Large-scale (>600–1100 km) left-lateral offsets during Late Jurassic–Early Cretaceous time have long been inferred to explain the distribution of terranes in the Canadian portion of the North American Cordillera, but identification of structures with the correct age, location, and magnitude to accommodate such large displacements remains a significant challenge. Previous studies along the coast of British Columbia identified an ~250-km-long system of orogen-parallel intra-arc sinistral shear zones that may have accommodated some of these predicted motions, but crosscutting relationships could only constrain an ~20-m.y.-long period of motion in the late Early Cretaceous at the end of shear zone activity (ca. 125–100 Ma). In this chapter, we present new mapping from the same region that more than doubles the documented extent of sinistral shear zones recognized by previous workers. Complementary structural analysis and U-Pb (zircon) geochronology on synkinematic and postkinematic intrusions confirm that most shear zones were active during Early Cretaceous time (ca. 112 Ma) and demonstrate that some were active during Late Jurassic to earliest Cretaceous time (ca. 165–143 Ma). This extended time of motion supports the hypothesis that large-scale (~800 km) sinistral motion occurred along the northern Cordilleran margin.

ABSTRACT The Santa Rosalía basin (Baja California Sur, México) contains a rich record of late Cenozoic volcanism, faulting, and sedimentation that provides a crucial constraint on the timing of marine flooding from the Pacific Ocean into the nascent Gulf of California oblique rift, yet the precise age of the basin is uncertain. Previous studies used reconnaissance paleomagnetic data and a 40Ar/39Ar age of 6.76 ± 0.90 Ma on the intrabasinal Cinta Colorada tuff to estimate a depositional age of ca. 7.2–6.3 Ma for the marine Boleo Formation and initial flooding of the central Gulf of California. Here, we present a large (n = 2091) detrital zircon U-Pb geochronology data set from the Boleo Formation that indicates a maximum depositional age of 6.35 ± 0.21 Ma for pumiceous sandstone at the base (below the basal limestone), a revised age of 5.86 ± 0.06 Ma for the Cinta Colorada tuff in the middle, and a maximum depositional age of 5.70 ± 0.21 Ma for the top. Detrital zircon age spectra suggest a local provenance for the Boleo Formation involving recycling from underlying Oligocene–Miocene strata in proximal source areas. Integration of detrital zircon ages with existing paleomagnetic data suggests that the lower ~30 m of the Boleo Formation accumulated during normal-polarity subchron C3An.1n (6.27–6.02 Ma), and the middle to upper Boleo Formation was deposited entirely during reverse-polarity chron C3r (6.02–5.24 Ma). We therefore reassign the depositional age span of the Boleo Formation to ca. 6.3–5.7 Ma. Although not preferred, a minimum-duration depositional model from ca. 6.1 to 5.8 Ma is also permissible if a consistently high sedimentation rate of ~0.4– 1.0 mm/yr is inferred. This revised younger age for the Boleo Formation implies marine incursion in the central Gulf of California at ca. 6.3 Ma, ~1 m.y. younger than previously thought. We envision that regional marine flooding occurred during a very short (<100 k.y.) event that inundated a narrow tectonic trough over a distance of at least ~1000 km along the plate boundary from the central Gulf of California to the Salton Trough and reaching into the present-day Lower Colorado River Valley. This study also demonstrates the utility of large-volume and large-n detrital zircon studies in establishing the ages of sedimentary successions deposited over very short time spans (<1 m.y.) and/or during relative lulls in magmatism and geomagnetic reversals.

Detrital zircon (DZ) U-Pb geochronology has improved the way geologists approach questions of sediment provenance and stratigraphic age. However, there is debate about what constitutes an appropriate sample size (i.e., the number of dates in a DZ sample, n), which depends on project objectives, sample complexity, and, critically, analytical budget. Additionally, there is ongoing concern about bias introduced by zircon grain size. We tested a recently developed rapid (3 s/analysis) data acquisition method by multicollector laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) that incorporates an automated selection routine and calculates two-dimensional grain geometry from polished sample surfaces. Eleven samples were analysed from below and above the Late Cretaceous (Campanian) basal Castlegate unconformity of the Book Cliffs, Utah, in a down-depositional-dip transect including Price, Horse, Tusher, and Thompson canyons. 12,448 new concordant dates were generated during two measurement sessions. Results are consistent with recent studies suggesting there is no major provenance change and little time (1-2 Myr) represented across the unconformity. Grain size and sample size both exert a strong control on sample dissimilarity. Age distributions constructed from subsamples of large grains are systematically less similar to whole samples; age distributions composed of small grains are overall more similar to whole samples. As such, North American sediment sources that produce large grains such as the Grenville and Yavapi-Mazatzal belts can bias age distributions if only large grains are analysed. A sample size of n = 100 is inadequate for characterizing age distributions as complex as those of the Book Cliffs, whereas a sample size of n = 300 provides good characterization. Sample size of n approximate to 1000 or more is unnecessary unless project objectives include scanning for subordinate age groups, such as when identifying the youngest grains for calculating a maximum depositional age (MDA). Dates used in MDA calculations acquired with rapid acquisition are best re-analysed with longer LA-ICP-MS acquisition methods or isotope dilution thermal ionization mass spectrometry for increased accuracy and precision. We include new MATLAB code and open-source software programs, DZpick and DZmda, for automated spot picking and calculating MDAs.

ABSTRACT Paleocene Lower Wilcox Group sedimentation rates are three times the Cenozoic average for the Gulf of Mexico region and are attributed to Laramide tectonism within the Laramide–Rocky Mountains region. These increased rates likely represent the erosion of easily weathered Phanerozoic strata that blanketed the Laramide-age basement-cored uplifts. Geologic observations and U-Pb geochronology are not sufficient to fully address this hypothesis alone, so we conducted 439 Lu-Hf isotopic analyses on detrital zircons from eight samples from the San Juan Basin and five samples from the Gulf of Mexico Basin. Focusing on the zircons younger than 300 Ma allowed us to make direct comparisons to the eight principal components that comprise the North American Cordilleran magmatic arc: (1) Coast Mountains batholith; (2) North Cascades Range; (3) Idaho batholith; (4) Sierra Nevada batholith; (5) Laramide porphyry copper province; (6) Transverse Ranges; (7) Peninsular Ranges; and (8) Sierra Madre Occidental. The εHf(t) results range from +8.9 to –27.0 for the San Juan Basin samples and from +13.0 to –26.6 for the Gulf of Mexico samples. Using the San Juan Basin samples as a proxy for the eroded Mesozoic cover that was shed from the Laramide uplifts, we show that much of the sediment entering the Gulf of Mexico through the Houston and Mississippi embayments during the late Paleocene was derived from reworked cover from the greater Laramide–Rocky Mountains region. However, the Gulf of Mexico samples also include a distinct juvenile suite (εHf[t] ranging from +13 to +5) of zircons ranging in age from ca. 220 to 55 Ma that we traced to the Coast Mountains batholith in British Columbia, Canada. This transcontinental connection indicates an extension to the headwaters of the previously defined paleo-Mississippi drainage basin from ca. 58 to 56 Ma. Therefore, we propose a through-going fluvial system (referred to here as the “Coast Mountains River”) that was routed from the Coast Mountains batholith to the Gulf of Mexico. This expands the previously defined paleo-Mississippi drainage basin area by an estimated 280,000 km2. Our comprehensive Hf isotopic compilation of the North American Cordilleran magmatic arc also provides a benchmark εHf(t) versus U-Pb age plot, which can be used to determine provenance of detrital zircons (85–50 Ma) at the scale of specific region(s) within the Cordillera based on their εHf(t) values.

The geodynamic process from the evolution of supercontinent has distinct isotope characteristics explorable using zircon Hf isotopic composition. Since Peninsular Malaysia associates with Gondwana dispersal and Pangaea formation, analyzing the U-Pb and Hf-isotopic content of its 500-200 Ma magmatic zircon could reveal the signal left by the transitional geodynamic process between the supercontinents. We collected two groups of magmatic rocks from West Malaya: Ordovician meta-volcanics (n = 8), and Triassic Main Range granitoid province (MRGP) (n = 6); and three groups from East Malaya: Carboniferous meta-volcanics (n = 2), Permian-Triassic Eastern granite province (EGP) (n = 6), and Permian-Triassic EGP volcanics (n = 8). Difference in magmatic zircon Hf isotopic crustal model ages uphold the previous rationale which separates Peninsular into two blocks: West Malaya (part of Sibumasu terrane) magmatic zircon Hf isotopic crustal model ages (Average T-DM2: 1.3 Ga) are older than East Malaya (part of Chanthaburi-Sukhothai-Lincang arc of Indochina terrane) (Average T-DM2: 0.9 Ga). During the final assembly of Gondwana from 500 to 450 Ma, West Malaya and East Malaya were at the outboard of Gondwana Proto-Tethys margin. The shift of East Malaya zircon Hf array towards higher epsilon Hf(t) (external orogenic system) after ca.370 Ma may infers Paleo-Tethys ocean broadening and East Malaya separation from Gondwana. The 370-350 Ma juvenile zircon Hf isotopic composition in East Malaya is a significant improvement over radiolarian age to show the broadening and subduction of the Paleo-Tethys ocean between the two terranes. After ca.280 Ma, East Malaya zircon Hf array shifted towards lower epsilon Hf(t) (internal orogenic system). Coinciding with the Indosinian collision at ca.230 Ma, crustal reworking signal increases in both blocks, signifying the end in Peninsular Malaysia Gondwana to Pangaea transitional geodynamic process. As the Paleo-Tethys segment was completely subducted after 230 Ma, the peninsular crustal thickening starts from 230 to 218 Ma. The post-collision phase would begin at ca.215 Ma. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

The Ross orogenic belt in Antarctica is one of several Neoproterozoic-early Palaeozoic orogens that crisscrossed Gondwana and are associated with Gondwana's assembly. We present new age data from the Queen Maud Mountains, Ross orogen, from areas that hitherto have lacked precise ages from the local plutonic rocks. The zircon U-Pb igneous crystallization ages (n = 7) and a hornblende 40Ar/39Ar cooling age (n = 1) constrain plutonism to primarily lie within the Cambrian to Ordovician. Cumulative zircon U-Pb crystallization age data yield polymodal age distributions (516 Ma, 506–502 Ma, and 488 Ma age peaks) that are similar to other areas of the Queen Maud-Horlick Mountains, consistent with regional magmatic flare-ups along the Pacific-Gondwana margin during these times. The ages of deformed plutons constrain deformation to the Cambrian (Series 2) to Ordovician (Lower), with some regions indicating a transition to post-tectonic magmatism and cooling at ~509-470 Ma. Collectively, the data indicate that the Queen Maud-Horlick Mountains share a similar petrotectonic history with other regions of the Pacific-Gondwana margin, providing new evidence that this tectonostratigraphic province is part of and not exotic to the larger igneous-sedimentary successions developed in the peri-Gondwana realm under a broadly convergent margin setting.

Accurate reconstruction of the Late Cretaceous paleogeography and tectonic evolution of the west- ern North American Cordilleran margin is required to resolve the long-standing debate over proposed large-scale, orogen-parallel terrane translation. The Nanaimo Basin (British Columbia, Canada) contains a high-fidelity record of orogenic exhumation and basin subsidence in the southwestern Canadian Cordillera that constrains the tectonic evolution of the region. Integration of detrital zircon U-Pb geochronology, conglomerate clast U-Pb geochronology, detrital muscovite 40Ar/39Ar thermochronology, and Lu-Hf isotopic analysis of detrital zircon defines a multidisciplinary provenance signature that provides a definitive linkage with sediment source regions north of the Sierra Nevada arc system (western United States). Analysis of spatial and temporal provenance variations within Nanaimo Group strata documents a bimodal sediment supply with a local source derived from the adjacent magmatic arc in the southern Coast Mountains batholith and an extra-regional source from the Mesoproterozoic Belt Supergroup and the Late Cretaceous Atlanta lobe of the Idaho batholith. Particularly robust linkages include: (1) juvenile (εHf >+10) Late Cretaceous zircon derived from the southern Coast Mountains batholith; (2) a bimodal Proterozoic detrital zircon signature consistent with derivation from Belt Supergroup (1700–1720 Ma) and ca. 1380 Ma plutonic rocks intruding the Lemhi subbasin of central Idaho (northwestern United States); (3) quartzite clasts that are statistical matches for Mesoproterozoic and Cambrian strata in Montana and Idaho (northwestern United States) and southern British Columbia; and (4) syndepositional evolved (εHf >−10) Late Cretaceous zircon and muscovite derived from the Atlanta lobe of the Idaho batholith. These provenance constraints support a tectonic restoration of the Nanaimo Basin, the southern Coast Mountains batholith, and Wrangellia to a position outboard of the Idaho batholith in Late Cretaceous time, consistent with proposed minimal-fault-offset models (<~1000 km).

Detrital zircon (DZ) U‐Pb laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has revolutionised the way geologists approach many Earth science questions. Although recent research has focused on rapid sample throughput, acquisition rates are limited to 100–300 analyses h−1. We present a method to acquire zircon U‐Pb dates at rates of 120, 300, 600 and 1200 analyses h−1 (30, 12, 6 and 3 s per analysis) by multi‐collector LA‐ICP‐MS. We demonstrate the efficacy of this method by analysing twelve zircon reference materials with dates from ~ 3465 to ~ 28 Ma. Mean offset from high‐precision dates increases with faster rates from 0.9% to 1.1%; mean random 1s uncertainty increases from 0.6% to 1.3%. We tested this new method on a sandstone sample previously characterised by large‐n DZ geochronology. Quantitative comparison shows increased correspondence among age distributions comprising > 300 dates. This new method holds promise for DZ geochronology because (a) it requires no major changes to hardware, but rather modifications to software; (b) it yields robust age distributions well‐suited for quantitative analysis and maximum depositional age calculations; (c) there is only a minor sacrifice of accuracy and measurement uncertainty; and (d) there is less burden to researchers in terms of time investment and analytical cost.

We present multitechnique U‐Pb geochronology and Hf isotopic data from zircon separated from rapakivi biotite granite within the Eocene Golden Horn batholith in Washington, USA . A weighted mean of twenty‐five Th‐corrected 206 Pb/ 238 U zircon dates produced at two independent laboratories using chemical abrasion‐isotope dilution‐thermal ionisation mass spectrometry ( CA ‐ ID ‐ TIMS ) is 48.106 ± 0.023 Ma (2 s analytical including tracer uncertainties, MSWD = 1.53) and is our recommended date for GHR 1 zircon. Microbeam 206 Pb/ 238 U dates from laser ablation‐inductively coupled plasma‐mass spectrometry ( LA ‐ ICP ‐ MS ) and secondary ion mass spectrometry ( SIMS ) laboratories are reproducible and in agreement with the CA ‐ ID ‐ TIMS date to within < 1.5%. Solution multi‐collector ICP ‐ MS ( MC ‐ ICP ‐ MS ) measurements of Hf isotopes from chemically purified aliquots of GHR 1 yield a mean 176 Hf/ 177 Hf of 0.283050 ± 17 (2 s , n = 10), corresponding to a εHf 0 of +9.3. Hafnium isotopic measurements from two LA ‐ ICP ‐ MS laboratories are in agreement with the solution MC ‐ ICP ‐ MS value. The reproducibility of 206 Pb/ 238 U and 176 Hf/ 177 Hf ratios from GHR 1 zircon across a variety of measurement techniques demonstrates their homogeneity in most grains. Additionally, the effectively limitless reserves of GHR 1 material from an accessible exposure suggest that GHR 1 can provide a useful reference material for U‐Pb geochronology of Cenozoic zircon and Hf isotopic measurements of zircon with radiogenic 176 Hf/ 177 Hf.

The Campanian Blackhawk Formation and Castlegate Sandstone are widely exposed along the Wasatch Plateau and Book Cliffs in east-central Utah, USA, and represent alluvial-deltaic and shoreline deposits in the Sevier foreland basin along the western margins of the Western Interior Seaway. This study presents detrital-zircon U-Pb ages (DZ U-Pb) from 31 samples in the Blackhawk-Castlegate succession so as to examine paleodrainage areas and sediment routing, the influence of autogenic processes on the stratigraphic record, and the timing of deposition. DZ U-Pb populations indicate the uppermost Blackhawk and lower Castlegate of the Wasatch Plateau represent multiple "parent" rivers with different drainage areas. However, there is no apparent change in provenance across the interpreted basal Castlegate unconformity, and the same parent rivers were responsible for deposition of the uppermost Blackhawk and lower Castlegate at each location. At the broader regional scale, paleodrainage systems included a series of smaller transverse fluvial systems that drained the proximal Sevier fold and thrust belt, as well as a longitudinal river system that flowed from the Mogollon Highlands in present-day central Arizona to the north-northeast. DZ U-Pb data from updip locations in the Wasatch Plateau provide faithful records of provenance for these parent systems, however downdip locations in the Book Cliffs record mixing of different parents, initially due to contributive patterns, where rivers with slightly different drainage areas merge in the downstream direction, then due to distributive patterns farther downdip on an alluvial-deltaic plain where backwater-mediated avulsions are common. Autogenic surface dynamics therefore produce DZ U-Pb daughter populations that complicate interpretations of tectonic and climatic forcing, and the influence of other external controls. Published age estimates for the Blackhawk-Castlegate succession have relied on correlation of chronologically constrained shallow-marine strata to presumed updip alluvial-deltaic equivalents. However, the youngest population of DZ U-Pb ages in the uppermost Blackhawk and the lower Castlegate through Bluecastle Tongue provide independently constrained maximum depositional ages (MDAs). MDAs for the upper Blackhawk and lower Castlegate are up to 2 Myr younger than previous inferences, but the Bluecastle Tongue MDA is consistent with previous constraints: deposition of the upper Blackhawk, formation of the interpreted basal Castlegate unconformity, and deposition of the lower Castlegate through Bluecastle Tongue represents an similar to 2 Myr period from ca. 77 to 75 Ma. The lack of provenance change across the basal Castlegate surface at any single location, combined with the short period of time it represents, suggests that this key sequence stratigraphic surface is not an unconformity but formed instead from autogenic scour by migratory channels.