Abstract Phanerozoic reactivations of basement fault zones are documented in 5000 m of basement core recovered from beneath the updip Atlantic Coastal Plain underlying the US Department of Energy Savannah River Site (SRS) in South Carolina. These basement fault zones are adjacent to the excised Rheic Ocean suture. Meta-intrusive rocks from c. 620 and 625 Ma contain a mylonitic fabric and intrude foliated mafic metavolcanic rocks. At c. 305 Ma, granulite facies orthogneisses were thrust over amphibolite facies meta-igneous rocks in the transpressive Tinker Creek Nappe. The overturned limb of the nappe localizes the Triassic Dunbarton Basin Border Fault. The border fault acted as a conduit for fluids in the Mesozoic and Cenozoic. At c. 220 ± 5 Ma, a potassium and silica metasomatic event affected the SRS basement. A propylitic event flushed reducing fluids through rocks as young as the Santonian. The remains of a Triassic sub-basin were identified in the northwesten part of the site. A Cretaceous and younger vein paragenesis overprints the previous events. More than 30 pseudotachylytes are found in the SRS basement and are preferentially localized on metasomatized Alleghanian chloritic fractures. Pseudotachylyte post-dates mineralized fractures. The Pen Branch Fault offsets the basement–Cretaceous unconformity and is present in c. 242 m of core between PBF-7-419 m and PBF-7-660.8 m. The Pen Branch Fault cross-cuts mineralized fractures and must post-date strike-normal zeolites.

Abstract The Neoarchean to Paleoproterozoic Betsiboka Suite of Madagascar mainly consists of granitoid orthogneiss and migmatite variably metamorphosed to amphibolite and granulite facies during the late Neoproterozoic. New U–Pb zircon data yield emplacement ages of 2512 ± 17, 2507 ± 17, 2493 ± 14 and 2485 ± 16 Ma. Zircon δ 18 O data indicate involvement of the continental crust during magma genesis. Mostly positive ε Hf ( t ) values plot slightly below the depleted-mantle curve. The isotopic data favour mixing between an older Archean crustal source and a juvenile depleted-mantle source. Possible local crustal candidates for the older component in the exposed Malagasy basement are Neoarchean mafic gneisses in the Tsaratanana Complex and the c. 3200 Ma Nosy Boraha orthogneiss in the Antongil Domain. The new isotopic data support interpretations from previous studies investigating the genesis of Tonian plutonic rocks advocating crustal melting of a Mesoarchean crustal source in the Antananarivo Domain. In addition, detrital zircon in the Proterozoic Ambatolampy Group and Itremo Group probably had a local source with the same isotopic composition as the Betsiboka Suite. Our study demonstrates the significance of multi-isotopic data from zircon for constraining the sources of detrital zircon grains in sedimentary provenance studies.

ABSTRACT Mesoproterozoic crust is widely exposed in the Grenville Province portion of northeastern Laurentia, where it is interpreted as an assemblage of two continental-arc segments separated by a composite arc belt (Quebecia) with island-arc remnants. A synthesis of the geologic context, types, and geochemical patterns of 1.5–1.35 Ga granitoids reveals a regional distribution in each segment, with dioritic to granitic plutonism variably associated with arc-related volcano-sedimentary belts in the south and inboard monzonitic to granitic plutonism in the north. In addition, belts of dioritic to granitic orthogneisses occupy intermediate positions in Quebecia and in the west. The inboard granites are consistently old in all segments (1.5–1.45 Ga), but the preserved volcano-sedimentary belts are older in the east and in Quebecia (1.5–1.45 Ga) and younger in the west (1.39? and 1.36 Ga), while the belts of orthogneisses show a large spread of ages at 1.45–1.37 Ga. Granitoids in the volcano-sedimentary belts and the orthogneisses include magnesian, calcic to calc-alkalic components to ferroan, alkali-calcic components. In contrast, the inboard plutons are dominantly ferroan and alkali-calcic to alkalic in the continental-arc segments, where they are locally associated with anorthosite-mangerite-charnockite-granite (AMCG) suites. Collectively, the different types of granitoid magmatism can be linked to an active margin, with subduction under northeastern Laurentia, involving arc building, arc rifting, back-arc opening and inboard extension, and amalgamation processes variably operating at different parts of the margin and at different times. In addition, the data provide a basis for comparison with other parts of the eastern to southwestern Laurentian margin in the 1.5–1.35 Ga time frame.

ABSTRACT The highest-grade Barrovian-type metamorphic rocks of the North American Cordillera exposed today are Late Cretaceous in age and found within an orogen-parallel belt of metamorphic core complexes for which the tectonic histories remain controversial. Thermobarometric studies indicate that many of these Late Cretaceous metamorphic assemblages formed at pressures of >8 kbar, conventionally interpreted as >30 km depth by assuming lithostatic conditions. However, in the northern Basin and Range Province, detailed structural reconstructions and a growing body of contradictory geologic data in and around the metamorphic core complexes indicate these metamorphic rocks are unlikely to have ever been buried any deeper than ~15 km depth (~4 kbar, lithostatic). Recent models controversially interpret this discrepancy as the result of “tectonic overpressure,” whereby the high-grade mineral assemblages were formed under superlithostatic conditions without significant tectonic burial. We performed several detailed studies within the Snake Range metamorphic core complex to test the possibility that cryptic structures responsible for additional burial and exhumation might exist, which would refute such a model. Instead, our data highlight the continued discordance between paleodepth and paleopressure and suggest the latter may have reached nearly twice the lithostatic pressure in the Late Cretaceous. First, new detrital zircon U-Pb geochronology combined with finite-strain estimates show that prestrain thicknesses of the lower-plate units that host the high-pressure mineral assemblages correspond closely to the thicknesses of equivalent-age units in adjacent ranges rather than to those of the inferred, structurally overridden (para) autochthon, inconsistent with cross sections and interpretations that assume a lower plate with a deeper origin for these rocks. Second, new Raman spectroscopy of carbonaceous material of upper- and lower-plate units identified an ~200 °C difference in peak metamorphic temperatures across the northern Snake Range detachment but did not identify any intraplate discontinuities, thereby limiting the amount of structural excision to motion on the northern Snake Range detachment itself, and locally, to no more than 7–11 km. Third, mapped geology and field relationships indicate that a pre-Cenozoic fold truncated by the northern Snake Range detachment could have produced ~3–9 km of structural overburden above Precambrian units, on the order of that potentially excised by the northern Snake Range detachment but still far short of expected overburden based on lithostatic assumptions. Fourth, finite-strain measurements indicate a shortening (constrictional) strain regime favorable to superlithostatic conditions. Together, these observations suggest that pressures during peak metamorphism may have locally reached ~150%–200% lithostatic pressure. Such departures from lithostatic conditions are expected to have been most pronounced above regions of high heat flow and partial melting, and/or at the base of regional thrust-bounded allochthons, as is characteristic of the spatial distribution of Cordilleran metamorphic core complexes during the Late Cretaceous Sevier orogeny.