William A. Matthews

A Pangean rim of fire: Reviewing the Triassic of western Laurentia

A synthesis of U-Pb detrital zircon data from the Triassic of western Laurentia is placed in the context of paleogeographic models for Pangea. We find that an emerging body of evidence supports the hypothesis that a Triassic subduction zone and continental magmatic arc system fringed western Laurentia starting in the southwestern United States, continuing northward along the Cordillera, including arc rocks of the Quesnel and Yukon-Tanana terranes, and extending further into the Arctic region. In the context of this geodynamic setting, the western interior basins of North America would have formed, probably by subduction dynamics, as a collage of backarc and retroarc foreland basins. The convergent tectonic model for western Laurentia is consistent with paleogeographic reconstructions that show a subduction zone encircling Pangea, called the Pangean rim of fire.

Timing of deep-water slope evolution constrained by large-n detrital and volcanic ash zircon geochronology, Cretaceous Magallanes Basin, Chile

Deciphering depositional age from deposits that accumulate in deep-water slope settings can enhance understanding of shelf-margin evolutionary timing, as well as controlling mechanisms in ancient systems worldwide. Basin analysis has long employed biostratigraphy and/or tephrochronology to temporally constrain ancient environments. However, due to poor preservation of index fossils and volcanic ash beds in many deepwater systems, deducing the timing of slope evolution has proven challenging. Here, we present >6600 new U-Pb zircon ages with stratigraphic information from an ~100-kmlong by ~2.5-km-thick outcrop belt to elucidate evolutionary timing for a Campanian– Maastrichtian slope succession in the Magallanes Basin, Chile. Results show that the succession consists of four stratigraphic intervals, which characterize four evolutionary phases of the slope system. Overall, the succession records 9.9 ± 1.4 m.y. (80.5 ± 0.3 Ma to 70.6 ± 1.5 Ma) of graded clinoform development punctuated by out-of-grade periods distinguished by enhanced coarse-grained sediment bypass downslope. Synthesis of our results with geochronologic, structural, and stratigraphic data from the basin suggests that slope evolution was largely controlled by an overall decline in basin subsidence from 82 to 74 Ma. In addition to providing insight into slope evolution, our results show that the reliability of zircon-derived depositional duration estimates for ancient sedimentary systems is controlled by: (1) the proportion of syndepositionally formed zircon in a strati-

Latest Neoproterozoic to Cambrian detrital zircon facies of western Laurentia

Late Neoproterozoic to Cambrian sandstone units are common in western Laurentia and record initial transgression of the craton after the formation of the western passive margin during the latest Neoproterozoic to earliest Cambrian. Detrital zircon measurements from 42 latest Neoproterozoic to Cambrian basal Sauk sequences and five older Neoproterozoic sandstone samples from a region extending from the Mexico–United States border to central British Columbia, Canada, are combined with previous results to characterize sediment source areas and dispersal systems. Detrital zircon populations in Neoproterozoic and Cambrian sedimentary rocks are divided into six facies based on a statistical comparison using multidimensional scaling. Detrital zircon facies are found in unique geographical regions reflecting proximity to the major tectonic provinces of Laurentia. Samples from northern regions are dominated by Archean and Paleoproterozoic zircons derived from Archean tectonic provinces and the orogenic belts that record the assembly of the Laurentian craton. More southerly sample locations show an increase in detrital zircons derived from younger Paleoproterozoic orogenic belts and early Mesoproterozoic intrusive suites. Detrital zircons from Grenville-aged sources are common in the south. The Transcontinental Arch, a feature interpreted to have controlled large-scale sediment dispersal patterns in the midto late Cambrian, likely played a major role in isolating the southern and northern signatures. Our data set can be used to test tectonic models for the Cordilleran orogen that invoke Jurassic or Cretaceous collision of a ribbon continent as the driving mechanism for orogenesis. Cambrian rocks of the Cassiar-Antler platform juxtaposed with North America during the hypothetical ribbon continent collision show the same geographic distribution of detrital zircon facies as similar-aged rocks from autochthonous and parautochthonous locations on the Laurentian margin. The concordance of detrital zircon facies across the proposed suture is a negative result for models that predict large dextral displacements, on the order of 2000 km, across the suture.

Record of orogenic cyclicity in the Alberta foreland basin, Canadian Cordillera

Jurassic–Cretaceous sedimentary rocks of the Alberta foreland basin are a key record of the evolution of the Canadian Cordillera. We test a recent model for cyclical development of Cordilleran orogenic systems using detrital zircon analysis of the major sandstone units deposited between 145 and 80 Ma exposed in the Rocky Mountain Foothills near Grande Cache, Alberta. The basin history is well constrained by decades of study, and the stratigraphy has been previously subdivided into tectonostratigraphic wedges. U-Pb data from 14 detrital zircon samples are included in this study. All the major magmatic provinces of North America are represented in each sample, with the relative proportions varying between samples. The samples are assigned to five groups with the aid of multidimensional scaling. Groups 1–3 are interpreted to record recycling from specific passive-margin units of western North America with varying input from the Cordilleran magmatic arc. Group 4 is interpreted to record recycling from sedimentary strata in the United States and dispersal by basin-axial fluvial systems. Group 5 is dominated by Mesozoic zircon grains interpreted to have originated in the Cordilleran magmatic arc. Detrital zircon age spectra do not form groups based on the tectonostratigraphic wedges from which they were sampled; rather, within each tectonostratigraphic wedge, they exhibit evolution from diverse age spectra to a less-diverse distribution of detrital zircon ages. We constructed a proxy for magmatic flux of the Cordilleran magmatic arc using detrital zircon ages younger than 200 Ma; it shows three modes at ca. 165, 115, and 74 Ma. These ages are considered high-flux episodes of magmatism that are linked to cyclical uplift and plateau formation in the orogen. This cyclical process is interpreted to: (1) control sedimentation rates in the foreland; (2) account for evolving provenance by altering catchments; and (3) be a plausible mechanism for the deposition of the tectonostratigraphic wedges in the Alberta foreland basin.

Late Paleozoic to Triassic arc magmatism north of the Sverdrup Basin in the Canadian Arctic: Evidence from detrital zircon U-Pb geochronology

Paleozoic and Mesozoic tectonic reconstructions of the Arctic regions have been a subject of debate in recent years. The Permian emergence of a landmass north of the Sverdrup Basin in the Canadian Arctic led to the shedding of northerly derived detritus, an event that followed volcanism and basin inversion pulses that began in the late Pennsylvanian. However, the mechanisms for these events and the Paleozoic to Mesozoic paleogeography of this region remain controversial. New detrital zircon U-Pb geochronology results from Permian to Lower Triassic strata from northern Axel Heiberg and Ellesmere islands constrain the magmatic events within this northern landmass and its implications for the tectonic regime of the Sverdrup Basin and adjacent domains. Permian to lowermost Triassic strata along the northern margin of the Sverdrup Basin contain zircons derived from Silurian to Devonian rocks (420–350 Ma), Timanian-aged basement (700–500 Ma), and a Permian syndepositional source (300–250 Ma). Coeval strata in the southern margin are dominated by zircons formed during the Taconic, Scandian, and post-Scandian phases of the Appalachian and Caledonian orogenies, respectively (480–400 Ma). The detrital zircon signatures of the analyzed strata on the northern margin of the Sverdrup Basin record continuous magmatism within the northern landmass from latest Carboniferous (ca. 300 Ma) to at least earliest Triassic (ca. 250 Ma) time. These results are indicative of ongoing subduction and development of a magmatic arc off the northern margin of Laurentia, with the Sverdrup Basin potentially located in the backarc region of a proto-Pacific convergent margin involving parts of Arctic Alaska, Chukotka, and the Chukchi Shelf. The hypothesized onset of subduction in latest Carboniferous time and closure of this backarc basin in the latest Permian to earliest Triassic provides an explanation for the shift in stress regimes in the Sverdrup Basin that led to basin inversion and volcanism episodes. Therefore, the data presented here supports a backarc to retroarc setting for the Sverdrup Basin and the possibility of a convergent margin regime for the northern edge of Laurentia during the late Paleozoic to Triassic, contrasting with the generally accepted rift and passive margin settings. LITHOSPHERE; v. 10; no. 3; p. 426–445; GSA Data Repository Item 2018093 | Published online 23 March 2018 https://doi.org/10.1130/L683.1

Cambrian Sauk transgression in the Grand Canyon region redefined by detrital zircons

The Sauk transgression was one of the most dramatic global marine transgressions in Earth history. It is recorded by deposition of predominantly Cambrian non-marine to shallow marine sheet sandstones unconformably above basement rocks far into the interiors of many continents. Here we use dating of detrital zircons sampled from above and below the Great Unconformity in the Grand Canyon region to bracket the timing of the Sauk transgression at this classic location. We find that the Sixtymile Formation, long considered a Precambrian unit beneath the Great Unconformity, has maximum depositional ages that get younger up-section from 527 to 509 million years old. The unit contains angular unconformities and soft-sediment deformation that record a previously unknown period of intracratonic faulting and epeirogeny spanning four Cambrian stages. The overlying Tapeats Sandstone has youngest detrital zircon ages of 505 to 501 million years old. When linked to calibrated trilobite zone ages of greater than 500 million years old, these age constraints show that the marine transgression across a greater than 300-km-wide cratonic region took place during an interval 505 to 500 million years ago—more recently and more rapidly than previously thought. We redefine this onlap as the main Sauk transgression in the region. Mechanisms for this rapid flooding of the continent include thermal subsidence following the final breakup of Rodinia, combined with abrupt global eustatic changes driven by climate and/or mantle buoyancy modifications.Extensive flooding of the North American continent during the Cambrian occurred more recently and more rapidly than previously thought, according to analyses of detrital zircons sampled from the Grand Canyon region.

A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada

The Late Jurassic to Early Cretaceous fill of the Western Interior foreland basin is characterized using geochronological data in order to assess the stratigraphic expression of wedge-top geomorphology, as controlled by sediment cover and denudation. In northern Montana, USA, and Alberta, Canada, wedge-top deposits are poorly preserved; however, their former presence may be inferred from the detrital record in the foreland basin. We present new U/Pb detrital zircon data from nine samples collected near Great Falls, Montana, augmented with field data. The stratigraphy at Great Falls is characterized by Late Jurassic marine and nonmarine deposits, which are truncated by a basinwide sub-Cretaceous unconformity. Aptian and lower Albian strata overlying the unconformity are dominated by nonmarine deposits, which transition up-section into a predominantly marine succession related to a major transgression of the Boreal Seaway in the Albian. Detrital zircon grains from Great Falls strata yield age spectra that can be subdivided into three groups using multidimensional scaling. Group 1 is characterized by diverse zircon populations, which are interpreted to record recycling of pre-Cordilleran sedimentary strata transported via foreland basin-axial river systems with headwaters in the southwestern United States. Group 2 is characterized by the dominance of Mesozoic detrital zircon grains, which are interpreted to record sediment dispersal by fluvial systems with headwaters in the Cordillera. Group 3 is intermediate between groups 1 and 2, based on its proportion of Mesozoic zircon grains. This group records a diversification of the provenance from one dominated by Cordilleran igneous rocks to include recycled sedimentary strata. New data are integrated with three other data sets from Montana and Alberta such that stratal thicknesses (a proxy for accommodation development) and provenance evolution can be compared across the basin. The detrital record in each area, which transitions from diverse provenance to predominantly Cordilleran through the entire stratigraphic section, can be linked to the burial of the pre-foreland strata in the wedge-top depozone. This record elucidates a period of evolution of the western margin of North America to a more Andean-type system with primary input to the basin from an active magmatic arc. INTRODUCTION Aggradation and denudation of proximal foreland basin deposits in the wedge-top depozone are first-order controls on sediment flux to more-distal parts of foreland basin systems (Ben-Avraham and Emery, 1973; DeCelles, 1994; DeCelles and Giles, 1996; Roddaz et al., 2005; Ross et al., 2005; Horton, 2018). Burial of the frontal toe of the orogen can heal complex topography and enhance direct sediment transfer from the orogenic hinterland to a basin. Conversely, denudation of these wedge-top sediments can lead to structural control of river pathways; the process also can expose older stratigraphy in the orogenic wedge, which can be reflected by provenance changes in the adjacent foreland basin (Ross et al., 2005; Lawton et al., 2010). Sediments in the wedge-top depozone accumulate on top of orogenic structures and are part of the frontal toe of the orogenic wedge (DeCelles and Giles, 1996). These synorogenic strata accumulate near the erosional/depositional surface and are characterized by low preservation potential such that the history of sedimentation in this zone is difficult to discern (e.g., Coogan, 1992; Frisch et al., 2001; McMechan et al., 2018). More distal parts of the foreland basin contain well-established records of orogenic processes (e.g., Heller and Paola, 1989; Ross et al., 2005; Quinn et al., 2016). Therefore, we hypothesize that the history of the wedge-top depozone can be elucidated by investigating strata in more-distal parts of the foreland basin. This is particularly relevant to understanding topographic evolution in fold-thrust belts that have been deeply incised during subsequent orogenesis and glaciation (Osborn et al., 2006). We present a detrital zircon data set from Western Interior basin strata exposed near Great Falls, Montana, USA. The units provide a unique window into the Late Jurassic–Early Cretaceous evolution of the foreland basin because outcrops of these strata are rare except within the fold-thrust belt, which is ~100 km west of the study area. New data are integrated with previously presented data sets from across the basin in order to consider the history of burial and exhumation of the frontal toe of the orogenic wedge (Fig. 1) ( Fuentes et al., 2011; Leier and Gehrels, 2011; Raines et al., 2013; Benyon et al., 2014; Blum and Pecha, 2014; Quinn et al., 2016). This study emphasizes the potential impact of wedge-top dynamics on sediment dispersal across foreland basins and the structural evolution of the orogen. GEOSPHERE GEOSPHERE; v. 14, no. 3 https://doi.org/10.1130/GES01606.1 12 figures; 1 table; 1 supplemental file CORRESPONDENCE: garrett .quinn@ ucalgary.ca; gquinn7@ gmail .com CITATION: Quinn, G.M., Hubbard, S.M., Putnam, P.E., Matthews, W.A., Daniels, B.G., and Guest, B., 2018, A Late Jurassic to Early Cretaceous record of orogenic wedge evolution in the Western Interior basin, USA and Canada: Geosphere, v. 14, no. 3, p. 1187–1206, https:// doi .org /10 .1130 /GES01606.1. Science Editor: Raymond M. Russo Received 24 August 2017 Revision received 21 February 2018 Accepted 16 April 2018 Published online 7 May 2018