John R. Hopper

Crustal structure of the southeast Greenland margin from joint refraction and reflection seismic tomography

We present results from a combined multichannel seismic reflection (MCS) and wideangle onshore/offshore seismic experiment conducted in 1996 across the southeast Greenland continental margin. A new seismic tomographic method is developed to jointly invert refraction and reflection travel times for a two-dimensional velocity structure. We employ a hybrid ray-tracing scheme based on the graph method and the local ray-bending refinement to efficiently obtain an accurate forward solution, and we employ smoothing and optional damping constraints to regularize an iterative inversion. We invert 2318 Pg and 2078 PmP travel times to construct a compressional velocity model for the 350-km-long transect, and a long-wavelength structure with strong lateral heterogeneity is recovered, including (1) ∼30-km-thick, undeformed continental crust with a velocity of 6.0 to 7.0 km/s near the landward end, (2) 30- to 15-km-thick igneous crust within a 150-km-wide continent-ocean transition zone, and (3) 15- to 9-km-thick oceanic crust toward the seaward end. The thickness of the igneous upper crust characterized by a high-velocity gradient also varies from 6 km within the transition zone to ∼3 km seaward. The bottom half of the lower crust generally has a velocity higher than 7.0 km/s, reaching a maximum of 7.2 to 7.5 km/s at the Moho. A nonlinear Monte Carlo uncertainty analysis is performed to estimate the a posteriori model variance, showing that most velocity and depth nodes are well determined with one standard deviation of 0.05–0.10 km/s and 0.25–1.5 km, respectively. Despite significant variation in crustal thickness, the mean velocity of the igneous crust, which serves as a proxy for the bulk crustal composition, is surprisingly constant (∼7.0 km/s) along the transect. On the basis of a mantle melting model incorporating the effect of active mantle upwelling, this velocity-thickness relationship is used to constrain the mantle melting process during the breakup of Greenland and Europe. Our result is consistent with a nearly constant mantle potential temperature of 1270–1340°C throughout the rifting but with a rapid transition in the style of mantle upwelling, from vigorous active upwelling during the initial rifting phase to passive upwelling in the later phase.

The effect of lower crustal flow on continental extension and passive margin formation

The great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting. To investigate how these and other factors affect rifting and the development of passive continental margins, we develop a simplified model of lithospheric extension. We consider the evolution of extensional deformation for a three-layer model lithosphere bounded laterally by much stronger lithosphere. The cold part of the crust and mantle are treated as thin brittle/plastic layers. The lower crust is approximated as a thin viscous channel. Each brittle/plastic layer can extend in only one location determined by the strength of the layer, shear of the lower crust, and buoyancy forces related to both crustal thickness variations and thermally induced density differences. The lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle/plastic layers are horizontally offset, a situation we term shear decoupling. As in previous studies, we see three distinct patterns, or modes, of extensional deformation that occur under different sets of model parameters: the core complex mode, the wide rift mode, and the narrow rift mode. Shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values. We are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions. We show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas. Predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins, including very wide and asymmetric margins.

Continental breakup and the onset of ultraslow seafloor spreading off Flemish Cap on the Newfoundland rifted margin

Prestack depth-migrated seismic reflection data collected off Flemish Cap on the Newfoundland margin show a structure of abruptly thinning continental crust that leads into an oceanic accretion system. Within continental crust, there is no clear evidence for detachment surfaces analogous to the S reflection off the conjugate Galicia Bank margin, demonstrating a first-order asymmetry in final rift development. Anomalously thin (3–4 km), magmatically produced oceanic crust abuts very thin continental crust and is highly tectonized. This indicates that initial accretion of the oceanic crust was in a magma-limited setting similar to present-day ultraslow spreading environments. Seaward, oceanic crust thins to <1.3 km and exhibits an unusual, highly reflective layering. We propose that a period of magma starvation led to exhumation of mantle in an oceanic core complex that was subsequently buried by deep-marine sheet flows to form this layering. Subsequent seafloor spreading formed normal, ∼6-km-thick oceanic crust. This interpretation implies large fluctuations in the available melt supply during the early stages of seafloor spreading before a more typical slow-spreading system was established.

Contrasting rifted margin styles south of Greenland: implications for mantle plume dynamics

Abstract We present new and reprocessed seismic reflection data from the area where the southeast and southwest Greenland margins intersected to form a triple junction south of Greenland in the early Tertiary. During breakup at 56 Ma, thick igneous crust was accreted along the entire 1300-km-long southeast Greenland margin from the Greenland Iceland Ridge to, and possibly ∼100 km beyond, the triple junction into the Labrador Sea. However, highly extended and thin crust 250 km to the west of the triple junction suggests that magmatically starved crustal formation occurred on the southwest Greenland margin at the same time. Thus, a transition from a volcanic to a non-volcanic margin over only 100–200 km is observed. Magmatism related to the impact of the Iceland plume below the North Atlantic around 61 Ma is known from central-west and southeast Greenland. The new seismic data also suggest the presence of a small volcanic plateau of similar age close to the triple junction. The extent of initial plume-related volcanism inferred from these observations is explained by a model of lateral flow of plume material that is guided by relief at the base of the lithosphere. Plume mantle is channelled to great distances provided that significant melting does not take place. Melting causes cooling and dehydration of the plume mantle. The associated viscosity increase acts against lateral flow and restricts plume material to its point of entry into an actively spreading rift. We further suggest that thick Archaean lithosphere blocked direct flow of plume material into the magma-starved southwest Greenland margin while the plume was free to flow into the central west and east Greenland margins. The model is consistent with a plume layer that is only moderately hotter, ∼100–200°C, than ambient mantle temperature, and has a thickness comparable to lithospheric thickness variations, ∼50–100 km. Lithospheric architecture, the timing of continental rifting and viscosity changes due to melting of the plume material are therefore critical parameters for understanding the distribution of magmatism.

Styles of extensional decoupling

Simplified models of continental lithospheric extension demonstrate that the strength of the lower crust is an important factor in the evolution of rifting. When the lower crust is strong, both the crust and the mantle lithosphere should extend in the same place. When the lower crust is weak, however, the upper crust can mechanically decouple from the lithospheric mantle during extension. Two distinct styles of extensional decoupling can be recognized. Diffuse decoupling occurs when the lower crust flows laterally in response to topographically induced pressure gradients. Offset decoupling occurs when stretching of the upper crust is horizontally displaced from mantle lithospheric stretching. We show that diffuse decoupling is expected for a range of possible crustal mineralogies, but that offset decoupling only occurs for extremely weak mineralogies.

The initiation of rifting at constant tectonic force: Role of diffusion creep

We have investigated the effect of diffusion creep on lithospheric extension using a one-dimensional numerical model that assumes a constant force is available to drive extension. The model is motivated by the fact that continental areas with average heat flow should be too strong to rift using standard estimates of lithospheric strength. However, such areas do rift and diffusion creep is a mechanism by which lithosphere may deform at a lower stress level than is required for the usually assumed dislocation creep. We consider the evolution of strain rate and temperature at the center of an idealized pure shear rift. The strain rate will either increase or decrease with time depending on whether lithospheric weakening or strengthening dominates as extension progresses. Given an initial thermal condition and an assumed lithospheric rheology, the applied force must be such that the initial strain rate is greater than some critical value for weakening to dominate. The force at which this condition is met we will term the critical force. Diffusion creep is more efficient at smaller grain sizes and the model results indicate that mantle grain sizes would have to be less than 1 mm for diffusion creep to significantly reduce the critical force. We speculate that during the initial stages of continental rifting, grain size reduction and diffusion creep deformation mechanisms may have an important effect on lithospheric strength.

Crustal structure at the SE Greenland margin from wide-angle and normal incidence seismic data

Abstract Results from a seismic refraction and reflection line along the southeast coast of Greenland give information on both the Precambrian structures on the Greenland continent and on the effects of the Tertiary breakup of the North Atlantic. Three seismic stations on the Greenland coast recorded the airgun shots from a 279-km reflection seismic line approximately 20 km offshore. The maximum offset recorded was 313 km. The wide-angle data show crustal thickness variation from 39 km in the south to 49 km in the north, with an 8- to 17-km-thick, high-velocity (7.5 km/s) layer at the base of the crust, interpreted as underplating related to the opening of the North Atlantic in the Tertiary. The boundary between the early Proterozoic Ketilidian orogen in the south and the Archaean block to the north show little variation in seismic velocities, and the reflection pattern suggests that the Archaean underlies the Ketilidian at depth. We see no evidence that the Julianehab Batholith at the boundary between rocks of the Ketilidian orogen and the North Atlantic block is a deep structure.

Lithospheric controls on melt production during continental breakup at slow rates of extension: Application to the North Atlantic

Rifted margins form from extension and breakup of the continental lithosphere. If this extension is coeval with a region of hotter lithosphere, then it is generally assumed that a volcanic margin would follow. Here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly. We find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than ? 50mmyr?1, the lithosphere acts as a lid to the hot material. The thermal anomaly cools significantly by conduction before having an effect on decompression melt production. If the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting. In the North Atlantic a series of extensional basins off the coast of northwest Europe and Greenland provide the required thinning. This observation suggests that volcanic margins that show slow rates of extension, only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere.

The Jan Mayen microcontinent: an update of its architecture, structural development and role during the transition from the Ægir Ridge to the mid-oceanic Kolbeinsey Ridge

Abstract We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinents Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the Ægir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinents southern flank. At C13n (33.2 Ma), spreading rates along the Ægir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinents NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the Ægir Ridge was abandoned between C7 and C6b (24–21.56 Ma).

Structure of the Flemish Cap margin, Newfoundland: insights into mantle and crustal processes during continental breakup

Abstract Seismic reflection and refraction data from the Flemish Cap margin off Newfoundland reveal the large-scale structure of a magma-starved rifted margin. There is little evidence for significant extensional deformation of the Flemish Cap, consistent with the hypothesis that it behaved as a microplate throughout the Mesozoic. The seismic data highlight important asymmetries at a variety of scales that developed during the final stages of continental breakup and the onset of oceanic sea-floor spreading. In strong contrast to the conjugate Galicia Bank margin, Flemish Cap shows: (1) an abrupt necking profile in continental crust, thinning from 30 km thick to 3 km thick over a distance of 80 km, and a narrow, less than 20 km-wide, zone of extremely thin continental crust; (2) no clear evidence for horizontal detachment structures beneath continental crust similar to the ‘S’ reflection; and (3) evidence for at least a 60 km-wide zone of anomalously thin oceanic crust that began accreting to the margin shortly after continental crustal separation. The oceanic crust averages only 3–4 km thick and in places is as thin as 1.3 km thick, although seismic layer 3 is missing where this occurs. The data suggest that there are large spatial and temporal variations in the available melt supply following continental breakup as oceanic sea-floor spreading becomes established. In addition, wide-angle data show that anomalously slow mantle P-wave velocities appear approximately where continental crust has thinned to 6–8 km thick, indicating that low-degree serpentinization begins where the entire crust has become embrittled.