper Ordovician–Lower Devonian(?) Clarence River Group [20, 23]. The pre-Mississippian basinal succession is locally in tectonic contact with the Whale Mountain allochthon, an imbricated package of upper Cambrian mafic volcanic rocks, carbonate, and phyllite [22, 23]. The platformal succession is comprised of mixed siliciclastic and carbonate strata of the Tonian Mount Weller Group [19], which is overlain by the ca. 719 Ma Kikiktat volcanic rocks [75], the Sturtian (ca. 660-717 Ma) Hula Hula diamictite, and an ~5 km thick Cryogenian–Devonian platformal carbonate succession named the Katakturuk Dolomite and Nanook Group [19, 76]. The platformal succession is tilted and unconformably overlain by Mississippian strata of the Endicott Group.

3. Methods

Fieldwork was conducted along the Porcupine River in Yukon during the summer of 2017 and in Alaska during the summers of 2018 and 2019. Carbonate samples were collected for carbon, oxygen and strontium isotope analyses and chert nodules were sampled for micropaleontological investigations. Siliciclastic rocks were sampled for detrital zircon U-Pb geochronology and Hf isotope geochemistry. Igneous rocks were collected for U-Pb zircon geochronology, major and trace element geochemistry, and strontium and neodymium isotope geochemistry. The coordinates for the collected samples are provided in Table 1, and the analytical methods are briefly summarized below and described in greater detail in the Supplemental Materials (available here). All of the raw data are also provided in the Supplemental Materials.

We analyzed 175 samples for carbonate carbon (δ13Ccarb) and oxygen (δ18Ocarb) isotopes from nine measured sections at Dartmouth College, as well as eleven samples from two measured sections for strontium (87Sr/86Sr) isotopes at Yale University (Figure 3; Table 1). We present data from five measured sections in the main text, but data from the four additional sections is reported in Table DR1. Nine sandstone samples exposed within and surrounding the PSZ were collected for detrital zircon U-Pb geochronologic and Lu-Hf isotopic analyses (Figure 3; Table 1). Samples were prepared using standard mineral separation procedures, co-mounted in epoxy with natural reference materials and imaged for cathodoluminescence (CL) at Dartmouth College and the University of Iowa. Detrital zircon U-Pb isotopes were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) at the University of Arizona Laserchron Center (ALC) following protocols outlined by Gehrels et al. [77, 78] and Gehrels and Pecha [79]. Reported uncertainties on the U-Pb isotopic data are at the 1σ level and include only measurement errors. The 206Pb/238U age is used if the 206Pb/238U age was <1200 Ma and the 207Pb/206Pb age if the 206Pb/238U age was >1200 Ma. Individual analyses were excluded from further consideration based on the following criteria: (1) >10% 2σ uncertainty in 206Pb/238U or 206Pb/207Pb age; (2) >10% discordance or >5% reverse discordance calculated as 206Pb/238U age divided by 206Pb/207Pb age for grains older than 500 Ma; and (3) grains younger than 500 Ma were rejected if the 2σ error ellipse did not overlap Concordia. These data are presented in normalized age probability plots generated using the Excel macro provided by G.E. Gehrels at the ALC.

Igneous zircons from sample KF17-12 (Figure 3; Table 1) were separated by standard procedures at Dartmouth College and CL-imaged and analyzed at the Stanford-U.S. Geological Survey Micro-Analysis Center at Stanford University. All of the CL images from sample KF17-12 are presented in Figure S1 of the Supplemental Materials. Zircon grains were simultaneously analyzed for U-Th-Pb isotopes and trace elements by Secondary Ion Mass Spectrometry (SIMS) on a sensitive high-resolution ion microprobe with reverse geometry (SHRIMP-RG) [8083]. Data reduction and ages were calculated using programs of Ludwig [84, 85],

Twenty igneous samples were collected for whole rock geochemistry from mafic intrusions and flows within and adjacent to the PSZ (Figures 2 and 3; Table 1). Major and trace elements were measured by Activation Laboratories in Ontario, Canada. Unaltered samples were selected for strontium isotopic analyses and measured on a GV IsoProbe-T multi-collector thermal ionization mass spectrometer (TIMS) at Massachusetts Institute of Technology. Neodymium isotopic data from the same sample powders were measured on a Thermo Triton Thermal Ionization Mass Spectrometer (TIMS) at the GEOTOP laboratories at UQAM in Montreal, Canada.

4. Results

4.1. Field Observations and Stratigraphic Assignments

The stratigraphic position and depositional relationships of pre-Mississippian rocks exposed along the PSZ remain uncertain due to extensive deformation (e.g., [14, 15]). Thick packages of mostly siliciclastic and carbonate strata exposed in the river corridor form fault-bounded blocks characterized by complex internal structures with evidence for multi-phase brittle-ductile deformation related to both sinistral and dextral strike-slip displacement (Figure 3) [15]. Despite these challenges, we have reconstructed a preliminary pre-Mississippian stratigraphic architecture of the PSZ by collecting detailed field observations and samples along a ca. 30-km transect across the international border. Given the extensive regional deformation and lack of continuous stratigraphic sections, we propose a suite of new informal units with names derived from local geographic locations (Figures 3 and 4). In addition, we recommend abandoning the term “Porcupine River Sequence” (sensu Brosgé et al., [62]; see also [67]), as it can easily be confused with previous terminology for paleogeographic and/or tectonic elements in northwestern Laurentia, such as the “Porcupine terrane” (e.g. [86]) or “Porcupine platform” (e.g. [87]). Here, we refer to pre-Mississippian strata deformed within the PSZ as the Ch’oodeenjìk succession, which is the Gwich’in name for the Porcupine River. The Ch’oodeenjìk succession is defined along the Porcupine River and its tributaries and corresponds to the area shown in Figure 3. Correlations north of the PSZ are less certain due to poor exposure.

4.1.1. Lahchah Formation (New)

The base of the Ch’oodeenjìk succession is marked by the ~300 m thick Lahchah formation, which is well exposed along Caribou Bar and Sunaghun creeks, as well as multiple unnamed N-S-trending tributaries of the Porcupine River (Figures 2 and 3). This unit is named after Lahchah Mountain in the NTS 116 N10 quadrangle. The Lahchah formation is inferred to underlie large areas surrounding the Old Crow Range between the Porcupine and Rapid rivers where it is intruded by felsic plutonic rocks of the Old Crow suite [72] (Figures 2, 3). The base of the Lahchah formation is locally faulted or poorly exposed; a faulted upper contact with the overlying Sunaghun formation (new name) is well exposed 2.7 km east of the Yukon-Alaska border (Figure S2A) where the Lahchah formation is thrust on top of member B of the Fred Creek formation (new name) (Figures 4 and 5(a)). The lower part of the Lahchah formation consists of yellow-weathering, fine- to medium-grained micaceous quartz arenite interbedded with brown to grey micaceous shale and siltstone (Figure 6(a)). The upper part is dominated by interbedded brown- to tan-weathering, light grey to brown, thin- to medium-bedded micaceous quartz arenite and wacke with brown to black silty shale. Quartz arenite in the upper part of the Lahchah formation is generally fine- to medium-grained and planar- to cross-laminated with locally preserved asymmetrical and symmetrical ripple marks. Locally, the Lahchah formation also contains intervals of interbedded maroon, green and purple micaceous shale.

The Lahchah formation is correlative to map unit pЄsq of Brosgé et al. [61], Pzs of Brosgé and Reiser [62], parts of unit Pu and Pq of Norris [27] and unit Pu1 of Norris and Dyke [64]. The Strangle Women Sequence of Brosgé and Reiser [62], which is exposed northeast of the Old Crow batholith, as well as poorly exposed siliciclastic rocks north of the Porcupine River that are intruded by the Old Crow suite are most likely part of the Lahchah formation (Figures 2, 3); however, the distinct lack of associated carbonate rocks and the general higher-grade of metamorphism might suggest differences that should be investigated in the future.

4.1.2. Sunaghun Formation (New)

The ca. 100–200 m thick Sunaghun formation comprises a homogenous siliciclastic unit almost exclusively comprised of tan- to white-weathering, medium- to thick-bedded, mature quartz arenite. It is well exposed in multiple locations along the northern and southern banks of the Porcupine River, and it is named after Sunaghun Creek, a S-flowing tributary of the Porcupine River that straddles the international border in the NTS 116 N11 quadrangle (Figure 3). The base of the Sunaghun formation is tectonized where observed, but it is inferred to be disconformable or conformable on the Lahchah formation due to the repeated occurrence of these formations in sequence and consistent bedding orientations across tectonized boundaries. White quartz arenite of the Sunaghun formation is commonly amalgamated, medium- to thick-bedded with dune- to ripple-scale trough and tabular cross-bedding (locally overturned) and local planar and convolute lamination (Figure 6(b)). Along Sunaghun Creek, the Sunaghun formation is pyritic with violet to pastel weathering colors. The unit generally forms semi-coherent fault blocks with well-preserved sedimentary structures, except where pulverized in tectonic breccias. The Sunaghun formation is correlative to map unit pЄq and pЄqs of Brosgé et al. [61], Pzq and Pzqa of Brosgé and Reiser [62], parts of unit Pu and Pq of Norris [27] and unit Pu2 of Norris and Dyke [64].

4.1.3. Caribou Bar Formation (New)

Siliciclastic strata of the Lahchah and Sunaghun formations are overlain by the carbonate-dominated Caribou Bar formation (Figures 5(b)-(d)), which is comprised of two informal members: 1) black calcareous shale/slate and thin-bedded black argillaceous lime mudstone of member A (Figure 6(c)); and 2) grey-weathering, thin- to medium-bedded, planar to nodular dolostone of member B (Figures 6(d), 6(e)). The base of the Caribou Bar formation is well exposed at the confluence of Caribou Bar Creek and the Porcupine River in the NTS 116 N07 quadrangle, as well as near the mouth of Darcy Creek, ~2.4 km east of the international border (Figure 3). One km west of the border, along the northern and southern banks of the Porcupine River, these strata appear to disconformably overlie white quartz arenite of the Sunaghun formation, although the contact is tectonized (Figure S2B). The top of the Caribou Bar formation is commonly poorly exposed or faulted, but locally these strata are overlain conformably by massive recrystallized dolostone of the Fred Creek formation (Figure 5(b); see also Figures S2C, S2D).

Member A of the Caribou Bar formation is commonly intensely folded and truncated by numerous faults, so any attempt to measure continuous stratigraphic sections is challenging; despite this, we measured two fault-bounded sections with clear up indicators to provide some insight into the internal sedimentology and chemostratigraphy of this unit. Section J1702 is dominated by blue-gray dolomitic lime mudstone with faint planar to crinkly lamination that transitions into massive lime mudstone and boundstone with small (<10 cm tall) domal to columnar stromatolites and abundant molar-tooth structures (Figure 7(a)) [88]. The upper part of this short section becomes increasingly dolomitized with local intervals of saddle dolomite, and the top is marked by a ~3 m thick package of medium-bedded dolorudstone with up to 5 cm long tabular intraclasts. Section J1703 of member A is almost completely dominated by interbedded calcareous black shale, lime mudstone and planar-laminated fine-grained grainstone with local intervals of microbial lamination and molar tooth structures (Figures 7(a)). This latter facies combination is the most common sedimentological expression of the Caribou Bar formation throughout the Porcupine River area.

Three abbreviated stratigraphic sections were also measured through fault blocks of member B of the Caribou Bar formation, which largely consists of white- to yellow-weathering doloboundstone, dolograinstone, and dolomudstone. In section J1701 (Figure 7(a)), the lower part of the unit is dominated by white doloboundstone with mudcracks, tepee structures [89] and irregular lenses of dark grey chert interbedded with laminated dolograinstone and massive dolomudstone. These strata transition up-section into medium-bedded, very fine-grained oolitic and peloidal dolograinstone with trough and hummocky cross-stratification. Section J1704 of member B (Figure 7(a)) is characterized by grey doloboundstone, dolomudstone and dolograinstone with domal stromatolites up to 0.3 m tall, abundant trough cross-bedding, and local tepee structures and mudcracks. Section KF17-6 (Figure 7(a)) is almost entirely comprised of medium- to thick-bedded oolitic dolograinstone with minor chert nodules and rare beds of sandy dolograinstone. At the mouth of Darcy Creek, member B contains black chert nodules with well-preserved rip-ups clasts of microbial mat fragments (sample KF17-56, location on Figure 3). Within the mat fragments and in the adjacent organic-rich matrix (Figure 8(a)-8(b)), multiple vase-shaped microfossils (VSMs; [52, 90, 91]) were identified. The VSMs are preserved as silicified tests and range in size from ca. 40 to 100 μm. Although the specific taxonomic assignment of these microfossils is not described here, the majority of the VSMs belong to the genus Bonniea (Figure 8(c)–8(e)) and Cycliocyrillium (Figure 8(f)-8(h)) [52, 90].

Observations from multiple localities along the Porcupine River suggest that bedded dolostone of member B commonly overlies finer-grained limestone strata of member A in a conformable and transitional contact; however, we have also recognized ca. 30 m thick blocks of stromatolitic dolostone in member A that resemble the dolomitic lithofacies of member B. Along strike, these stromatolitic blocks occasionally coalesce to form continuous >100 m thick bodies of bedded dolostone units. Although many of these dolostone blocks are fault-bounded and/or displaced, they may reflect stratiform change between stromatolitic bioherms and bounding lime mudstone facies. This interbedded relationship and the transitional contact between the two members are particularly evident along the northern and southern banks of the Porcupine River approximately 1 km west of the border (Figure 5(d)).

Member A of the Caribou Bar formation is correlative to map unit pЄsl of Brosgé et al. [61] and member B is correlative to map unit pЄld of Brosgé et al. [61]; the Caribou Bar formation as a whole can be correlated with unit Pzl of Brosgé and Reiser [62], parts of unit Pu of Norris [27] and unit Pu3 of Norris and Dyke [64].

4.1.4. Fred Creek Formation (New)

The Fred Creek formation consists of two informal members whose aggregate thickness is ~50-250 m: member A – an orange-weathering and massive recrystallized vuggy dolostone unit with rare intervals of silicified oolitic dolograinstone, intraclast rudstone, and stromatolitic doloboundstone; and member B – a thin- to medium-bedded dolomudstone, doloboundstone, and dolosiltstone unit. These strata are named after Fred Creek in the USGS Coleen B-1 quadrangle (Figure 3). The contact between the Fred Creek and Caribou Bar formation is exposed approximately 5.6 km west of the international border on the northern bank of the Porcupine River, and the internal Fred Creek member contact is well exposed ca. 2.5 km west of the international border (Figure 5(a)). Carbonate strata of member A of the Fred Creek formation are commonly massive and characterized by intense tectonic brecciation that prohibits measurement of continuous stratigraphic sections and the identification of internal lithofacies. Despite this, member A appears in consistent stratigraphic order above the Caribou Bar formation along the Porcupine River corridor. Locally, dolostone units of member B contain brown chert nodules, laterally linked domal and columnar stromatolites (Figure 6(f)), laminated doloboundstone, and trough cross-bedded silty dolograinstone intervals. In various locations, the upper part of the Fred Creek formation hosts pink- to red-weathering beds of carbonate lithic wacke or dolograinstone (Figure 5(a)), which are composed of well-rounded grains of re-worked carbonate in an iron oxide-rich matrix.

Member A of the Fred Creek formation is correlative to map unit pЄud of Brosgé et al. [61] and Pzcd of Brosgé and Reiser [62], while member B is in part correlative with unit Da of Brosgé et al. [61] and Pd of Brosgé and Reiser [62]. Combined, these strata are correlative to part of unit Pu of Norris [27] and unit Pu4 of Norris and Dyke [64].

4.1.5. Darcy Creek Formation (New)

The Darcy Creek formation is well exposed along Darcy Creek (Figure 3; NTS 116 N07), a small tributary to the Porcupine River ~2.4 km east of the international border. Exposures of this formation are limited throughout the study area but are exposed along an unnamed N-S-trending creek 5 km east of the border, as well as in a prominent shear zone described previously in von Gosen et al. [15] along the southern bank of the Porcupine River ~18 km east of the border. The Darcy Creek formation depositionally overlies the Caribou Bar formation at three localities (Figures 3 and 5(c), 5(e)), including one location where it rests with profound erosional relief on member A (Figure 5(c)). Its upper contact has not been recognized. These strata consist of a basal, poorly exposed beige- to white-weathering clast-supported cobble conglomerate that is up to ~4 m thick and dominated by subrounded to subangular clasts of dolostone and chert. This basal conglomerate is overlain by interbedded blue- to light-grey calcareous siltstone and sandstone and chert pebble conglomerate. The upper part of the Darcy Creek formation is comprised of an ~20–40 m thick succession of interbedded maroon to brick red mudstone, siltstone, and matrix-supported conglomerate with sparse angular to subrounded clasts of chert and dolostone (Figures 5(e), 6(g), 6(h)). The Darcy Creek formation has not been previously distinguished from other pre-Mississippian map units within the Porcupine River corridor.

4.1.6. Mafic Igneous Rocks of the Porcupine River Area and Keele Range

Mafic igneous rocks exposed in the study area can be divided into two groups: 1) highly deformed dikes and sills that intrude strata of the Lahchah, Sunaghun, Caribou Bar and Fred Creek formations along the Porcupine River corridor (Figure 3); and 2) mafic flows/tuffs, volcaniclastic strata, and various dikes and sills exposed to the south and southeast of the Porcupine River and within the Keele Range (Figure 2). Mafic intrusions in the Porcupine River corridor are up to 5 m thick and cut across thick exposures of the pre-Mississippian Ch’oodeenjìk succession (Figure 5(f)), although they more commonly occur as highly deformed isolated tectonic blocks. The majority of these mafic rocks are highly weathered and sheared, but well preserved fine- to medium-grained pockets with amphibole and plagioclase laths up to 3 cm long are locally observed. One outcrop near Nothlah Hill, ~22 km east of Old Crow, consists of fine-grained amphibolite with well-developed foliation and mineral stretching lineation marked by elongated feldspar [15] (Figure S2E). In contrast to these highly deformed rocks within the PSZ, mafic rocks of the Keele Range are undeformed and consist of m-scale dikes and sills, as well as >100 m thick exposures of basalt flows, breccias, tuffs, and volcaniclastic strata (Figure S2F).

4.2. Analytical Results

4.2.1. Carbon, Oxygen and Strontium Isotope Chemostratigraphy

δ13Ccarb and δ18Ocarb values from the Caribou Bar formation range from -3.7 to 3.9‰ and -0.5 to 10.4‰, respectively, with little variation between individual measured sections (Figure 7(a); Table DR1). The maximum deviation of δ13Ccarb values from the mean for each measured section does not exceed 1.4‰ (and 2.7‰ for δ18Ocarb). In sections J1701 and J1704 of member B of the Caribou Bar formation, δ13Ccarb values are depleted and oscillate around -1.3 to -2.3‰, while mean δ18Ocarb values are -5.1 to -5.4‰. In section J1702 of the Caribou Bar formation member A, δ13Ccarb and δ18Ocarb values are close to -0.1 and -7.7‰, respectively, whereas in section J1703, δ13Ccarb and δ18Ocarb values hover around 3.0 and -8.4‰, respectively. Finally, in section KF17-6 of the Caribou Bar formation member B, δ13Ccarb values are positive with a mean around 2.2‰; δ18Ocarb values in this section oscillate around -1.3‰. There is no correlation between δ18Ocarb and δ13Ccarb values in any individual measured sections (maximum r2 = 0.06, Figure 7(b)).

In member A of the Caribou Bar formation, strontium concentration data vary from 155 to 1477 ppm (J1702) and 209 to 417 ppm (J1703) (Figure 7(c); Table DR2). Here, we apply a cutoff concentration value of 250 ppm as an indication of reliable 87Sr/86Sr ratios (e.g. [9294]). Five samples from section J1702 have Sr concentrations higher than that value and yielded a narrow range of 87Sr/86Sr isotopic values between 0.70636 and 0.70652 (Figure 7(d), average of 0.70642). Similarly, four samples from section J1703 yielded 87Sr/86Sr isotopic values between 0.70700 and 0.70714 (Figures 7(c), 7(d); average of 0.70704).

4.2.2. U-Pb Detrital Zircon Geochronology and Hf Isotope Geochemistry

(1) Pinguicula Group (?), Keele Range. Sample KF17-48 is a quartz arenite collected from the Keele Range, ~60 km south of the Porcupine River (Figure 2). The unit was originally mapped as the Paleoproterozoic Quartet Group by Norris [59] and subsequently reassigned to the Mesoproterozoic–Neoproterozoic Pinguicula Group [14]. In thin section, the sample is dominated by rounded to subrounded grains of quartz with sutured contacts, as well as rare chert lithics. The cement is composed of amorphous silica, which also forms veins cutting through the sample (Figure S3A). Zircon grains are rounded to subrounded and range from ~40–190 μm in diameter. Major U-Pb age peaks occur at ca. 1050, 1140, 1445, and 1630 Ma with significant populations around 1000–1230 (25%), 1370–1500 (28%) and 1550–1700 Ma (22%) (Figure 9; Table DR3). The youngest concordant zircon yields an age of 940 ± 11 Ma, and the youngest population is 1019 ± 10 Ma (weighted average; n =5; MSWD =0.4; error is reported at 2σ based on the youngest 1σ grain cluster where n >3 grains [95]).

(2) Lahchah Formation, Porcupine River. Sample KF 17-9 is a lithic arenite collected from the Lahchah formation along an unnamed N-S-trending creek ~500 m from its confluence with the Porcupine River (Figure 3). In thin section, the sample is dominated by carbonate grains with floating angular to subangular quartz grains. Chert lithics constitute a minor component of the sample, and many of the carbonate grains are coated by iron oxides or are recrystallized. Parts of the sample contain chalcedony between detrital grains (Figure S3B). Zircon grains are subrounded to rounded with grains ranging in diameter from ~35–160 μm. Major U-Pb age peaks occur at ca. 1100, 1460, and 1630 Ma, with the majority of ages falling within 1000–1530 (67%) and 1550–1765 Ma (24%, Figure 9). Six grains produced Tonian ages in the range of 911 to 999 Ma. The youngest concordant grain yields an age of 911 ± 14 Ma, and the youngest population is ca. 994 ± 9 Ma (n =5; MSWD =0.9).

Sample KF17-59 is a quartz arenite collected from an outcrop of the Lahchah formation along Caribou Bar Creek (Figure 3). In thin section, the sample is dominated by subangular to subrounded grains of quartz with minor chert and muscovite. The interstitial spaces between quartz grains are filled with sericite, clay minerals and minor chalcedony cement (Figure S3C). Zircon grains are rounded to subrounded and range in diameter from ~40–140 μm. Major U-Pb age peaks occur at ca. 1075, 1380, and 1450 Ma, with major populations ranging from 1070–1270 (25%), 1325–1540 (41%) to 1590–1880 Ma (17%) (Figure 9). The youngest concordant zircon yields an age of 1032 ± 12 Ma, and the youngest population is 1045 ± 8 Ma (n =5; MSWD =0.6).

Sample KF17-40 is a quartz wacke collected from the Lahchah formation along the northern edge of the Mount Schaeffer pluton (Figure 3). Quartz grains in this sample are aligned along weakly defined foliation planes, and the majority of grains are subangular to subrounded with many grains displaying irregular boundaries. Muscovite is in general aligned within the foliation plane, while biotite forms either elongated crystals perpendicular to the foliation plane or subhedral grains overprinting the dominant fabric. The matrix is composed of clay minerals, sericite and fine-grained biotite (Figure S3D). Based on the presence of metamorphic biotite, this sample was likely metamorphosed during emplacement of the Old Crow suite [72]. Zircon grains are rounded to subangular and range in diameter from ~50–180 μm. Significant U-Pb age peaks occur at ca. 1060, 1365, 1445, and 1635 Ma, with major populations spanning ca. 1000–1250 (31%), 1250–1500 (38%) and 1560–1780 Ma (16%) (Figure 9). Six grains yield ages in the range of 892 to 983 Ma. The youngest concordant zircon grain yields an age of 754 ± 9 Ma, and the youngest population is 975 ± 13 Ma (n =4; MSWD =0.3).

(3) Fred Creek Formation, Porcupine River. Sample K1805 is a lithic wacke collected 2.7 km west of the international border from an ~2 m thick red-weathering horizon in member B of the Fred Creek formation (Figure 3). In thin section, this sample is dominated by rounded to subrounded grains of recrystallized detrital carbonate clasts with floating grains of angular quartz. The matrix is composed of carbonate micrite mixed with quartz, iron oxides and organic matter (Figure S3E). Zircon grains are rounded to subrounded and range from ~40–100 μm in diameter. Major age peaks occur at ca. 1160, 1325, 1440, 1650 and 1850 Ma with the majority of ages being Mesoproterozoic to late Paleoproterozoic (1000 to 2500 Ma, 90%) (Figure 9). This sample also has a minor Tonian age population (777 to 997 Ma; 5%, Figure 9); the youngest concordant zircon yields an age of 777 ± 7 Ma, and the youngest population is 812 ± 9 Ma (n =3; MSWD =1.1).

(4) Lahchah Formation(?), Old Crow Uplift. Sample KF17-43 is a micaceous quartz wacke collected ~42 km north of the Porcupine River within the Old Crow Uplift from strata of the Lahchah formation(?) (Figures 2 and 3). In thin section, quartz grains are aligned within the foliation plane with evidence for recrystallization, while muscovite and biotite crystals are randomly oriented. The matrix is dominated by clay minerals or sericite and fine-grained biotite (Figure S3F). Zircon grains are subrounded to subangular and range in size from ~30–155 μm. A major U-Pb age peak occurs at ca. 1570 Ma, with 70% of the U-Pb ages falling within the 1400–1700 Ma range. Subordinate peaks occur around ca. 1260, 1480, and 1750 Ma (Figure 9). The youngest concordant grain yields an age of 974 ± 11 Ma, and the youngest population is 1127 ± 12 Ma (n =5; MSWD =0.5).

(5) Sunaghun Formation(?), Old Crow Uplift. Sample 13WW38 was originally reported in Colpron et al. [14] and is a texturally mature fine-grained quartz arenite collected ca. 12 km northeast of the Old Crow village (Figure 2). Major U-Pb age peaks occur at ca. 1180, 1450, 1650 and 1850 Ma, with significant populations of 1000-1240 (29%) and 1300-1500 Ma (41%) grains and subordinate Paleoproterozoic and Archean populations (Figure 9). The youngest concordant zircon yields an age of 976 ± 19 Ma, and the youngest population is ca. 1004 ± 16 Ma (n =5; MSWD =1.1).

Sample KF17-42 is a quartz arenite collected ~40 km northeast of the Porcupine River from what is tentatively correlated to the Sunaghun formation(?) exposed within the Old Crow Uplift (Figure 2). In thin section, the sample is dominated by subangular grains of quartz, many display undulatory extinction and have sutured contacts. Chert lithics constitute a minor component of the sample, along with a few muscovite grains, and the matrix is dominated by clay minerals and sericite (Figure S3G). Zircon grains are rounded to subangular and range in size from ~50–200 μm. Major U-Pb age peaks occur at ca. 1160, 1450, and 1620 Ma, with significant populations of 1060–1275 (33%) and 1275–1535 Ma (48%) grains and minor early Paleoproterozoic and Archean populations (Figure 9). The youngest concordant zircon yields an age of 986 ± 17 Ma, and the youngest population is ca. 1054 ± 11 Ma (n =5; MSWD =0.7).

(6) Darcy Creek Formation, Porcupine River. Sample KF17-17 is a matrix-supported pebble conglomerate collected from the Darcy Creek formation along a small unnamed creek ~4 km east of the international border (Figure 3). In thin section, the sample is dominated by subrounded to subangular grains of carbonate and chert, while the matrix is composed of fine-grained carbonate and iron oxides (Figure S3H). Zircon grains are rounded to subangular and range in diameter from ~50–200 μm. Major U-Pb age peaks occur at ca. 1080, 1410 and 1630 Ma, with significant populations around 980–1225 (29%), 1300–1530 (20%) and 1550–1700 Ma (15%, Figure 9). Neoproterozoic ages constitute 10% of the sample population with Tonian zircons ranging from 790 to 999 Ma and three Ediacaran zircons in the range of 626-653 Ma. Paleozoic ages constitute 5% of the dated zircon grains with U-Pb ages ranging from 379 to 470 Ma. The youngest concordant grain yields an age of 379 ± 6 Ma and the youngest U-Pb age population is 427 ± 5 Ma (n =4; MSWD =1.2).

Sample KF17-14 is a matrix supported chert pebble conglomerate collected from strata unconformably overlying the Caribou Bar formation member B along the northern bank of the Porcupine River ~7 km west from the confluence with Caribou Bar Creek (Figure 5(c)). In thin section, the sample is dominated by rounded to subrounded clasts of carbonate lithics and peloids, as well as angular to subangular grains of quartz, some of which are embayed. The matrix is composed of carbonate micrite with spar (Figure S3I). Zircon grains are angular to rounded and range in size from ~50–170 μm. Major U-Pb age peaks occurs at ca. 775, 1150, 1470, and 1875 Ma, while significant populations cluster around 670–835 (35%), 1000–1500 Ma (36%) and 1600-2000 Ma (19%; Figure 9). The youngest concordant grain yields a U-Pb age of 618 ± 4 Ma and the youngest U-Pb age population is 682 ± 5 Ma (n =5; MSWD =0.6).

Samples 13WW40 and KF17-32 were collected from a broad zone of highly deformed shale, phyllite, sandstone, and carbonate exposed on the southern bank of the Porcupine River, ~1.2 km west from the confluence with Caribou Bar creek (Figures 2 and 3). This outcrop was studied in detail by von Gosen et al. [15] due to well-preserved indicators of strike-slip displacement. These samples come from layers of gritty sandstone to pebble conglomerate within a medium- to thick-bedded succession of maroon shale, siltstone and dolomitic sandstone. As reported in Colpron et al. [14], major U-Pb age peaks in sample 13WW40 occur at ca.1090, 1350, 1450 and 1650 Ma, with significant populations around 1000-1200 (24%), 1250-1500 (37%), 1610-1780 Ma (26%; Figure 9). The youngest single concordant grain with a U-Pb age of 385 ± 8 Ma (reported in Table DR3) was originally considered suspect and not robust enough to revise the assumed Neoproterozoic depositional age for the unit.

Sample KF17-32 was collected from a pebbly sandstone horizon at the same locality to test this interpretation. In thin section, the sample is dominated by rounded quartz grains and chert lithics with a bimodal size distribution ranging between ca. 400 to 1000 μm and ca. 50 to 150 μm (Figure S3J). Zircon grains are rounded to subrounded and range in size from ~50–200 μm. Major U-Pb age peaks occur at ca. 1160, 1370, 1450, and 1640 Ma, with significant populations around 1000–1520 (66%) and 1610–1770 Ma (20%; Figure 9). Two grains are Tonian (973 ± 10 and 954 ± 10 Ma), two are Silurian (423 ± 5 and 433 ± 6 Ma) and four are Devonian (366 ± 5 to 402 ± 6 Ma) in age. There are two distinctly younger zircon grains, one that is 263 ± 3 Ma and one that is 51 ± 1 Ma; we speculate these young ages may be a result of hydrothermal alteration or Pb-loss during deformation. We use a weighted average of 368 ± 7 Ma (MSWD =0.3) from the two youngest Devonian grains as the maximum depositional age, which is consistent with the single youngest grain in sample 13WW40.

(7) Tuttle Formation, Keele Range. Sample 13WW44 was reported in Colpron et al. [14] and is a chert-pebble conglomerate collected ~57 km southeast of Old Crow village. The sample comes from a horizon of conglomerate that overlies the Upper Devonian Imperial Formation. This unit was initially mapped as Kekiktuk Conglomerate(?) by Norris [27], but it was later assigned to the Tuttle Formation by Norris [96] and Richards et al. [97] and back to Kekiktuk Conglomerate(?) by Gordey & Makepeace [65]. In order to avoid confusion with North Slope subterrane terminology (see discussion), we follow Richards et al. [97] in assigning it to the Tuttle Formation. Major U-Pb age peaks occur at ca. 378, 1090, 1470 and 1840 Ma, with significant populations around 375–425 (8%), 1000–1500 (39%), 1600–1670 (9%) and 1780–2100 Ma (24%) and a prominent Archean population. The youngest concordant grain yields a U-Pb age of 375 ± 9 Ma and the youngest U-Pb age population is 377 ± 6 Ma (n =5; MSWD =0.1).

(8) Zircon Hf Isotope Geochemistry. Lu-Hf isotopic analyses were performed on 118 detrital zircon grains representing the major U-Pb age populations described above (Figure 9; Table DR4). Tonian grains from the Lahchah and Sunaghun formations have intermediate εHf(t) values of -4.5 to -3.9, while Mesoproterozoic grains show generally intermediate to juvenile εHf(t) compositions ranging from -2.4 to +10.2 (Figure 9(b)). In contrast, Paleoproterozoic grains have more moderate to evolved compositions with εHf(t) values as low as -15.1. In the overlying carbonate-dominated strata of the Fred Creek formation, Mesoproterozoic grains are relatively juvenile with εHf(t) values of +6.8 to +8.5. Similar to the underlying strata, Paleoproterozoic grains are more evolved with εHf(t) values up to -9.6. Sample KF17-48 from the Keele Range displays a similar range of εHf(t) values, with Mesoproterozoic grains ranging from +1.9 to +9.8 and Paleoproterozoic grains ranging from -1.0 to +3.2 (Figure 9(b)). The εHf(t) composition of Mesoproterozoic to Paleoproterozoic detrital zircons from the Darcy Creek formation is comparable to that of the underlying strata. Neoproterozoic grains show a wide range of values. εHf(t) values of ca. 629, 842, and 927 Ma zircon grains from sample KF17-17 are -7.1, +4.0, and -2.1, respectively, while ca. 714-790 Ma grains from sample KF17-14 have more intermediate to juvenile εHf(t) compositions ranging from +1.6 to +8.9. Finally, four Paleozoic grains from the Darcy Creek formation yield εHf(t) values of -13.7 to +3.2 (Figure 9(b)).

4.2.3. U-Pb Igneous Zircon Geochronology

SHRIMP-RG U-Pb ages were obtained for 18 igneous zircons separated from sample KF17-12. This sample comes from a coarse-grained and deformed diabase dike forming a ca. 5 m wide intrusion within black shale and limestone of the Caribou Bar formation along an unnamed creek ~2 km from its confluence with the Porcupine River (Figures 3, 5(f)). In thin section, sample KF17-12 contains >1 mm long phenocrysts of sericitized twinned plagioclase, ~1 mm long pseudomorphs of chlorite replacing amphibole, and embayed quartz. Although several have relatively high uncertainties, all analyses define simple systematics with no indication of inherited components or Pb-loss. All 18 analyses define a weighted mean 206Pb/238U age of 380 ± 4 Ma (Figure 10(a) and 10(b); n =18; MSWD =0.7) and a Concordia age of 380 ± 4 Ma (95% confidence; MSWD =1.1) (Table DR5). These zircons have similar Hf concentrations, U/Yb and Gd/Yb ratios, and REE patterns (Figure 10(c)–10(e); Table DR5), as well as positive Ce and Sm and negative Eu anomalies, all of which are consistent with igneous crystallization of zircon in the presence of plagioclase (e.g., [98]).

4.2.4. Igneous Whole-Rock Geochemistry

Twenty samples of mafic igneous rocks, forming both intrusions and flows, were analyzed from outcrops exposed along the trace of the PSZ, including Nothlah Hill, as well as in the adjacent Keele Range (Figures 2, 3, Table DR6). We excluded one sample (KF17-50) from a dike intruding the Ch’oodeenjìk succession because of its high loss on ignition (LOI) value (5.8 wt%), as well as its anomalous Al2O3 and Sr values (19.7 wt% and 1303 ppm, respectively). Most of our geochemical assessment is based on incompatible trace elements, which are typically not affected by alteration or low-grade metamorphism (e.g., [99, 100]).

The majority of the analyzed samples plot within the basalt field of a Zr/Ti and Nb/Y plot [100], with four samples falling within the basaltic andesite or andesite field (Figure 11(a)). Samples with the most primitive compositions, as indicated by low Zr contents, generally show higher Ni contents (Figure 11(b)), and all of the samples plot along clinopyroxene or orthopyroxene fractionation trajectories, with sample KF17-12 plotting at the terminus of the evolution trend. Similarly, Sc contents decrease with increasing Zr in these samples (Figure 11(c)), which is also consistent with pyroxene fractional crystallization trends. The sample suite as a whole plots along a linear Eu anomaly trend when plotted against Zr, which is consistent with plagioclase fractionation (Figure 11(d)). Noticeably, samples from the Keele Range are generally more evolved and alkaline, forming clusters towards the end of the evolution trajectories (Figure 11); despite this, it is difficult to separate these mafic rocks from the deformed intrusions within the PSZ corridor, except for the anomalous sample KF17-12.

All of the analyzed samples are enriched in light rare earth elements (LREE) with respect to primitive mantle, and they display trends that are consistent with some degree of crustal contamination (Figure 11(e)). Sample KF17-12 shows the highest LREE/HREE enrichment, which is indicated by a La/Yb ratio normalized to primitive mantle of 14.2 compared to a whole sample suite average of 3.5. Samples from the Keele Range show elevated La[PM]/Yb[PM] values (average of 5.2) and those from the Nothlah Hill record lower La[PM]/Yb[PM] ratios (average of 2.4). All of the samples display a negative Nb anomaly, and most of the samples show a negative Ti anomaly, with the most significant deviation recorded in sample KF17-12 (Figure 11(e)). On a bivariate plot of Th/Yb vs. Nb/Yb (Figure 12(a)), samples from Porcupine River and surrounding area form an array with consistently elevated Th/Yb values (0.5–2.8 with a maximum of 9.7 for sample KF17-12), which is indicative of high magma-crust interaction related to crustal contamination or wedge melting above a subduction zone [101]. This is also reflected in the overall elevated concentrations of REEs (Figure 12(b)). The Ti and V compositions of these samples (Figure 12(C)) show a wide range of values plotting around the Ti/V ratio of 20, with individual values ranging from 14–41.

Initial 87Sr/86Sr and εNd(t) values for these igneous rocks were calculated at both 720 and 380 Ma based upon previously suggested ties to the ca. 720 Ma Franklin Large Igneous Province (LIP) (see [14]) and the newly obtained ca. 380 Ma date from sample KF17-12 in this study (Figures 12(d) and 12(e); Tables DR7 and DR8). The age-corrected 87Sr/86Sr ratio in sample KF17-12 is 0.72007, which is slightly elevated compared to the remaining samples that yield values between 0.70995 and 0.71636 (380 Ma) or 0.70787 to 0.71265 (720 Ma) (Figure 12(d)). These samples also yield εNd(t) values of -9.5 to -2.4 (380 Ma) or -7.0 to -1.5 (720 Ma), with depleted mantle model ages ranging from 2.3 to 3.3 Ga. In contrast, the εNd(t) value from sample KF17-12 is -11.1, with a depleted mantle model age of 1.9 Ga (Figure 12(e); model ages are reported in Table DR7).

5. Discussion

5.1. Age and Provenance of the Ch’oodeenjìk Succession

As highlighted previously, highly deformed sedimentary units exposed along the Porcupine River of Yukon and Alaska have been assigned to the Proterozoic [14, 27, 58, 61, 64], early Paleozoic [62, 67] or kept undivided [73]. Here, we separate the Neoproterozoic Ch’oodeenjìk succession into a lower siliciclastic-dominated succession composed of the Lahchah and Sunaghun formations and an upper carbonate-dominated succession comprising the Caribou Bar and Fred Creek formations (Figure 4). We report a ca. 975 Ma maximum deposition age for the Lahchah formation and a ca. 1054 Ma maximum deposition age for the Sunaghun formation, which are consistent with previous U-Pb data reported by Colpron et al. [14]; however, the presence of zircon grains as young as 911 ± 14 Ma (1σ) in sample KF17-9 and 754 ± 9 Ma (1σ) in sample KF17-40 from the Lahchah formation indicates this entire succession is likely late Tonian in age.

Tonian depositional ages for these strata are also provided from the overlying Caribou Bar formation, whose 87Sr/86Sr isotopic data yield a narrow range between 0.70636 and 0.70714 (average of 0.70670; n =9; Figure 7(d)). These 87Sr/86Sr values are characteristic of the late Tonian, when global marine carbonate 87Sr/86Sr ratios increased from ~0.7055 to >0.7060 around 840–850 Ma and stayed below or around ~0.7070 until the end of the ca. 660–717 Ma Sturtian glaciation [102104]. Although we do not observe significant internal variation in δ13Ccarb data from individual measured sections of the Caribou Bar formation, there is variation within the overall formation between -3.7 to 3.9‰. The negative δ13Ccarb values may correspond to either the ca. 810 Ma Bitter Springs negative carbon isotope excursion [94, 105] or the ca. 735 Ma Islay anomaly [91, 102, 106], although this is speculative given the lack of detailed stratigraphic control. Despite this, the presence of VSMs and molar tooth sedimentary structures in the Caribou Bar formation also confirms a Tonian depositional age for these strata, since VSMs are distributed globally in marine basins between ~730–790 Ma [90, 91, 107] and molar tooth structures are unique to Paleoproterozoic–Tonian carbonate deposits [88]. These age constraints are also consistent with the presence of late Tonian (ca. 780 to 820 Ma) detrital zircon grains within the overlying Fred Creek formation (Figure 9); thus, together these data suggest that the Lahchah, Sunhagun, Caribou Bar and Fred Creek formations all have firm late Tonian depositional ages.

Detrital zircon U-Pb data from these Tonian strata are very similar to one another, except for sample KF17-43 (Figure 9, see below). They are generally characterized by prominent Mesoproterozoic age populations (ca. 1000–1500 Ma) with subordinate populations of Paleoproterozoic (1600–2000 Ma) and Neoarchean (2500–2800 Ma) grains. Proximal and roughly age-equivalent Tonian siliciclastic strata are exposed in the Tatonduk and Coal Creek inliers of the Yukon block (Fifteenmile and Mount Harper groups), the Mackenzie and Wernecke Mountains (Hematite Creek, Katherine and Little Dal groups of the Mackenzie Mountain Supergroup; [108]), the Minto, Brock and Coppermine inliers of Northwest Territories and Victoria Island (Rae, Reynolds Point and Holman groups of the Shaler Supergroup; [108]), and the North Slope subterrane of Alaska and Yukon (Mount Weller Group; [19]) (Figure 1). The provenance of the Lahchah and Sunaghun formations resembles that of the Hematite Creek, Katherine, and Little Dal groups [14, 108, 109], the Fifteenmile and Mount Harper groups [12], and the Nelson Head Formation of the Shaler Supergroup [108] (Figure 13(a)). The main difference between Tonian strata of the Shaler Supergroup and Ch’oodeenjìk succession is a greater proportion of ca. 1800–2000 Ma zircons in the Reynolds Point and Holman groups, which are hypothesized to reflect their proximity to the ca. 1820-1890 Ma Great Bear magmatic arc [108, 110113]. The εHf(t) values from the Ch’oodeenjìk succession broadly overlap data from the Mount Weller Group (Figure 13(b)) [19], but equivalent data from the northwestern Canadian successions are needed to enable further comparisons based on Hf isotopic data.

Despite these broad similarities in provenance to Tonian strata of the Yukon block and the Mackenzie and Wernecke Mountains, the Ch’oodeenjìk succession does contain rare nearly syn-depositional Tonian (ca. 780-820 Ma) zircon age populations (Figure 13(a)) that are virtually absent in strata of the Mackenzie Mountains Supergroup or the Fifteenmile and Mount Harper groups [12, 108, 109]. In contrast, these Tonian zircon populations do exist in small proportions within the Mount Weller Group [19] and the Kilian and Kuujjua formations of the upper Shaler Supergroup [114]. Intriguingly, Macdonald et al. [115] reported a ca. 811 Ma tuff from the Reefal assemblage of the Fifteenmile Group in the Coal Creek inlier (Figure 1), but detrital zircon grains of this age are notably absent in bounding siliciclastic strata of the Fifteenmile and Mount Harper groups (Figure 13(a)) [12]; this suggests a limited role for this poorly understood magmatic event to provide a source of regionally significant detrital zircons in the Yukon block and therefore does not provide a distinct provenance link with the Ch’oodeenjìk succession. In fact, the source of this tuffaceous horizon is enigmatic given the lack of magmatism of this age in autochthonous Laurentia (e.g., [46]). Similar ca. 780–820 Ma detrital zircon age populations have been observed in age-equivalent strata of southern Siberia [116, 117], and felsic magmatism of this age has been reported from northern Mongolia [118, 119] and South China [120122]; slightly older Tonian magmatic rocks are also reported within the Kalak Nappe Complex of northern Scandinavia (ca. 825–840 Ma; [123125], Scotland (ca. 830–870 Ma; [126129]), Farewell terrane of central Alaska (830-860 Ma; [130]) and the Seward Peninsula region of the Arctic Alaska–Chukotka microplate (ca. 865 Ma; [71]).

The anomalous sample KF17-43 was collected north of the Old Crow batholith (Figure 2) and has a significantly different detrital zircon U-Pb age distribution from other samples in the Ch’oodeenjìk succession, as well as the other Tonian successions in northwestern Laurentia (Figure 13(a)). In particular, it contains a prominent peak around 1570 Ma, which falls within the North American magmatic gap [131]. Similar age zircons (~1550–1600 Ma) were reported from samples in the Escape Rapids and Wynniatt formations of the Shaler Supergroup (Figure 13(a)) – these were interpreted as being derived via long-distance transport from the Grenville Province of eastern Laurentia (i.e., the Pinware terrane) or a non-Laurentian source region north of the Amundsen Basin [108, 114]. Younger ca. 1500 Ma zircon populations were also reported from the Mesoproterozoic–Tonian(?) PR1 unit exposed in the Coal Creek inlier, where they were interpreted as being derived from the Mt. Isa inlier of northeastern Australia [132]. We posit the older ca. 1570 Ma age population in sample KF17-43 favors a similar source region to Tonian strata of the Shaler Supergroup. As highlighted previously, this sample was collected from an isolated outcrop belt north of the Porcupine River that is locally intruded by the Old Crow batholith; thus, the exact stratigraphic position of this sample with respect to the better characterized Ch’oodeenjìk succession described herein is still uncertain, and it remains a possibility that this sample is from a fault-bounded sliver of para-autochthonous or allochthonous rocks between the North Slope subterrane and Ch’oodeenjìk succession.

5.2. Age and Provenance of the Darcy Creek Formation

Tonian strata of the Ch’oodeenjìk succession are unconformably overlain by texturally immature siliciclastic rocks of the Darcy Creek formation, which herein yielded a Late Devonian (Famennian) maximum depositional age of 368 ± 7 Ma (Figure 9). This Upper Devonian or younger unit was previously assumed to be part of the older Neoproterozoic succession [14], but our recognition of a profound erosional contact with the Caribou Bar formation (Figure 5(c)), coupled with the appearance of younger Neoproterozoic and Paleozoic detritus in all of the Darcy Creek samples (Figure 9), supports separation of these strata from the Ch’oodeenjìk succession. There are no previous descriptions of this profound unconformity from the Porcupine River area, but based on geological relationships both north and south of the PSZ, there are three regionally significant post-Famennian unconformities that may provide insight into the depositional age of the Darcy Creek formation: 1) the “pre-Mississippian” unconformity of the North Slope subterrane, where the Carboniferous Endicott and Lisburne groups rest on various polydeformed Neoproterozoic to early Paleozoic units (e.g., [68] and references therein); 2) the sub-Permian unconformity of the northern Yukon block, where Upper Carboniferous(?) to Lower Permian strata of the Jungle Creek Formation and its equivalents rest on deformed to concordant Proterozoic to Lower/Middle Carboniferous strata ([97], and references therein); or 3) the widespread sub-Jurassic and/or sub-Cretaceous unconformities that characterize both the North Slope subterrane and Yukon block and are related to the Brookian and Cordilleran orogenies, respectively (e.g., [68, 133, 134]). Given lack of post-Devonian zircons, which are common in the Arctic Alaska-Chukotka microplate (e.g., [14, 135, 136]), we suggest that Darcy Creek formation is most likely Upper Devonian–Carboniferous in age and therefore occupies a similar stratigraphic position to the Endicott Group of the North Slope subterrane.

Based on its sedimentological characteristics and detrital zircon U-Pb age populations, the Darcy Creek formation was locally derived from recycling of the underlying Tonian succession; however, these strata also host younger Neoproterozoic and Paleozoic detritus from unknown source regions exotic to northwestern Laurentia (Figure 9(a)). For example, ca. 600–835 Ma zircon age populations in the Darcy Creek formation have only been reported in northwestern Laurentia from strata post-dating the arrival of the Devonian–Carboniferous Ellesmerian clastic wedge (e.g., [137]). Locally, the Upper Devonian Imperial Formation and the overlying Famennian–Tournaisian Tuttle Formation of northern Yukon contain small populations of ca. 500–700 Ma zircon grains [137, 138] (Figure 13(c)). Similarly, late Neoproterozoic detrital zircon ages occur in Middle to Upper Devonian strata of the Ellesmerian clastic wedge in the Canadian Arctic Islands, most prominently in the Frasnian–Famennian Parry Islands Formation [139]. Small populations of ca. 500–700 Ma zircon also occur in Ordovician–Devonian syn-orogenic strata of the Clarence River Group of the North Slope subterrane [20, 23].

In contrast to most samples from the Darcy Creek formation, sample KF17-14 contains significant Tonian detrital zircon ages between ca. 750 and 810 Ma (Figure 13(c)). This age population is virtually absent in all of the comparable clastic wedge successions mentioned above, but it does occur in smaller proportions within Devonian(?) metasedimentary rocks of the Schist Belt [69] and the Seward Peninsula region [140] of the southwestern subterranes of the Arctic Alaska-Chukotka microplate (Figure 13(c)). Other detrital matches include minor ca. 785 Ma zircon populations in the early Paleozoic Donjek [141] and Banks Island assemblages [142] of the Alexander terrane, and in Ordovician and older units of the Farewell terrane [130, 143]. Highly localized ca. 780 Ma mafic dikes, sills, and minor volcanic rocks have been reported in the Mackenzie Mountains and elsewhere in western Laurentia associated with the Gunbarrel large igneous province (LIP; e.g., [144, 145]); however, only negligible Gunbarrel LIP-related detrital zircon age populations have been reported in Neoproterozoic strata of Death Valley and Utah (e.g., [146, 147]) and detrital zircons of this age are virtually absent in autochthonous strata of the Yukon block and Mackenzie/Wernecke Mountains [12]. In addition, the short-lived nature of this event does not correspond well with the broad spread of ages between 750-810 Ma that are preserved in sample KF17-14 (Figure 13(c)).

Due to the presence of ca. 500–740 Ma igneous rocks and abundant detrital zircons in metasedimentary units of the southwestern subterranes of the Arctic Alaska-Chukotka microplate [69, 140, 148151] this composite terrane is often invoked as a source of Neoproterozoic detritus to the Ellesmerian clastic wedge [8, 137139] (Figure 13(c)). The presence of these Cryogenian–Ediacaran igneous ages and the similarities in provenance to portions of northern Baltica led to the proposed restoration of the Arctic Alaska-Chukotka microplate adjacent to the ca. 550–610 Ma Timanian orogen [152] of northern Scandinavia and Russia (e.g. [2, 71, 140, 148, 149]) or more broadly, parts of this composite terrane to an extensive Neoproterozoic arc system fringing Baltica, Siberia and Laurentia ([35]; see also [129, 153157]). In this context, the ca. 750-810 Ma age populations from the Darcy Creek formation, as well as the rare Tonian grains in older strata of the Ch’oodeenjìk succession, could potentially be linked to this long-lived active margin. It is also noteworthy that large populations of ca. 770 Ma detritus are present in Neoproterozoic strata of southern Siberia [158] and Mongolia [116], ca. 785 Ma felsic plutons are present in the Tuva-Mongolia Massif [159], and ca. 720-790 Ma granites in the Yenisey Ridge of Western Siberia [160, 161], so Siberian source regions are also plausible for these Tonian zircon grains.

The ultimate derivation of Neoproterozoic detritus in the Darcy Creek formation from the North Atlantic or Siberian realms via the Arctic Alaska-Chukotka microplate or other circum-Arctic terranes is further supported by the presence of exotic ca. 420–470 Ma zircon grains (Figure 9). These early Paleozoic U-Pb age populations are abundant in strata of the Ellesmerian clastic wedge (Figure 13(c); [8, 137139]), as well as the Clarence River Group of the North Slope subterrane (e.g., [20]) and a wide variety of Devonian(?) metasedimentary units throughout the southwestern subterranes of the Arctic Alaska–Chukotka microplate (e.g., [35, 69, 151]). The Ordovician-Silurian signature reflects ties to arc magmatism in the circum-Arctic realm, which is locally preserved in the Pearya terrane [162], Doonerak arc of the Arctic Alaska–Chukotka microplate ([35] and references therein), and the Descon Formation of the Alexander terrane (e.g., [163, 164]).

The prominent ~420–470 Ma detrital zircon age population is also reported in the Mississippian Kekiktuk Conglomerate of the Endicott Group [8, 14, 35, 165, 166], which consists of an overlap assemblage on the North Slope subterrane following assembly of the Arctic Alaska-Chukotka microplate (e.g., [12]) (Figure 13(c)). The Kekiktuk Conglomerate often contains a distinct ca. 360-380 Ma age population that has been linked to the local weathering of Upper Devonian granitic rocks exposed throughout the North Slope subterrane [8, 14, 35, 72]. The conglomerate sample in the northern Keele Range (13WW44 of Colpron et al. [14]; Figure 9) yields a similar U-Pb age distribution to published Kekiktuk samples, as well as data from the Darcy Creek formation (Figure 13), but it also records a prominent ca. 1800–2000 Ma age population that is more similar to the underlying Imperial Formation (Figure 13(c)) [137, 138], supporting its assignment to the Tuttle Formation [97] and differentiating it from the Darcy Creek formation. Although Norris [96] and Richards et al., [97] recognized differences in stratigraphy across the PSZ, they extended the North Slope subterrane nomenclature of the Endicott and Lisburne groups into the northern Yukon block based on perceived lithological similarities. Here, we recommend terminating North Slope subterrane nomenclature at the PSZ for three reasons: 1) the PSZ is a significant crustal-scale tectonic boundary and there is currently no evidence that the Endicott or Lisburne groups present an overlap assemblage on the Yukon block; 2) Carboniferous strata of the northern Yukon block are >400 km away from the type areas of the Endicott and Lisburne groups in the central Brooks Range of Alaska [165, 167]; and 3) this terminology gives the false impression that the northern Yukon block was affected by similar “pre-Mississippian” penetrative deformation to the North Slope subterrane. In conclusion, we recommend differentiating the Endicott Group of the North Slope subterrane from the Darcy Creek formation of the PSZ, both of which are distinct from age-equivalent Upper Devonian–Carboniferous units on the Yukon block (i.e., Imperial, Tuttle, Ford Lake, Hart River, Blackie and Ettrain formations; [96, 97]). The ca. 420-470 and 600–835 Ma detritus present in the Darcy Creek formation constitutes significantly smaller populations in Devonian strata of the Yukon block or is completely absent (Figure 13(c)). This suggests closer proximity of the Darcy Creek formation to sources in the Arctic Alaska–Chukotka microplate or circum-Arctic terranes derived from Baltica or Siberia.

5.3. Age and Significance of Mafic Magmatism in the Porcupine River Corridor

Mafic intrusions exposed in the Porcupine River corridor and Keele Range were initially mapped together as Proterozoic based on their intrusive relationships with hypothesized Proterozoic strata, as well as the lack of intrusions in the overlying early Paleozoic units [27, 58, 59]. Based on a comparison to known regional mafic volcanic rocks, Colpron et al. [14] tentatively correlated these rocks with either the ca. 780 Ma Gunbarrel LIP [144, 145, 168], the ca. 720 Ma Franklin LIP [53, 75, 169, 170], or early Paleozoic volcanic rocks exposed in the northwestern Selwyn basin (e.g., [40, 171, 172]).

Mafic intrusions from the Porcupine River corridor are generally basaltic or andesitic based on their Zr/Ti and Nb/Y ratios (Figure 11(a)), but sample KF17-12 is a clear outlier in terms of its trace element composition. This sample also has a uniquely pronounced Ti anomaly, displays significantly higher LREE fractionation with trends better resembling those of the felsic Old Crow plutonic suite, as well as higher 87Sr/86Sr and lower εNd(t) values compared to the remaining samples (Figure 12). In addition, sample KF17-12 plots in a distinct cluster with the Old Crow suite on a Th/Yb versus Nb/Yb diagram, while all of the remaining samples from the Porcupine River area form an array overlapping the composition of volcanic rocks associated with the Franklin LIP (Figure 12(c)) [75, 170]. For example, the majority of the magmatic and volcanic rocks from the Porcupine River corridor and Keele Range overlap with the Tatonduk suite of the Tatonduk inlier and Kikiktat volcanic rocks of the North Slope subterrane, both of which are local expressions of the Franklin LIP (Figures 1, 12) [75, 170]. Although the geochemistry of the ca. 780 Ma Gunbarrel LIP is somewhat similar (Figure 12) [168], it has significantly higher Ti/V ratios and lower Th/Nb ratios than the samples from the Porcupine River area.

Mafic rocks intruding the Ch’oodeenjìk succession of the Porcupine River corridor do differ from potential age-equivalent rocks of the Keele Range, most prominently in their outcrop expressions. For example, mafic rocks exposed along the Porcupine River are exclusively comprised of sills and dikes, whereas the igneous rocks exposed south of the PSZ are dominated by thick exposures of basaltic flows, breccias, and tuffs with prominent volcaniclastic intervals and rare dikes and sills. The latter are similar to volcanic expressions of the Franklin LIP throughout the Yukon block and North Slope subterrane (e.g., [75, 170, 173]), although the Pleasant Creek volcanics are associated with a prominent dike swarm in the Tatonduk inlier [170, 174]. As highlighted above, the geochemical differences between these two regions of the Porcupine River area are subtle, and correlation to either the Tatonduk suite [170] or the Kikiktat volcanic rocks [75] based on the available geochemistry or petrology is permissible. Importantly, samples from the Keele Range display similar trace element distributions, Th/Yb, and Nb/Yb ratios to the Tatonduk suite, while samples from the Porcupine River corridor more closely resemble the Kikiktat volcanic rocks (Figure 12). The heterogeneous composition of these igneous suites would be expected for widespread LIP-related volcanism [175] and may reflect different origins and/or amounts of crustal assimilation during emplacement of the Franklin LIP.

Our SIMS U-Pb zircon age from sample KF17-12 overlaps in age with the adjacent Late Devonian Old Crow plutonic suite (ca. 368–375 Ma; [72]). Zircons from the mafic intrusion and Old Crow suite display similar trace element ratios (Figure 10(c), 10(d)) and broadly indicate contamination from a crustal component. For example, the covariance of prominent Eu anomalies and similar Hf concentrations in these data are consistent with crystallization of zircon under oxidized conditions during plagioclase fractionation in a continental arc setting (Figure 10(c)) [176]. Thus, the age, geochemistry, and geographic proximity of these features all point to a co-genetic origin for the dated intrusion and Old Crow suite, while the geochemistry and setting of the remaining intrusions point to their association with the older Cryogenian Franklin LIP.

5.4. Paleogeographic and Tectonic Significance

The most fundamental question this study aimed to address is whether deformed Proterozoic and Paleozoic rocks of the PSZ belonged to the Yukon block or the North Slope subterrane (Figure 1). Based on its stratigraphic architecture, provenance, and magmatic history, we propose that the newly defined Ch’oodeenjìk succession of the PSZ most likely represents a para-autochthonous and peri-Laurentian crustal fragment that differs slightly from both of these tectonic blocks. This is primarily based upon the following observations: 1) the abrupt stratigraphic mismatches between the Ch’oodeenjìk succession and adjacent exposures of the Mesoproterozoic–Tonian(?) Pinguicula or Hematite Creek groups of the northernmost Yukon block and the Firth River, Neruokpuk, and Clarence River groups of the North Slope subterrane (Figure 2); 2) the presence of nearly syn-depositional Tonian zircon in the Ch’oodeenjìk succession that is consistent with age-equivalent strata in the Shaler Supergroup of Victoria Island [108, 114] and the Mount Weller Group of the North Slope subterrane [19] but distinctly lacking in the Yukon block [12]; 3) the unusual ca. 1570 Ma U-Pb age population in the Lahchah formation(?) sample north of the Old Crow batholith that is exotic to Tonian strata of the Yukon block [12] but similar to small age populations recorded in the upper Shaler Supergroup [108, 114]; 4) the different outcrop expression of Franklin LIP volcanic rocks in the Ch’oodeenjìk succession compared to immediately adjacent correlative volcanics in the northern Keele Range of the Yukon block; 5) the presence of Upper Devonian mafic and felsic intrusions in the Ch’oodeenjìk succession (Figure 2), which are present in the North Slope subterrane (e.g., [72]) but absent in autochthonous rocks of the Yukon block except locally in the Earn Group of central Yukon [177]; and 6) the presence of a profound erosional unconformity beneath the Upper Devonian–Carboniferous(?) Darcy Creek formation, which is notably lacking in the adjacent Yukon block [97] but widespread throughout the North Slope subterrane (e.g., [68]). Given the ambiguity in the data outlined above and the acknowledgement that the Ch’oodeenjìk succession is situated within the polyphase PSZ, we suggest the most parsimonious interpretation is to distinguish this region as a separate tectonic entity.

During the late Tonian, most paleogeographic reconstructions place Siberia (and North China) adjacent to the northern margin of Laurentia during the break-up of Rodinia (Figure 14(a)) [19, 122, 178184]. We speculate that the Ch’oodeenjìk succession represents a displaced fragment of a Tonian extensional basin formed between Laurentia and Siberia (and/or North China) during the break-up of Rodinia, similar to that proposed for the North Slope subterrane [19]. Such an interpretation is consistent with the presence of ca. 780-820 Ma U-Pb zircon age populations in the Ch’oodeenjìk succession, Shaler Supergroup, and Mount Weller Group, as well as within coeval strata in southern Siberia [116, 117]. Extension between Siberia and Laurentia is marked by the emplacement of the ca. 720 Ma Franklin LIP (e.g. [169]), which is recorded by the abundant mafic dikes intruding the Ch’oodeenjìk succession along the PSZ (Figure 14(a)).

The Old Crow plutonic suite of the Porcupine River area has been interpreted as a subduction-related continental arc or back-arc magmatic suite that was emplaced within the North Slope subterrane outboard of the Laurentian margin [72]. In contrast, Lane [185] and Lane and Mortensen [73] suggested this magmatic suite intruded the Yukon block and pointed to magmatic underplating as a potential factor in pluton emplacement that led to subsequent rifting. The presence of Late Devonian magmatism in the North Slope subterrane and Ch’oodeenjìk succession points to a common tectonic history between the two regions. Recent tectonic models for the North Slope subterrane posit large-scale Silurian–Devonian strike-slip translation from a position adjacent to northeastern Laurentia (e.g., [12, 13, 23]). We propose that this displacement was synchronous with translation of the Ch’oodeenjìk succession along the PSZ [15], most likely from a position outboard of the western Canadian Arctic Islands (Figure 14(b)). Strike-slip displacement was likely in part synchronous with emplacement of the Old Crow magmatic suite [72]. We suggest that deposition of the coarse-grained Darcy Creek formation occurred during and/or shortly after this strike-slip translation, with sediment being recycled from the Tonian sedimentary succession and exotic detritus ultimately derived from the assembly and translation of the Arctic Alaska-Chukotka microplate and other circum-Arctic terranes across the northern margin of Laurentia. In this model, the Ch’oodeenjìk succession may have been juxtaposed with the Arctic Alaska–Chukotka microplate in the middle Paleozoic.

Alternatively, the Ch’oodeenjìk succession may represent a highly deformed northern continuation of the Yukon block, which was sheared but not significantly displaced during middle Paleozoic and potentially younger strike-slip translation along the PSZ (Figure 14). In this model, the Ch’oodeenjìk succession would represent a previously undocumented Proterozoic inlier in northern Yukon. Critically, in all of these and future tectonic models for the evolution of northwestern Laurentia, the PSZ must be considered a deep-seated crustal boundary with various amount of distributed shear that was likely reactivated multiple times during the Paleozoic–Cenozoic evolution of the Arctic.

6. Conclusions

Highly deformed Proterozoic and early Paleozoic strata exposed within the Porcupine Shear Zone (PSZ) of the northern Yukon-Alaska border region represent a newly defined sedimentary succession, referred to herein as the Ch’oodeenjìk succession. This succession is comprised of the newly defined lower siliciclastic-dominated Lahchah and Sunaghun formations (informal) and the upper carbonate-dominated Caribou Bar and Fred Creek formations (informal). Detrital zircon maximum depositional ages, carbonate carbon and strontium isotopic data, and microfossils all indicate that Ch’oodeenjìk succession is early Neoproterozoic (Tonian) in age. The provenance of this succession indicates a Laurentian origin, with detrital zircon U-Pb age populations recording broad similarities to the Mackenzie Mountains and Shaler supergroups and Fifteenmile and Mount Harper groups of northwestern Laurentia and differences with the Mount Weller Group of the North Slope subterrane. However, small Tonian U-Pb age populations in the Ch’oodeenjìk succession, along with the presence of rare ca. 1570 Ma non-Laurentian signatures, favor deposition in an extensional basin formed between Laurentia and Siberia during the breakup of the supercontinent Rodinia. The Ch’oodeenjìk succession was subsequently intruded by mafic dikes during the emplacement of the ca. 720 Ma Franklin LIP. During the early Paleozoic, the Ch’oodeenjìk succession was likely juxtaposed with the North Slope subterrane outboard of the Canadian Arctic Islands along the PSZ. This displacement was potentially synchronous with emplacement of the Late Devonian (ca. 368 to 380 Ma) Old Crow plutonic suite into the Ch’oodeenjìk succession. Syn-tectonic Late Devonian or younger sedimentation along the PSZ is recorded by the newly defined Darcy Creek formation (informal), which overlies the Tonian sedimentary succession with profound erosional unconformity and records detrital zircon signatures consistent with both local sedimentary reworking and delivery from exotic Neoproterozoic and early Paleozoic source regions that can be linked to Arctic Alaska-Chukotka microplate and/or other circum-Arctic terranes.

Data Availability

All the data used in this study are incorporated within the article and in the supplementary material provided along with the article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

We thank the Vuntut Gwich’in community in Old Crow for providing access to outcrops along the Porcupine River. This project was funded by National Science Foundation (NSF) Tectonics grants EAR-1624131 and 1624130, awarded to JVS and WCM, respectfully. The Yukon Geological Survey provided additional logistical support. Support for geochronological analyses was provided by NSF EAR-1649254 awarded to the University of Arizona LaserChron facility. Additional support was provided by an ExxonMobil Student Research Grant awarded to KF through Geological Society of America and from the Undergraduate Research and Advising (UGAR) program at Dartmouth College for CLN and RSM. MT received additional support from the Geological Society of America. Kirk Sweetsir of Yukon Air Service and Alfredo Camacho of the U.S. Fish and Wildlife Service were instrumental in providing access to the field area in the Arctic National Wildlife Refuge and HeliDynamics supported helicopter field work around Old Crow. We are grateful to Marwan Wartes and Karri Sicard of the Alaska Division of Geological and Geophysical Surveys (DGGS) who provided logistical support in Fairbanks. Steve Israel greatly contributed to field work and sample collection in 2017. We thank Thi Hao Bui and Galen Halverson of McGill University for their assistance with Nd isotopic work, Jahan Ramezani of Massachusetts Institute of Technology for help with igneous 87Sr/86Sr isotopic work and Eric Bellefroid of Yale University for carbonate 87Sr/86Sr isotopic work. Xiahong Feng and Jennifer Howley from Dartmouth College helped collect carbonate carbon isotopic data, and Tyler Allen assisted in mineral separation. Finally, we are grateful to Keith Dewing, two anonymous reviewers, and Associate Editor Tamer S. Abu-Alam for providing feedback that helped improve this manuscript. This is Yukon Geological Survey contribution #050.

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