Subduction zone backarcs, mobile belts, and orogenic heat

Hot Cordilleran hinterland promoted lower crust mobility and decoupling of Laramide deformation

The Late Cretaceous to Paleogene Laramide orogen in the North American Cordillera involved deformation >1,000 km from the plate margin that has been attributed to either plate-boundary end loading or basal traction exerted on the upper plate from the subducted Farallon flat slab. Prevailing tectonic models fail to explain the relative absence of Laramide-aged (ca. 90-60 Ma) contractional deformation within the Cordillera hinterland. Based on Raman spectroscopy of carbonaceous material thermometry and literature data from the restored upper 15-20 km of the Cordilleran crust we reconstruct the Late Cretaceous thermal architecture of the hinterland. Interpolation of compiled temperature data (n = 200) through a vertical crustal column reveals that the hinterland experienced a continuous but regionally elevated, upper-crustal geothermal gradient of >40 °C/km during Laramide orogenesis, consistent with peak metamorphic conditions and synchronous peraluminous granitic plutonism. The hot and partially melted hinterland promoted lower crust mobility and crust-mantle decoupling during flat-slab traction.

UHT Metamorphism Peaking Above 1100 °C with Slow Cooling: Insights from Pelitic Granulites in the Jining Complex, North China Craton

The peak temperature and duration of ultrahigh-temperature (UHT) metamorphism are critical to identify and understand its tectonic environment. The UHT metamorphism of the Jining complex in the Khondalite Belt, North China Craton is controversial on the peak temperature, time and tectonic setting. A representative sapphirine-bearing granulite sample is selected from the classic Tianpishan outcrop for addressing the metamorphic evolution and timing. The rock is markedly heterogeneous on centimetre scale and can be divided into melanocratic domains rich in sillimanite (MD-s) or rich in orthopyroxene (MD-o), and leucocratic domains (LD). On the basis of detailed petrographic analyses and phase equilibria modelling using THERMOCALC, all three types of domains record peak temperatures of 1120–1140 °C and a series of post-peak cooling stages at 0·8–0·9 GPa to the fluid-absent solidus (∼890 °C), followed by sub-solidus decompression. The peak temperature for MD-s is constrained by the coexistence of sillimanite-I + sapphirine + spinel + quartz, where sillimanite-I contains densely exsolved aciculae of hematite, yielding reintegrated Fe2O3 contents up to 2·1–2·3 wt %. The post-peak cooling evolution involves the sequential appearance of K-feldspar, sillimanite-II + garnet, orthopyroxene and biotite, where sillimanite-II is exsolution-free and contains variable Fe2O3 contents of 1·3–1·8 wt %. The peak temperature for MD-o is constrained by the sapphirine + orthopyroxene assemblage, where orthopyroxene has a maximum AlIV of 0·22 (Al2O3 = 9·5 wt %) in the core. The cooling evolution involves the sequential appearance of K-feldspar, garnet and biotite, and the decreasing AlIV (0·22→0·17) from core to rim in orthopyroxene. The peak temperature for LD is constrained by the inferred K-feldspar-absent assemblage and the maximum anorthite content of 0·11 in K-feldspar. The cooling evolution involves the crystallization of segregated melts, exsolution of supra-solvus ternary feldspars and growth of biotite. The Al in orthopyroxene, Fe2O3 in sillimanite and anorthite in K-feldspar are good indicators for constraining extreme UHT conditions although they depend differently on bulk-rock compositions. In-situ SHRIMP U–Pb dating of metamorphic zircon indicates that the UHT metamorphism may have occurred at >1·94 Ga and the cooling under UHT conditions lasted over 40 Ma. The extreme UHT metamorphism in the Jining complex is interpreted to be triggered by the advective heating of intraplate hyperthermal mafic magmas together with a plume-related hot mantle upwelling, following an orogenic crustal thickening event.

The evolution of the continental crust and the onset of plate tectonics

The Earth is the only known planet where plate tectonics is active, and different studies have concluded that plate tectonics commenced at times from the early Hadean to 700 Ma. Many arguments rely on proxies established on recent examples, such as paired metamorphic belts and magma geochemistry, and it can be difficult to establish the significance of such proxies in a hotter, older Earth. There is the question of scale, and how the results of different case studies are put in a wider global context. We explore approaches that indicate when plate tectonics became the dominant global regime, in part by evaluating when the effects of plate tectonics were established globally, rather than the first sign of its existence regionally. The geological record reflects when the continental crust became rigid enough to facilitate plate tectonics, through the onset of dyke swarms and large sedimentary basins, from relatively high-pressure metamorphism and evidence for crustal thickening. Paired metamorphic belts are a feature of destructive plate margins over the last 700 Myr, but it is difficult to establish whether metamorphic events are associated spatially as well as temporally in older terrains. From 3.8-2.7 Ga, suites of high Th/Nb (subduction-related on the modern Earth) and low Th/Nb (non-subduction-related) magmas were generated at similar times in different locations, and there is a striking link between the geochemistry and the regional tectonic style. Archaean cratons stabilised at different times in different areas from 3.1-2.5 Ga, and the composition of juvenile continental crust changed from mafic to more intermediate compositions. Xenon isotope data indicate that there was little recycling of volatiles before 3 Ga. Evidence for the juxtaposition of continental fragments back to ~2.8 Ga, each with disparate histories highlights that fragments of crust were moving around laterally on the Earth. The reduction in crustal growth at ~ 3 Ga is attributed to an increase in the rates at which differentiated continental crust was destroyed, and that coupled with the other changes at the end of the Archaean are taken to reflect the onset of plate tectonics as the dominant global regime.

Seismic evidence for a mantle suture and implications for the origin of the Canadian Cordillera

The origin of the North American Cordillera and its affinity with the bounding craton are subjects of contentious debate. The mechanisms of orogenesis are rooted in two competing hypotheses known as the accretionary and collisional models. The former model attributes the Cordillera to an archetypal accretionary orogen comprising a collage of exotic terranes. The latter, less popular view argues that the Cordillera is a collisional product between an allochthonous ribbon microcontinent and cratonic North America. Here we present new seismic evidence of a sharp and structurally complex Cordillera-craton boundary in the uppermost mantle beneath the southern Canadian Cordillera, which can be interpreted as either a reshaped craton margin or a Late Cretaceous collisional boundary based on the respective hypotheses. This boundary dips steeply westward underneath a proposed (cryptic) suture in the foreland, consisent with the predicted location and geometry of the mantle suture, thus favoring a collisional origin.

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'.

Chapter 4 Cenozoic rifting, passive margin development and strike-slip faulting in the Andaman Sea: a discussion of established v. new tectonic models

The Andaman Sea evolved from near-pure extension (WNW–ESE) during the Late Palaeogene, to highly oblique extension (NNW–SSE) during the Neogene, to strike-slip-dominated deformation (Late Miocene–Recent). These changes in extension direction and deformation style probably reflect the switch from slab rollback-driven extension to India coupling with Myanmar and driving oblique extension/dextral strike-slip. The East Andaman, Mergui–North Sumatra and Martaban Shelf basins, along with the Alcock and Sewell rises and Central Andaman Basin (CAB), were all involved with this deformation which became increasingly focused on the CAB and the rises with time. Possible revisions to traditional models for the Andaman Sea include: (1) the Alcock and Sewell rises are hyper-extended continental or island arc crust, not Miocene oceanic crust; (2) the East Andaman Basin is predominantly underlain by strongly necked to hyper-extended continental crust, not oceanic crust; or (3) CAB oceanic crust is of Miocene, not Pliocene–Recent age. At present a number of major issues can be addressed but not fully resolved, including: (1) the distribution, timing, volume and origin of magmatism in the basins; (2) the causes and significance of strong crustal reflections imaged on 2D and 3D seismic data; (3) implications for crustal thinning geometries, upper crustal extensional patterns and distribution of igneous intrusions for current models of passive margin development (i.e. volcanic v. non-volcanic margins), and how the back-arc setting modifies these models. Elements of both volcanic and non-volcanic margins are present in the East Andaman Sea, with well-developed necking of continental crust (perhaps due to dry mafic, granulite facies lower crust) and extensive igneous intrusions in the lower and middle crust.

Subsidence and uplift by slab-related mantle dynamics: a driving mechanism for the Late Cretaceous and Cenozoic evolution of continental SE Asia?

Continental SE Asia is the site of an extensive Cretaceous–Paleocene regional unconformity that extends from Indochina to Java, covering an area of c. 5 600 000 km2. The unconformity has previously been related to microcontinental collision at the Java margin that halted subduction of Tethyan oceanic lithosphere in the Late Cretaceous. However, given the disparity in size between the accreted continental fragments and area of the unconformity, together with lack of evidence for requisite crustal shortening and thickening, the unconformity is unlikely to have resulted from collisional tectonics alone. Instead, mapping of the spatial extent of the mid–Late Cretaceous subduction zone and the Cretaceous–Paleocene unconformity suggests that the unconformity could be a consequence of subduction-driven mantle processes. Cessation of subduction, descent of a northward dipping slab into the mantle, and consequent uplift and denudation of a sediment-filled Late Jurassic and Early Cretaceous dynamic topographic low help explain the extent and timing of the unconformity. Sediments started to accumulate above the unconformity from the Middle Eocene when subduction recommenced beneath Sundaland.

Australia–SE Asia collision: plate tectonics and crustal flow

The Sundaland core of SE Asia is a heterogeneous assemblage of Tethyan sutures and Gondwana fragments. Its complex basement structure was one major influence on Cenozoic tectonics; the rifting history of the north Australian margin was another. Fragments that rifted from Australia in the Jurassic collided with Sundaland in the Cretaceous and terminated subduction. From 90 to 45 Ma Sundaland was largely surrounded by inactive margins with localized strike-slip deformation, extension and subduction. At 45 Ma Australia began to move north, and subduction resumed beneath Sundaland. At 23 Ma the Sula Spur promontory collided with the Sundaland margin. From 15 Ma there was subduction hinge rollback into the Banda oceanic embayment, major extension, and later collision of the Banda volcanic arc with the southern margin of the embayment. However, this plate tectonic framework cannot be reduced to a microplate scale to explain Cenozoic deformation. Sundaland has a weak thin lithosphere, highly responsive to plate boundary forces and a hot weak deep crust has flowed in response to tectonic and topographic forces, and sedimentary loading. Gravity-driven movements of the upper crust, unusually rapid vertical motions, exceptionally high rates of erosion, and massive movements of sediment have characterized this region.

To what extent does terrestrial life "follow the water"?

Terrestrial life is known to require liquid water, but not all terrestrial water is inhabited. Thus, liquid water is a necessary, but not sufficient, condition for life. To quantify the terrestrial limits on the habitability of water and help identify the factors that make some terrestrial water uninhabited, we present empirical pressure-temperature (P-T) phase diagrams of water, Earth, and terrestrial life. Eighty-eight percent of the volume of Earth where liquid water exists is not known to host life. This potentially uninhabited terrestrial liquid water includes (i) hot and deep regions of Earth where some combination of high temperature (T > 122 degrees C) and restrictions on pore space, nutrients, and energy is the limiting factor and (ii) cold and near-surface regions of Earth, such as brine inclusions and thin films in ice and permafrost (depths less than approximately 1 km), where low temperatures (T < -40 degrees C), low water activity (a(w) < 0.6), or both are the limiting factors. If the known limits of terrestrial life do not change significantly, these limits represent important constraints on our biosphere and, potentially, on others, since approximately 4 billion years of evolution have not allowed life to adapt to a large fraction of the volume of Earth where liquid water exists.

Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians

During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma).

The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395–c. 375 Ma is tentatively attributed to slab-breakoff.

Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

Accretionary orogens through Earth history

Accretionary orogens form at intraoceanic and continental margin convergent plate boundaries. They include the supra-subduction zone forearc, magmatic arc and back-arc components. Accretionary orogens can be grouped into retreating and advancing types, based on their kinematic framework and resulting geological character. Retreating orogens (e.g. modern western Pacific) are undergoing long-term extension in response to the site of subduction of the lower plate retreating with respect to the overriding plate and are characterized by back-arc basins. Advancing orogens (e.g. Andes) develop in an environment in which the overriding plate is advancing towards the downgoing plate, resulting in the development of foreland fold and thrust belts and crustal thickening. Cratonization of accretionary orogens occurs during continuing plate convergence and requires transient coupling across the plate boundary with strain concentrated in zones of mechanical and thermal weakening such as the magmatic arc and back-arc region. Potential driving mechanisms for coupling include accretion of buoyant lithosphere (terrane accretion), flat-slab subduction, and rapid absolute upper plate motion overriding the downgoing plate. Accretionary orogens have been active throughout Earth history, extending back until at least 3.2 Ga, and potentially earlier, and provide an important constraint on the initiation of horizontal motion of lithospheric plates on Earth. They have been responsible for major growth of the continental lithosphere through the addition of juvenile magmatic products but are also major sites of consumption and reworking of continental crust through time, through sediment subduction and subduction erosion. It is probable that the rates of crustal growth and destruction are roughly equal, implying that net growth since the Archaean is effectively zero.