Intraplate Tectonic Setting Research Articles

Supracrustal intraplate thickening of Variscan basement due to Alpine foreland compression: Results from the superdeep well KTB (Bohemian Massif, Germany)

Until its final depth of 9101 m the superdeep well KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland) drilled through Variscan basement rocks which suffered strong brittle‐ductile and brittle deformation during late‐ and post‐Variscan times. Investigations of faults and mineralized tension gashes revealed the following deformation stages: (1) Upper Carboniferous subvertical tension gashes, (2) Upper Carboniferous reverse faults, (3) Cretaceous subhorizontal tension gashes and reverse faults, (4) ?Neogene normal faults. Clear depth‐dependent variations of metamorphic conditions of the post‐Variscan deformation structures are obvious, whereas the Upper Carboniferous structures show almost no variations with depth. This refers to the metamorphic index minerals as well as to the deformation fabrics in quartz. We explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust. The fault geometry indicates an antiformal stack, the frontal ramp of which was drilled by KTB at 7000 m depth. There is strong evidence that this frontal ramp, referred to as the Franconian Lineament at the surface, rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle‐ductile boundary layer of quartz‐bearing rocks. The considerable amount of supracrustal vertical thickening above the Franconian Lineament results from repeated movements along this detachment since Upper Carboniferous times. It seems reasonable to suppose that in intraplate tectonic settings the brittle‐ductile boundary layer is prone to form a long‐lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust.

Litogeochemistry of paleoproterozoic orthogranulites from the Rio Preto (MG) - Vassouras (RJ) region, Central Ribeira Belt, southeastern Brazil

The granulites of the Juiz de Fora Complex (JFC) integrate the pre-1.8 Ga basement of the Intermediate Thrust Sheet of the Ribeira Belt's central segment. This tectonic domain is characterized by intense tectonic imbrication of the JFC granulites with Meso- to Neoproterozoic metasedimentary units. In the Rio Preto (MG)-Vassouras (RJ) region, the JFC comprises three lithological associations: orthogranulites, orthogneisses and paragranulites which crop out as several thrust sheets formed by intense shortening during the Brasiliano Orogeny. The main contacts are characterized by the transformation of the granulites into amphibolite facies mylonites. The geochemistry of the orthogranulites leads to the identification of two tectonic associations. One comprises two calc-alkaline series compositionally similar to rocks of modern magmatic arcs, while the other contains two groups of basic rocks, respectively the tholeiitic series and alkaline to transitional basalts, both of intraplate extensional tectonic setting. The relationships between these distinct tectonic environments need further geochronological background, as the Brasiliano shortening obliterated the original paleogeography of this important Paleoproterozoic segment of the Ribeira belt.

Are Coronae Restricted to Venus?: Corona-Like Tectonovolcanic Structures on Earth

Coronae may not be tectonovolcanic features ‘unique to Venus’ because both the processes that lead to corona formation, and their final tectonovolcanic output (formation of domes, plateaus, extensional rings, etc.), are also found on Earth. Large-scale corona formation processes on Earth may be restricted (because of plate motion) but not absent. The same applies to resurfacing processes. We here suggest that at least, the early stages of corona formation can be recognized in intraplate tectonic settings on Earth. The African plate displays many Cenozoic examples of plume-related domal uplifts and volcanism (e.g., Hoggar, Tibesti, Darfur, Ethiopia). Furthermore, the east African rift system (EARS) around lake Victoria displays many striking features that resemble those of the Venus coronae associated with extensional belts. Among these are the following: (1) an overall elliptical shape; (2) the existence of a mantle plume (Kenya plume) centered beneath lake Victoria; (3) a central plateau (east African plateau); (4) an external extensional belt (the EARS east and west branches); (5) doming processes (Kenya dome); and last but not least (6) volcanism.

Geochronology of Cretaceous granites and metasedimentary basement on Edward VII Peninsula, Marie Byrd Land, West Antarctica

Rb-Sr ages of Swanson Formation on Edward VII Peninsula, West Antarctica, indicate a late Ordovician age, 421–432 Ma for regional metamorphism. K-Ar ages of 113–440 Ma, reflect a second thermal metamorphism during emplacement of widespread Cretaceous granites. Rb-Sr ages of five monzogranite/syenogranitic plutons of Byrd Coast Granite are in the range 95–105 Ma (initial 87Sr/86Sr ratios 0.710–0.715) and represent stages of crystallization of anorogenic granite (A subtype). These correlate with Byrd Coast Granite (100–110 Ma) farther east in Marie Byrd Land, and with Cretaceous granitoids on the Campbell Plateau and in southern New Zealand. K-Ar mica/hornblende and fission-track apatite/zircon ages indicate that regional cooling began c. 90–100 M.y. ago immediately after granite emplacement. Uplift continued throughout the peninsula during the period 55–100 Ma (late Cretaceous-early Tertiary), associated with regional uplift in the rift-drift stages of Gondwana break-up at the South-west Pacific spreading centre. Apatite fission track ages show that during late Cretaceous-early Tertiary time the peninsula behaved as two blocks. The Alexandra Mountains were exhumed 20 m.y. before the Rockefeller Mountains and are possibly separated by a fault active initially in the mid-Cretaceous or earlier, and later reactivated in the late Cretaceous-early Tertiary. An averaged uplift rate (50–100 Ma) of 0.025 mm yr−1 is characteristic of the inferred intraplate tectonic setting.

Preservation Potential of Ultrabasic Volcanic Sand in an Arid Intracontinental Setting: Will the Hopi Buttes Maar-diatreme Field be Preserved in the Rock Record?

ABSTRACT Petrographic study of modern sand samples collected near Hopi Buttes volcanic field (northeastern Arizona) underscores the low preservation potential of mafic minerals and ultrabasic volcanic detritus even in an arid, intraplate tectonic setting. The Miocene-Pliocene Hopi Buttes volcanic field has been interpreted as a maar-diatreme and scoria cone field in which phreatomagmatic eruptions took place in a playa lake basin. The tephra, necks, dikes, flows, and maar crater fill are now sources of sediment found in local ephemeral streams. Once the volcanic field is eroded away, the main record of its existence will be as a provenance component in the sand it is now shedding. The locally derived volcanic components include monocrystalline pyroxene, glass shards, scoria, and a narrow range of ultrabasic volcanic rock fragments. Thin-section point counts (450 grains) on four sieved sand fractions from each of 7 sites downstream (0, 2, 4, 8, 12, 21, and 31 km) from one margin of the volcanic field provided the following results. (1) The proportion of volcanogenic to total sand grains decreases from an initial value of about 40% at a rate of about 0.8% per kilometer downstream from the source rocks over the first 30 km of stream length. (2) Dilution by sand eroded from other local source rocks, rather than physical grain-size reduction or chemical dissolution, drives the diminution of volcanogenic grain proportions. Detritus sourced from the Hopi Buttes volcanic field has a very low preservation/recognition potential, despite the arid climate and stable tectonic setting far removed from a plate margin. It follows that maar-diatreme volcanism is generally underrepresented in the sedimentary record.

Uplift and cooling pathways derived from fission track analysis and mica dating: a review

Definition of time and temperature pathways for episodes of lithospheric movement provides not simply a chronological framework for crustal dynamism but also permits estimation of rates of crustal cooling and uplift. Important aspects of such pathways are the constraints provided for timing of both plate collision and lithospheric extension. Classically Rb-Sr and K-Ar mica ages have been used to delineate rates of cooling and exhumation in the Central Alps, by comparison of the measured ages with estimates of temperatures for the retention of daughter isotopes. Similar use of fission track apatite and zircon ages has provided data for lower temperature intervals (∼ 100 and ∼ 200°C respectively). Recent detailed studies of the annealing kinetics of fission tracks in apatite yield more precise estimates of cooling rate and permit predictive modelling of age and length parameters for given T,t pathways. In continental collision zones, fast episodic uplift in the western Alps can be contrasted with contemporaneous monotonic uplift in the Central Alps. Additional examples may be seen in the Tibet-Himalayan orogenic belt, in the southern Alps of New Zealand and in the Bolivian Andes. In divergent teceonic regimes, the record of uplift associated with rifting has been recorded by fission track ages in the southeastern Australian margin and around the Red Sea. In an intra-plate tectonic setting, our current fission track reconnaissance study in the British Isles is revealing a hitherto unrecognised thermal history for crystalline and sediment alike.

The origin of alkaline magmas in an intraplate setting near a subduction zone: the Ngatutura Basalts, North Island, New Zealand

Abstract The Ngatutura Basalts are one of a series of Pliocene-Quaternary alkalic basalt volcanic fields in North Island, New Zealand. They are situated in an intraplate tectonic setting behind the currently active Taupo Volcanic Zone, and 300 km above the subducting slab. The volcanic field consists of 16 small-volume monogenetic volcanic centres composed mainly of eroded scoria cones and lava flows, that occupy an extensional tectonic environment characterized by NE-striking block faulting. In some cases the faults have controlled the localization of volcanic vents. The lavas have restricted compositions, ranging from hawaiites to nepheline hawaiites, and are characterized by enriched LILE, LREE, and HFS elements, with particularly high Nb and Ta, low Ba/Nb, and high Zr/Y and Ce N /Yb N ratios. Nepheline hawaiites are slightly more differentiated than hawaiites and have higher Ce N /Yb N ratios. Petrogenetic modelling suggests that the range in composition was mainly controlled by fractional crystallization of olivine, clinopyroxene, and minor plagioclase and titanomagnetite, which is consistent with the modal phenocryst abundances. Fractionation is explained by side-wall crystallization and flowage differentiation during rapid ascent, rather than gravitative settling in a magma chamber. Ngatutura magmas were probably derived from an enriched garnet lherzolite source within the low-velocity mantle. The process of source enrichment is speculative but our preferred model calls on metasomatizing fluids in the low-velocity zone. There is no geochemical evidence for any influence of the subducted slab on their composition, even though they overlie the Pacific plate subduction zone. This implies that the extent of subduction-related contamination in the mantle wedge is not pervasive, but is confined to a limited region overlying the subducted slab. Also, the “deep mantle plume” responsible for alkalic magmatism must have originated above the slab, because it seems unlikely that such a plume could have occurred at a deeper level and penetrate the slab without some evidence. This therefore limits the depth of origin of these “deep mantle plumes” to less than 300 km.

An empirical model for metamorphic evolution of the Archaean Slave Province and adjacent Thelon Tectonic Zone, north-western Canadian Shield

Summary On the basis of P-T estimates derived from metamorphic assemblages and the geological setting before and after peak metamorphism, Archaean thermal evolution is attributed to overthickening of previously thinned sialic crust and overlying supracrustal basins. The extensional phase increases geothermal gradients and forms relatively ductile zones within which subsequent compression transports supracrustal rocks to metamorphic pressures, contributes to the rise of isotherms to high levels in the crust, and causes the moderate uplift and erosion required to bring low- P -high- T rocks to the surface of crust 35 km thick. Proterozoic upper amphibolite- and granulite-facies metamorphism and plutonism in the western Thelon Tectonic Zone is attributed to a similar thermal evolution 500 Ma later. Continued convergence across a long linear belt of overthickened, partially melted crust caused hot upper and middle crustal-level rocks to be thrust on to the Slave Province and formation of a medium pressure overprint on the Archaean. The empirical models derived for the region generate low- P -high- T metamorphism without excessive overthickening or major heat transfer by granitic magmatism. For both terrains, metamorphic data, considered in their geological context, are consistent with an ensialic intra-plate tectonic setting rather than an ensimatic, subduction-related interplate setting.

Petrochemistry and tectonic significance of Carboniferous volcanic rocks in New Brunswick

Carboniferous volcanic rocks from the New Brunswick Platform in the Maritimes Basin are divided into three age groups. Late Tournaisian to early Visean volcanic rocks are tholeiitic basalts and andesites that, in southern New Brunswick, are inter-bedded with abundant calc-alkalic rhyolite. Late Visean to Namurian volcanic rocks consist of an interbedded sequence of alkalic basalts and trachyandesites. Late Westphalian volcanic rocks change in composition up section from trachyte to peralkalic rhyolite. All three age groups display petrochemical features indicative of an intraplate tectonic setting. The volcanic geochemistry is consistent with the development of the Maritimes Basin either as a failed rift formed along the margin of a late Paleozoic ocean or as a rhomb graben formed within a transcurrent zone; the former model is preferred. The change in basaltic composition from tholeiitic to alkalic apparently coincided with a decrease in rate of extension between the Tournaisian and Namurian. Local peralkalic volcanism occurred during regional sagging of the basin as extension ceased and basement rocks cooled in the Late Carboniferous.