Vertical Tectonics Research Articles

Archean diapirism recorded by vertical sheath folds in the core of the Yalgoo Dome, Yilgarn Craton

In Archean granite-greenstone terrains, both vertical and horizontal tectonic processes can generate the typical dome-and-keel structural geometries. One of the classic Archean domes interpreted to result from a horizontal tectonic regime is the Yalgoo Dome in the Yilgarn Craton of Western Australia. In this contribution, we reassess the structural evolution of the Yalgoo Dome by detailing the structures exposed in the migmatitic core. The core contains multi-scale domes-and-basins. These structures were previously interpreted as the result of superposed folding. Our structural mapping shows that the dome-and-basin patterns result from vertical sheath folds developed in a single, progressive deformation event (D1). This first event most likely resulted from diapiric ascent of a buoyant partially molten tonalitic crust and was overprinted by two contractional episodes (D2–D3) both reflecting bulk EW shortening. The structural evolution proposed here points towards a shift from an early history of vertical tectonics dominated by diapirism to a regional regime dominated by horizontal tectonics.

Geological archive of the onset of plate tectonics.

Plate tectonics, involving a globally linked system of lateral motion of rigid surface plates, is a characteristic feature of our planet, but estimates of how long it has been the modus operandi of lithospheric formation and interactions range from the Hadean to the Neoproterozoic. In this paper, we review sedimentary, igneous and metamorphic proxies along with palaeomagnetic data to infer both the development of rigid lithospheric plates and their independent relative motion, and conclude that significant changes in Earth behaviour occurred in the mid- to late Archaean, between 3.2 Ga and 2.5 Ga. These data include: sedimentary rock associations inferred to have accumulated in passive continental margin settings, marking the onset of sea-floor spreading; the oldest foreland basin deposits associated with lithospheric convergence; a change from thin, new continental crust of mafic composition to thicker crust of intermediate composition, increased crustal reworking and the emplacement of potassic and peraluminous granites, indicating stabilization of the lithosphere; replacement of dome and keel structures in granite-greenstone terranes, which relate to vertical tectonics, by linear thrust imbricated belts; the commencement of temporally paired systems of intermediate and high dT/dP gradients, with the former interpreted to represent subduction to collisional settings and the latter representing possible hinterland back-arc settings or ocean plateau environments. Palaeomagnetic data from the Kaapvaal and Pilbara cratons for the interval 2780–2710 Ma and from the Superior, Kaapvaal and Kola-Karelia cratons for 2700–2440 Ma suggest significant relative movements. We consider these changes in the behaviour and character of the lithosphere to be consistent with a gestational transition from a non-plate tectonic mode, arguably with localized subduction, to the onset of sustained plate tectonics.This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics'.

Pre-rift sedimentation of the Lomonosov Ridge, Arctic Ocean at 84°N – A correlation to the complex geologic evolution of the conjugated Kara Sea

The Arctic Ocean region plays, and has played in the geological past, a key role for Earth’s climate and oceanic circulation and their evolution. Studying the Lomonosov Ridge, a narrow submarine continental ridge in the central Arctic Ocean, is essential to answer fundamental questions related to the complex tectonic evolution of the Arctic basins, the glacial history, and the details of known paleoceanographic changes in the Cenozoic. In this study, we present a new seismic dataset that provides insights into the sedimentary structures along the ridge, their possible origin, age and formation. We compare the structure and stratigraphy of the deeper parts of the ridge between 83°N and 84°30′N to its conjugate, the Severnaya Zemlya Archipelago at the Eurasia margin. We propose that some sediment sequences directly underlying the prominent HARS (High Amplitude Reflector Sequence) formed well before the ridge separated from the Barents and Kara shelves and represent a prolongation of the North Kara Terrane, most likely part of the Neoproterozoic Timanide orogen. Towards Siberia along the Lomonosov Ridge, we interpret the HARS to be underlain by Upper Proterozoic-Lower Paleozoic metasedimentary material that is correlated to metamorphic complexes exposed on Bol’shevik Island. Northward, this unit descends and gives way to a foreland sedimentary basin complex of presumed Ordovician/Devonian age, which underwent strong deformation during the Triassic/Jurassic Novaya Zemlya orogeny. The transition zone between these units might mark a conjugate continuation of the Eurasian margin’s Bol’shevik-Thrust Zone. A prominent erosional unconformity is observed over these strongly deformed foreland basins of the Eurasian and Lomonosov Ridge margins, and is conceivably related to vertical tectonics during breakup or a later basin-wide erosional event.

Mantle Lithosphere Rheology, Vertical Tectonics, and the Exhumation of (U)HP Rocks

AbstractNumerical modeling results indicate that mantle lithosphere rheology can influence the pressure‐temperature‐time (P‐T‐t) trajectories of continental crust subducted and exhumed during the onset of continental collision. Exhumation of ultrahigh‐pressure (~35 kbar)/high‐temperature (~750°C) metamorphic rocks is more prevalent in models with stronger continental mantle lithosphere (e.g., dry), whereas high‐pressure (~9–22 kbar)/low‐temperature (350°C–630°C) metamorphic rocks occur in models with weaker rheology (e.g., hydrated) for the same layer. In the latter case, the buried crustal rocks can remain encased in ablatively subducting mantle lithosphere, reach only moderate temperatures, and exhume by dripping/detachment of the lithospheric root. In this transition from subduction to a dripping style of “vertical tectonics,” burial and exhumation of crustal rocks are driven without imposed far‐field plate convergence. The model results are compared against thermobarometric P‐T estimates from major (ultra)high‐pressure metamorphic terranes. We propose that the exhumation of high‐pressure/low‐temperature metamorphic rocks in Tavşanlı and Afyon zones in western Anatolia may be caused by viscous dripping of mantle lithosphere suggesting a weaker continental mantle lithosphere, whereas (ultra)high‐pressure exhumation (e.g., Dabie Shan‐eastern China and Dora Maira‐western Alps) may be associated with plate‐like subduction. In the latter case, the slab is much stronger and deformation is localized to the subduction interface along which rocks are buried to >100 km depth before they are exhumed to the near surface.

Crustal shortening and thickening in Neoarchean granite-greenstone belts: A case study from the link between the ∼2.7 Ga Elu and Hope Bay belts, northeast Slave craton, Canada

The Elu Link between the ∼2.7 Ga Hope Bay and Elu belts in the northeast Bathurst Block of the Slave craton comprises supracrustal and intrusive rocks variably deformed by three tectono-metamorphic events (D1–D3). The geometry of D1 structures formed during prograde metamorphism is uncertain, because of subsequent overprint. D2 occurred in two stages predating (D2a) and postdating (D2b) peak metamorphism. D1 and D2a were thrusting events inferred from peak metamorphic pressures of ∼6.7 kbar (670 MPa) retained by a garnet orthogneiss. The latter is diagnostic of thrust tectonism in Archean granite-greenstone belts with no characteristic thrust faults. Unlike D2a, D2b was a vertical general flattening event prevailing during the formation of magmatic domes and interdomal folds that form the main strain patterns of the belts. This was followed by the formation of buckled F3 folds associated with D3 vertical constriction. The switch from thrust to vertical tectonics during peak metamorphism and subsequent deformation resulted in intense recrystallization that explains the poor preservation and scarcity of early-formed shears, including thrust zones. A tectonic process, combining D1+D2a thrust stacking, sagduction, and vertical stretching during D2b and D3, is suggested to explain crustal thickening in the Elu Link and terrains of similar ages.

Anomalous subsidence history of the West Siberian Basin as an indicator for episodes of mantle induced dynamic topography

Vertical intraplate motions are not easily related to plate tectonics. Understanding their underlying mechanism is further complicated by poor constraints on the magnitude and timing of these motions. Western Siberia is a prominent intraplate area affected by vertical tectonics. Shortly after the assemblage of Pangaea, the region experienced substantial uplift followed by a long period of subsidence that led to formation of the world's largest sedimentary basin–the West Siberian Basin. Its sedimentary cover has been extensively explored and described in the Soviet literature. Here we digitise compilations of stratigraphic data to calculate the basin's subsidence history by backstripping analysis. Our results confirm the prolonged and high amplitude regional subsidence that has been noted before and constitutes the basin's first-order vertical motion. However, our backstripping results also reveal a secondary mode of shorter spatial and temporal scales induced by the migration of the maximum subsidence across the basin. Importantly, the generally slow subsidence of the basin was interrupted by uplift events in the early and late Cretaceous and in the Middle Oligocene. While the secondary mode of vertical motion is not easily understood in terms of traditional subsidence models restricted to lithospheric cooling after stretching, it is consistent with rapid uplift events that have been reported for other locations, such as elevated passive continental margins. We discuss the geodynamic implications and conclude that the basin may hold important constraints on dynamic topography induced by sublithospheric mantle flow processes.

Large-Scale Crustal-Block-Extrusion During Late Alpine Collision

The crustal-scale geometry of the European Alps has been explained by a classical subduction-scenario comprising thrust-and-fold-related compressional wedge tectonics and isostatic rebound. However, massive blocks of crystalline basement (External Crystalline Massifs) vertically disrupt the upper-crustal wedge. In the case of the Aar massif, top basement vertically rises for >12 km and peak metamorphic temperatures increase along an orogen-perpendicular direction from 250 °C–450 °C over horizontal distances of only <15 km (Innertkirchen-Grimselpass), suggesting exhumation of midcrustal rocks with increasing uplift component along steep vertical shear zones. Here we demonstrate that delamination of European lower crust during lithosphere mantle rollback migrates northward in time. Simultaneously, the Aar massif as giant upper crustal block extrudes by buoyancy forces, while substantial volumes of lower crust accumulate underneath. Buoyancy-driven deformation generates dense networks of steep reverse faults as major structures interconnected by secondary branches with normal fault component, dissecting the entire crust up to the surface. Owing to rollback fading, the component of vertical motion reduces and is replaced by a late stage of orogenic compression as manifest by north-directed thrusting. Buoyancy-driven vertical tectonics and modest late shortening, combined with surface erosion, result in typical topographic and metamorphic gradients, which might represent general indicators for final stages of continent-continent collisions.

Neoarchean tectonics: Insight from the Baijiafen ductile shear zone, eastern Anshan, Liaoning Province, NE China

The North China Craton is one of the major Archean to Paleoproterozoic cratons in the world and oldest craton in China, which preserves a large amount of ancient basement and abundant structures showing the early earth tectonics. The controversy over the Archean tectonic regimes has lasted several decades centering around horizontal and vertical tectonics, the two classical tectonic models for Archean times. Thus, more studies of the early crustal growth and tectonic evolution are requisite for better understanding geodynamic regimes in the early Precambrian. This study provides an example for revealing of Archean tectonics. The NWN-trending, ENE-dipping Baijiafen ductile shear zone is located in the eastern Anshan of the northeastern North China Craton and mainly comprises two types of gneisses, including the Chentaigou porphyritic granitic gneiss and the Baijiafen trondhjemitic gneiss. In this study, we have carried out a detailed study on the macrostructure, microstructure and fabric characteristics of the two main types of deformed gneisses within the shear zone. The ribbon structures formed by intensely elongated quartz grains are widespread in these gneisses. Well-developed mineral stretching lineations and asymmetric fabrics indicate an ESE-directed downward shearing. The quartz c-axis fabric patterns obtained by electron backscatter diffraction technique imply low to middle temperature non-coaxial deformation with active rhomb 〈a〉 slip and basal 〈a〉 slip. Deformation behaviors of minerals and quartz crystallographic preferred orientations demonstrate that the rocks underwent mylonitization at a temperature of 400–500°C under greenschist facies metamorphic conditions. Dislocation creep is the main rock deformation mechanism within the shear zone. Finite strain measurement results suggest that the strain types of the shear zone are generally related to elongate-plane deformation, and the tectonites change from L-S- to LS-type across the shear zone. The strain intensity increases obviously towards east, and the Baijiafen gneiss unit is located within the high-strain zone. Kinematic vorticity values of gneisses indicate that the deformed rocks were produced by steady-state simple-shear dominated general shear. Based on abundant structural evidence and previous studies, we infer that the deformation of the Baijiafen ductile shear zone may have resulted from the downward adjoining rock flow during the sagduction of the banded iron formations and synchronous Archean granite dome emplacement, supporting a vertical tectonic regime in Archean times.

Tectonic aspects of Precambrian metallogeny of gold and uranium

A cause-and-effect relation is established between historical metallogeny of gold and uranium and extraterrestrial factors (impact events, evolution of the distance between Earth and Moon, rotation geodynamics), which significantly affected the Early Precambrian tectonic evolution of our planet. It is shown by the example of the complex Witwatersrand deposit that the Precambrian polygenetic Au and U deposits of the quartz–pebble type were formed within a near-equatorial epi-Archean supercontinent and were related to extraterrestrial factors under a rotation regime of the plume vertical tectonics. The beginning of breakup of the epi-Archean supercontinent in the Paleo- and Mesoproterozoic (2.0 ± 0.3 Ga) was related to the abrupt decrease in the velocity of the Earth’s axial rotation followed by the dominant regime of subhorizontal plate tectonics and formation of rich U deposits of the nonconformity type (which are structurally related to the horizontal inertial detachments at the contacts of the consolidated crust) and Meso- and Neoproterozoic sedimentary complexes.

Characterization of transpressive deformation in shear zones of the Archean North Caribou greenstone belt (NW Superior Province) and the relationship with regional metamorphism

The 2.7–3.0Ga North Caribou greenstone belt (NCGB), host to the Musselwhite BIF-hosted gold deposit, possesses abundant shear zones on its northern margins, which appear to have formed under amphibolite facies conditions. Protracted deformation and regional metamorphism are coeval with widespread magmatism and accretion events during crustal amalgamation of the Western Superior Province, and are responsible for folding the ore-hosting BIF and channeling fluids. The importance of shear zones in behaving as conduits for fluids during the tectonic evolution of the NCGB is not well known and their relationship with metamorphism is equivocal, yet higher-grade, syn- to post-tectonic metamorphic minerals seem to correlate with loci of higher strain. Structural analyses support oblique transpressive collision that produced steeply-dipping planar and shallowly-plunging linear fabrics with dominant dextral kinematics, that trend broadly parallel to the doubly arcuate shape of the belt. Electron backscatter diffraction analyses were conducted on strategic samples across one shear zone in order to characterize crustal conditions during transpressive deformation. The Dinnick Lake shear zone cuts through mafic metavolcanics and at its core is an L-tectonite granite composed of recrystallized quartz. Whole rock geochemistry shows little variation in Ca, Na, Mg and K (often used as indicators of hydrothermal alteration) from surrounding less deformed units, suggesting deformation in a dry environment. Microstructural analysis indicates subgrain rotation recrystallization and deformation by prism a- and c-slip in quartz, as well as aligned hornblende that suggest deformation temperatures above 500°C. Quartz in mafic rocks along the margins of the shear zone also exhibits a basal a-slip component, indicating a slight decrease in strain or temperature. Although the NCGB exhibits some first-order evidence of vertical tectonism (dome and keel geometries), the dominant strain record within shear zones is that of horizontal (oblique transpressive) displacement. This is in agreement with other greenstone belts in the Western Superior Province where vertical tectonism and horizontal tectonism were coeval.

Generation of felsic crust in the Archean: A geodynamic modeling perspective

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.

Qingyuan high-grade granite–greenstone terrain in the Eastern North China Craton: Root of a Neoarchaean arc

The Qingyuan high-grade granite–greenstone terrain is one of the first recognized granite–greenstone terrains in the Eastern North China Craton. Similar to other Archaean terrains, its tectonic environment is debated: was it an intra-continental rift or a modern analogy of arc? Occurrence and petrogenesis of major igneous series, the ultramafic–mafic and felsic volcanic rocks (>2510Ma) and the plutons of the quartz diorite (2570–2510Ma), TTG (2570–2510Ma) and quartz monzodiorite (2510–2490Ma) series in Xinbin area are investigated. The mafic intrusives and volcanic rocks have high MgO content (5.4–7.5wt.%) and Mg-number (48–61). They show slightly depleted to flat trace element patterns. The ultramafic rocks (serpentinite) could be genetically related to the mafic rocks. The meta-dacite–rhyolite is adakitic with enriched light and middle REEs and LILEs, but significantly depleted HFSEs. The quartz diorite has high Mg-number (60–64), moderately enriched light and middle REEs and LILEs. The TTG shows distinct light but moderate middle REE-enrichment, prominent Nb–Ta-depletion but Zr–Hf-enrichment (Zr/Sm>100). The quartz monzodiorite has moderate light and middle REE-enrichment, significant Nb–Ta-depletion, and negative Eu-/Sr-anomalies. The TTG has more depleted Sr–Nd isotopes (εNdt=+2–+6; 87Sr/86Srt=~0.700) than all the others (εNdt=0–+2; 87Sr/86Srt=0.701–703). Their petrogenesis can be explained by an Archaean-style subduction defined as a mantle wedge–absent flat–‘hot’-subduction with significant vertical tectonism in the overriding slab: the ultramafic–mafic rocks were originated from primitive mantle; the meta-dacite–rhyolite was originated from the eclogite facies overriding crust; the quartz diorite was a mixture of melts from mantle and the overriding crust; the TTG was from the subducting slab under amphibolite to amphibole-bearing eclogite facies; and the quartz monzodiorite was from the subducting slab after the derivation of the TTG. The Qingyuan terrain represents remnants of the root of a Neoarchaean arc: its anticlockwise P–T-path recorded the initiation of the arc to its cession and exhumation/cratonization at ~2480Ma.

Reply to “Paleo- to Mesoarchean polymetamorphism in the Barberton granite-greenstone belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt: Discussion” by M. Brown

Were modern-type subduction-accretion–like processes already operative in the Archean, or did a hotter Archean Earth with consequently weaker lithosphere prevent this and favor gravitationally driven vertical tectonics? Brown’s comment on [Cutts et al. (2014)][1] weighs in on this central

Role of fluids in the exhumation of the Southern Marginal Zone of the Limpopo Complex, South Africa

The Limpopo Complex (LC) of southern Africa displays structural, metamorphic, and geochronological evidence for a complex exhumation-controlled evolutionary history that followed after burial and attainment of peak metamorphic conditions (P>9kbar, T>900°C) in the root zone (Central Zone, CZ) of the orogen before ∼2.72Ga. The Southern Marginal Zone (SMZ) of the LC, in addition, provides evidence for thrust-controlled emplacement of a hot granulite facies nappe onto the adjacent low-grade granite greenstone terrane of the northern Kaapvaal Craton (NKVC) in the interval ∼2.69–2.62Ga. A tectono-metamorphic scenario in which initial near vertical channeling (exhumation) at ∼2.69Ga was transferred into horizontal channeling (thrusting) at ∼2.67–2.64Ga is interpreted to have commenced with in situ partial melting of crustal rocks in the root zone of the orogen (CZ). This resulted in formation of melt-weakened crust (migmatized gneisses) that with continuous shortening led to vertical transfer of lower crustal material, accompanied by fluid-fluxed decompression melting that resulted in production of large volumes of leucocratic granitic material. This material detached to form buoyant diapirs that accompanied the emplacement at ∼2.65Ga of mega sheath folds at the middle crustal level throughout the entire CZ of the LC. This early stage of near-vertical exhumation is reflected (i) by the fact that linear elements of sheath folds and shear zones plunge consistently at steep angles to the SW throughout the entire CZ, and (ii) by decompression-cooling (DC) segments of P–T paths constructed for metapelitic rocks. DC paths in the far northern part of the SMZ that are associated with the steep north-dipping portion of the Hout River Shear Zone (HRSZ) traversed from P∼8.5kbar, T∼850°C to P∼6.5kbar, T=720°C. Early near-vertical exhumation transferred into horizontal channel flow that controlled emplacement at the middle crustal level of the granulite facies nappe (SMZ). Horizontal channeling (thrusting) is recorded in metapelitic granulite by near-isobaric cooling (P∼6kbar, T∼700–550°C) segments of earlier DC P–T paths that are associated with the shallow north-dipping portion of the HRSZ. Devolitilization of underthrusted greenschist due to thrusting hot granulite over cool greenschist released large volumes of high temperature aqueous fluids from the underthrusted rocks that constantly infiltrated the overriding granulite during southwards migration of the nappe. Immiscible CO2-rich and brine fluids thus derived persistently infiltrated into and reacted with cooling granulite and triggered the main pulse of anatexis at P>7.5kbar, T>750°C. Similar fluids established a regional retrograde isograd and associated zone of 4500km2 retrograde hydrated granulite at P∼6kbar, T=600–630°C located in the hanging wall of the shallow north-dipping HRSZ that bounds the SMZ in the south. Shear zone-hosted metasomatic alteration including formation of lode-gold deposits occurred at T=600–630°C, P∼6kbar in the zone of rehydrated granulite, and at higher P–T conditions in the granulite zone. The granulite facies SMZ of the LC thus preserves clear evidence for a direct link between persistent fluid flux and fluid-rock interaction as an integral component of a conveyor-like tectonic mechanism driven by vertical tectonics that operated from the core region of the orogen (CZ). This mechanism explains the important link between the thrust-controlled emplacement of the SMZ onto the NKVC and deep crustal fluid flow.

Bouse Formation in the Bristol basin near Amboy, California, USA

Limestone beds underlain and overlain by alluvial fan conglomerate near Amboy, California, are very similar in many respects to parts of the Bouse Formation, suggesting that an arm of the Pliocene Bouse water body extended across a wide part of the southern Mojave Desert. The deposits are north of the town of Amboy at and below an elevation of 290 m, along the northern piedmont of the Bristol “dry” Lake basin. The Amboy outcrops contain the Lawlor Tuff (4.83 Ma), which is also found in an outcrop of the Bouse Formation in the Blythe basin near Buzzards Peak in the Chocolate Mountains, 180 km southeast of Amboy. Bouse exposures near Amboy are ∼3.4 m thick, white, distinctly bedded, with limestone and calcareous sandstone as well as stromatolite mounds; we interpret these as nearshore deposits. The Bouse at Amboy contains ostracodes, diatoms, and mollusks that indicate saline lake or estuarine environments with an admixture of fresh-water forms. Along with wading bird tracks and a spine from a marine fish, these fossils suggest that the deposits formed in saline waters near a fresh-water source such as a perennial stream. Beds of the outcrop dip southward and are 113 m above the surface of Bristol Playa, where similar age sediments are buried 270+ m deep, indicating significant faulting and vertical tectonics in this part of the Eastern California Shear Zone during the past 5 m.y. Confirmation of the Bouse Formation at Amboy strengthens previous assignments to the Bouse Formation for mudstones in driller logs at Danby “dry” Lake, California, and suggests that areally extensive arms of the Bouse water body were west of the Blythe basin. The Bristol basin arm of the lower Bouse basin probably was restricted from the main water body by narrow passages, but Bouse sediment there is similar to that in the Blythe basin, suggesting generally similar water chemistry and environmental conditions. Examining the degree to which Bouse deposits in the western arms differed from Bouse deposits in the Blythe basin offers an approach to test whether the southernmost Bouse water body was deposited in an estuarine or lacustrine setting.

Active simultaneous uplift and margin-normal extension in a forearc high, Crete, Greece

The island of Crete occupies a forearc high in the central Hellenic subduction zone and is characterized by sustained exhumation, surface uplift and extension. The processes governing orogenesis and topographic development here remain poorly understood. Dramatic topographic relief (2–6 km) astride the southern coastline of Crete is associated with large margin-parallel faults responsible for deep bathymetric depressions known as the Hellenic troughs. These structures have been interpreted as both active and inactive with either contractional, strike-slip, or extensional movement histories. Distinguishing between these different structural styles and kinematic histories here allows us to explore more general models for improving our global understanding of the tectonic and geodynamic processes of syn-convergent extension. We present new observations from the south–central coastline of Crete that clarifies the role of these faults in the late Cenozoic evolution of the central Hellenic margin and the processes controlling Quaternary surface uplift. Pleistocene marine terraces are used in conjunction with optically stimulated luminesce dating and correlation to the Quaternary eustatic curve to document coastal uplift and identify active faults. Two south-dipping normal faults are observed, which extend offshore, offset these marine terrace deposits and indicate active N–S (margin-normal) extension. Further, marine terraces preserved in the footwall and hanging wall of both faults demonstrate that regional net uplift of Crete is occurring despite active extension. Field mapping and geometric reconstructions of an active onshore normal fault reveal that the subaqueous range-front fault of south–central Crete is synthetic to the south-dipping normal faults on shore. These findings are inconsistent with models of active horizontal shortening in the upper crust of the Hellenic forearc. Rather, they are consistent with topographic growth of the forearc in a viscous orogenic wedge, where crustal thickening and uplift are a result of basal underplating of material that is accompanied by extension in the upper portions of the wedge. Within this framework a new conceptual model is presented for the late Cenozoic vertical tectonics of the Hellenic forearc.

Nonvolcanic tectonic islands in ancient and modern oceans

Most oceanic islands are due to excess volcanism caused by thermal and/or compositional mantle melting anomalies. We call attention here to another class of oceanic islands, due not to volcanism but to vertical motions of blocks of oceanic lithosphere related to transform tectonics. Sunken tectonic islands capped by carbonate platforms have been previously identified along the Vema and Romanche transforms in the equatorial Atlantic. We reprocessed seismic reflection lines, did new facies analyses and 87Sr/86Sr dating of carbonate samples from the carbonate platforms. A 50 km long narrow paleoisland flanking the Vema transform, underwent subsidence, erosion, and truncation at sea level; it was then capped by a 500 m thick carbonate platform dated by 87Sr/86Sr at ∼11–10 Ma. Three former islands on the crest of the Romanche transverse ridge are now at ∼900 m bsl; they show horizontal truncated surfaces of oceanic crust capped by ∼300 m thick carbonate platforms, with 10–6 Ma Sr isotopic ages. These sunken islands formed due to vertical tectonics related to transtension/transpression along long‐offset slow‐slip transforms. Another tectonic sunken island is Atlantis Bank, an uplifted gabbroic block along the Atlantis II transform (SW Indian Ridge) ∼700 m bsl. A modern tectonic island is St. Peter and St. Paul Rocks, a rising slab of upper mantle located at the St. Paul transform (equatorial Atlantic). “Cold” tectonic islands contrast with “hot” volcanic islands related to mantle thermal and/or compositional anomalies along accretionary boundaries and within oceanic plates, or to supra‐subduction mantle melting that gives rise to islands arcs.

Diapirism and sagduction as a mechanism for deposition and burial of “Timiskaming-type” sedimentary sequences, Superior Province: Evidence from detrital zircon geochronology and implications for the Borden Lake conglomerate in the exposed middle to lower crust in the Kapuskasing uplift

Neoarchean “Timiskaming-type” sedimentary sequences are the youngest supracrustal rocks in many Archean greenstone belts. Detrital zircon ages from such a sequence in the northwestern Superior Province match the known ages of volcanism and plutonism in the surrounding area. They show a marked change in zircon provenance from the lower to upper part of the sequence, in a clear pattern that can be best explained by a model involving basin formation by diapirism and sagduction. In this model, detritus were derived from down-stratigraphy erosion above rising granitoid domes and deposited in a sinking synclinal keel between them. These new data thus support the presence of vertical tectonism in the Superior Province at a time in which abundant evidence for horizontal tectonism has been documented. Diapirism and sagduction provides a mechanism to bring supracrustal rocks deep into the crust below the synclinal keel, without involving crustal thickening and potentially during crustal extension. This can readily explain the observation in the Kapuskasing uplift that the Borden Lake conglomerate, a Timiskaming-type sedimentary rock, was buried up to 30km deep in the crust and experienced upper amphibolite to granulite-facies metamorphism very shortly after its deposition.

Emerged fossil corals on the coast of northwestern Hainan Island, China: Implications for mid-Holocene sea level change and tectonic uplift

Fossil corals are widespread on the coast of northwestern Hainan Island, China. Most of these corals are exposed during low tide levels, indicating that a higher sea level may have existed during their life time. The radiocarbon data introduced by this paper in combination with those from other sources show that the corals were alive mainly during the mid- to late-Holocene. Mid-Holocene sea levels 1–3 m higher than the present level were confirmed for adjacent coasts; however, the emerged corals on the investigated coast are not necessarily indicators of a higher eustatic sea level. They do predict relative sea levels of 1.5–3.4 m when using 1 m below the tidal datum as the upper limit of coral growth. However, using the Mean Lower Low Water as the coral’s upper growth limit, the relative sea level on the non-volcanic coast was almost as high as the present level, whereas that on the volcanic coast was 0.4–2.0 m higher than present. Therefore, the exposure of these dead coral heads and their discrepancy in elevation may be the result of tectonic uplift caused by volcanic activities. From these results, the highest sea level of upward coral growth must be defined first when using fossil corals to reconstruct past sea levels. Meanwhile, vertical tectonics have to be taken into account as a significant factor when conducting high-resolution sea level reconstruction, although the South China coast is located in a “tectonically stable” region.

Synchronous vertical and horizontal tectonism at late stages of Archean cratonization and genesis of Hemlo gold deposit, Superior craton, Ontario, Canada

The Hemlo gold deposit (∼700 t Au; northwestern Ontario, Canada) is hosted in a shear zone in the Hemlo greenstone belt, which is a synclinal keel between two granitoid domes. The dome-and-keel structure formed by diapirism and sagduction (vertical tectonism) that was synchronous with regional horizontal shearing (horizontal tectonism). Mineralization occurred during shearing, and mineralizing fluids probably had a magmatic source that is interpreted to be related to syntectonic mantle-derived granodiorite (sanukitoid) intrusions. The gold deposit, the intrusions, and the hosting shear zone all developed within a regime characterized by synchronous vertical and horizontal tectonism. They were all possibly linked to a range of processes associated with the accretionary growth and stabilization of the craton, in particular slab breakoff and the associated extensional orogenic collapse following terrane accretion.