Subducted lithosphere under South America from multifrequency p wave tomography

We analyze mantle structure under South America in the DETOX-P1 seismic tomography model, a global-scale, multifrequency inversion of teleseismic P waves. DETOX-P1 inverts the most extensive data set of broadband, waveform-based traveltime measurements to date, complemented by analyst-picked traveltimes from the ISC-EHB catalog. The mantle under South America is sampled by ∼665,000 cross-correlation traveltimes measured on 529 South American broadband stations and on 5,389 stations elsewhere. By their locations, depths, and geometries, we distinguish four high-velocity provinces under South America, interpreted as subducted lithosphere (“slabs”). The deepest (∼1,800–1,200 km depth) and shallowest (<600 km) slab provinces are observed beneath the Andean Cordillera near the continent’s northwest coast. At intermediate depths (1,200–900 km, 900–600 km), two slab provinces are observed farther east, under Brazil, Bolivia and Venezuela, with links to the Caribbean. We interpret the slabs relative to South America’s paleo-position over time, exploring the hypothesis that slabs sank essentially vertically after widening by viscous deformation in the mantle transition zone. The shallowest slab province carries the geometric imprint of the continental margin and represents ocean-beneath-continent subduction during Cenozoic times. The deepest, farthest west slab complex formed under intra-oceanic trenches during late Jurassic and Cretaceous times, far west of South America’s paleo-position adjoined to Africa. The two intermediate slab complexes record the Cretaceous transition from westward intra-oceanic subduction to eastward subduction beneath South America. This geophysical inference matches geologic records of the transition from Jura-Cretaceous, extensional “intra-arc” basins to basin inversion and onset of the modern Andean arc ∼85 Ma

Potential for Kutcho Creek Volcanogenic Massive Sulphide Mineralization in the Northern Cache Creek Terrane: A Progress Report

International audienc

Metamorphic, structural and stratigraphic evolution of the Telkwa Formation/: Zymoetz River area (NTS 103 I/8 and 93L/5), near Terrace, British Columbia

Bibliography: p. 119-128.Late Triassic to Hid-Jurassic Hazelton Group volcanics underlie much of north-central British Columbia and are dominated by Late Triassic to Early Jurassic Telkwa Formation strata. The Telkwa Formation is important (a) from an economic standpoint as a host to precious metal mineralization; and (b) from a tectonic history perspective as a sequence overlapping allocthonous terranes amalgamated in the Late Triassic, and as a monitor of geologic evolution since that time. Telkwa Formation strata rest unconformably atop deformed Permian carbonates, which are among the oldest rocks in the Zymoetz River study area on the southern margin of the Bowser Basin. Subdivision of the Telkwa Formation into a lower intermediate to mafic member, variegated middle member, and an intermediate to felsic and tuffaceous upper member, enables correlation amongst five structural domains which comprise an eastward-dipping homocline. A western intrusive-dominated domain is bounded to the east by west-directed thrust faults involving Permian strata. The remainder of the study area is divided into four domains, primarily by major west-side-down normal faults interpreted to have 2-4km of stratigraphic throw. Limits of a domain along the southern margin of the area are poorly constrained. Deformation within eastern domains is mainly by smaller-scale (<700m) block faulting. Metamorphism of Telkwa Formation rocks generally increases in grade from zeolite to prehnite-pumpellyite facies with depth of burial, but also displays characteristics of contemporaneous and later hydrothermal events. Fluid inclusion composition and thermobarometry is generally consistent with the stability limits of observed authigenic minerals, as well as pressures inferred from the estimated thickness of overlying stratigraphy. Contact metamorphism at the base of the Permian carbonate occurred during Early Eocene intrusion. Garnet-biotite exchange thermometry, wollastonite equilibria, modelled conductive heating, fluid inclusion data, and estimates of the pressure due to overlying strata are all consistent with contact metamorphic P-T conditions of approximately 550°C and Pe20 approaching PFLuro-2Kb. Geothermal gradients within the middle and upper members prior to Eocene intrusion range from 30 to 45°C/km based on fluid inclusion data. Maximum geothermal gradients during and/or following Late Cretaceous to Eocene intrusive events are estimated from laumontite breakdown as between 45 and 85°C/km, but are not of regional extent. A geothermal gradient of 60°C/km is in agreement with local transitions from prehnite-pumpellyite facies to greenschist (epidote-actinolite) facies at the base of the Telkwa Formation. Uplift rates since the early Tertiary based on a 6 to 7km thick succession of Telkwa Formation volcanics averaged approximately 0.1 mm/year

Mantle and geological evidence for a Late Jurassic−Cretaceous suture spanning North America

Crustal blocks accreted to North America form two major belts which are separated by a tract of collapsed Jura-Cretaceous basins extending from Alaska to Mexico. Evidence of oceanic lithosphere that once underlay these basins is rare at the earth’s surface. Most of the lithosphere was subducted, which accounts for the general difficulty of reconstructing oceanic regions from surface evidence. However, this seafloor was not destroyed; it remains in the mantle beneath North America and is visible to seismic tomography, revealing configurations of arc-trench positions back to the breakup of Pangea. The double uncertainty of where trenches ran and how subducting lithosphere deformed while sinking in the mantle is surmountable, owing to the presence of a special-case slab geometry. Wall-like, linear slab belts exceeding 10,000 km in length appear to trace out intra-oceanic subduction zones that were stationary over tens of millions of years, and beneath which lithosphere sank almost vertically. This hypothesis sets up an absolute lower-mantle reference frame. Combined with a complete Atlantic spreading record that paleo-positions North America in this reference frame, the slab geometries permit detailed predictions of where and when ocean basins at the leading edge of westward-drifting North America were subducted, how intra-oceanic subduction zones were overridden, and how their associated arcs and basement terranes were sutured to the continent. An unconventional paleogeography is predicted in which mid- to late Mesozoic arcs grew in a long-lived archipelago located 2000-4000 km west of Pangean North America (while also consistent with the conventional view of a continental arc in early Mesozoic times). The Farallon Ocean subducted beneath the outboard (western) edge of the archipelago, whereas North America converged on the archipelago by westward subduction of an intervening, major ocean, the Mezcalera-Angayucham Oceans. The most conspicuous geologic prediction is that of an oceanic suture which must run along the entire western margin of North America. It formed diachronously between ~155 Ma and ~50 Ma, analogous to diachronous suturing of southwest Pacific arcs to the northward migrating Australian continent today. We proceed to demonstrate that this suture prediction fits the spatiotemporal evidence for the collapse of at least 11 Middle Jurassic to Late Cretaceous basins wedged between the Intermontane and Insular-Guerrero superterranes, about half of which are known to contain mantle rocks. These relative late suturing ages run counter to the Middle Jurassic or older timing required and asserted by the prevailing, Andean-analogue model for the North American Cordillera. We show that the arguments against late suturing are controvertible, and we present multiple lines of direct evidence for late suturing, consistent with geophysical observations. We refer to our close integration of surface and subsurface evidence from geology and geophysics as “tomotectonic analysis”. It provides a stringent test for currently accepted tectonic models and offers a blueprint for similar, continental-scale investigations in other accretionary orogens

Atlin TGI, Part III; Geological Setting and Style of Mineralization at the Joss'alun Occurrence

International audienc

Subducted Lithosphere Under South America From Multifrequency P Wave Tomography

International audienc

Geology and new mineralization in the Joss'alun belt, Atlin area

International audienc

GSA Bulletin: Structural and kinematic evolution of the Yukon-Tanana upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly: Discussion and Reply: Discussion

We welcome the efforts of Hansen and Dusel-Bacon (1998) in synthesizing sparse and apparently inconsistent structural data sets from widely separated areas of the poorly exposed Yukon-Tanana Uplands. However, we consider that credible extrapolation of such a synthesis southward to include a 300 × 800 km region of the Canadian Cordillera is unwarranted. We clarify here points of terrane definition and discuss the implications for tectonic models for the Yukon-Tanana terrane in the Canadian Cordillera. TERRANE AND ASSEMBLAGE CLARIFICATIONS The emplacement of allochthonous terranes (upper plate) over parautochthonous North America (lower plate) has been known for decades from the recognition of obvious klippen of Slide Mountain and YukonTanana terranes resting on North American strata (e.g., Sylvester allochthon and St. Cyr and Stewart Lake klippen, The metamorphic-structural focus of the Hansen and Dusel-Bacon study diverts attention from the fundamental geological criteria of terrane definition: rock types and protoliths, ages and contact relationships. Ideas of Hansen and Dusel-Bacon (1998) appear to be an outgrowth of those presented b