Late Proterozoic Rifting Research Articles

10.1007/s11491-008-1013-7

For the first time, the structure of the sedimentary basins of the Late Proterozoic rift system in the White Sea is characterized based on a set of new marine geological geophysical data such as the results of the common depth point seismic method, gravity and magnetic data, and seismoacoustics. The main tectonic structures in the topography of the heterogeneous basement within the basin of the White Sea are distinguished and described. A structural tectonic scheme of the basement surface is presented. The thicknesses of the sediments are estimated and the stratigraphic confinement of the seismic units recognized is done.

Structure of the basins of the White Sea rift systems

For the first time, the structure of the sedimentary basins of the Late Proterozoic rift system in the White Sea is characterized based on a set of new marine geological geophysical data such as the results of the common depth point seismic method, gravity and magnetic data, and seismoacoustics. The main tectonic structures in the topography of the heterogeneous basement within the basin of the White Sea are distinguished and described. A structural tectonic scheme of the basement surface is presented. The thicknesses of the sediments are estimated and the stratigraphic confinement of the seismic units recognized is done.

Structural evolution of a major Appalachian salient-recess junction: Consequences of oblique collisional convergence across a continental margin transform fault

Most orogenic belts exhibit broadly arcuate regional map-view trends of structural patterns. Commonly, the inflection between opposing curves (recesses and salients) is a transverse zone marking a linear array of along-strike contrasts in regional structure and stratigraphy. Analysis of such transverse zones can provide insight into the processes affecting orogenic curvature. Curvature is especially distinctive along the west flank of the Appalachians, where the tightest salient/recess juncture is a thinned-skinned transverse zone in Georgia, separating the Alabama recess and Tennessee salient, and extending entirely across the width of deformed Laurentian cover rocks. This zone's most notable feature is a large oblique hanging-wall ramp within the frontal metamorphic allochthon formed during Alleghanian collisional events. The basal decollement steps several kilometers stratigraphically upward across this ramp from Proterozoic basement and its overlying Late Proterozoic rift sequence (Tennessee salient), southwestward into basal Cambrian and younger rocks (Alabama recess). This ramp resulted from oblique sinistral displacement of the allochthon against an earlier, east-facing, dextral continental margin transform fault that initially separated adjacent rifted margins of opposite polarity. Differences in the distribution, thickness, and facies of units indicate that important variations in basin geometry existed across the curvature inflection before Paleozoic deformation, suggesting that the orogenic curvature in part results from an inherited basin architecture. Translation of the allochthon over the ramp caused deflection of the tectonic transport trajectory, major cross folding, and rotation of earlier structures. Associated out-of-plane deformation propagated outward across the foreland, producing the array of thinned-skinned transverse structures aligned into the transverse zone.

WINCHESTER, J. A., PHARAOH, T. C. & VERNIERS, J. (eds) 2002. Palaeozoic Amalgamation of Central Europe. Geological Society Special Publication no. 201. vi + 353 pp. London, Bath: Geological Society of London. Price £85.00, US $142.00; members' price £42.50, US $71.00; AAPG members' price £51.00, US $85.00 (hard covers). ISBN 1 86239 118 1

The diversity and complexity of European geology owes much to Palaeozoic events. The late Proterozoic rifting of Laurentia from Gondwana heralded a procession of smaller continental fragments, each separating from the high southern latitude of Gondwana and drifting northwards to accrete to a more equatorial Laurentia. Our present understanding of the resulting collage of terranes was nurtured in Western Europe and developed by correlating further west, to the North American Appalachians. In the past twenty …

The basement versus the no-basement hypotheses for folding within the Appalachian plateau detachment sheet

Within the Appalachian plateau detachment sheet of southwestern Pennsylvania tectonic thickening is a central element to the growth of anticlines in a Silurian-Devonian section containing a three-tiered mechanical stratigraphy (that is, a basal detachment zone, a lower imbrication zone, and an upper wedge zone). The detachment zone is predominantly within disturbed shale of the Silurian Vernon Formation which sits above a disturbed surface on the Lockport dolomite. Large-scale anticline growth is, in part, a consequence of small-scale blind imbrication of the more competent mechanical layers in the lower portion of the detachment sheet (the imbrication zone). Here, salt within the Silurian Syracuse Formation hosts secondary detachment responsible for imbrication and the development of triangle zones in the core of the anticlines. Some fold amplification is also accomplished by extensive, smaller-scale thrust wedging and concomitant tectonic thickening of the less competent Devonian section in the upper portion of the detachment sheet (the wedge zone). These spatially periodic anticlines are separated by horizontally bedded synclines characterized by the absence of thrust wedging and fault imbrication across all three zones. Tectonic thickening continued behind the foreland propagation of the detachment-tip line within the Vernon shale as indicated by a systematic hinterland increase in the cross-sectional width and amplitude of the anticlines. On the basis of seismic images, each detachment sheet anticline is situated above prominent, periodically-spaced, pre-Alleghanian structures in the footwall of the detachment sheet. These footwall structures arise from a combination of thrust imbrication at the depth of the Ordovician Trenton Group and high-angle, basement-involved faulting at the depth of the Cambrian Gatesburg Formation. Some of the high-angle fault displacement is a consequence of Alleghanian inversion on faults associated with the extensional Rome Trough and other basement structures developed during the Late Proterozoic rifting of the eastern margin of Laurentia. Evidence for tectonic inversion is present in seismic reflection images that show buttress anticlines in the Cambro-Ordovician carbonate section. The superposition of detachment-sheet anticlines above footwall structures strongly suggests that the foreland transport was disrupted by topographic bumps on the base of the detachment zone. Bending of the detachment sheet over these irregularities may have promoted the spatially periodic collapse and concomitant tectonic thickening in the over-riding sheet as it was pushed laterally toward the foreland. A strain hardening of the detachment zone part of the detachment sheet at the topographic irregularities permitted the growth of double-sided tapered wedges (that is, each anticline). The overall structural profile of the detachment sheet is consistent with the growth of a Coulomb wedge whose low basal friction is interrupted by patches of strain hardening.

La série du PII–III de l'Anti-Atlas occidental (Sud marocain) : un olistostrome à la base de la couverture post-panafricaine (PIII) du Protérozoı̈que supérieur

Abstract At the base of the so-called PII–III series (Late Proterozoic) in the Moroccan western Anti-Atlas, many lenticular quartzitic bodies are embedded within conglomeratic layers. Field observations presented here suggest that the PII–III is an olistostrome that is either conformably or disconformably covered by the PIII Series. On the other hand, the Hercynian deformation is the only compression to have affected the PII–III and PIII. The emplacement of the chaotic ‘PII–III’ series registered the onset of the Late Proterozoic rifting in the Anti-Atlas.

Cover stratigraphy and structure of the southernmost external basement massifs in the Appalachian Blue Ridge; evidence for two-stage late Proterozoic rifting

Cover stratigraphy and structure of the southernmost external basement massifs in the Appalachian Blue Ridge; evidence for two-stage late Proterozoic rifting

Geochemistry and provenance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New England

The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a sandstone‐shale flysch succession deposited in the foreland of the Taconian Orogen. Petrographic, major and trace element, and Nd–Pb isotopic data provide substantial constraints on its provenance. Lack of K‐feldspar and paucity of plagioclase, in addition to the dominance of sedimentary rock fragments, indicate that the source was dominated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same. No evidence of an ophiolitic or volcanic component was observed. Interpretation of Nd isotopic characteristics are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of the Austin Glen Member had a long‐term history of light rare earth element (LREE) enrichment (average TDM = 1·8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (ɛNd = –8·1 ± 0·6; 1 s.d.; n = 23) at 450 Ma, the approximate depositional age, indicating either a single source or very well‐mixed sources. 207Pb/204Pb ratios are variable but within the range of Pb isotopic compositions typically described as Grenvillian. The range of 207Pb/204Pb is greater than expected for the range of 206Pb/204Pb and suggests an additional component of Pb, possibly introduced during diagenesis. The immediate source of the Austin Glen Member may have been the accretionary prism that developed as older sediments of the Laurentian margin were scraped off the basin floor, incorporated within the accretionary prism and shed into the basin. No evidence indicating the arrival of an undifferentiated island arc or continental fragment during the Taconian Orogeny has been found. The data acquired during this study can be explained almost exclusively by Grenville Province source components but with possible additional contributions from older Laurentian terranes and Late Proterozoic rift volcanics that are not readily quantified but likely to have been minor. Accordingly, we conclude that the Taconian Orogeny in New England involved either: (1) a continental arc that involved exclusively Laurentia; (2) collision of a continental block with identical geochemical characteristics as Laurentia; or (3) essentially no detritus from any exotic colliding block (island arc or continental fragment) reached the foreland basin at the time of Austin Glen deposition.

Late Proterozoic rift faults and basement—cover relationships within the Ben More thrust sheet, NW Scotland

Recently proposed models for the tectonic evolution of Northern Scotland, involving lithospheric extension during the late Precambrian, are supported by new discoveries and interpretations of early normal faults within the Moine Thrust Belt. Such structures control the preser­vation of Torridonian sediments beneath the sub-Cambrian unconform­ity. The Ben More thrust ramp from basement into the Cambrian cover may have been controlled by the early faults. Major synclines within the Ben More thrust sheet appear as tightenedhalf-grabens and thus are composite structures. This may require a re-examination of the field relationships between these folds and the apparently syn-tectonic alkaline intrusions that have been used to date thrust activity. The Ben More thrust sheet of the Moine Thrust Belt contains the most easterly outcrops of Torridonian Supergroup (late Proterozoic) in Scotland (Fig. 1). These sediments and their Lewisian basement are unconformably overlain by the Cambrian quartzites of the Eriboll Sandstone Group. The stratigraphic relationships of these units, with the two un­conformities (Cambrian on Torridonian, Torridonian on Lewisian) discordant so that the Cambrian sediments overstep from Toridonian onto Lewisian basement, have been known for over a century. However, the nature of the overstep and its implications for the Precambrian arrangement of Torridonian and Lewisian rocks, is less established. Soper & Barber (1979) inferred the presence of major upright folds with wavelength in excess of 30 km so that Torridonian sediments lie preserved in synclines while the crests of the anticlines are eroded beneath the sub-Cambrian unconformity. This model for Precambrian basement–cover relationships has

Deep-water fault-controlled sedimentation, Arctic Norway and Russia: response to Late Proterozoic rifting and the opening of the Iapetus Ocean

The Late Proterozoic (late Riphean) sections of Varanger Peninsula, northern Norway, and the Rybachi Peninsula, Kola Peninsula. Russia, both show an overall progradational succession from deep-marine to slope environments, with the Varanger sections passing up into shallow-marine and then fluviatile deposits. The oldest exposed parts of both successions are correlated along-strike over a minimum distance of c. 75 km, and the turbidite systems are interpreted as fragments of the sedimentary infill of the same rift basin, defined here as the Varanger–Rybachi Basin, which fringed the Fennoscandian Shield in Late Proterozoic time. Sediment transport patterns show lateral input and axial deflection to more eastward-flowing currents. Proximal-to-distal and axial-to-lateral changes in the sedimentary facies associations are placed within deep-marine slope and basin-floor turbidite systems. In Arctic Russia. the basin-bounding Rybachi-Sredni Fault Zone probably broke the seafloor to shed breccias along a fault scarp, possibly even as a subaerial fault zone and associated with fan-deltas. In contrast, to the west in northern Norway, the Trollfjord–Komagelv Fault Zone probably acted as a concealed basin-margin fault zone which controlled the location and pattern of deep-water sedimentation, offshore from a major fluvially dominated delta system. The Late Proterozoic development and subsequent infilling of this basin is interpreted as having formed in response to the opening of the Iapetus Ocean. possibly associated with mantle plume activity farther west. The Late Proterozoic Barents Sea Basin may have been analogous to the late Jurassic and Palaeogene northern North Sea, in which sand-prone turbidite systems accumulated predominantly by deep-marine sediment transport away from a developing ocean basin (North Atlantic) associated initially with thermally induced lithospheric extension and uplift (late Jurassic), followed by mantle-plume activity centred over Iceland during Paleocene time causing the development of a thermal-sag basin and uplift over the plume.

Implied basement-tectonic control on deposition of Lower Carboniferous carbonate ramp, southern Cordillera, Canada

cambrian (Kanasevich et al., 1969) and preThe Mount Head embayment is a regional downwarp along the west coast of Lower Middle Devonian (Norris and Price, 1966; Carboniferous western Canada that developed by differential subsidence, which we recog- Benvenuto and Price, 1979). nize from lithostratigraphic and biostratigraphic patterns. Palinspastic restoration of the Mount Head embayment illustrates that subsidence domains are geometrically coincident with previously identified tectonic elements of the autochthonous basement. Thus, it ap- METHODS pears that basement-tectonic elements controlled sediment accommodation and accumu- Geologic observations were recorded on lation within the Mount Head embayment. The overall shape of this embayment is probably palinspastically restored maps and cross setinherited from the arcuate shape of the Late Proterozoic cratonic rift margin. Within the tions and to basement-tectonic Mount Head embayment, several northeast-southwest-trending Archean and Proterozoic domains defined on aeromagnetic and gravbasement-tectonic elements intersect the autochthonous cratonic margin at high angles. ity maps (Brandley, 1993). The palinspastic Carboniferous piano-key-like structural reactivation of these tectonic elements produced base map is modified from Gibson,s (1985) oriented trends of differential subsidence that partitioned the Mount Head embayment into map of displacement vectors for Jurassic two subbasins (Crowsnest and Kananaskis depocenters) separated by a more positive area rocks. The magnitudes of the vectors were (Highwood high). Moreover, our data imply that Precambrian basement structure noted by others under the Plains and Rocky Mountain fold and thrust belt continued across the altered to reflect displacements at the MisRocky Mountain trench to the west, and that tectonic control on sedimentation noted by sissippian level documented by Bally et al. others for Precambrian and pre-Middle Devonian rocks extends clearly into overlying (1966), Price (1981), and Price and Fermor Carboniferous deposits. (1985), university theses, and Geological Survey of Canada maps and sections across

Trace element and Sr and Nd isotopic composition of mantle xenoliths from the Big Pine Volcanic Field, California

Mantle xenoliths from the Quaternary Big Pine volcanic field of eastern California provide direct evidence for the presence of isotopically enriched mantle beneath the Sierran Province of the western United States. Sr and Nd isotopic compositions determined on five bulk rock and five clinopyroxene separates are negatively correlated; εNd values range from +13.8 to −3.4, and 87Sr/86Sr ratios range from 0.7022 to 0.7065. The measured Sr and Nd contents and Ce/Yb ratios of mantle xenoliths in the Big Pine volcanic field are well correlated with Sr and Nd isotopic compositions, and these xenoliths define an 820‐m.y.‐old Sm‐Nd isochron. The Big Pine mantle xenoliths have similar textures and mineral compositions, and we interpret the large range in isotopic and trace element compositions to be a result of cryptic mantle metasomatism. The timing of this mantle metasomatic event is unclear but a maximum Late Proterozoic age can be inferred from the 820 Ma Sm‐Nd isochron. If the Sm‐Nd isochron is interpreted as a result of recent metasomatism then the metasomatising agent was similar in composition to the Big Pine basalts. If the isochron is interpreted to have age significance then this mantle metasomatism was associated with late Proterozoic rifting of the western margin of North America. Although production of isotopically enriched mantle is generally attributed to ancient subduction, these data are inconsistent with such an interpretation.

Hydrothermal origin of Late Proterozoic bedded chert at Gusui, Guangdong, China: petrological and geochemical evidence

ABSTRACTThe Late Proterozoic bedded chert from Gusui in Guangdong Province, southern China, is characterized by bedded, laminated, massive and pseudobrecciated structures. The chert is depleted in TiO2, Al2O3 and most trace elements. However, it is enriched in Ba, As, Sb, Hg, and Se. In Al‐Fe‐Mn ternary diagrams, it falls into the ‘hydrothermal field’. Factor analysis shows that many elements show up in the principal trace element factor suggesting their enrichment results from leaching of the country rock by hydrothermal solutions. The chert has low REE concentrations and displays an REE pattern consistent with a hydrothermal origin. It may have formed in a Late Proterozoic rift or extension zone developed within the Yunkai continental margin back‐arc basin, with a fault system linking it to an unknown heat source at depth.

Extensional and contractional deformation in the Blue Ridge Province, Virginia

Basement rocks of the Blue Ridge anticlinorium in central and northern Virginia were involved in two temporally distinct deformation episodes. The older event is characterized by a locally pervasive low-strain fabric and high-strain shear zones. A preferred grain shape orientation in recovered quartz, feldspar, and biotite, combined with a poorly developed symmetric quartz c -axis pattern, defines the low-strain fabric. Kinematic indicators are typically symmetric, but where asymmetric, they consistently indicate top down-to-the-southeast extensional motion. Discrete shear zones are within and close to Late Proterozoic plutons; kinematic evidence fron these zones records large noncoaxial strains also with a down-to-the-southeast extensional shear sense. Feldspar microstructures range from cataclastic fractures to complete recovery, suggesting that deformation occurred at greenschist to amphibolite grade. Both low-strain foliations and high-strain shear zones are cut by Paleozoic thrust-related tectonites. On the basis of kinematic evidence and spatial association with Late Proterozoic plutons, cross cutting relations with thrust-related structures, and regional geologic relations, we suggest that extensional deformation is related to Late Proterozoic rifting of the Laurentian margin. The kinematic histories of the Rockfish Valley fault zone and related shear zones appear to be relatively simple and consistent with Paleozoic west- to northwest-directed thrusting. The Rockfish Valley fault zone does not separate distinctly different basement terranes, and thus its displacement is thought to be small. Mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars, under lower to middle greenschist-facies conditions. Solution transfer processes were an important deformation mechanism in these tectonites. Thrust- related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands, suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust. The Rockfish Valley fault zone appears to predate the formation of the South Mountain cleavage in the Valley and Ridge province, but it may be part of a lengthy, progressive, thrust-related deformation.

Dnieper-Donets palaeorift

The Devono-Carboniferous intra-cratonic Dnieper-Donets palaeorift transects the southwestern part of the East European platform in a NW-SE direction. It is characterized by linear bounding faults penetrating much of the crust, substantial crustal thinning and syn-rift volcanic activity. The Dnieper-Donets rift developed on cold Precambrian shield-type continental crust. Its rifting stage spans early Frasnian to Late Visean times. During its post-rift evolution an elongate, saucer-shaped basin subsided, the axis of which coincides with the trace of the underlying rift. The Palaeozoic Donets Rift is superimposed on a Late Proterozoic rift. A thermal perturbance of the asthenosphere is thought to have provided the driving mechanism for the development of the Palaeozoic Dnieper-Donets Rift. Rapid thinning of the subcrustal lithosphere in response to advecting asthenospheric melt was coupled with tensional fracturing of the crust and melting of its lower part. This caused an upward displacement of the Moho discontinuity. Post-rift subsidence of the Dnieper-Donets graben was governed by lithospheric cooling. The southeastern parts of this rift system were inverted during the Uralian orogeny.

Late Proterozoic Rift Control on the Shape of the Appalachians: The Pennsylvania Reentrant

The Pennsylvania reentrant, the most prominent deviation in the trend of the Appalachians, is the product of Late Proterozoic rifting. The Peters Creek Formation, Pennsylvania-Maryland Piedmont, contains rift-gene rated, deep-water turbidite deposits of Late Proterozoic-Cambrian(?) age. These rocks are an extension of the Westminster terrane and lie well to the northeast of the southern Appalachian Late Proterozoic-Cambrian rift basin (Lynchburg-Chilhowee Group basin). The basin into which the Peters Creek Formation was deposited may have connected the southern rift basin with one to the north. The preservation of the Peters Creek Formation and other age equivalent units within the Pennsylvania reentrant indicates that the New York promontory acted as a buttress to Paleozoic orogenic activity.

Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent

Evidence supports the hypothesis that the Laurentian and East Antarctic-Australian cratons were continuous in the late Precambrian and that their Pacific margins formed as a conjugate rift pair. Both margins extend for approximately 40° of latitude. They have a similar rift history throughout their length—i.e., Late Proterozoic rifting and Early Cambrian carbonate platform development. A geometrically acceptable computer-generated reconstruction for the latest Precambrian juxtaposes and aligns the Grenville front that is truncated at the Pacific margin of Laurentia and a closely comparable tectonic boundary in East Antarctica that is truncated along the Weddell Sea margin. These may prove to be critical, perhaps even unique, piercing points for relating the northern and southern continents. Geologic and paleomagnetic evidence also suggests that the Atlantic margin of Laurentia rifted from the proto-Andean margin of South America in earliest Cambrian time. Early Phanerozoic sea-floor spreading that isolated Laurentia from South America and East Antarctica-Australia in an Eocambrian supercontinent appears to balance convergence along the Mozambique suture which resulted in final amalgamation of the smaller Gondwana supercontinent at ∼500 Ma.

Geochemical patterns related to major tectono-stratigraphic units in the Precambrian of northern Scandinavia and Greenland

Abstract Large areas of the Fennoscandian Shield (Norway, Sweden, Finland) and the Laurentian Shield (Greenland, Canada) have been covered by regional geochemical surveys based on low-density sampling of various surficial materials. Till and sediment, organic matter and moss from streams were collected at an average density of 1 site per 30 km 2 over 250,000 km 2 of northern Fennoscandia. The geochemical patterns reflect both the major Precambrian tectono-stratigraphic units, and also distinguish sub-units, e.g., Archaean gneiss units, ultramafic predominance within greenstone belts, and granites enriched in incompatible elements. In Greenland, stream sediment was collected over 35,000 km 2 of Archaean and Proterozoic rocks at a density of 1 site per 20–30 km 2 in the west, and at 1 site per 2–5 km 2 in the south. Stream sediment data display geochemical boundaries that coincide with recently defined tectono-stratigraphic units in Archaean gneisses. The Proterozoic of South Greenland is enriched in U and other incompatible elements, resembling the Fennoscandian Shield. A major Nb province outlines alkaline magmatism associated with Late Proterozoic rifting. Widely scattered isotopic data and plate-tectonic models have been used to correlate the evolution of the Fennoscandian and Laurentian shields. The regional geochemical data distinguish major boundaries and provinces, will aid in further correlation, and help to unravel crustal evolution.

Subsurface Structure of the Eastern Arkoma Basin (1)

Analysis of 425 km of seismic reflection data in the eastern Arkoma basin reveals a structure and history quite different from those previously reported for the Arkoma basin. The eastern Arkoma basin has three structural styles that formed in the following chronological order: deep, steep normal faults; shallow listric normal faults; and thrust faults. The origin of the structural styles and the Paleozoic history of the eastern Arkoma basin may be summarized as follows. Late Proterozoic or Early Cambrian rifting was followed by deposition of Cambrian through Upper Mississippian strata on a passive plate margin. The Mississippian-Pennsylvanian boundary marks a time of major down-to-the-south normal faulting with coincident folding of the footwall blocks and truncation of t e faulted and folded terrain by a pre-Morrowan (Mississippian-Pennsylvanian) unconformity. The unconformity slopes southward and steepens locally across the erosionally truncated footwall blocks. During the Pennsylvanian, down-to-the-south listric normal faults cut the Atokan and Morrowan sections and merged with, but did not displace, the steeper segments of the sub-Morrowan unconformity. Surface folds north of the Ross Creek thrust are rollover anticlines overlying these listric normal faults. Major deformation in the eastern Arkoma basin terminated with emplacement of the Ross Creek thrust in the Late Pennsylvanian.

High TiO 2 metadiabase dikes of the Hudson Highlands, New York and New Jersey; possible late Proterozoic rift rocks in the New York recess

<h3>ABSTRACT</h3> The ongoing COVID-19 pandemic represents an unprecedented global health crisis. Here, we report the identification of a synthetic nanobody (sybody) pair (Sb#15 and Sb#68) that can bind simultaneously to the SARS-CoV-2 spike-RBD and efficiently neutralize pseudotyped and live-viruses by interfering with ACE2 interaction. Two spatially-discrete epitopes identified by cryo-EM translated into the rational design of bispecific and tri-bispecific fusions constructs, exhibiting up to 100- and 1000-fold increase in neutralization potency. Cryo-EM of the sybody-spike complex further revealed a novel <i>up-out</i> RBD conformation. While resistant viruses emerged rapidly in the presence of single binders, no escape variants were observed in presence of the bispecific sybody. The multivalent bispecific constructs further increased the neutralization potency against globally-circulating SARS- CoV-2 variants of concern. Our study illustrates the power of multivalency and biparatopic nanobody fusions for the development of clinically relevant therapeutic strategies that mitigate the emergence of new SARS-CoV-2 escape mutants.