Tectonic settings influence the geochemical and microbial diversity of Peru hot springs

Tectonic processes control hot spring temperature and geochemistry, yet how this in turn shapes microbial community composition is poorly understood. Here, we present geochemical and 16 S rRNA gene sequencing data from 14 hot springs from contrasting styles of subduction along a convergent margin in the Peruvian Andes. We find that tectonic influence on hot spring temperature and geochemistry shapes microbial community composition. Hot springs in the flat-slab and back-arc regions of the subduction system had similar pH but differed in geochemistry and microbiology, with significant relationships between microbial community composition, geochemistry, and geologic setting. Flat-slab hot springs were chemically heterogeneous, had modest surface temperatures (up to 45 °C), and were dominated by members of the metabolically diverse phylum Proteobacteria. Whereas, back-arc hot springs were geochemically more homogenous, exhibited high concentrations of dissolved metals and gases, had higher surface temperatures (up to 81 °C), and host thermophilic archaeal and bacterial lineages.

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.

Illuminating subduction zone rheological properties in the wake of a giant earthquake

Deformation associated with plate convergence at subduction zones is accommodated by a complex system involving fault slip and viscoelastic flow. These processes have proven difficult to disentangle. The 2010 M _w 8.8 Maule earthquake occurred close to the Chilean coast within a dense network of continuously recording Global Positioning System stations, which provide a comprehensive history of surface strain. We use these data to assemble a detailed picture of a structurally controlled megathrust fault frictional patchwork and the three-dimensional rheological and time-dependent viscosity structure of the lower crust and upper mantle, all of which control the relative importance of afterslip and viscoelastic relaxation during postseismic deformation. These results enhance our understanding of subduction dynamics including the interplay of localized and distributed deformation during the subduction zone earthquake cycle.

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.

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.

The southern Mexico block: main boundaries and new estimation for its Quaternary motion

Numerous studies, mainly based on structural and paleomagnetic data, consider southern Mexico as a crustal block (southern Mexico block, SMB) uncoupled from the North American plate with a southeast motion with respect to North America, accommodated by extension through the central Trans-Mexican volcanic belt (TMVB). On the other hand, the accommodation of this motion on the southeastward boundary, especially at the Cocos–Caribbean–North American triple junction, is still debated.

The boundary between the SMB and the North American plate is constituted by three connected zones of deformation: (1) left-lateral transtension across the central TMVB, (2) left-lateral strike-slip faulting along the eastern TMVB and Veracruz area and (3) reverse and left-lateral strike-slip faulting in the Chiapas area. We show that these three active deformation zones accommodate a counterclockwise rotation of the SMB with respect to the North American plate. We specially discuss the Quaternary motion of the SMB with respect to the surrounding plates near the Cocos–Caribbean–North American triple junction. The model we propose predicts a Quaternary counterclockwise rotation of 0.45°/Ma with a pole located at 24.2°N and 91.8°W.

Finally we discuss the geodynamic implications of this counterclockwise rotation. The southern Mexico block motion is generally assumed to be the result of slip partitioning at the trench. However the obliquity of the subduction is too small to explain slip partitioning. The motion could be facilitated by the high thermal gradient and gravitational collapse that affects central Mexico and/or by partial coupling with the eastward motion of the Caribbean plate.