41. TECTONISM AND VOLCANISM AT THE SOUTHEAST GREENLAND RIFTED MARGIN: A RECORD OF PLUME IMPACT AND LATER CONTINENTAL RUPTURE1H.C. Larsen2 and A.D. Saunders3 |
ABSTRACTDuring 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 (Iceland-Greenland 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-m-thick 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) 0.225 km3/k.y./km rift length (Continental Succession) to 1.333 km3/k.y./km rift length (Oceanic Succession). Excess temperature of the mantle source is estimated to approximately 100°C for both successions. The main SDRS wedge is interpreted to represent fully igneous crust that formed rapidly (half-rate 4.4 cm/yr) along a subaerially exposed rift like the present-day Iceland rift zone. The featheredge of the SDRS overlying continental crust subsided significantly later than the main wedge that was transgressed by a shallow sea almost immediately after its formation. Erosion of the rift flank (Continental Succession) was (locally) deep shortly after breakup, and the Eocene witnesses erosion of a former, large and deeply weathered volcanic cover over the continental margin. Regional, kilometer-scale margin uplift took place after breakup, possibly during the mid-Tertiary, where significant crustal exhumation of the continental margin is taking place. However, the onset of North Atlantic and Greenland glaciation recorded at Site 918 and dated to approximately 7 Ma led to further erosion of remnants of the formerly much wider volcanic cover of the inner shelf and coastal region. Significant thinning of the continental crust takes place within an only 25-km-wide zone seaward from Site 917. Thinning of the upper crust took place through seaward rotation of fault blocks along landward-dipping normal faults extending to mid-crustal depths much like the structures displayed by the East Greenland coastal flexure and dike swarm. Significant parts of the crustal thinning took place between eruption of the Continental and Oceanic Successions. We explain observations on mantle sources, primary melting depths, timing and location of volcanism, and magmatic flux rates with a model invoking a plume head in the order of 108 km3 in volume, and rapidly ascending (in the order of m/yr) and spreading (in the order of 0.5 m/yr) below the lithosphere within an upper mantle with a non-Newtonian (nonlinear) rheology. Upon impact, the former, more bulbous-shaped, plume head is spreading into a thin (in the order of 50 km thick) sheet with a diameter of about 2000 km and filling in the topographic relief at the bottom of the lithosphere. This was completed by 61 Ma and caused regional North Atlantic uplift evident at Site 917 (basal unconformity between Lower Series and pre-rift sediments). Decompression melting of plume material and frictionally heated ambient asthenosphere (and possible lithosphere) take place at thin lithospheric spots. Opening of the Northeast Atlantic rift takes place at or shortly after magnetic Chron C25n (56 Ma) and allows the hot plume sheet to spread along the rift, which becomes floored by hot mantle for as much as 2700 km from south of Greenland to the Barents Sea. The developing plate boundary offers a continuously growing lithospheric thin spot. Plume mantle is flowing, decoupled from the underlying asthenosphere, at rates estimated to between 11 and 22 cm/yr, laterally toward the rift and fills the void between the parting plates. The relief at the bottom of the lithosphere thus controls the thickness of the plume layer below the SDRS rift, and allows for a melting column of the order 100 km thick, which, at 15%–20% degree of melting, is able to generate the close to 20-km-thick SDRS crust at the observed high rates. Eventually, the plume reservoir is exhausted, and normal temperature asthenosphere mantle starts to rise (and melt) below the rift. |
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