Abstract:
The Newfoundland-Iberia rift is a type example of a nonvolcanic rift. Rifting occurred in two phases: Late Triassic into Early Jurassic, and Late Jurassic through Early Cretaceous. During the first phase, extension occurred in a wide rift mode without continental separation. During the second phase, extension initially was spatially and temporally variable but eventually focused at future distal margins where it was succeeded by seafloor spreading. Second-phase spreading appears to have been concentrated in three episodes: (1) Late Jurassic-Berriasian rifting that culminated in separation of continental crust in the southern half of the rift, (2) Valanginian-Hauterivian rifting of continental crust in the northern rift that culminated in separation of continental crust there and was coincident with rifting of subcontinental mantle lithosphere in the southern rift, and (3) Barremian-Aptian rifting of mantle lithosphere that was at least partially subcontinental. We suggest that rising asthenosphere breached the mantle lithosphere and led to seafloor spreading near the Aptian/Albian boundary. This event is marked by a prominent seismic stratigraphic horizon ("Aptian event") in the sedimentary record of both margins. Extension and exhumation of mantle lithosphere in episodes 2 and 3 is not easily characterized as either continental rifting or seafloor spreading, and we propose that it be referred to as "transitional extension." Transitional extension during episode 3 (and possibly also during episode 2) was accompanied by intraplate extension of exhumed mantle and by minor magmatism, probably because a well defined plate boundary characterized by significant magmatic accretion had not been established and horizontal in-plane tensile stress was elevated throughout the plates. However, during the later part of episode 3 there appears to have been increasing magmatism and even formation of relatively normal ocean crust, at least locally. During the transition between episodes 2 and 3, the southern part of the rift was affected by magmatism related to a mantle plume that formed the Southeast Newfoundland Ridge, J Anomaly Ridge, Madeira-Tore Rise, and probably part of Gorringe Bank at the southern edge of the rift. M-series magnetic anomalies are observed over the peridotite basement exhumed during episodes 2 and 3. Compared to their counterparts in ocean crust of the main North Atlantic Basin to the south, anomalies older than approximately M1 are poorly developed, and our modeling suggests that they are best explained by magnetization of serpentinites rather than igneous rocks. The high-amplitude J Anomaly ( approximately M1-M0) is strongest at the southern edge of the rift, but the amplitude decreases northward to near the southern margin of Galicia Bank and is relatively constant farther to the north. The character of the anomaly most likely reflects the spatial distribution of magma that emanated from the plume at the southern margin of the rift. Significant postrift magmatism on the Newfoundland margin was expressed at Site 1276 as two diabase sills that intruded Aptian-Albian sediments. The upper sill dates to approximately 105 Ma (middle Albian) and the lower sill is approximately 98 Ma (early Cenomanian). The upper sill is intruded at the stratigraphic level of the Aptian event (U reflection in the Newfoundland Basin). Widespread distribution and high amplitude of the U reflection in the basin suggest that sills may be pervasive at this stratigraphic level, which may have been a level of hydrostatic equilibrium for intruding magmas. There is no known magmatism of comparable age and extent on the Iberia margin. The source of the Newfoundland magmas is postulated to be the Madeira and Canary hotspots. The Newfoundland Basin is thought to have passed over these hotspots at approximately 100-90 Ma before they were crossed by the Mid-Atlantic Ridge and subsequently affected the southernmost part of the Iberia plate and northwestern edge of the African plate. Key features of the sedimentary record in the Newfoundland-Iberia rift are similar to those of the main North Atlantic Basin to the south, although there are variations at individual drill sites that are related to, for example, input of shallow-water clastic debris and seafloor paleodepth. The sedimentary record cored to date reflects the following: 1. Tithonian-Berriasian shallow-water carbonate platforms, together with rift basins that accumulated detrital and hemipelagic sediments; 2. Valanginian-Hauterivian deposition of carbonate-rich sediments in rift basins above the calcite compensation depth (CCD); 3. Barremian-Cenomanian hemipelagic deposition below the CCD and under dysoxic to anoxic conditions, with the occurrence of at least six oceanic anoxic events; 4. Turonian-Paleocene deposition of reddish to multicolored sediments on a generally well oxygenated seafloor below the CCD; winnowing of fines from sediments at Site 1276 suggests strong abyssal circulation during Turonian to earliest Campanian time; recurrence of dark to black shales in the lower to middle Paleocene section indicates stagnant circulation and low-oxygen conditions in the deepest basins; and 5. Late Paleocene to Oligocene deposition of hemipelagic and pelagic (calcareous and siliceous) sediments, with the CCD at intermediate levels; initiation of strong abyssal circulation that has persisted to the present appears to have begun near the Eocene/ Oligocene boundary.