We present evidence that the drowning of the -150 m coral reef around Hawaii was caused by rapid sea-level rise associated with meltwater pulse 1A (MWP-1A) during the last deglaciation. New U/Th and C-14 accelerator mass spectrometry dates, combined with reinterpretation of existing radiometric dates, constrain the age of the coral reef to 15.2-14.7 ka (U/Th age), indicating that reef growth persisted for 4.3 k.y. following the end of the Last Glacial Maximum at 19 ka. The drowning age of the reef is roughly synchronous with the onset of MWP-1A between 14.7 and 14.2 ka. Dates from coralline algal material range from 14 to 10 cal ka (calibrated radiocarbon age), 1-4 k.y. younger than the coral ages. A paleoenvironmental reconstruction incorporating all available radiometric dates, high-resolution bathymetry, dive observations, and coralgal paleobathymetry data indicates a dramatic rise in sea level around Hawaii ca. 14.7 ka. Paleowater depths over the reef crest increased rapidly above a critical depth (30-40 m), drowning the shallow reef-building Porites corals and causing a shift to deep-water coralline algal growth, preserved as a crust on the drowned reef crest.

The upper surface of the Albian platform limestone on many Mid-Pacific Mountains (MPM) guyots, as shown by multibeamsonar, bathymetric data, and seismic-reflection data from where pelagic sediments have buried the Albian platform limestone, shows that the top of the limestone sequence has been sculptured by streams, by waves, and by dissolution during an episode of emergence to heights of as much as 180 m above sea level.Seismic-reflection profiles show that the surface is arched over the buried volcanic basement, reflecting progressive differential compaction of the limestone section over erosional relief on the basement.This compaction began during early stages of burial of limestone layers and continued after final drowning of the platform.On some guyots, differential erosion left a prominent perimeter rim several tens of meters high.This atoll-like erosional form was drowned in post-Albian, pre-mid-Turonian time, with no further accumulation of platform limestone.Petrographic and stable-isotopic studies of samples recovered in drill holes in the upper parts of the platform limestone on Allison and Resolution guyots show dissolution cavities tens of meters below the top of the limestone, some filled with pelagic sediments and others still open and lined with stalagmite-like calcite structures and cements showing evidence of accumulation in vadose environments.

Neptunian dykes and sills in Jurassic carbonate platform strata of the Betic region of Spain occupy spaces created by wholly mechanical fracturing and displacement of the host strata, and later filled by pelagic sediments from above or by precipitation of calcite from circulating solutions. In some places, joint-bounded blocks of platform carbonates have been wholly removed, possibly by sliding down submarine slopes, leaving a staircase topography, commonly Fe-encrusted, that was subsequently filled by pelagic sediments. Other cavities that formed during Cretaceous times were developed by dissolution and current erosion in moderately deep submarine environments, and then filled by pelagic sediments from above.None of the cavities hosted by either platform or pelagic strata contain evidence for their formation by dissolution in a subaerial environment. The Jurassic and Cretaceous history of subsidence of the Betic margins is thus simpler than in versions requiring repeated emergence to form subaerial karstic cavities.

The results of drilling on Resolution and Allison guyots, in the Mid-Pacific Mountains (MPM), document a long history of volcanism, subsidence, and accumulation of Hauterivian-Albian shallow-water carbonate sediments.Mid-Cretaceous emersion was followed by subsidence and accumulation of pelagic sediments.Basement beneath Resolution Guyot is subaerial flows of alkalic basalt, with radiometric dates averaging 127.6 ±2.1 Ma, emplaced at about 14°S.Overlying shallow-water carbonates are 1620 m thick.Sediments at Site 866, 2 km inward from the platform edge, were deposited in shallow subtidal to intertidal depths, with intermittent subaerial exposure.At Site 867, 0.5 km from the platform edge, beach and storm deposits are common, and at Site 868,0.1 km from the edge, sponges and rudists in life positions indicate a platform-margin environment.At Allison Guyot, alkalic basalt sills were cored at the bottom of Hole 865A.Seismic profiles suggest as much as 600 m of sediments underlie the sills, which have 40 Ar/ 39 Ar radiometric dates averaging 110.7 ±1.2 Ma.The drilled strata, 730 m thick, extend from near the base to nearly the top of the Albian.The section begins with about 200 m of clayey limestone deposited in quiet, swampy waters.Upward, clays gradually disappear, reflecting the burial of volcanic hills as the seamount subsided.The rest of the series is wackestone deposited in subtidal to intertidal environments.Core and logging data at both guyots show shallowing-upward cycles of 3 to 10 m thick.Fourier analysis yields estimates of about 100 ka as the most common frequency.Longer-term fluctuations in sea level are suggested by facies successions at a decameter scale.Diagenesis was dominated by dissolution of aragonite, and cements are now marine, low-magnesium calcite.Pore-water data show the entire succession to be open to modern seawater.Dolomite dominates in the lower 400 m of carbonate strata at Resolution Guyot, and Sr-isotope data suggest much of it formed 15 to 20 m.y. after deposition.Compaction of limestone over buried basement topography proceeded apace with deposition and continued after drowning of the guyots.During the latest Albian, a fall in relative sea level of nearly 200 m exposed limestone strata on both guyots to subaerial and wave erosion, but whether the cause was tectonic or eustatic is not yet known.By mid-Turonian time, the guyots had re-submerged, but only pelagic sediments accumulated.Why no further shallow-water sediments accumulated on the guyots is a mystery.They were at about 8°S (Resolution) to 11°S (Allison) at the time of emergence.Upper Cretaceous pelagic sediments are preserved only in cavities within Albian limestone.Eocene and Paleocene sediments on Allison Guyot, about 120 m thick, were deposited at near-equatorial latitudes.The Lower Cretaceous platform is variably encrusted with phosphorite and ferromanganese oxides, even where buried beneath pelagic sediments.Primary control on acoustic-wave velocity is from diagenetic changes in density.Chaotic reflections around the guyot edges may be caused by the thick ferromanganese pavements, rather than by massive reefs.Drilling showed that the perimeter rims are not framework reefs, but erosional features carved from sand shoals and lagoonal sediments.A significant residual remains in the free-air gravity anomalies over Resolution Guyot after subtracting topographic effects.Drilling results imply that most of this excess mass can be explained by the density contrast between dolomitized limestone and low-density sediments surrounding the edifice.Inversion of magnetic anomalies over MPM seamounts gives a wide geographic scatter in paleomagnetic pole location.Poles near the geomagnetic pole can be attributed to induced magnetization.Some seamounts have complex anomalies perhaps explained by a complex magnetization structure containing magnetic reversals.

The Ontong Java Plateau, a large, deep-water carbonate plateau in the western equatorial Pacific, is an ideal location for studying responses of carbonate sedimentation to the effects of changing paleoceanographic conditions.These carbonate responses are often reflected in the physical properties of the sediment, which in turn control the appearance of seismic reflection profiles.Seismic stratigraphy analyses, correlating eight reflector horizons to each drill site, have been conducted in an attempt to map stratigraphic data.Accurate correlation of seismic stratigraphic data to drilling results requires conversion of traveltime to depth in meters.Synthetic seismogram models, using shipboard physical properties data, have been generated in an attempt to provide this correlation.Physical properties, including laboratory-measured and well-log data, were collected from sites drilled during Deep Sea Drilling Project Legs 30 and 89, and Ocean Drilling Program Leg 130, on the top and flank of the Ontong Java Plateau.Laboratory-measured density is corrected to in-situ conditions by accounting for porosity rebound resulting from removal of the sediment from its overburden.The correction of laboratory-measured compressional velocity to in situ appears to be largely a function of increases in elastic moduli (especially shear rigidity) with depth of burial, more than a function of changes in temperature, pressure, or density (porosity rebound).Well-log velocity and density data for the ooze intervals were found to be greatly affected by drilling disturbance; hence, they were disregarded and replaced by lab data for these intervals.Velocity and density data were used to produce synthetic seismograms.Correlation of seismic reflection data with synthetic data, and hence with depth below seafloor, at each drill site shows that a single velocity-depth function exists for sediments on the top and flank of the Ontong Java Plateau.A polynomial fit of this function provides an equation for domain conversion: Depth (mbsf) = 44.49+ 0.800(traveltime[ms]) + 3.308 × 10" 4 (traveltime[ms] 2 ) Traveltime (ms) = -35.18+ 1.118(depth[mbsf]) -1.969 × KT* (depth[mbsf] 2 )Seismic reflection profiles down the flank of the plateau undergo three significant changes: (1) a drastic thinning of the sediment column with depth, (2) changes in the echo-character of the profile (development of seismic facies), and (3) loss of continuous, coherent reflections.Sediments on the plateau top were largely deposited by pelagic processes, with little significant postdepositional or syndepositional modification.Sediments on the flank of the plateau are also pelagic, but they have been modified by faulting, erosion, and mass movement.These processes result in disrupted and incoherent reflectors, development of seismic facies, and redistribution of sediment on the flank of the plateau.Seismic stratigraphic analyses have shown that the sediment section decreases in thickness by as much as 65% between water depths of 2000 m water depth (at the top of the plateau) and 4000 m (near the base of the plateau).Thinning is attributed to increasing carbonate dissolution with depth.If this assumption is correct, then changes in the relative thicknesses of seismostratigraphic units at each drill site are indicative of changes in the position of the lysocline and the dissolution gradient between the lysocline and the carbonate compensation depth.We think that a shallow lysocline in the early Miocene caused sediment thinning.A deepening of the lysocline in the late-early Miocene caused relative thickening at each site.Within the middle Miocene, a sharp rise in lysoclinal depth occurs, concurrent with a steepening of the dissolution gradient.These events result in sediment thinning at all four sites.The thicker sections in the late Miocene likely correspond to a deepening of the lysocline, and a subsequent rise in the lysocline again hinders accumulation of sediment in the very late Miocene and Pliocene.

Site surveys conducted in conjunction with Leg 130 on the Ontong Java Plateau reveal a strong seismic reflector at 0.8 to 1.0 s below the seafloor that drilling at Sites 803 and 807 confirmed is Cretaceous basalt.This reflector is generally smooth, except for the northeastern margin of the plateau, where it forms a series of small, irregularly shaped depressions.Correlatable reflectors present at the bottom of the depressions are also present on the adjacent highs, suggesting that these depressions are original volcanic topography.A strong sub-basalt reflector occurs on many seismic profiles on the northeastern portion of the plateau.This reflection may be caused by a density and velocity contrast between pillow lavas and flood basalt flows or it may result from interbedded sediment and thus may represent significant lulls in volcanic activity.The presence of sub-basalt reflectors near Site 803 may indicate that later volcanic episodes occurred there, in contrast to Site 807, where this reflector was not observed and where older basalt ages were obtained.

During Leg 143, 12 holes located on 6 sites allowed study of (1) the development, growth and drowning of two Cretaceous atolls in the NW-Pacific, (2) the Upper Cretaceous and Tertiary archipelagic apron adjacent to Bikini Atoll in Marshall Islands and (3) the shallow-water drilling capability of Joides-Resolution. About 3,800 m of sediment and basalt were cored with water-depths varying from 38 to 4,800 m. A new history of evolution of Early Cretaceous guyots in the Mid-Pacific Mountains is now proposed.

Leg 143 drilled on the summits of two guyots in the Mid-Pacific Mountains (Sites 865, 866, and 867/868), the archipelagic apron adjacent to an atoll-guyot pair in the Marshall Islands (Site 869), and within the lagoon of a modern Marshall Islands atoll (Site 870).Scientific party objectives were to study the development, growth, and drowning of Cretaceous Pacific carbonate platforms, to examine their record of sea-level change, and to decipher the tectonic and volcanic history of these edifices.At Site 870, the purpose was mainly a test of the shallow-water drilling capability of JOIDES Resolution.Deep holes drilled into the Cretaceous lagoonal limestones of Allison (Hole 865A) and Resolution (previously named Huevo; Holes 866A and 866B) Guyots yielded thick, shallow-water limestone caps that record the histories of the guyots from the submergence of the volcanic pedestal through the final drowning of the carbonate platform.Both guyots have alternated between subsidence and uplift, with Resolution Guyot having an additional chapter in its history owing to its greater age.The carbonate platform on Resolution formed during the Barremian, on a small, rapidly subsiding volcanic pedestal.It quickly accumulated about 1350 m of carbonate sediments by early to middle Aptian time.After a hiatus of perhaps as great as 10-15 m.y., during which time the carbonate platform either drowned with currents sweeping the summit clear of pelagic sediment or was uplifted and eroded, approximately 270 m of late Albian sediments accumulated above.Allison Guyot formed during late Albian time and it also accumulated a late Albian shallow-water limestone cap, 731 m of which were drilled on Leg 143.Despite the rapid subsidence of both guyots, their lagoonal facies sediments indicate very shallow water throughout most of the carbonate platform histories.Furthermore, the limestone sections are in many places characterized by meter-scale facies shifts that imply short-period cycles of emergence and submergence.Cores from both guyot summits also show evidence of dissolution and mineralization (by manganese and phosphate), indicating emergence and karsting at some time after the late Albian.Pelagic sediments infilling dissolution cavities from Hole 866B imply that this event occurred before mid-Turonian time.Drilling results and guyot morphology indicate that the relative sea-level drop was at least 160 m.Holes drilled into (867A and 867B) and next to (868A) the perimeter mound surrounding the summit on Resolution Guyot failed to find the expected abundant reefal material, suggesting that these mounds, commonly seen on guyot profiles, are not necessarily reefs like those on Cenozoic Leg 143 Preliminary Report Page 8 atolls.These results point out important differences between Cretaceous and Cenozoic atolls -the latter were probably more open platforms with low relief.An entirely different geologic history was recorded at Site 869, on the apron adjacent to the atollguyot pair, Pikinni and Wodejebato.At this site, surprisingly little shallow-water debris was encountered.Instead, an abundance of volcaniclastic material was delivered to the site by turbidity currents, grain flows, and mass flows from late Cenomanian to Maastrichtian time.Especially large influxes during the Cenomanian and Campanian imply the existence of nearby land and shallow-water carbonate shoals at that time.During the Cenozoic, volcanism ceased and pelagic sedimentation prevailed, interrupted by turbidity currents carrying debris from shallow water.

This volume consists of papers presented at the M. T. Halbouty Continental Margins Conference, held at the Tremont House in Galveston, Texas, February 5-9, 1989. The conference was sponsored by the American Association of Petroleum Geologists and the Departments of Geophysics and Oceanography of Texas A&M University. Convenors were Joel Watkins, Gregory Mountain, and Feng Zhiqiang.We gratefully acknowledge the support of the following companies: Amoco Production Company, Chevron Oil Field Research Company, Enserch Exploration Inc., Exxon Company International, Howell Corporation, Maxus Energy Corporation, Mitchell Energy and Development Corporation, Mobil Oil Corporation, Pennzoil Exploration and Production Company, Phillips Petroleum Company, Primary Fuels Inc., Standard Oil Production Company, Shell Oil Company, Texaco Overseas Holdings Inc., and Unocal Corporation. Financial support provided by these companies underwrote the travel expenses of many foreign participants and U.S. students as well as some of the costs incurred in the production of this volume. Without this generous support, neither the conference nor this memoir would have been possible.Anita Fickey, Michele Beal, Debbie Waits, and Crissy Ponzio of the Texas A&M Geophysics Department staff made arrangements, retyped manuscripts, and performed a myriad of administrative chores necessary for organizing the conference and editing the manuscripts. Their always cheerful assistance smoothed the road to publication.One of the worst ice storms in the history of southeast Texas descended on the area immediately before the start of the conference. For a period of time, the only road to and from the mainland was closed because of the severeicing

ABSTRACTNeptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow‐water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half‐grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow‐based gravity‐driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal‐sediment stratigraphy and cross‐cutting dykes. Stable isotopic data on internal sediments and early‐formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.

During Mesozoic times, the Pacific Ocean occupied almost an entire hemisphere, plus a great wedge of ocean that lay between eastern Gondwana and Laurasia and extended westward from the Pacific into the gradually widening Tethys sea in the Mediterranean, Atlantic, and Gulf of Mexico. As the Atlantic widened, the Pacific narrowed apace, and the Pacific Plate expanded at the expense of bordering plates. During the Late Jurassic and Cretaceous, connections between the Atlantic and Pacific, via passageways between small continental plates in Central America, gradually widened, permitting a globe-circling, mainly west-flowing current system in low latitudes.Except for tectonic slices preserved along the adjacent American and Asian continents, all pre-Jurassic Pacific oceanic sediments have been subducted. Middle and Late Jurassic oceanic crust is inferred from magnetic data, and one drill hole reached Middle Jurassic crust.Pacific Cretaceous pelagic sedimentary rocks are typically nannofossil limestone above the calcite compensation depth and brown, zeolitic clay and radiolarian mudstone below. Chert is common as nodules in both limey and clayey sediments, and siliceous sediments appear to have been deposited over a wide band of latitudes in the Cretaceous. Lower Cretaceous black shale is present on several plateaus, at paleodepths of 500-1500 m, but contemporaneous sediments in adjacent basins are oxidized.Cretaceous vulcanism was on a stupendous scale: in the western equatorial Pacific, a region of about 4 x 10(6) km2, centered on the Ontong Java Plateau, is covered almost entirely by mid-plate tholeiitic flood basaltic flows and sills, probably emplaced mainly during the earliest Aptian. Emplacement of these huge masses in a relatively short time may have had important consequences not only for global sea level, but also for the chemistry of the oceans and atmosphere.A striking link to Tethyan facies in the other hemisphere is the presence of shallow-water platform and reefal sediments on the summits of many Pacific Cretaceous seamounts. Several dozen Early Cretaceous seamounts built to heights above sea level, and doubtless many more existed on crust now subducted along the margins of the Pacific. As the seamounts subsided, shallow-water rudist/coral platforms gradually covered them, eventually forming atolls. The platforms are the Pacific expression of the ''Urgonian'' platforms of the Tethys. Many-perhaps most-of the Pacific Early Cretaceous platforms drowned in the latest Albian, and are guyots today. New volcanic chains rose later in the Cretaceous, but there is a gap in rudist reef development from the Cenomanian through the Santonian. Campanian and Maestrichtian rudist reefs flourished in equatorial latitudes, but hermatypic reefs appear to have been absent during the Paleocene.

The timing of flood basalt volcanism associated with formation of the Ontong Java Plateau (OJP) is estimated from paleomagnetic and paleontologic data. Much of OJP formed rapidly in less than 3 million years during the early Aptian, at the beginning of the Cretaceous Normal Polarity Superchron. Crustal emplacement rates are inferred to have been several times those of the Deccan Traps. These estimates are consistent with an origin of the OJP by impingement at the base of the oceanic lithosphere by the head of a large mantle plume. Formation of the OJP may have led to a rise in sea level that induced global oceanic anoxia. Carbon dioxide emissions likely contributed to the mid-Cretaceous greenhouse climate but did not provoke major biologic extinctions.

The Japanese Guyots, Wake Guyots, and Mid‐Pacific Mountains are part of a broad area of Cretaceous volcanism in the western Pacific termed the “Darwin Rise.” Based on Seabeam bathymetric data we classify these drowned volcanic islands as: type “A,” those that advanced to the atoll stage before final submergence; type “B,” those that drowned at the barrier reef stage; and type “V,” those with little or no reef material on their volcanic summits. Widespread evidence for karst topography extending to depths of 200 m on the summits of A and B guyots sheds new light on events leading to the synchronous extinction of reefs on the Darwin Rise in the mid‐Cretaceous. We propose that after the formation of the reefs on the A and B guyots, the entire region was elevated at approximately the Aptian‐Albian boundary (113 Ma) to form a superswell similar to that existing now in French Polynesia. The type V guyots formed on this anomalously shallow lithosphere. The demise of the reefs was the direct result of the rise of this superswell, although climate factors may have prevented reef recolonization following its later subsidence.

Stimulated by the wealth of frontier exploration data and deep seismic surveys about the North Atlantic margins, this publication was crafted to provide a comprehensive analysis of North Atlantic extension. The 40 papers in this volume are divided into 6 sections: concepts, North Atlantic perspectives, North American margins, European-African margins, North Sea and Barents Shelf, and analogs. This book is concerned primarily with the circum-North Atlantic data base. It is largely biased toward presentation and interpretation of data rather than being model driven. The book includes comparative stratigraphic columns for basins of the North Atlantic margins.

The principal findings of drilling on the Galicia margin during ODP Leg 103, supplemented by sampling from the submersible Nautile and calibration of seismic-reflection profiles, are:1. Peridotite, exposed in a ridge at the foot of the margin, is clinopyroxene-bearing spinel harzburgite, more than 90% serpentinized and cut by veins of calcite.During the rifting and lithospheric stretching stage of margin formation, these rocks ascended to the seafloor from a depth of about 30 km, where the temperature was about 1250°C.The rocks record the successive effects of partial melting, stretching, serpentinization, and fracturing.2. The sequence of sedimentary strata in a typical tilted fault block of the margin comprises, in ascending order: a.An unknown thickness (probably > 500 m) of undated sandstone containing volcanic detritus and interbedded with shelly dolomite.(Known only from Nautile samples.)b.A few meters thickness of conglomerate of low-grade clastic metasedimentary clasts, resting on fragments of al tered rhyolite.This rhyolite may not be part of a conglomerate but part of the Hercynian basement.c.About 400 m of Jurassic (Tithonian) shallow-water carbonate rocks, including about 100 m of limestone inter bedded with sandstone and claystone, overlain by about 250 m of intensely fractured dolomite.The strata up to this level are classified as "pre-rift," but may have been deposited during an episode of pre-Cretaceous faulting.d.A syn-rift sequence, more than 1 km thick in the deepest part of the half-graben, consisting of about 40 m of Valanginian calpionellid marlstone overlain by Valanginian-Aptian turbidite sandstone, claystone, and hemipelagic limestone interbedded with debris-flow beds rich in shallow-water bioclasts.At least one angular unconformity inter nally divides the syn-rift sequence.e. Albian-Recent post-rift strata, thickest in the half-grabens and thin to absent over the upper edges of the fault blocks.These strata were not systematically explored during Leg 103.3. The prominent deep seismic reflector, "S," seen on profiles on the western part of the margin, lies within or be neath continential crust.In spite of the significant advances made possible by the drilling during Leg 103, a number of unanswered important questions remain as obstacles to our building a comprehensive geodynamic model for the evolution of the Galicia mar gin:1.The mechanism of emplacement of the ridge of mantle peridotite at the foot of the margin and the respective roles of asthenospheric diapirism and detachment faulting in the uplift and unroofing of the peridotite.2. The extent on the seafloor of peridotite west of the peridotite ridge bounding the margin.3. The nature of the regional deep seismic reflector termed horizon S, interpreted as the seismic signature of a de tachment fault.4. The stratigraphy of pre-Valanginian ("pre-rift") strata on the margin and a possible episode of Jurassic or older rifting and crustal thinning.5. The possible role of hydrothermal fluids rising along the rift faults in the diagenesis (dolomitization and silicification) of pre-and syn-rift sediments.6.The regional tectonic significance of widespread unconformities within the rift-stage sediments and their relation to the inception of seafloor spreading along adjacent parts of the North Atlantic margins.

Structural styles in the southern Alps reflect the mechanisms that extended the crust to form a Jurassic passive margin. The southern Alps consist of an array of half-grabens, each with a west-tilted floor bounded on the west by an east-dipping master fault. Antithetic faults divide each half-graben into a western depocenter and an eastern ramping marginal plateau, shallower and less subsident. The half-grabens, from west to east, are M. Nudo-Arbostora, M. Generoso-Albenza, West Sebino-Botticino, East Sebino-Trento, and Belluno-Friuli. Typical antithetic faults, commonly with further Cenozoic displacements, include the Ballino and Carnian Prealps fault systems. Timing of half-graven formation shows an eastward-younging age progression, suggesting that extension propagated by steps into unextended crust over much of early and middle Liassic time. The continent-facing dip of faults in the southern Alps is the reverse of the dip predicted by simple symmetrical extension models with only ductile extension in the lower lithosphere. However, it agrees with models in which low-angle shearing cuts through the entire lithosphere and results in structural asymmetry of conjugate margins. The authors suggest that the polarity of the observed shallow structures is the consequence of a gently east-dipping master shear cutting through the lithosphere at depth beneath the Brianconnais-southernmore » Alps systems.« less