Stephen B. Lucas

Intraoceanic tectonics and the development of continental crust: 1.92–1.84 Ga evolution of the Flin Flon Belt, Canada

The Flin Flon Belt is a complex (“Amisk collage”) of distinct tectono-stratigraphic assemblages that was brought together at a relatively early stage in the tectonic evolution of the Paleoproterozoic Trans-Hudson orogen. Four main tectono-stratigraphic assemblage types are recognized: isotopically juvenile oceanic arc (1.90–1.88 Ga), ocean floor (ca. 1.90 Ga), oceanic plateau/ocean island, and isotopically evolved arc (1.92–1.90 Ga). Oceanic arc assemblages include tholeiitic, calc-alkaline, and lesser shoshonitic volcanic and volcaniclastic rocks, as well as turbidites and arc-rift volcanic rocks. The tectono-stratigraphic assemblages were juxtaposed in an accretionary complex (Amisk collage) at ca. 1.88–1.87 Ga, probably as a result of arc-arc collision/collisions. Accretionary collage structures are largely obliterated by subsequent deformation and metamorphic events, but can be inferred where cut by calc-alkaline plutons related to a 1.866–1.838 Ga arc. Coeval subaerial volcanism is recorded in ca. 1.87–1.85 Ga calc-alkaline to shoshonitic volcaniclastic sequences. Unroofing of the accretionary collage, development of a paleosol, and deposition of continental (alluvial-fluvial) sedimentary rocks occurred ca. 1.85–1.84 Ga, coeval with the waning stages of post-accretion arc magmatism. Stabilization of the Flin Flon Belt by 1.85–1.84 Ga as part of a Philippines- or Japan-like microcontinent is attributed to both tectonic and magmatic thickening of the lithosphere.

Structure of a Paleoproterozoic continent-continent collision zone: a LITHOPROBE seismic reflection profile across the Trans-Hudson Orogen, Canada

Abstract An ~ 800 km reflection seismic profile across the Trans-Hudson Orogen, northern Saskatchewan and Manitoba, images crustal-scale tectonic imbrication in an unprecedented picture of Paleoproterozoic crustal accretion and continent-continent collisional tectonism. The profile is crudely symmetric about a crustal-scale culmination in the western part of an accreted juvenile collage (Reindeer Zone). Geologic and isotopic data suggest that this culmination is cored by microcontinental Archean basement. West of the culmination, highly reflective juvenile crustal elements dip westward into the lower crust, beneath the Wathaman Batholith and Archean continental crust of Hearne craton. To the east, strong reflections in the juvenile Reindeer Zone crust and reworked Archean foreland of the Thompson belt have eastward dips persisting into the middle crust and extending beneath the Superior craton. A continuous reflection Moho, well-defined for > 500 km in the western part of the profile, shows marked relief (12- > 15 s), including a prominent root below the crustal culmination. These imaged structures give evidence of substantial crustal shortening and thickening via large-scale imbrication consistent with collisional orogeny. W-dipping structures below the Wathaman Batholith and reworked Hearne craton may reflect subduction polarity in this part of the orogen. However, geological evidence suggests that E-dipping structures below Superior craton are largely related to late/post-collisional deformation, rather than to prior oceanic subduction polarity.

Geochemistry of 1.9 Ga MORB- and OIB-like basalts from the Amisk collage, Flin Flon Belt, Canada: Evidence for an intra-oceanic origin

Subaqueously-erupted basalts that occur in kilometre-scale allochthons within the 1.9 Ga Flin Flon Belt, Canada, appear to have been generated at oceanic ridges and possibly oceanic plateaus, remote from Archean cratons. The ocean-floor basalts fall into two categories: (1) N-type, resembling N-MORBs and Mariana-type back-arc basin basalts (depleted to flat REE patterns, high ZrNb, variable ThNb, and initial ϵNd = + 3.3 to + 5.4); (2) E-type, resembling transitional and plume MORBs (slightly enriched REE patterns, lower ZrNb, initial ϵNd = +3.1 to +4.5). In the largest and best-studied allochthon, the Elbow-Athapapuskow ‘assemblage,’ mixing between depleted (N-MORB) and enriched (OIB) sources or melts, coupled with variable addition of a subduction LILE component, can explain the chemical variations in the basalts. Zircon U-Pb dates of 1904 ± Ma for a syn-volcanic diabase sill and 1901 +6-5 Ma for a gabbro-peridotite cumulate complex demonstrate that crystallization of the ‘ocean-floor’ basalts overlapped with, in part, eruption of the tectonically juxtaposed 1.90-1.88 Ga arc volcanic rocks. The Elbow-Athapapuskow allochthon is interpreted as back-arc basin crust that developed simultaneously with Flin Flon arc magmatism. Subaerially erupted basalts that chemically resemble tholeiitic OIBs (8–14% MgO, relative HREE depletion, initial ϵNd = +2.2 to +3.4) occur in tectonic contact with the Elbow-Athapapuskow assemblage. The OIBs may have been generated by deeper (garnet residue) melting of enriched mantle tapped during extension in the Elbow-Athapapuskow back-arc basin, and were possibly erupted onto a remnant arc. Deeper mantle melting is also indicated by the presence of the LREE-enriched oceanic plateau-like basalts of the Sandy Bay assemblage. The back-arc, 01B, and plateau volcanic assemblages were jux-taposed against ca. 1.9 Ga arc assemblages in a Philippines-like intraoceanic accretionary complex by 1.87 Ga.

Paleoproterozoic (1.90 1.86 Ga) arc volcanism in the Flin Flon Belt, Trans-Hudson Orogen, Canada

Geochemical and isotopic (Nd, Sr) data are reported on Paleoproterozoic (1904–1864 Ma), maficintermediate (<63% SiO2), arc metavolcanic rocks from the Flin Flon greenstone belt, Manitoba and Saskatchewan. Major element criteria permit subdivision of the rocks into tholeiitic (TH), calc-alkaline (CA), alkaline, and boninitic (BO) magma series. Subaqueously erupted, TH and related CA basalt-basaltic andesite, and rare high-Ca boninites dominated between 1904 Ma and 1890 Ma. The TH rocks are similar to modern island are tholeiites, having low high-field-strength element (HFSE) and rare earth element (REE) abundances, and chondrite-normalized light REE depletion to slight enrichment. The boninites have even lower HFSE and REE abundances (1–2X chondrites). Along with their extreme ratios of refractory incompatible elements (e.g., high Al/Ti, Ti/Zr, low Ti/V, Zr/Y), these features indicate that the arc mantle source was strongly depleted, probably residual after MORB or back-arc basin basalt extraction. Elevated Th/Yb, Ba/La, La/Nb values, and the spread in Nd isotopic compositions (initial ɛNd=−0.4 to +4.8) suggest recycling of small amounts (0–8%) of Archean and possibly older Proterozoic crust via sediment subduction and, locally, intracrustal contamination. Calcalkaline andesite-rhyolite and rare shoshonite and trachyandesite, erupted between 1890 Ma and 1864 Ma, are more strongly light REE enriched and have comparatively higher HFSE abundances, and higher Zr/Y and Nb/Y values. The rocks have strong arc trace element signatures (e.g., high Th/Nb, La/Nb), and initial ɛNd values (+2.3 to +4.6) indicate that depleted mantle contributions to the magmas continued to be dominant. The geochemistry and geology of these younger volcanic rocks suggest a mature island arc setting in which the arc lithosphere was thicker than in the previous period, and a more fertile sub-arc mantle source was tapped. The pre-1890 Ma volcanism occurred in one or more separate arcs, probably characterized by rapid subduction of oceanic lithosphere, relatively thin, tholeiitic arc crust, and extensive backarc basin formation. In contrast, post-1890 Ma volcanism is dominantly calc-alkaline to (rarely) alkaline, and is interpreted to reflect crustal thickening due to longterm growth of arc edifice(s) and tectonic thickening associated with intraoceanic arc-arc (>1870 Ma) collision and subsequent intra-arc deformation.

Three-dimensional collisional structure of the Trans-Hudson Orogen, Canada

Abstract The three-dimensional structure of the eastern Trans-Hudson Orogen (THO), part of a Paleoproterozoic continent-continent collision zone in central North America, is revealed through a network of LITHOPROBE seismic reflection profiles. The seismic images are interpreted to delineate a series of stacked thrust sheets essentially confined to the crust. E-W profiles show strong, E-dipping reflections extending throughout the crust while N-S profiles record events outlining antiformal and synformal structures. This allows the identification of decollements that may have localized along pre-existing structures (e.g. possible basin-bounding extensional faults) and at major rheological boundaries (e.g. basement-cover contact, upper-middle crust transition). The present topographic surface displays oblique crustal sections with 10–15 km of structural relief, generated during post-collisional, intracontinental transpression of THO as a result of crustal-scale folding and faulting.

Syn-tectonic magmatism and the development of compositional layering, Ungava Orogen (northern Quebec, Canada)

Abstract Layer- and foliation-parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now-exhumed middle crustal sections of both Archean and Paleoproterozoic age in the northern Ungava peninsula, Quebec (Canada). In the Archean Superior Province, diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite-facies conditions coeval with voluminous granitic magmatism. The Paleoproterozoic Narsajuaq arc contains evidence for contemporaneous magmatism, transpressional deformation and granulite-facies metamorphism prior to its collision with the Superior Province basement. In both plutonic domains, regional compositional layering is defined by (1) metre to kilometre-scale alternation of generally well foliated tonalite and quartz diorite bodies; and (2) centimetre to kilometre-scale, variably deformed granitic veins and sheets that lie parallel to layering/tectonic foliation in the host rocks. Syn-tectonic intrusion of a substantial portion of the veins along extension fractures sub-parallel to layering/foliation (i.e. at high angle to the regional shortening direction) is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures. Compositional layering generated and/or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust.

Seismic reflection images of high‐angle faults and linked detachments in the Trans‐Hudson Orogen

Postcollisional (1.8–1.7 Ga) intracontinental deformation in the Trans-Hudson Orogen (Canada) produced a series of orogen-parallel high-angle faults and folds. In seismic reflection profiles, the faults are imaged by subvertical zones of diffractions and truncated reflections that extend to 4–8 s (12–24 km). The folded and faulted upper part of the crust is underlain by laterally coherent shallow-dipping reflections that are locally bounded by discrete, highly reflective zones. These zones are interpreted as detachments (shear zones) and can be traced from the upper to lower crust, where some of them appear to pass into laterally continuous reflections that define the Moho. Two distinct regimes of postcollisional crustal deformation are inferred from the seismic images: high-angle faulting and lateral block extrusion in the upper crust and low-angle ductile shearing in the mid/lower crust. The surface geology indicates that the faults resulted in southwest (orogen-parallel) extrusion of the orogens internal zone relative to the bounding Archean Hearne and Superior cratons. Faulting was concurrent with the development of upright folds with trends that are subparallel to the extrusion direction. The seismic images suggest that the high-angle fold/fault structures are kinematically linked to low-angle detachments represented by laterally coherent, highly reflective zones. The detachment shear zones are inferred to have a top-to-the-southwest sense of shear associated with a subhorizontal, northeast-southwest extension direction, parallel to those observed for 1.83–1.80 Ga collisional shear zones exposed in major postcollisional fold culminations. Long-lived orogen-parallel extension is interpreted as a consequence of the boundary conditions imposed by the northeast trend of both the Superior and Hearne margins.

Tectonic evolution of the Superior Boundary Zone from coincident seismic reflection and magnetotelluric profiles

The Superior Boundary Zone (SBZ) forms the northwestern margin of the Archean Superior craton and constitutes a tectonic foreland of the ∼1.8 Ga Trans-Hudson Orogen. The Superior Boundary Fault (SBF) separates the SBZ from the adjacent Reindeer Zone, a collage of Paleoproterozoic juvenile intraoceanic rocks. Lithoprobe seismic reflection and magnetotelluric data were acquired along two profiles crossing the SBZ in an attempt to better constrain the deformation and crustal geometry resulting from Trans-Hudson orogeny. Analysis and interpretation of spatially coincident regional seismic and magnetotelluric data acquired along the southern 200 km profile indicate the following: (1) the Reindeer Zone accretionary collage forms an east dipping, eastward steepening, crustal-scale tectonic stack of moderately conductive rocks near the SBZ. (2) The SBZ is characterized at shallow depths ( 1.88-1.81 Ga, through lithospheric delamination at ∼1.82-1.80 Ga to a steep transpressive plate boundary at 1.80-1.72 Ga.

Early partitioning of Quebec: Microcontinent formation in the Paleoproterozoic

The distribution of allochthonous versus parautochthonous carbonate platforms, combined with the timing of initial continental-rift magmatism versus the timing of subsequent rifting of the continental margin, provides tectonostratigraphic evidence for the formation and isolation of the oldest documented microcontinent, perhaps as a consequence of the impingement of a mantle plume onto a cratonic margin during the Paleoproterozoic. Initial rifting of the Archean nucleus of North America in eastern Canada is constrained to have been diachronous between 2.17 and 2.03 Ga. Renewed rifting of a segment of the continental margin, 292 to 120 m.y. later, was accompanied by the emplacement of ultramafic layered sills, the accumulation of komatiitic and alkalic basalts, the deposition of banded iron formations and the isolation of a microcontinent and its 1.93 Ga continental shelf succession (subsequently accreted to the telescoped continental margin during collisional orogenesis).

Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada

Lithoprobe acquired a 190-km-long deep seismic reflection profile across the Superior boundary zone at the eastern margin of the Paleoproterozoic Trans-Hudson orogen. The profile is located ∼250 km south of a major promontory along the ancient margin of the Superior craton and complements earlier profiles closer to the promontory. The new data image the remnants of a craton-verging thrust belt, younger than 1864 Ma, within the Superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement. Across the surface suture zone, a dip reversal to east-dipping reflections occurs within the adjacent Reindeer zone, where a crustal-scale imbricate stack is imaged. Reindeer zone lower crust and upper mantle extend eastward beneath the Superior craton margin for 40–100 km. Preservation of a foreland thrust belt within the Superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the Thompson promontory.