Seismic reflection models of continental crust based on metamorphic terrains

Summary. Laboratory seismic velocity measurements on rock samples from metamorphic terrains, coupled with geologic cross sections, provide the basis for synthetic seismic reflection profiles for various types of continental crust. Results from greenstone belts, mylonite zones and partial cross sections of continental crust indicate that lithologic heterogeneity and geometrical factors control crustal reflection characteristics.     

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

We report results from the Seismic Wide-Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the information from a conventional deep multichannel reflection profile and a coincident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide-angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide-angle data, and an excellent match was obtained. It was possible to fine-tune velocities in less well-resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth-convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward-dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Ocean.     

Determination of the lower edges of magnetized bodies by using geothermal data

The determination of the lower edges of magnetized bodies in the Earth's crust is a complex geophysical problem, although these values can be estimated by using geothermal data. An analysis of the temperature regime and location of the lower edges of magnetized bodies has been carried out for the geosynclinal region of the southern Caucasus and the area joining the ancient platform with the Arabian Shield in Israel. Geothermal calculations for Israel have been performed for three models of the thermal regime for the Earth's crust and upper mantle. The process of ultrabasic rock serpentinization is accompanied by the transformation of iron suboxide to iron oxide. Both these processes run under identical thermodynamic conditions within an average temperature interval of 200°-400°C. The Curie surface controls the position of lower edges only in fault zones where oxidation conditions hold up to great depths.     

The Acre Basin basement (NW Brazil) and the transition from the intracratonic to retroarc foreland basin system

The understanding of the crustal transition between orogenic zones and cratonic portions in distal regions of foreland basins has received increasing attention, but the analysis is often hampered by the sedimentary cover. Despite the peculiar location of the Acre Basin, specifically between the Amazonian Craton and the sub-Andean zone, local basement studies are still scarce due to lacking seismic data and exploratory wells. Therefore, this work aims to map basement depths, estimate crustal compositions and identify the main depocenters, structures and limits of Acre Basin using an integrated analysis to understand better the region lithospheric evolution, its relationship with the Amazonian Craton and its positioning within the Andean orogeny. For that, we used well, 2D seismic reflection, airborne and ground gravity and magnetic data as well as the EMG2008. Tilt Depth estimates indicate basement depths between 500 and 7800 m and larger sedimentary thicknesses in the northern portion. Additionally, we modelled groups of potential sources between 0.1 and 22 km and Moho depths between 26 and 37 km. Compositionally, the upper crust consists dominantly of meta-sedimentary and low-grade metamorphic rocks and granites, indicating that the sub-Andean and Acre Basins share a similar basement. Thus, there are indications that the basement of the Acre Basin is essentially formed by the Sunsás province in the Amazonian Craton. However, local differences in basement depth, magnetic susceptibility and exploratory potential led to the subdivision into Divisor and Xapuri sub-basins, north and south of the Fitzcarrald Arch, respectively. Finally, it was possible to establish the limits of the Andean orogeny influence in the Acre Basin and delimit the area of the Western Amazon Foredeep installed during the Neogene.     

Electrical conductivity and crustal structure beneath the central Hellenides around the Gulf of Corinth (Greece) and their relationship with the seismotectonics

A deep magnetotelluric sounding (MTS) investigation in the western part of the Gulf of Corinth has revealed a complex electrical image of the crustal structure. The geotectonic structure of the Parnassos unit and the Transition zone in the central Hellenides, overthrusting the Pindos zone both towards the west and towards the south, has been clearly identified by its higher resistivity and its intrinsic anisotropy related to the N–S strike of the Hellenides range. Subsequent N–S extension of the Gulf introduced another heterogeneous anisotropy characteristic that corresponds to E–W-trending normal faults on both sides of the Gulf. The 2-D modelling of the MTS results reveals the existence of a relatively conductive layer about 4 km thick at a depth greater than 10 km in the middle crust. It corresponds to a ductile detachment zone suggested by microseismic and seismic studies ( King et al . 1985 ; Rigo et al . 1996 ; Bernard et al . 1997a ). It may be attributed to the phyllite series lying between the allochthonous Hellenic nappes and the autochthonous Plattenkalk basement. Towards the east, under the Pangalos peninsula, approaching the internal Hellenides, the detachment zone could root deeply into the lower crust.
Some strong local electrical anomalies are observed, reaching the conductive layer in the middle crust, such as that under the Mamousia fault and under the front of the overthrust of the Transition zone on the Pindos zone. Other anomalies affect only the shallower zones such as that beneath the Helike fault and in the Psaromita peninsula. These shallower anomalies provide complementary information to the study of spatial and temporal variations of the seismic anisotropy in relation to the short- and long-term tectonic activity of the Gulf ( Bouin et al . 1996 ; Gamar et al . 1999 ).     

Preliminary deep reflection studies in the Arunta Block, Central Australia

Summary. In 1985, near-vertical incidence reflection profiling was carried out across the Arunta Block in Central Australia. This region consists of exposed Proterozoic metasediments, granites and granulites. There is usually a limited sedimentary coverage generated by deep weathering. The seismic sections for the deep crust are markedly different from those previously recorded in Eastern Australia where there is extensive sedimentary cover. One of the striking features is the presence of energy with frequencies as high as 100 Hz at two-way times of 5-6 s. Reflections are found throughout the crust, and there is no zone that can be characterised as non-reflective. The strongest reflectors commonly lie in the intervals around 4-6 s and 8–11 s and display significant dip. Individual shot records show fairly rapid variations in amplitude and waveform within a reflection band and the correlation between records from adjacent shots can also be somewhat limited. Such features are not well suited to the application of standard processing techniques designed for subhorizontal structures, and call into question the utility of conventional stacking. The character of the reflections changes markedly with varying frequency which suggests that they arise by interference phenomena, probably associated with laterally varying lamellar structures.     

Lithoprobe seismic reflection profiling across Vancouver Island: results from reprocessing

Summary. Multichannel seismic reflection sections recorded across Vancouver Island have revealed two extensive zones of deep seismic reflections that dip gently to the northeast, and a number of moderate northeasterly dipping reflections that can be traced to the surface where major faults are exposed. Based on an integrated interpretation of these data with information from gravity, heat flow, seismicity, seismic refraction, magnetotelluric and geological studies it is concluded that the lower zone of gently dipping reflections is due to underplated oceanic sediments and igneous rocks associated with the current subduction of the Juan de Fuca plate, and that the upper zone represents a similar sequence of accreted rocks associated with an earlier episode of subduction. The high density/high velocity material between the two reflection zones is either an underplated slab of oceanic lithosphere or an imbricated package of mafic rocks. Reprocessing of data from two of the seismic lines has produced a remarkable image of the terrane bounding Leech River fault, with its dip undulating from >60° near the surface to 20° at 3 km depth and ∼38° at 6 km depth.     

The synthetic seismic expression of the Messinian salinity crisis from onshore records: Implications for shallow‐ to deep‐water correlations

The onshore–offshore correlation of sedimentary successions is a common problem in basin analysis, but it becomes critical for the full understanding of the Messinian salinity crisis (MSC), a complex array of palaeoenvironmental events which affected the Mediterranean basin at the end of the Miocene. The outcrop records show that the Messinian stratigraphic architectures may be highly complex as the deposits of the different MSC evolutionary stages can be lithologically similar and separated by erosional surfaces and/or morphostructural highs. The correct definition of the nature and stratigraphic position of Messinian deposits in offshore areas through seismic data may be almost impossible, especially where core data are sparse. To bridge the gap between onshore and offshore records, we have built synthetic seismic sections from well‐constrained outcrop successions. Our results provide useful insights and warnings for the interpretation of offshore data, pointing out that MSC units having different age, nature and depositional settings, may show similar seismic facies and geometries. Conversely, the same deposit may result in different seismic facies, either with parallel and high‐amplitude reflections or even transparent or chaotic due to interference patterns of seismic reflections related to dominant frequency. It follows that a correct interpretation of the nature and age of deep‐seated Messinian deposits can only be obtained through the integration of seismic and core data, and considering the onshore record. The application of our approach to the Balearic Promontory results in an alternative interpretation with respect to previous models. We show that this offshore area has good analogues in the onshore of the Betic Cordillera and includes both shallow and intermediate depth sub‐basins that underwent a strong post‐Messinian subsidence.     

Magnetotelluric interpretation of crustal and mantle structure in the Grenville Province

Summary Accurate determinations of depths and conductivities of electrical structures in shield regions are often difficult because of the inhomogeneity of the uppermost crust. A magnetotelluric (MT) station (BAT) in the Grenville Province of the Precambrian Shield in eastern Canada has been in operation since 1975 for time-dependency studies of electrical resistivity changes related to earthquakes. The MT response of the station displays low skew with small to moderate anisotropy. One-dimensional inversion of the apparent resistivity and phase reveals two well-defined conductors in the crust, one at 10 km and the second at the base of the crust. The latter has a resistivity less than 50 Ω m. These results are substantiated by three additional MT stations located up to 40 km distant.
Data from other new MT stations and from stations previously published in the literature are compared with two-dimensional computer model results and with the three-dimensional analogue scale model results of Dosso et al. While additional data for periods less than 100 s would be desirable the results from a number of the MT stations are not inconsistent with a widespread occurrence of a conducting zone at the base of the crust in the Grenville. The inversion analysis also indicates the existence of a conductor at some depth greater than 100 km with a resistivity less than 30 Ω m. This may coincide with a seismic low-velocity zone observed in the mantle under the Canadian Shield.     

The transition from subduction to continental collision: crustal structure in the North Canterbury region, New Zealand

The North Canterbury region marks the transition from Pacific plate subduction to continental collision in the South Island of New Zealand. Details of the seismicity, structure and tectonics of this region have been revealed by an 11-week microearthquake survey using 24 portable digital seismographs. Arrival time data from a well-recorded subset of microearthquakes have been combined with those from three explosions at the corners of the microearthquake network in a simultaneous inversion for both hypocentres and velocity structure. The velocity structure is consistent with the crust in North Canterbury being an extension of the converging Chatham Rise. The crust is about 27 km thick, and consists of an 11 km thick seismic upper crust and 7 km thick seismic lower crust, with the middle part of the crust being relatively aseismic. Seismic velocities are consistent with the upper and middle crust being composed of greywacke and schist respectively, while several lines of evidence suggest that the lower crust is the lower part of the old oceanic crust on which the overlying rocks were originally deposited.
The distribution of relocated earthquakes deeper than 15 km indicates that the seismic lower crust changes dip markedly near 43S. To the south-west it is subhorizontal, while to the north-east it dips north-west at about 10. Fault-plane solutions for these earthquakes also change near 43S. For events to the south, P -axes trend approximately normal to the plate boundary (reflecting continental collision), while for events to the north, T -axes are aligned down the dip of the subducted plate (reflecting slab pull). While lithospheric subduction is continuous across the transition, it is not clear whether the lower crust near 43S is flexed or torn.     

Seismic modeling of Tertiary sandstone clinothems, Spitsbergen

Two-dimensional seismic modelling has been undertaken on an overall progradational succession of sloping mudstone and sandstone units from the Palaeogene of Spitsbergen. The modelling shows that the main geometric features of the section would be resolved at 1500 m depth (with frequencies below 60 Hz, which is common in seismic data at these depths). However, interference between the base and top of lithological units gives lateral amplitude variations and discrepancies between the seismic image and the geometrical model. This is particularly prominent in low-frequency models. Terminations of reflectors, resembling toplap and onlap, may be interpreted, but are artefacts of the general convergence of lithological units present in the geometrical model. The geological section causes a seismic pattern resembling sigmoid progradational seismic facies. Two-dimensional seismic modelling is an efficient tool in bridging the gap between outcrop observations and subsurface data. Hence, modelled outcrop sections are important as reference points' for improved seismic stratigraphic interpretation.     

Crustal underplating and its implications for subsidence and state of isostasy along the Ninetyeast Ridge hotspot trail

Recent seismic field work has revealed high lower-crustal velocities under Ninetyeast Ridge, Indian Ocean, indicating the presence of crustal underplating ( Grevemeyer et al . 2000 ). We used results from Ocean Drilling Program (ODP) drill cores and cross-spectral analysis of gravity and bathymetric data to study the impact of the underplating body on the subsidence history and the mode of isostatic compensation along Ninetyeast Ridge. Compared with the adjacent Indian basin, the subsidence of Ninetyeast Ridge is profoundly anomalous. Within the first few millions of years after crustal emplacement the ridge subsided rapidly. Thereafter, however, subsidence slowed down significantly. The most reliable model of isostasy suggests loading of a thin elastic plate on and beneath the seafloor. Isostatic compensation of subsurface loading occurs at a depth of about 25 km, which is in reasonably good agreement with seismic constraints. Subsurface loading is inherently associated with buoyant forces acting on the lithosphere. The low subsidence may therefore be the superposition of cooling of the lithosphere and uplift due to buoyant material added at the base of the crust. A model including prolonged crustal growth in the form of subcrustal plutonism may account for all observations.     

Late Archaean to Palaeoproterozoic geotherms in the Kaapvaal craton, South Africa: constraints on the thermal evolution of the Witwatersrand Basin

The c. 2.97–2.71 Ga Witwatersrand Basin located in the Kaapvaal craton of South Africa represents a remnant of a large Late Archaean sedimentary basin that hosts the world's premier gold deposit within a series of conglomerate horizons. Evidence of postdepositional gold mobility within these conglomerates associated with hydrothermal–metamorphic activity has led to speculation about the Late Archaean to Palaeoproterozoic geothermal gradients in the basin. We use surface heat flow and heat production data from rocks in the basin and its environs in order to calculate detailed temperature profiles for the central Kaapvaal craton that show that the steady state crustal geotherm during the Late Archaean and Palaeoproterozoic was relatively cool at 15–20 K km^?1. The geotherm in the upper crustal strata is also largely unaffected by substantial increases in the heat flow into the base of the crust. Consequently, regional greenschist facies metamorphism of the basin sediments could only have been achieved during a transient thermal event that advected heat into the upper crust. The most likely candidate for this is the Bushveld magmatic event at 2.06 Ga.     

Crustal and uppermost mantle structure in southern Africa revealed from ambient noise and teleseismic tomography

Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s^–1 at 0–10 km depths to 3.86 km s^–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.     

Crustal thickening under the palaeo-meso-Proterozoic Delhi Fold Belt in northwestern India: evidence from deep reflection profiling

The results of deep reflection profiling studies carried out across the palaeo-meso-Proterozoic Delhi Fold Belt (DFB) and the Archaean Bhilwara Gneissic Complex (BGC) in the northwest Indian platform are discussed in this paper. This region is a zone of Proterozoic collision. The collision appears to be responsible for listric faults in the upper crust, which represent the boundaries of the Delhi exposures. In these blocks the lower crust appears to lie NW of the respective surface exposures and the reflectivity pattern does not correspond to the exposed blocks. A fairly reflective lower crust northwest of the DFB exposures appears to be the downward continuation of the DFB upper crust. The poorly reflective lower crust under the exposed DFB may be the westward extension of the BGC upper crust at depth. Thus, the lower crust in this region can be divided into the fairly reflective Marwar Basin (MB)-DFB crust and a poorly reflective BGC crust. Vertically oriented igneous intrusions may have disturbed the lamellar lower-crustal structure of the BGC, resulting in a dome-shaped poorly reflective lower crust whose base, not traceable in the reflection data, may have a maximum depth of about 50 km, as indicated by the gravity modelling.
The DFB appears to be a zone of thick (45-50 km) crust where the lower crust has doubled in width. This has resulted in three Moho reflection bands, two of which are dipping SE from 12.5 to 15.0 s two-way time (TWT) and from 14.5 to 16.0 s TWT. Another band of subhorizontal Moho reflections, at ≈ 12.5 s TWT, may have developed during the crustal perturbations related to a post-Delhi tectonic orogeny. The signatures of the Proterozoic collision, in the form of strong SE-dipping reflections in the lower crust and Moho, have been preserved in the DFB, indicating that the crust here has not undergone any significant ductile deformation since at least after the Delhi rifting event.     

MT and reflection: an essential combination

Summary. At many localities in the world there have been coincident comprehensive electromagnetic (EM) studies and seismic reflection profiles conducted. Unfortunately, over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information. This paper is an attempt to convince the reader that a collocated magnetotelluric (MT) study should, in almost every case, be made wherever a seismic reflection survey is undertaken. Examples are shown from six studies in which the EM results aided the geological/tectonic interpretations of the seismic sections.
Also, difficulties with the MT technique are discussed, and the interpretations of conducting zones within the lower crust are examined. Finally, a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust.     

A hypothesis for the seismogenesis of a double seismic zone

The seismogenesis of a double seismic zone, in particular the lower layer of a double seismic zone, has not been adequately explained in the literature. On the basis of seismic data and geothermal structures along three well-studied cross-sections in the Kuril-Kamchatka and Japan subduction zones, we investigate the temperature/pressure conditions associated with seismogenic structures of the double seismic zones. the corresponding T/P loci seem to suggest that earthquakes observed in the lower layer and in the lower part (below approximately 130 ± 20 km) of the top layer of a double seismic zone were caused by metastable phase transition-a mechanism similar to that responsible for deep-focus earthquakes only at lower temperature/pressure conditions. Under this hypothesis, the wedge-shaped configuration of a double seismic zone is interpreted to represent the loci of the kinetic boundary of the phase transition. According to theoretical/experimental studies and the constraints imposed by our observations, a likely candidate for such a phase transition is the metastable Al-rich enstatite decomposing into the assemblage of Al-poor enstatite plus garnet. Earthquakes in the upper part of the top layer were most probably due to conventional mechanisms such as dehydration of subducted materials and/or facies change from basalt to eclogite. That the top layer involves more than one seismogenic mechanism is also implied by the distinct behaviour of seismicity in the vicinity of 130 ± 20 km. Because the presence of deviatoric stress is critical to the reaction rate of a metastable phase transition, it is inferred that single seismic zones are also caused by the same mechanisms, except that the implicit layer of a supposed double seismic zone is missing, due to the insufficient amount of appropriate metastable minerals or to the lack of appropriate deviatoric stresses in the source region.     

Metamorphism of the Mullerneset Formation, St. Jonsfjorden, Svalbard

The metamorphism of upper greenschist facies metasediments exposed in the extreme southwestern portion of St. Jonsjorden, Svalbard, is described. The rocks form part of the Mullerneset Formation of the late Precambrian age Kongsvegen Group and constitute a portion of the central-western Spitsbergen Cale-donides. Four deformations (D, -D4) and two metamorphic episodes (Mi and M2) have affected the rocks of the Mullerneset area. Mi was a prograde event which was initiated prior to the onset of the Di and continued through this deformation. Pre-Dt metamorphism reached biotite grade whereas garnet grade was attained syn-Di. M2 was a lower-middle greenschist facies metamorphism associated with D2. The results of quantitative geothermometry in the pelitic rocks show that peak Mi metamorphic temperatures decrease southwards across the field area from about 540°C to 510°C. Geobarometry and estimates of depth of burial indicate that Mi pressures were in the range of 5–7 kb. The data are consistent with geothermal gradients in the range of 21 ± 4°C/km to 24 ± 5°C/km. M2 metamorphic conditions are not precisely determinable but temperatures and pressures were probably less than those attained during Mi. It is suggested that the rocks of central-western Spitsbergen were originally deposited in an aulacogen before the initiation of Caledonian diastrophism.     

The reflection seismic survey of project TIPTEQ—the inventory of the Chilean subduction zone at 38.2° S

We describe results of an active-source seismology experiment across the Chilean subduction zone at 38.2°S. The seismic sections clearly show the subducted Nazca plate with varying reflectivity. Below the coast the plate interface occurs at 25 km depth as the sharp lower boundary of a 2–5 km thick, highly reflective region, which we interpret as the subduction channel, that is, a zone of subducted material with a velocity gradient with respect to the upper and lower plate. Further downdip along the seismogenic coupling zone the reflectivity decreases in the area of the presumed 1960 Valdivia hypocentre. The plate interface itself can be traced further down to depths of 50–60 km below the Central Valley. We observe strong reflectivity at the plate interface as well as in the continental mantle wedge. The sections also show a segmented forearc crust in the overriding South American plate. Major features in the accretionary wedge, such as the Lanalhue fault zone, can be identified. At the eastern end of the profile a bright west-dipping reflector lies perpendicular to the plate interface and may be linked to the volcanic arc.     

Rapid outer marginal collapse at the rift to drift transition of passive margin evolution,with a Gulf of Mexico case study

Interpretation of long‐offset 2D depth‐imaged seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading and thermal subsidence of the margin. This collapse post‐dates rifting and stretching of the crust, but occurs roughly ten times faster than thermal subsidence of young oceanic crust, and thus is tectonic and pre‐dates the ‘drift stage’. We term this middle stage of margin development ‘outer margin collapse’, and it accords with the exhumation stage of other authors. Outer continental margins, already thinned by rifting processes, become hanging walls of crustal‐scale half grabens associated with landward‐dipping shear zones and zones of low‐shear strength magma at the base of the thinned crust. The footwalls of the shear zones comprise serpentinized sub‐continental mantle that commonly becomes exhumed from beneath the embrittled continental margin. At magma‐poor margins, outer continental margins collapse and tilt basinward to depths of about 3 km subsea at the continent–ocean transition, often deeper than the adjacent oceanic crust (accreted later between 2 and 3 km). We use the term ‘collapse’ because of the apparent rapidity of deepening (<3 Myr). Rapid salt deposition, clastic sedimentation (deltaic), or magmatism (magmatic margins) may accompany collapse, with salt thicknesses reaching 5 km and volcanic piles 1525 km. This mechanism of rapid salt deposition allows mega‐salt basins to be deposited on end‐rift unconformities at global sea level, as opposed to deep, air‐filled sub‐sea depressions. Outer marginal collapse is ‘post‐rift’ from the perspective of faulting in the continental crust, but of tectonic, not of thermal, origin. Although this appears to be a global process, the Gulf of Mexico is an excellent example because regional stratigraphic and structural relations indicate that the pre‐salt rift basin was filled to sea level by syn‐rift strata, which helps to calibrate the rate and magnitude of collapse. We examine the role of outer marginal detachments in the formation of East India, southern Brazil and the Gulf of Mexico, and how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading. We suggest that backstripping estimates of lithospheric thinning (beta factor) at outer continental margins may be excessive because they probably attribute marginal collapse to thermal subsidence.