Andrew T. Calvert

Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history

Abstract The Qinling orogen preserves a record of late mid-Proterozoic to Cenozoic tectonism in central China. High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) assembled the Yangtze craton, including the lower Qinling unit, into Rodinia during the ∼1.0 Ga Grenvillian orogeny. The lower Qinling unit then rifted from the Yangtze craton at ∼0.7 Ga. Subsequent intra-oceanic arc formation at ∼470–490 Ma was followed by accretion of the lower Qinling unit first to the intra-oceanic arc and then to the Sino-Korea craton. Subduction then imprinted a ∼400 Ma Andean-type magmatic arc onto all units north of the northern Liuling unit. Oblique subduction created Silurian–Devonian WNW-trending, sinistral transpressive wrench zones (e.g., Lo-Nan, Shang-Dan), and Late Permian–Early Triassic subduction reactivated them in dextral transpression (Lo-Nan, Shang-Xiang, Shang-Dan) and subducted the northern edge of the Yangtze craton. Exhumation of the cratonal edge formed the Wudang metamorphic core complex during dominantly pure shear crustal extension at ∼230–235 Ma. Post-collisional south-directed shortening continued through the Early Jurassic. Cretaceous reactivation of the Qinling orogen started with NW–SE sinistral transtension, coeval with large-scale Early Cretaceous crustal extension and sinistral transtension in the northern Dabie Shan; it presumably resulted from the combined effects of the Siberia–Mongolia—Sino-Korean and Lhasa–West Burma—Qiangtang–Indochina collisions and Pacific subduction. Regional dextral wrenching was active within a NE–SW extensional regime between ∼60 and 100 Ma. An Early Cretaceous Andean-type continental magmatic arc, with widespread Early Cretaceous magmatism and back-arc extension, was overprinted by shortening related to the collision of Yangtze–Indochina Block with the West Philippines Block. Strike–slip and normal faults associated with Eocene half-graben basins record Paleogene NNE–SSW contraction and WNW–ESE extension. The Neogene(?) is characterized by normal faults and NNE-trending sub-horizontal extension. Pleistocene(?)–Quaternary NW–SE extension and NE–SW contraction comprises sinistral strike–slip faults and is part of the NW–SE extension imposed across eastern Asia by the India–Asia collision.

Ultrarapid exhumation of ultrahigh-pressure diamond-bearing metasedimentary rocks of the Kokchetav Massif, Kazakhstan?

The diamond-bearing, ultrahigh-pressure Kokchetav Massif recrystallized at eclogite-facies conditions deep in the mantle at 180-km depth at 535F3 Ma, and yet new 40 Ar/ 39 Ar ages suggest that it may have been exhumed to crustal depths (as indicated by closure of mica to Ar loss) by f529 Ma. These data indicate a possible exhumation rate of tens of kilometers per million years, i.e., a rate that is comparable to rates of horizontal plate motion and subduction. D 2003 Elsevier B.V. All rights reserved.

Quaternary faulting history along the Deep Springs fault, California

New geologic mapping, structural studies, geochronology, and diffusion erosion modeling along the Deep Springs fault, California, shed light on its Quaternary faulting history. The Deep Springs fault, a 26km-long, predominantly north-northeaststriking, west-northwestdipping normal fault bounding the eastern side of Deep Springs Valley, cuts Jurassic batholithic rocks nonconformably overlain by middle Miocene to Pleistocene stream gravels, coarse-grained sand, tuffaceous sand, unwelded to partially welded tuff, and Bishop ash, as well as Quaternary coarse- to finegrained alluvial fan deposits. The 40 Ar/ 39 Ar geochronology yields ages of 3.09 0.08 Ma for the unwelded tuff and 753 4k a for the Bishop ash. Holocene debris flows, a landslide, and recent alluvium bury the youngest fault scarp. The Deep Springs fault is characterized by multiple fault planes and fault scarps that become progressively younger toward the basin. The dip of the fault plane varies from 20 to 87 and fault-plane and striation measurements indicate an average orientation of N23E, 47NW, and N77E, 49SW, respectively. The offset of Bishop ash and underlying tuff across the Deep Springs fault indicates horizontal extension and vertical slip rates of 0.7 and 0.9 mm/yr, respectively, since the eruption of the Bishop Tuff, and 0.2 and 0.2 mm/yr, respectively, since the eruption of the unwelded tuff. If the vertical slip rate since the eruption of the Bishop Tuff has remained constant through time, then slip along the Deep Springs fault began ca. 1.7 Ma. Younger fault scarps to the west of the bedrock fault cut Quaternary deposits; scarp offset ranges from 0.8 to 17.5 m and scarp slope angle ranges from 8 to 37. Topographic profiling of the smallest, least eroded fault scarps, with an average surface offset of 2.7 m, indicates that these scarps developed as the result of a single earthquake and ruptured an 20km-long segment of the fault. Radiocarbon analyses on detrital charcoal, located in the footwall block of one of these scarps, yield an age of 1.960 0.055 ka. Diffusion erosion modeling of these fault scarps yields an elapsed time of 1.7 0.5 k.y. since these fault scarps formed. Making reasonable assumptions about the depth of this earthquake and shear modulus, we estimate a moment magnitude, MW 7.0, for this earthquake. The Deep Springs fault is one of several displacement-transfer normal faults that define a zone of distributed deformation between subparallel right-lateral strike-slip faults east of the Sierra Nevada that make up the northern part of the eastern California shear zone. The young age and recent earthquake activity along the Deep Springs fault are consistent with a model proposed for the kinematic evolution of this part of the eastern California shear zone.

Cretaceous to Recent extension in the Bering Strait region, Alaska

A key issue presented by the geology of northern Alaska concerns the demise of the Brooks Range going west toward the Bering Strait region. The main Brookian tectonic and stratigraphic elements continue into the Russian Far East, but the thick crustal root and high elevations that define the modern physiographic Brooks Range die out approaching the Bering and Chukchi shelves, which form an unusually broad area of submerged continental crust. Structural, geochronologic, and apatite fission-track data indicate that at least three episodes of extension may have affected the crust beneath the Bering Strait region, in the middle to Late Cretaceous, Eocene-early Oligocene, and Pliocene(?)-Recent. This extension may explain the present thinner crust of the region, the formation of extensive continental shelves, and the dismemberment and southward translation of tectonic elements as they are traced from the Brooks Range toward Russia. Evidence for these events is recorded within a gently tilted 10- to 15-km thick crustal section exposed on the western Seward Peninsula. The earliest episode is documented at high structural levels by the postcollision exhumation history of blueschists. Structural data indicate exhumation was accomplished in part by thinning of the crust during north-south extension bracketed between 120 and 90 Ma by 40Ar/39Ar and U-Pb ages. The Kigluaik Mountains gneiss dome rose through the crust during the later stages of this extension at 91 Ma. Similar gneiss domes occur within a broad, discontinuous belt of Cretaceous magmatism linking interior Alaska with northeast Russia; mantle-derived melts within this belt likely heated the crust and facilitated extension. Apatite fission-track ages indicate cooling below ≈120–85°C occurred sometime between 100 and 70 Ma, and the area subsequently resided at shallow crustal depths (<3–4 km) until the present. This suggests that denudation of deep levels of the crust by erosion and/or tectonism was mostly completed by the Late Cretaceous and thus that the present-day thinner crust of the Bering Strait region developed primarily in the Cretaceous. Regional tilting and at least several more kilometers of local erosion followed in Eocene-early Oligocene time as documented by fission-track ages from deeper levels of the crustal section exposed in the Kigluaik Mountains. This event is generally coeval with development of the offshore transtensional Hope and Norton Basins which flank the Seward Peninsula to the north and south. Modern seismicity, active normal faults, and basin-range topography document Pliocene(?) to Recent north–south extension across the region. Fission-track data indicate that exhumation during this period was quite limited, less than 2–3 km. This inferred history of protracted extension in the Bering Strait region stands in sharp contrast to well-documented Cretaceous and Tertiary north–south shortening in interior Alaska. This contrast in tectonic histories suggests a model in which contraction and westward extrusion of crustal fragments from interior Alaska by strike-slip faulting were accommodated by north–south extension in the Bering Strait region. This resulted in the counterclockwise rotation of extruded crustal blocks, the extensional thinning of the western part of the Brooks Range crustal root, and the formation of transtensional basins and unusually broad continental shelves between Alaska and Russia.

Age of the Lava Creek supereruption and magma chamber assembly at Yellowstone based on 40Ar/39Ar and U‐Pb dating of sanidine and zircon crystals

The last supereruption from the Yellowstone Plateau formed Yellowstone caldera and ejected the >1000 km3 of rhyolite that composes the Lava Creek Tuff. Tephra from the Lava Creek eruption is a key Quaternary chronostratigraphic marker, in particular for dating the deposition of mid Pleistocene glacial and pluvial deposits in western North America. To resolve the timing of eruption and crystallization history for the Lava Creek magma, we performed (1) 40Ar/39Ar dating of single sanidine crystals to delimit eruption age and (2) ion microprobe U-Pb and trace-element analyses of the crystal faces and interiors of single zircons to date the interval of zircon crystallization and characterize magmatic evolution. Sanidines from the two informal members composing Lava Creek Tuff yield a preferred 40Ar/39Ar isochron date of 631.3 ± 4.3 ka. Crystal faces on zircons from both members yield a weighted mean 206Pb/238U date of 626.5 ± 5.8 ka, and have trace element concentrations that vary with the eruptive stratigraphy. Zircon interiors yield a mean 206Pb/238U date of 659.8 ± 5.5 ka, and reveal reverse and/or oscillatory zoning of trace element concentrations, with many crystals containing high U concentration cores that likely grew from highly evolved melt. The occurrence of distal Lava Creek tephra in stratigraphic sequences marking the Marine Isotope Stage 16–15 transition supports the apparent eruption age of ∼631 ka. The combined results reveal that Lava Creek zircons record episodic heating, renewed crystallization, and an overall up-temperature evolution for Yellowstones subvolcanic reservoir in the 103−104 year interval before eruption.

Eruptive history of Neapolitan volcanoes: constraints from 40Ar–39Ar dating

The city of Naples can be considered part of the Campi Flegrei volcanic field, and deposits within the urban area record many autochthonous pre- to post-caldera eruptions. Age measurements were carried out using 40 Ar– 39 Ar dating techniques on samples from small monogenetic vents and more widely distributed tephra layers. The 40 Ar– 39 Ar ages on feldspar phenocrysts yielded ages of c . 16 ka and 22 ka for events older than the Neapolitan Yellow Tuff caldera-forming eruption (15 ka), and ages of c . 40 ka, 53 ka and 78 ka for events older than the Campanian Ignimbrite caldera-forming eruption (39 ka). The oldest age obtained is 18 ka older than previous dates for pyroclastic deposits cropping out along the northern rim of Campi Flegrei. The results of this study allow us to divide the Campi Flegrei volcanic history into four main, geochronologically distinct eruptive cycles. A new period, the Paleoflegrei, occurred before 74–78 ka and has been proposed to better discriminate the ancient volcanism in the volcanic field. The eruptive history of Campi Flegrei extends possibly further back than this, but the products of previous eruptions are difficult to date owing to the lack of fresh juvenile clasts. These new geochronological data, together with recently published ages related to young volcanic edifices located in the city of Naples (Nisida volcano, 3.9 ka) testify to persistent activity over a period of at least 80 ka, with an average eruption recurrence interval of ~555 years within and adjacent to this densely populated city.

Diapiric ascent and cooling of a sillimanite gneiss dome revealed by 40Ar/39Ar thermochronology: the Kigluaik Mountains, Seward Peninsula, Alaska

Abstract The upper amphibolite to granulite facies Kigluaik gneiss dome cooled rapidly in late Cretaceous time as it rose through the crust and was emplaced against the older blueschist-greenschist facies Nome Group. 40Ar/39Ar data from metamorphic rocks reveal prolonged moderately rapid cooling histories that vary somewhat with structural depth and geographical position. Hornblende ages (Tc c. 535°C) range from 86 to 82 Ma, mica ages (Tc c. 300–400°C) range from 85 to 83 Ma, and K-feldspar spectra yield age gradients that record cooling from c. 300° to c. 150°C between 82 and 65 Ma. These high-precision cooling ages on hornblende, white mica and biotite, and detailed temperature-time curves obtained from diffusion modelling of K-feldspar age spectra allow us to reconstruct the 3D geometry of isotherms during cooling and local reheating of the gneiss dome. Isotherms were parallel to lithological contacts during high-temperature cooling, then became subhorizontal in present-day coordinates as layering was domed by c. 84 Ma. Low-temperature cooling (300–150°C) of the gneiss dome is asymmetrical, with the north side cooling several million years before the south side. The contact between high-grade gneisses of the Kigluaik Mountains and surrounding lower-grade rocks is not a fault, but rather a steep metamorphic field gradient where closely spaced Barrovian isograds and partially reset mica ages document progressive thermal overprint of the older blueschist-greenschist facies Nome Group rocks. These combined geological and thermochronological data are most compatible with a model wherein the Kigluaik gneiss dome rose diapirically from mid-crustal levels between 91 Ma and c. 82 Ma and cooled through low temperatures differentially as it was tilted gently southward between 83 and 65 Ma.

Mammoth Mountain and its mafic periphery—A late Quaternary volcanic field in eastern California

The trachydacite complex of Mammoth Mountain and an array of contemporaneous mafic volcanoes in its periphery together form a discrete late Pleistocene magmatic system that is thermally and compositionally independent of the adjacent subalkaline Long Valley system (California, USA). The Mammoth system first erupted ca. 230 ka, last erupted ca. 8 ka, and remains restless and potentially active. Magmas of the Mammoth system extruded through Mesozoic plutonic rocks of the Sierra Nevada batholith and extensive remnants of its prebatholith wall rocks. All of the many mafic and silicic vents of the Mammoth system are west or southwest of the structural boundary of Long Valley caldera; none is inboard of the caldera’s buried ring-fault zone, and only one Mammoth-related vent is within the zone. Mammoth Mountain has sometimes been called part of the Inyo volcanic chain, an ascription we regard inappropriate and misleading. The scattered vent array of the Mammoth system, 10 × 20 km wide, is unrelated to the range-front fault zone, and its broad nonlinear footprint ignores both Long Valley caldera and the younger Mono-Inyo range-front vent alignment. Moreover, the Mammoth Mountain dome complex (63%–71% SiO 2 ; 8.0%–10.5% alkalies) ended its period of eruptive activity (100–50 ka) long before Holocene inception of Inyo volcanism. Here we describe 25 silicic eruptive units that built Mammoth Mountain and 37 peripheral units, which include 13 basalts, 15 mafic andesites, 6 andesites, and 3 dacites. Chemical data are appended for nearly 900 samples, as are paleomagnetic data for ∼150 sites drilled. The 40 Ar/ 39 Ar dates (230–16 ka) are given for most units, and all exposed units are younger than ca. 190 ka. Nearly all are mildly alkaline, in contrast to the voluminous subalkaline rhyolites of the contiguous long-lived Long Valley magma system. Glaciated remnants of Neogene mafic and trachydacitic lavas (9.1–2.6 Ma) are scattered near Mammoth Mountain, but Quaternary equivalents older than ca. 230 ka are absent. The wide area of late Quaternary Mammoth magmatism remained amagmatic during the long interval (2.2–0.3 Ma) of nearby Long Valley rhyolitic eruptions.

Evolution of the intra-arc Taupo-Reporoa Basin within the Taupo Volcanic Zone of New Zealand

The spatial and temporal distributions of volcaniclastic deposits in arc-related basins reflect a complex interplay between tectonic, volcanic, and magmatic processes that is typically difficult to unravel. We take advantage of comprehensive geothermal drill hole stratigraphic records within the Taupo-Reporoa Basin (TRB), and integrate them with new 40 Ar/ 39 Ar age determinations, existing age data, and new mapping to develop a four-dimensional model of basin evolution in the central Taupo Volcanic Zone (TVZ), New Zealand. Here, exceptional rhyolitic productivity and high rates of extensional tectonism have resulted in the formation of at least eight calderas and two subparallel, northeast-trending rift basins, each of which is currently subsiding at 3 to 4 mm/yr: the Taupo fault belt (TFB) to the northwest and the TRB to the southeast (the main subject of this paper). The basins are separated in the northeast by a high-standing, fault-controlled range termed the Paeroa block, which is the focus of mapping for this study, and in the southwest by an along strike alignment of smaller scale faults and an associated region of lower relief. Stratigraphic age constraints within the Paeroa block indicate that a single basin (∼120 km long by 60 km wide) existed within the central TVZ until 339 ± 5 ka (Paeroa Subgroup eruption age), and it is inferred to have drained to the west through a narrow and deep constriction, the present-day Ongaroto Gorge. Stratigraphic evidence and field relationships imply that development of the Paeroa block occurred within 58 ± 26 k.y. of Paeroa Subgroup emplacement, but in two stages. The northern Paeroa block underwent uplift and associated tilting first, followed by the southern Paeroa block. Elevations (>500 m above sea level) of lacustrine sediments within the southern Paeroa block are consistent with elevations of rhyolite lavas in the Ongaroto Gorge, the outlet to the paleolake in which these sediments were deposited, and indicate that the Paeroa block has remained relatively stable since development. East of the Paeroa block, stratigraphic relationships show that movement along the Kaingaroa Fault zone, the eastern boundary of the central TVZ, is associated with volcano-tectonic events. Stratigraphic and age data are consistent with rapid formation of the paired TRB and TFB at 339 ± 5 ka, and indicate that gradual, secular rifting is punctuated by volcano-tectonic episodes from time to time. Both processes influence basin evolution.

Age, geochemical composition, and distribution of Oligocene ignimbrites in the northern Sierra Nevada, California: implications for landscape morphology, elevation, and drainage divide geography of the Nevadaplano

To gain a better understanding of the topographic and landscape evolution of the Cenozoic Sierra Nevada and Basin and Range, we combine geochemical and isotopic age correlations with palaeoaltimetry data from widely distributed ignimbrites in the northern Sierra Nevada, California. A sequence of Oligocene rhyolitic ignimbrites is preserved across the modern crest of the range and into the western foothills. Using trace and rare earth element geochemical analyses of volcanic glass, these deposits have been correlated to ignimbrites described and isotopically dated in the Walker Lane fault zone and in central Nevada (Henry et al., 2004, Geologic map of the Dogskin mountain quadrangle; Washoe County, Nevada; Faulds et al., 2005, Geology, v. 33, p. 505–508). Ignimbrite deposits were sampled within the northern Sierra Nevada and western Nevada, and four distinct geochemical compositions were identified. The majority of samples from within the northern Sierra Nevada have compositions similar to the tuffs of Axehandle Canyon or Rattlesnake Canyon, both likely sourced from the same caldera complex in either the Clan Alpine Mountains or the Stillwater Range, or to the tuff of Campbell Creek, sourced from the Desatoya Mountains caldera. New 40Ar/39Ar age determinations from these samples of 31.2, 30.9, and 28.7 Ma, respectively, support these correlations. Based on an Oligocene palinspastic reconstruction of the region, our results show that ignimbrites travelled over 200 km from their source calderas across what is now the crest of the Sierra Nevada, and that during that time, no drainage divide existed between the ignimbrite source calderas in central Nevada and sample locations 200 km to the west. Palaeoaltimetry data from Sierra Nevada ignimbrites, based on the hydrogen isotopic composition of hydration water in glass, reflect the effect of a steep western slope on precipitation and indicate that the area had elevations similar to the present-day range. These combined results suggest that source calderas were likely located in a region of high elevation to the east of the Oligocene Sierra Nevada, which had a steep western slope that allowed for the large extent and broad distribution of the ignimbrites.