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The timing of the processes occurring in the lithosphere during the ascent and eruption of kimberlites has been challenging to ascertain. Here, we present a novel application of paired (U-Th)/He and U/Pb geochronology on megacrystic zircon to date both the crystallization of the megacryst suite and the host kimberlite eruption. Unlike Pb, He is not retained in the crystal at temperatures >250 ◦C. For megacrystic zircon that crystallizes in the mantle, He retention begins upon kimberlite eruption such that the zircon (U-Th)/He (ZHe) dates should record kimberlite emplacement. We collected cathodoluminescence, trace element, U/Pb, and (U-Th)/He data for nine megacrystic zircon from five kimberlites: Diavik and Panda (Canada), Juína and Batovi-9 (Brazil), and Monastery (South Africa). For all zircon except from Diavik, ZHe and U/Pb dates overlap within uncertainty, indicating megacryst crystallization during or shortly before kimberlite eruption, and suggesting a genetic link between the kimberlite magma and megacryst suite. For Diavik zircon, the ZHe dates are ~2.5 Gyr younger than the U/Pb dates, documenting that the megacrysts and the host kimberlite magma are genetically unrelated. ZHe dates from three of the kimberlites overlap with eruption ages interpreted from other methods. However, for Panda and Monastery, our paired ZHe-U/Pb dates differ from previous results, consistent with multiple eruptions over ~5 Myr and implying that protracted eruptive phases may be more common than previously thought. Our work shows that (U-Th)/He dating of megacrystic zircon accurately dates kimberlite emplacement, and when coupled with U/Pb dating can yield insights into megacryst-kimberlite relationships. 

The Northern Madison Range is one of several exposures in SW Montana that record tectonism associated with the 1.78–1.72 Ga Big Sky orogeny. Studies show that the exposed 1.7 Ga crustal levels were as deep as 40 km at the NW end of the range and potentially as shallow as 10 km at the SE end. In documenting the internal structure of this oblique section, we report on the newly recognized Hell Roaring Creek shear zone. This amphibolite facies structure is 2–3 km wide, NE–SW-striking and steeply dipping. Stretching lineations have nearly down-dip orientations in most lithologies, but shallowly plunging orientations in the quartz-rich units. Dextral shear sense indicators are consistently observed on the sub-horizontal surfaces. Observations suggest that the steep fabric elements were partly inherited from an earlier episode of SE-vergent thrusting. The shear zone is interpreted to exhibit strain partitioning during transpression, apparently due to the rheological contrast among the lithologies and the heterogeneity from inherited structures. Geochronological data support a 1.74 Ga age for the shear zone and a 2.55 Ga age for the earlier episode of shearing. Dextral shear along present-day NE-striking structures late during the Big Sky orogeny may reflect evolving convergence directions of accreting terranes from the west. 

The ~2,000-km-long Central Range of New Guinea is a hotspot of modern carbon sequestration due to the chemical weathering of igneous rocks with steep topography in the warm wet tropics. These high mountains formed in a collision between the Australian plate and ophiolite-bearing volcanic arc terranes, but poor resolution of the uplift and exhumation history has precluded assessments of the impact on global climate change. Here, we develop a palinspastic reconstruction of the Central Range orogen with existing surface geological constraints and seismic data to generate time–temperature paths and estimate volumes of eroded material. New (U-Th)/He thermochronology data reveal rapid uplift and regional denudation between 10 and 6 Mya. Erosion fluxes from the palinspastic reconstruction, calibrated for time with the thermochronological data, were used as input to a coupled global climate and weathering model. This model estimates 0.6 to 1.2 °C of cooling associated with the Late Miocene rise of New Guinea due to increased silicate weathering alone, and this CO₂sink continues to the present. Our data and modeling experiments support the hypothesis that tropical arc-continent collision and the rise of New Guinea contributed to Neogene cooling due to increased silicate weathering. 

The Snowball Earth hypothesis predicts global ice cover; however, previous descriptions of Cryogenian (720-635 Ma) glacial deposits are limited to continental margins and shallow marine basins. The Tavakaiv (Tava) sandstone injectites and ridges in Colorado, USA, preserve a rare terrestrial record of Cryogenian low-latitude glaciation. Injectites, ridges, and chemically weathered crystalline rock display features characteristic of fluidization and pervasive deformation in a subglacial environment due to glacial loading, fluid overpressure, and repeated sand injection during meltwater events. In situ hematite U-Pb geochronology on hematite-quartz veins, which crosscut and are cut by Tava dikes, constrain sand injection at ~690-660 Ma. We attribute early Tava sand injection episodes to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation. A modern analog is provided by the Ross Embayment of Antarctica, where rift-related faults border sediment-filled basins, overpressurized fluids circulate in confined aquifers below ice, and extensive preglacial topography is preserved. Field evidence and geochronology in Colorado further highlight that deep chemical weathering of Proterozoic bedrock and denudation associated with the Great Unconformity predate Cryogenian injection of fluidized sand, consistent with limited glacial erosion. 

Volcanic highlands in the tropics increased global weatherability ~0.9–1.6 Ma before onset of Cryogenian Snowball Earth. 

Abstract. Here we examine the landscape of New Zealand'sMarlborough Fault System (MFS), where the Australian and Pacific plates obliquelycollide, in order to study landscape evolution and the controls on fluvialpatterns at a long-lived plate boundary. We present maps of drainageanomalies and channel steepness, as well as an analysis of the plan-vieworientations of rivers and faults, and we find abundant evidence ofstructurally controlled drainage that we relate to a history of drainagecapture and rearrangement in response to mountain-building and strike-slipfaulting. Despite clear evidence of recent rearrangement of the western MFSdrainage network, rivers in this region still flow parallel to older faults,rather than along orthogonal traces of younger, active strike-slip faults.Such drainage patterns emphasize the importance of river entrenchment,showing that once rivers establish themselves along a structural grain,their capture or avulsion becomes difficult, even when exposed to newweakening and tectonic strain. Continued flow along older faults may alsoindicate that the younger faults have not yet generated a fault damage zonewith the material weakening needed to focus erosion and reorient rivers.Channel steepness is highest in the eastern MFS, in a zone centered on theKaikōura ranges, including within the low-elevation valleys of main stemrivers and at tributaries near the coast. This pattern is consistent with anincrease in rock uplift rate toward a subduction front that is locked on itssouthern end. Based on these results and a wealth of previous geologicstudies, we propose two broad stages of landscape evolution over the last 25 million years of orogenesis. In the eastern MFS, Miocene folding above blindthrust faults generated prominent mountain peaks and formed major transverserivers early in the plate collision history. A transition to Pliocenedextral strike-slip faulting and widespread uplift led to cycles of riverchannel offset, deflection and capture of tributaries draining across activefaults, and headward erosion and captures by major transverse rivers withinthe western MFS. We predict a similar landscape will evolve south of theHope Fault, as the locus of plate boundary deformation migrates southwardinto this region with time.