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In intraplate areas where regional tectonic strain is accommodated by the reactivation of pre-existing structures, the level of seismic hazard associated with faults maybe underestimated due to the poor surface expression of faults, scattered earthquake distribution, and long earthquake recurrence intervals. The cause(s) of seismicity in eastern Canada remains unresolved. This is partially because surface expressions of faults have been eroded during glacial and deglacial periods and in the Ottawa-Bonnechere Graben (OBG) are undetectable until a seismic event. Morphotectonic analysis has been widely applied to assess relative tectonic activity in various geological settings. To establish whether active uplift is occurring and to investigate the spatial distribution of relative uplift rates, 131 bedrock drainage basins in OBG were analysed. The aim of this was to (1) test the applicability of geomorphic indices for quantifying active deformation, (2) quantify the spatiotemporal distribution of relative uplift rates, and (3) explore the implications for faulting mechanisms, deformation styles, and ultimately regional seismic hazard. We measured valley floor width-to-height ratio (Vf), basin elongation ratio (Re), basin hypsometric integral, and normalized channel steepness index ([k.sub.sn]). The results show that high relative uplift rates exist in all the six bedrock escarpment sections investigated, suggesting that they are presumably related with regional broad wavelength uplift (epeirogeny) caused by a complex interaction between far-field tectonic stress and glacial isostatic adjustment (GIA). Our analysis showed that [V.sub.f], Re, and [k.sub.sn] reveals no considerable spatial diferences in high relative uplift rates, consistent with the Canadian Base Network GPS uplift rates. Highlights * Geomorphic indices are applied in a low relief and low-medium seismicity area. * Geomorphic indices reveal a broad wavelength regional uplift and show relative tectonic activity in the OBG. * Relatively high geomorphic indices values provide longer term deformation history compared to global positioning system (GPS) records. * Landscape formation and river incision is driven by a complex interaction between GIA and far-field tectonic stresses. * Relative uplift rate results show similar patterns to published GPS uplift rates. * Possible link between relative tectonic activity and current seismicity. Keywords: Ottawa-Bonnechere Graben, western Quebec seismic zone, morphotectonic analysis, intraplate seismicity, regional uplift, Canada, 1. Introduction Eastern Canada is characterized as a stable continental region (SCR) which has a relatively lower seismicity rate and longer earthquake recurrence intervals compared to active plate tectonic zones/boundaries [...]

The small volume, Mesozoic alkali gabbro intrusions of the Budgell Harbour and Dildo Pond stocks and associated alkaline lamprophyre dykes in Notre Dame Bay in Newfoundland are an example of onshore magmatism that may be associated with North Atlantic Ocean opening. Chemical abrasion isotope dilution thermal ionization mass spectrometry U–Pb dating of zircon from Budgell Harbour Stock drill core samples yielded a weighted average 206Pb/238U age of 147.9 ± 0.5 Ma (95% confidence interval, mean square weighted deviation = 0.10). Five 40Ar/39Ar laser step-heating, single phlogopite grain analyses from the Budgell Harbour and Dildo Pond stocks, plus a lamprophyre dyke, yielded ages ranging from 146.3 ± 0.2 to 149.5 ± 0.5 Ma. The data demonstrate a ca. 148 Ma (Jurassic and Tithonian) alkaline magmatic event in Newfoundland, contemporaneous with rifting and offshore basin formation. These new age data and published determinations from magmatic rocks on conjugate margins and adjacent regions reveal long-lived episodic magmatism in the embryonic North Atlantic rift environment. We propose a new model whereby magmatism occurred in Notre Dame Bay at the convergence of older crustal-scale faults and localized Moho depth variations that may have triggered lithospheric mantle melting through isothermal, distal, and edge-driven upwelling related to regional extension. [ABSTRACT FROM AUTHOR]

There is currently no active fault map for the intraplate western Quebec seismic zone (WQSZ) in eastern Canada, and consequently, no detailed finite-fault source models, which are critical for seismic hazard assessments in this region with a rapidly growing population. While previous numerical stress modelling studies have shown that mostly NNW–SSE to NW–SE-striking faults exhibit the highest potential for reactivation under the present-day tectonic stress field, such modelling is unable to take into account the interaction of faults and earthquakes. This study attempts to identify possible future rupture zones using Coulomb stress analysis. We explore the static stress transfer caused by the 1935 MW 6.1 Témiscaming, 1944 MW 5.8 Cornwall–Massena, and 2013 MW 4.7 Ladysmith earthquakes, which are proximal to faults in the WQSZ that exhibit a relatively high reactivation potential, to determine whether these faults have an increased potential for failure. The significance of Coulomb stress changes (ΔCFS) observed on the nearby "receiver" faults varied widely. Among the events analyzed in this study, only the 1935 MW 6.1 Témiscaming earthquake caused extensively positive ΔCFS (≥0.1 bar) on its receiver fault. The areal extent of the receiver fault that has been promoted to failure suggests that earthquakes with a comparable magnitude to the 1935 event can be triggered. This work is the first attempt to provide a physical basis for seismic hazard assessment input parameters in the WQSZ based on the results of numerical stress modelling. [ABSTRACT FROM AUTHOR]

The onshore exposures adjacent to modern, offshore passive continental margins may preserve evidence of deformation from the pre-, syn-, and post-rift phases of continental breakup that allow us to investigate the processes associated with and controlling rifting and breakup. Here, we characterize onshore brittle deformation and pre-rift basement metamorphic mineral fabric from onshore Labrador in Eastern Canada in the Palaeoproterozoic Aillik Domain of the Makkovik Province. Stress inversion (1) was applied to these data and then compared to (2) numerical models of hybrid slip and dilation tendency, (3) independent calculations of the regional geopotential stress field, and (4) analyses of palaeo-stress in proximal regions from previous work. The stress inversion shows well-constrained extensional deformation perpendicular to the passive margin, likely related to pre-breakup rifting in the proto-Labrador Sea. Hybrid slip and dilatation analysis indicates that inherited basement structures were likely oriented in a favorable orientation to be reactivated during rifting. Reconstructed geopotential stresses illuminate changes of the ambient stress field over time and confirm the present paleo-stress estimates. The new results and numerical models provide a consistent picture of the late Mesozoic-Cenozoic lithospheric stress field evolution in the Labrador Sea region. The proto-Labrador Sea region was characterized by a persistent E-W (coast-perpendicular) extensional stress regime, which we interpret as the pre-breakup continental rifting that finally led to continental breakup. Later, the ridge push of the Labrador Sea spreading ridge maintained this general direction of extension. We see indications for anti-clockwise rotation of the direction of extension along some of the passive margins. However, extreme persistent N-S-oriented extension as indicated by studies further north in West Greenland cannot be confirmed. [ABSTRACT FROM AUTHOR]

Significant poly-phase deformation occurs in adjacent continental domains prior to, simultaneously with, and after continental breakup. Constraining this deformation is essential for understanding the regional development and the mechanisms driving and controlling rifting and breakup. Here, we primarily use published constraints to construct deformable plate tectonic models for the East African conjugate margins of Madagascar and Somalia/Kenya/Tanzania using GPlates, an open source plate tectonic modelling environment. The aim of this work is to test both the capability of the GPlates deformable modelling approach and the published model inputs, including time and location of necking zones and earliest oceanic crust. The results demonstrate that inclusion of independent micro-continental fragments (e.g. Comoros microcontinent and the Bur region in Somalia) and locally defined limits of continental crust, generally produce results more akin to observations. The results also allude towards a complex poly-phase progression of rifting that led to the formation of continental fragments surrounding the Western Somali Basin. These deformation episodes are likely linked to changes in plate kinematics and had significant consequences for the age and crustal nature of the Somali margin. Finally, we suggest that the extreme compressional deformation observed offshore Madagascar and Mozambique along the Davie Ridge is also the result of Mesozoic plate motions. [ABSTRACT FROM AUTHOR]

The non-volcanic Newfoundland margin offshore eastern Canada formed after a period oflithospheric stretching and rifting which began in the Late Triassic to Early Cretaceous. Thiswas followed by continental breakup resulting in separation of the Grand Banks and Iberia,and northern Newfoundland and Ireland in the Aptian-Albian. Despite the Newfoundlandmargin being considered as a non-volcanic, or magma-poor, passive margin, a sequence ofMesozoic-Cenozoic magmatic rocks have been documented on- and offshore Newfoundlandthat have been proposed to be contemporaneous with rifting and breakup. Theseinclude Mesozoic alkaline intrusive rocks in northern-central Newfoundland, referredto in previous work as the Notre Dame Bay Magmatic Province (NDBMP), thatcomprise multiple kilometre-scale gabbroic intrusions in addition to widespread,genetically-related, metre-scale lamprophyre dykes. Here, we describe the results of acomprehensive study into the petrology, geochemistry and geochronology of these intrusivebodies. Specifically, we conducted U-Pb dating on zircon and 40Ar/39Ar dating onphlogopite from several of the intrusive bodies. Our analyses confirm that a substantial,and geodynamically significant, magmatic event occurred on the Newfoundlandmargin at ca. 147.9 Ma (Late Jurassic, Tithonian), contemporaneous with significantrifting, and the formation of the offshore petroliferous rift basins. Comparison ofthis new Late Jurassic age with geochronological determinations from adjacentMesozoic-Cenozoic rift-related magmatic rocks (e.g., the North Atlantic Igneous Provinceand the Central Atlantic Magmatic Province) reveals a picture of intermittent, episodic,rifting dominating the embryonic rift environment. This is contrary to the classiccontinuous, 'unzipping’ of the proto-North Atlantic previously interpreted. Instead, aprogressive and intermittent 'unsnapping’ of the proto-North Atlantic is favoured. [ABSTRACT FROM AUTHOR]