Tectonism and volcanism at the southeast Greenland rifted margin: a record of plume impact and later continental rupture

Abstract

During Ocean Drilling Program Leg 152, Sites 914 through 919 were drilled on the southeast Greenland Margin along a transect from the middle shelf into the adjacent deep-water Irminger Basin 500 km south of the Iceland hot-spot track (IcelandGreenland Ridge). Sites 915 through 918 penetrated the entire cover of postrift sediments, and three of these four sites sampled the volcanic basement of the seaward-dipping reflector sequences (SDRS). The landward featheredge of the SDRS was drilled at the most landward site, Site 917, where a 779.5-m-thick, south to southeastward dipping volcanic section was found to overlie steeply dipping, marine pre-rift sediments of unknown, possibly Cretaceous, age. The more seaward Site 915 (later deepened by Site 990) recovered two lava flows that are stratigraphically located above Site 917 and located within the oldest part of the main SDRS wedge. The central part of the SDRS wedge was penetrated at Site 918 in the Irminger Basin where a 120-mthick lava section was recovered below 1189 m of postrift sediments. A few sills and dikes were sampled. All other igneous units recovered were subaerially erupted and deposited lavas, some of which may have flowed over wet ground or into shallow water. Lavas at Site 917 display compositions ranging from picrite over olivine basalt to basalt, dacite, and acid tuffs. The younger lavas at Site 915 and Site 918 have a quite uniform composition similar to depleted Icelandic tholeiites. Two major successions are defined: the older Continental Succession and the younger Oceanic Succession. The Continental Succession comprises the Lower and Middle Series lavas at Site 917, and the Oceanic Succession comprises the Uppers Series lavas at Site 917 and the main SDRS series lavas at Sites 915 (and Site 990) and Site 918. The Oceanic Succession is separated from the Continental Succession by an unconformity and a thin sediment horizon. The Oceanic Succession shows strongly decreasing (to absent) continental contamination, a primary melting depth extending to a shallower level (spinel field), and, apart from the initial picritic Site 917 Upper Series, a much more uniform composition and a less depleted source than the Continental Sucession. The age of the reversely magnetized Continental Succession is approximately 61−60 Ma, possibly slightly older, and is related to magnetic Chron C26r (possibly C27r). The main part of the Oceanic Succession is 56−53 Ma (magnetic Chron C24r), with the oldest part extending into C25r. Except for six of the youngest lava flows in the Lower Series, the Continental Succession had a depleted (ambient asthenosphere?) mantle source, and magmas underwent considerable fractionation in crustal magma chambers. The Oceanic Succession, except for the initial Site 917 Upper Series, had a slightly less depleted mantle source. Tentative estimates of eruption rates vary from 1/11 k.y. within the Continental Succession to 1/670 yr within the main SDRS wedge. Likewise, the total magmatic flux is estimated to vary from (maximum) 0.22...

Crustal breakup and continent-ocean transition at South Atlantic conjugate margins

“…In this setting, the reconstructed conjugate magnetic anomaly map indicates overall symmetry at the Pelotas-Walvis conjugate basins, whereas, asymmetry is observed and is characterized by a narrow/wide margin configuration farther south. Along the NE Atlantic margin, a similar magnetic asymmetry has been explained by an asymmetric seafloor spreading [e.g., Larsen and Saunders, 1998] or by stretched and intruded continental crust, where the asymmetry is caused by the initial continental stretching rather than the subsequent seafloor spreading [e.g., White and Smith, 2009].…”

The evolution of the North Atlantic Igneous Province and the opening of the NE Atlantic rift

“…This correlation is further strengthened by a significant (although not abrupt) change in lava geochemistry towards the top of these formations, continuing into the overlying lavas sequences. While it is currently difficult to identify the exact timing of, and any possible tectonic linkage to, this change in lava geochemistry, the seaward dipping reflectors (SDRS) in the Irmiger Basin appear to have been emplaced during this period, with the switch to the Oceanic Succession, detailed by Larsen & Saunders (1998), having occurred at the start of Interval 3. The nature of the link between the geochemical change in lava composition, the widespread unconformity and the onset of SDRS emplacement still requires resolution.…”

“…A thin basalt bed in the oldest sandstone units of ODP Hole 918, which overlie the main SDRS basalts, yielded an age of 52.1 + 0.8 Ma, younger than the nannofossil zonation for the host sediment (Wei 1998). Larsen & Saunders (1998) used these data to suggest an intrusive relationship for the thin basalt unit with relatively continuous sedimentation from cessation of SDRS eruption. This interpretation appears to overemphasize the accuracy of calibration of isotopic ages to the bio-and magnetostratigraphical timescale, and a nannofossil age determination based on fossil absence data.…”

Interaction of rifting and hot horizontal plume sheets at volcanic margins

“…These numerical model results are compared with new studies of volcanic rifted margins, particularly those from the east Greenland margin [Larsen and Saunders, 1998], where the volume and distribution of igneous crust is clearly defined with respect to the Iceland hotspot. The excessively thick igneous crust appears to have been generated over a short time interval, a few million years.…”