Passive Continental Margins: Evidence for a Prebreakup Deep Crustal Metamorphic Subsidence Mechanism

Passive continental margins in the Australian region provide evidence of very extensive sedimentation and basement subsidence both before and after the time of continental breakup. Postbreakup sedimentation is largely marine, extends to deep water and follows an exponential or thermal cooling (concentration) accumulation pattern. Here, late stage prebreakup sedimentation occurs within faulted troughs or grabens, and is described as rift phase. All prebreakup sedimentation is non marine to shallow marine, and available vitrinite reflectance data suggests deposition in a relatively low heatflow environment. Continental crustal refraction velocities in the range 7.1 to 7.5 kms/sec. are observed beneath those margins studied, and the refractor rises markedly towards the continent-ocean boundary. The refractor is interpreted as the greenschist-amphibolite metamorphic facies boundary which has risen in response to increasing thermal conditions up until breakup time. A quantitative basement subsidence and palaeoheatflow model has been constructed in which a simple temperature anomaly deep within the lithosphere during the prebreakup period gives rise to the observed complex subsidence pattern plus low vitrinite reflectance values

Continental margin fault pattern mapped south-west of Ireland

A SIMPLE model for continental basement structures at rifted continental margins comprises large fault blocks which trend approximately parallel to, and step down towards, the continental–ocean boundary (for example, see ref. 1). These blocks may be cut by faults which strike across the margin, and, in many theoretical discussions, are shown as being separated from the true oceanic crust by an intermediate zone (see transitional crust of Fig. 3, ref. 2). On many rifted margins these features are deeply buried by young sediments and cannot be stutied in detail. On Goban Spur (Fig. 1), a marginal plateau south-west of Ireland, the young sediment cover is abnormally thin, however, and we have been able to map in detail a 150 km wide continental basement fracture pattern of horsts and grabens using a simple seismic reflection system (160 inch3 air-gun and two-channel hydrophone array). We also suggest a location for the continent–ocean boundary between the Spur and Porcupine Abyssal Plain. There are few previously published data from Goban Spur relevant to our study, although valuable sampling3 and geophysical3–5 results have been obtained north and south of the area

Accumulation of raft lipids in T-cell plasma membrane domains engaged in TCR signalling.

Activating stimuli for T lymphocytes are transmitted through plasma membrane domains that form at T-cell antigen receptor (TCR) signalling foci. Here, we determined the molecular lipid composition of immunoisolated TCR activation domains. We observed that they accumulate cholesterol, sphingomyelin and saturated phosphatidylcholine species as compared with control plasma membrane fragments. This provides, for the first time, direct evidence that TCR activation domains comprise a distinct molecular lipid composition reminiscent of liquid-ordered raft phases in model membranes. Interestingly, TCR activation domains were also enriched in plasmenyl phosphatidylethanolamine and phosphatidylserine. Modulating the T-cell lipidome with polyunsaturated fatty acids impaired the plasma membrane condensation at TCR signalling foci and resulted in a perturbed molecular lipid composition. These results correlate the accumulation of specific molecular lipid species with the specific plasma membrane condensation at sites of TCR activation and with early TCR activation responses

HORIZONTAL SHEAR-FLOW IN THE MANTLE BENEATH THE TONGA ARC

In the Tonga subduction zone, a systematic shear deformation, in which deep material moves relatively to the south, is superimposed on the mode of down-dip shortening which is characteristic of the region. We interpret this observation, together with a tectonic reconstruction, in terms of the interaction between the subducting lithospheric slab and a horizontal shear flow in the mantle. © 1986 Nature Publishing Group