An evaluation of Mesozoic rift-related magmatism on the margins of the Labrador Sea: Implications for rifting and passive margin asymmetry

Abstract

The Labrador Sea is a small (~900 km wide) ocean basin separating southwest Greenland from Labrador, Canada. It opened following a series of rifting events that began as early as the Late Triassic or Jurassic, culminating in a brief period of seafloor spreading commencing by polarity chron 27 (C27; Danian) and ending by C13 (Eocene-Oligocene boundary). Rift-related magmatism has been documented on both conjugate margins of the Labrador Sea. In southwest Greenland this magmatism formed a major coast-parallel dike swarm as well as other smaller dikes and intrusions. Evidence for rift-related magmatism on the conjugate Labrador margin is limited to igneous lithologies found in deep offshore exploration wells, mostly belonging to the Alexis Formation, along with a postulated Early Cretaceous nephelinite dike swarm (ca. 142 Ma) that crops out onshore, near Makkovik, Labrador. Our field observations of this Early Cretaceous nephelinite suite lead us to conclude that the early rift-related magmatism exposed around Makkovik is volumetrically and spatially limited compared to the contemporaneous magmatism on the conjugate southwest Greenland margin. This asymmetry in the spatial extent of the exposed onshore magmatism is consistent with other observations of asymmetry between the conjugate margins of the Labrador Sea, including the total sediment thickness in offshore basins, the crustal structure, and the bathymetric profile of the shelf width. We propose that the magmatic and structural asymmetry observed between these two conjugate margins is consistent with an early rifting phase dominated by simple shear rather than pure shear deformation. In such a setting Labrador would be the lower plate margin to the southwest Greenland upper plate.

The Role of Inherited Lithospheric Heterogeneities in Defining the Crustal Architecture of Rifted Margins and the Magmatic Budget During Continental Breakup

“…The rifted margins of the Labrador Sea are classified as magma poor in the south (Chian et al, ) and as magma rich in the north (Keen et al, ). Syn‐rift to breakup magmatism is found onshore and offshore southwest Greenland and Labrador (Larsen et al, ; Peace et al, ; Tappe et al, ). Its distribution is, however, highly asymmetric with only minor Mesozoic magmatic intrusions exposed in the Labrador side, mainly in the Makkovik domain, compared to the widespread Jurassic to Cretaceous igneous rocks outcropping along the SW Greenland side (Peace et al, ; Tappe et al, ).…”

“…Syn‐rift to breakup magmatism is found onshore and offshore southwest Greenland and Labrador (Larsen et al, ; Peace et al, ; Tappe et al, ). Its distribution is, however, highly asymmetric with only minor Mesozoic magmatic intrusions exposed in the Labrador side, mainly in the Makkovik domain, compared to the widespread Jurassic to Cretaceous igneous rocks outcropping along the SW Greenland side (Peace et al, ; Tappe et al, ). Offshore rift‐related magmatism is observed in wells on the Labrador shelf (no wells were drilled on the conjugate SW Greenland shelf), where basalt sequences are associated with the Lower Cretaceous Alexis formation deposited during the early phase of rifting (Umpleby, ).…”

Two‐Branch Break‐up Systems by a Single Mantle Plume: Insights from Numerical Modeling

“…(a) Phase, (b) temperature and (c) strain rate plots of model 7 (Moho temperature of 500°C, extension half‐rate of 10 mm/yr, plume shift of 375 km). This model develops a “two‐branch” break‐up mode and bears strong similarities with the geodynamical evolution in the North Atlantic domain (d, schematic representation): (1) the first branch forms in the left part of the model, corresponding to the strong crust, similar to Greenland craton that will eventually separate Greenland and Canada (Peace et al, ); and (2) the second branch forms 6 Myr later close to the inherited structure, comparable to the break‐up of the Caledonian orogeny eventually separating Greenland and Norway (Lundin & Doré, ). The pink color on the schematic profiles refers to newly formed oceanic lithosphere.…”

“…Here the old and rigid lithosphere of the Greenland craton (Kerr et al, ) was underlain by a single mantle anomaly (the Iceland mantle plume) before rifting started in the Labrador Sea (Lundin & Doré, ; Rogozhina et al, ). The old craton was subjected to plume‐activated continental rifting in the Late Triassic or Jurassic followed by seafloor spreading with the oldest accepted magnetic anomaly being of Danian (~64 Ma) age (Chalmers et al, ) (although older anomalies are still a matter of debate, Peace et al, ). Note, however, that the opening of the mostly a magmatic Labrador Sea might have started before the mantle plume impacted the lithosphere beneath West Greenland (Larsen & Saunders, ).…”

“…For our set of reference models (Figure 3a), a two-branch system forms when the mantle anomaly has a lateral displacement of 350-375 km towards the stronger half of the model domain with respect to the rheological contact. The two branches develop consecutively, with roughly 10 Myr delay, with either "structural inherited" break-up first, followed by "plume-centered" (displacement (1) the first branch forms in the left part of the model, corresponding to the strong crust, similar to Greenland craton that will eventually separate Greenland and Canada (Peace et al, 2016); and (2) the second branch forms 6 Myr later close to the inherited structure, comparable to the break-up of the Caledonian orogeny eventually separating Greenland and Norway (Lundin & Doré, 2002). The pink color on the schematic profiles refers to newly formed oceanic lithosphere.…”