ALBERT

Description: Subduction and exhumation of ultrahigh-pressure (UHP) metamorphic terranes are typically envisaged as short-lived processes associated with the transition from oceanic subduction to continent-continent collision. Norway’s Western Gneiss Region, by comparison, is a giant, late-orogenic UHP terrane that underwent protracted residence at UHP conditions during the Scandian phase of the Caledonian orogeny followed by relatively slow exhumation. Here, we use two-dimensional numerical thermal-mechanical models to explore the tectonics of orogens of this type and the associated controls on the size of their UHP terranes and the duration of UHP metamorphism. The models have four tectonic phases designed to capture the main stages of the Caledonian evolution: oceanic subduction and microcontinent accretion; continental margin subduction; plate quiescence; and postorogenic extension during plate divergence. Contrasting styles of exhumation are explored by varying the strength of the margin crust and investigating melt-induced weakening. The tectonic and metamorphic evolution of the Western Gneiss Region is consistent with a model in which continental margin crust was subducted beneath a thick orogenic wedge where it underwent metamorphism at (U)HP conditions for at least 15 million years (Myr) as subduction ended. The buoyant Baltican crust must have been especially strong in order to have stayed coupled to the underlying lithosphere during this phase, perhaps reflecting its refractory composition and/or a lack of fluids. Subsequent exhumation of the Western Gneiss Region can be explained by orogen-scale extension resulting from minor (~100 km) plate divergence, with removal of the orogenic wedge by combined top-to-the-hinterland transport, normal faulting, and erosion. We conclude that large, long-duration UHP terranes are fundamentally different from transient smaller ones. The latter are often explained by the paradigm of buoyant exhumation. This paradigm is incomplete, but both types can be explained by control of the system by the exhumation number (ratio of buoyancy force to basal traction). By implication, the existence of this type of large UHP terrane is a consequence of the high strength of the subducted crust.

Description: (Ultra)high-pressure [(U)HP] rocks form and exhume from deep within subduction channels, but subsequent horizontal transport in the shallower orogenic crust makes it difficult to reconstruct their tectonic histories. We use a conceptual framework and numerical models to show that buoyant exhumation from within a subduction conduit formed during one-sided subduction may lead to emplacement of (U)HP rocks into either the lower plate (prowedge) or upper plate (retrowedge) of an orogen, depending on whether the upper plate crust deforms or acts as a backstop during exhumation. Both modes may operate at different positions or different times within an orogen, leading to emplacement of (U)HP rocks into both plates without changing subduction geometry. We propose that retrotransport during exhumation may explain some (U)HP rocks (e.g., Liverpool Land) situated in the upper plate of the Greenland-Norwegian Caledonides.

Description: The Alexander terrane is an unusual tectonic fragment in the North American Cordillera in that it contains a long and very complete stratigraphic record, including sedimentary or volcanic rocks representing every period and nearly every epoch between Neoproterozoic and Late Triassic time. The terrane is also unusual in that the southern portion of the terrane experienced arc-type magmatism during Neoproterozoic–early Paleozoic time, whereas the northern portion of the terrane consists mainly of Paleozoic shelf-facies strata. This long and diverse history provides opportunities to reconstruct the evolution and displacement history of the terrane, and specifically test the prevailing interpretation that the terrane formed in the paleo-Arctic realm. This study presents U-Pb geochronologic data and Hf isotopic information for detrital zircons from arc-type rocks in the southern portion of the terrane. Information has been generated from seven samples of Ordovician through Devonian age, complementing the information available from previous studies of Ordovician through Triassic strata. Together, these data sets yield a robust record of the magmatic history of the southern Alexander terrane, with dominant age groups of 640–550 Ma, 490–400 Ma, 380–340 Ma, and 310–275 Ma (dominant ages of 579, 441, 361, and 293 Ma). There are few pre–640 Ma grains in any of the samples. Hf isotope compositions of the detrital zircons are exceptionally juvenile, with most epsilon Hf (t) values between +15 and +5. Collectively, the available geologic, U-Pb geochronologic, and Hf isotopic evidence suggests that the southern Alexander terrane formed within a juvenile Neoproterozoic–early Paleozoic arc system, with little continental influence, whereas the northern portion of the terrane formed in proximity to a continental landmass that experienced similar Neoproterozoic–early Paleozoic ages of continental-affinity magmatism. Our data are consistent with previous suggestions that the Alexander terrane resided in the paleo-Arctic realm during early Paleozoic time, with the northern portion of the terrane adjacent to Baltica and the Caledonides, and the southern portion of the terrane forming further offshore as a juvenile north-facing oceanic arc.

Description: 〈span〉〈div〉Abstract〈/div〉Thrust zones in a deep-water fold-thrust belt, offshore northwestern Borneo, display prominent reflections that can be mapped through a three-dimensional seismic volume. Unlike fault-plane reflections obtained from thrusts in other systems, these have positive polarity. Well data show that the reflections in the Borneo data set originate from fault-bound sandstone slices, with porosity-occluding calcite cement, entrained along the thrust zone. The thrust zone can support elevated fluid pressures beneath the fault with the cemented sandstones. Multiple sandstone slices indicate complex patterns of thrust zone localization, perhaps a common feature for deformation in sedimentary multilayers typical of many deep-water depositional successions.〈/span〉

Description: The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P - T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.

Description: The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P - T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.

Description: 〈span〉〈div〉Abstract〈/div〉Observations highlight the complex tectonic, magmatic, and geodynamic phases of the Cenozoic post-collisional evolution of the Himalayan-Tibetan orogen and show that these phases migrate erratically among terranes accreted to Asia prior to the Indian collision. This behavior contrasts sharply with the expected evolution of large, hot orogens formed by collision of lithospheres with laterally uniform properties. Motivated by this problem, we use two-dimensional numerical geodynamical model experiments to show that the enigmatic behavior of the Himalayan-Tibetan orogeny can result from crust-mantle decoupling, transport of crust relative to the mantle lithosphere, and diverse styles of lithospheric mantle delamination, which emerge self-consistently as phases in the evolution of the system. These model styles are explained by contrasting inherited mantle lithosphere properties of the Asian upper-plate accreted terranes. Deformation and lithospheric delamination preferentially localize in terranes with the most dense and weak mantle lithosphere, first in the Qiangtang and then in the Lhasa mantle lithospheres. The model results are shown to be consistent with 11 observed complexities in the evolution of the Himalayan-Tibetan orogen. The broad implication is that all large orogens containing previously accreted terranes are expected to have an idiosyncratic evolution determined by the properties of these terranes, and will be shown to deviate from predictions of uniform lithosphere models.〈/span〉

Description: 〈span〉Observations highlight the complex tectonic, magmatic, and geodynamic phases of the Cenozoic post-collisional evolution of the Himalayan-Tibetan orogen and show that these phases migrate erratically among terranes accreted to Asia prior to the Indian collision. This behavior contrasts sharply with the expected evolution of large, hot orogens formed by collision of lithospheres with laterally uniform properties. Motivated by this problem, we use two-dimensional numerical geodynamical model experiments to show that the enigmatic behavior of the Himalayan-Tibetan orogeny can result from crust-mantle decoupling, transport of crust relative to the mantle lithosphere, and diverse styles of lithospheric mantle delamination, which emerge self-consistently as phases in the evolution of the system. These model styles are explained by contrasting inherited mantle lithosphere properties of the Asian upper-plate accreted terranes. Deformation and lithospheric delamination preferentially localize in terranes with the most dense and weak mantle lithosphere, first in the Qiangtang and then in the Lhasa mantle lithospheres. The model results are shown to be consistent with 11 observed complexities in the evolution of the Himalayan-Tibetan orogen. The broad implication is that all large orogens containing previously accreted terranes are expected to have an idiosyncratic evolution determined by the properties of these terranes, and will be shown to deviate from predictions of uniform lithosphere models.〈/span〉