Showing papers in "Geological Society of America Bulletin in 2006"

the long-term strength of continental lithosphere: "jelly sandwich" or "crème brûlée"?

Abstract: There has been much debate recently concern ing the long-term (i.e., >1 m.y.) strength of continen tal lithosphere. In one model, dubbed jelly sandwich, the strength resides in the crust and mantle, while in another, dubbed creme brulee, the mantle is weak and the strength is limited to the crust. The different models have arisen because of conflicting results from elastic thickness and earthquake data. We address the problem here by first reviewing elastic thickness estimates and their relationship to the seismogenic layer thickness. We then explore, by numerical thermomechanical model ing, the implications of a weak and strong mantle for structural styles. We argue that, irrespective of the actual crustal strength, the creme-brulee model is unable to explain either the persistence of mountain ranges or the integrity of the downgoing slab in collisional systems. We conclude that while the creme-brulee model may apply in some tectonic settings, a more widely applica ble model is the jelly sandwich.

Contrasting Late Carboniferous and Late Permian–Middle Triassic intrusive suites from the northern margin of the North China craton: Geochronology, petrogenesis, and tectonic implications

Abstract: Two contrasting intrusive suites have been identified from the northern margin of the North China craton: a Late Carboniferous dioritegranodiorite suite mainly made up of quartz diorite, diorite, granodiorite, tonalite, and hornblende gabbro, and a Late Permian–Middle Triassic suite of granitoid intrusions consisting of monzogranite, syenogranite, and quartz monzonite. Plutons from the Late Carboniferous suite exhibit variable SiO2 contents and calc-alkaline or high-K calc-alkaline, metaluminous geochemical features. Most have low negative whole-rock ϵNd(T) values (where T is the crystallization age) of −17.1 to −11.5 and zircon ϵHf(T) values of −38.3 to −11.2, indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials. However, an older pluton in the suite exhibits higher ϵNd(T) values of −11.5 to −9.9, Nd model ages of 1.82–1.64 Ga, lower initial 87Sr/86Sr ratios of 0.7046–0.7048, and it contains some zircon grains that are characterized by high negative to positive zircon ϵHf(T) values of −8.7 to 1.2, indicating strong involvement of juvenile materials derived from the lithospheric mantle. The Late Carboniferous plutons are interpreted as subduction-related and to have been emplaced in an Andean-style continental-margin arc during the southward subduction of the paleo–Asian oceanic plate beneath the North China craton. Rocks from the Late Permian–Middle Triassic intrusive suite display geochemical signatures ranging from highly fractionated I-type to A-type. They exhibit higher zircon ϵHf(T) values of −14.9 to −6.7, whole-rock ϵNd(T) values of −10.6 to −8.8, and younger Hf and Nd model ages than most of the Late Carboniferous plutons, indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle- and crust-derived melts. They are interpreted as postcollisional/postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break-off and subsequent sinking after final collision and suturing of the Mongolian arc terranes with the North China craton. These two contrasting intrusive suites suggest that the final closure of the paleo–Asian Ocean and collision between the Mongolian arc terranes and the North China craton occurred during the Late Permian, and these events were followed by postcollisional/postorogenic extension, large-volume magmatism, and significant continental growth. No significant syncollisional crustal thickening, high-pressure metamorphism, or S-type granitoid magmatism occurred during the collision process.

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

Abstract: High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.

Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models

Abstract: We present a new geologic map of western Nepal and three balanced regional cross sections in the Himalayan thrust belt. The minimum shortening between the South Tibetan detachment and the Main Frontal thrust is 485–743 km and suggests that total Himalayan shortening may exceed 900 km. All rocks involved in the thrust belt are of upper crustal affinity, implying that a comparable length of Indian lower crust and mantle lithosphere was subducted beneath Tibet. Major structural features are the Subhimalayan thrust system, Lesser Himalayan imbricate zone, Dadeldhura thrust sheet, Lesser Himalayan duplex, Ramgarh thrust sheet, Main Central thrust sheet, and a north-dipping normal-sense shear zone, possibly related to the South Tibetan detachment. These structures are continuous along the entire Nepalese portion of the Himalayan thrust belt. New 40 Ar/ 39 Ar ages from the Ramgarh thrust zone, Greater Himalayan rocks, and the lower part of the Tethyan sequence support a kinematic model in which major thrust systems in Nepal propagated southward from early Miocene time onward. The geometry and kinematic history of the thrust belt in western Nepal are not compatible with recent models for southward ductile extrusion of Greater Himalayan rocks in a mid-crustal channel. Instead, the thrust belt in western Nepal behaved like a typical forward propagating thrust system, involving unmetamorphosed, brittlely deformed rocks in its frontal part and ductilely deformed, higher-grade metamorphic rocks in its hinterland region. Although our results do not support published versions of the channel flow model, they provide additional geological and geo-chronological data that will assist future attempts to develop geodynamic models for the Himalayan-Tibetan orogenic system.

Large kinetic isotope effects in modern speleothems

Abstract: The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions refl ect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m 2 area in Harrison’s Cave, Barbados, West Indies. Only some of the plate δ 13 C values and none of the plate δ 18 O values correspond to equilibrium values, indicating signifi cant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects. On each plate, speleothem calcite was deposited with distinct δ 18 O and δ 13 C compositions that increase progressively away from the growth axis, with up to 6.6‰ 13 C and 1.7‰ 18 O enrichments. The positive δ 13 C

Geochronology and stratigraphy of late Pleistocene lake cycles on the southern Bolivian Altiplano: Implications for causes of tropical climate change

Abstract: Large paleolakes (∼33,000–60,000 km2) that once occupied the high-altitude Poopo, Coipasa, and Uyuni Basins in southern Bolivia (18–22°S) provide evidence of major changes in low-latitude moisture. In these now-dry or oligosaline basins, extensive natural exposure reveals evidence for two deep-lake and several minor-lake cycles over the past 120 k.y. Fifty-three new U-Th and 87 new 14C dates provide a chronologic framework for changes in lake level. Deposits from the "Ouki" deep-lake cycle are extensively exposed in the Poopo Basin, but no deep lakes are apparent in the record between 98 and 18.1 ka. The Ouki lake cycle was ∼80 m deep, and nineteen U-Th dates place this deep-lake cycle between 120 and 98 ka. Shallow lakes were present in the terminal Uyuni Basin between 95 and 80 ka (Salinas lake cycle), at ca. 46 (Inca Huasi lake cycle), and between 24 and 20.5 ka (Sajsi lake cycle). The Tauca deep-lake cycle occurred between 18.1 and 14.1 ka, resulting in the deepest (∼140 m) and largest lake in the basin over the past 120 ka. Multiple 14C and U-Th dates constrain the highest stand of Lake Tauca along a topographically conspicuous shoreline between 16.4 and 14.1 ka. A probable post-Tauca lake cycle (the Coipasa) produced a ≤55-m-deep lake that is tentatively dated between 13 and 11 ka. We suggest that paleolakes on the Bolivian Altiplano expanded in response to increased moisture in the Amazon and enhanced transport of that moisture onto the Altiplano by strengthened trade winds or southward displacement of the Intertropical Convergence Zone (ITCZ). Pole-to-equator sea-surface temperature (SST) and atmospheric gradients may have influenced the position of the ITCZ, affecting moisture balance over the Altiplano and at other locations in the Amazon Basin. Links between the position of the ITCZ and the ca. 23 ka precessional solar cycle have been postulated. March insolation over the Altiplano is a relatively good fit to our lake record, but no single season or latitude of solar cycling has yet to emerge as the primary driver of climate over the entire Amazon Basin. Temperature may influence Altiplano lake levels indirectly, as potentially dry glacial periods in the Amazon Basin are linked to dry conditions on the Altiplano. Intensification of the trade winds associated with La Nina–like conditions currently brings increased precipitation on the Altiplano, and deep-lake development during the Tauca lake cycle coincided with apparently intense and persistent La Nina–like conditions in the central Pacific. This suggests that SST gradients in the Pacific are also a major influence on deep-lake development on the Altiplano.

Regional structure and kinematic history of the Sevier fold-and-thrust belt, central Utah

Abstract: Christie-Blick and Anders highlight worthwhile aspects of the debate over the magnitude of Cenozoic crustal extension associated with the Sevier Desert detachment of central Utah. We welcome this opportunity to expand the necessarily limited treatment of the Sevier Desert detachment in [DeCelles and

Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washington

Abstract: Deciphering the magmatic history of continental magmatic arcs, in general, and the growth history of individual intrusions, in particular, is key to understanding the complex history of magma generation, segregation, and transport that define the dynamics of crustal growth. We utilize high precision U-Pb geochronology to resolve a detailed magmatic history from two composite intrusions, the 2–4 kbar Mount Stuart batholith and the 7–10 kbar Tenpeak pluton, emplaced in the Cretaceous North Cascades arc. This temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts. U-Pb zircon crystallization ages were obtained from 12 samples of the Mount Stuart batholith and 8 samples of the Tenpeak intrusion, representing the range of compositional diversity and geographical extent. These dates indicate that the Mount Stuart batholith was constructed over a ∼5.5 m.y. time period that was punctuated by four intervals of high magma flux. The durations of the high-flux periods are short (a few hundred thousand years) relative to the duration of the batholith. The consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt-rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time-transgressive magma fabrics. In contrast, the Tenpeak intrusion was constructed over a ∼2.6 m.y. time period, with magma influx distributed throughout the intrusive history and texturally distinct magma bodies. The Tenpeak intrusion lacks distinct age domains, which suggests that any magma reservoir was smaller in size and potentially more ephemeral. Although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally, neither pluton bears resemblance to a purely end-member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little, if any, interaction. In particular, ages from the youngest domain of the Mount Stuart batholith indicate that a melt-rich magma reservoir of ≥520 km 3 existed over a 170 ± 90 k.y. time span.

Erosion of steepland valleys by debris flows

Abstract: Episodic debris flows scour the rock beds of many steepland valleys. Along recent debris-flow runout paths in the western United States, we have observed evidence for bedrock lowering, primarily by the impact of large particles entrained in debris flows. This evidence may persist to the point at which debris-flow deposition occurs, commonly at slopes of less than ∼0.03–0.10. We find that debris-flow–scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river-incision models. Much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows. The degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions. On the basis of these observations, we hypothesize that valley slope is adjusted to the long-term frequency of debris flows, and that valleys scoured by debris flows should not be modeled using conventional bedrock river-incision laws. We use field observations to justify one possible debris-flow incision model, whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris-flow sources. The model predicts that increases in incision rate caused by increases in flow event frequency and length (as flows gain material) downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes, and stronger, less weathered bedrock. These adjustments lead to a spatially uniform lowering rate. Although the proposed expression leads to equilibrium long-profiles with the correct topographic signature, the crudeness with which the debris-flow dynamics are parameterized reveals that we are far from a validated debris-flow incision law. However, the vast extent of steepland valley networks above slopes of ∼0.03–0.10 illustrates the need to understand debris-flow incision if we hope to understand the evolution of steep topography around the world.

Cenozoic exhumation and deformation of northeastern Tibet and the Qinling: Is Tibetan lower crustal flow diverging around the Sichuan Basin?

Abstract: Apatite fission-track thermochronology data elucidate the cooling/exhumation history of the Qinling (Qin Mountains), which contain a Paleozoic−Mesozoic orogenic collage north of the Sichuan Basin and northeast of the Tibetan Plateau. In particular, we examine the extent to which the Qinling were affected by the rising plateau. The northern and eastern Qinling show continuous cooling and slow exhumation since the Cretaceous. In contrast, in the southwestern Qinling, rapid cooling initiated at 9−4 Ma, a few million years later than in the eastern Tibetan Plateau. A compilation of major Cenozoic faults in the eastern Tibetan Plateau and the Qinling, and their kinematic and dynamic characterization, shows that deformation in the Qinling has predominantly been strike slip. Active sinistral and dextral strike-slip faults delineate an area of eastward rock flow and bound the area of rapid late Cenozoic cooling outlined by apatite fission-track thermochronology. These data can be interpreted to indicate that lower crustal flow has been diverted around the Longmen Shan and beneath the southwestern Qinling, causing active plateau uplift in this area. Alternatively, northeastern Tibet may be growing eastward faster in the western Qinling than the entire South China Block is extruding to the east.

Temporal and spatial records of active arc-continent collision in Taiwan: A synthesis

Abstract: Well-documented stratigraphy and clearly defined geodynamics in Taiwan, where some of the best records on arc-continent collision have been preserved, offer a unique example for the study of collision belts worldwide. The oblique arc-continent collision in Taiwan caused a simultaneous and sequential migration of four tectonic processes. Beginning from 16 to 15 Ma, subduction of the South China Sea oceanic crust beneath the Philippine Sea plate resulted in volcanism in the Coastal Range and formation of an accretionary prism in the Central Range. Beginning in the latest Miocene–earliest Pliocene, the subduction was followed by initial arc-continent collision, as supported by the following: unroofing and erosion of the deformed accretionary prism, and deposition of sediments thus derived in the adjacent accretionary forearc (5 Ma) and slope basins (4 Ma); waning of volcanism (north, 6–5 Ma; south, 3.3 Ma); buildup of fringing reefs on the gradually quiescent volcanoes (north, 5.2 Ma; south, 2.9 Ma); arc subsidence by strike-slip faulting and the development of pull-apart intra-arc basins (north, 5.2–3.5 Ma; south, 2.9–1.8 Ma); thrusting of forearc sequences to generate a collision complex starting from 3 Ma; and clockwise rotation of the arc-forearc sequences (north, 2.1–1.7 Ma; south, 1.4 Ma). The collision propagated southward and reached southern Taiwan by 5 Ma, as evidenced by the successive deformation of the associated accretionary wedge en route. Afterward, the advanced arc-continent collision stage appeared in the earliest Pleistocene, as marked by the westward thrusting and accretion of the Luzon arc-forearc against the accretionary wedge (north, 1.5 Ma; south, 1.1 Ma) and exhumation of the underthrust Eurasian continent rocks (north, 2.0–1.0 Ma; south, 1.0–0.5 Ma). The final stage of the tectonic process, arc collapse-subduction, began by 1 Ma off the northern Coastal Range. The geologic records compiled and presented in this study strongly support the scenario of a continuous southward migration of tectonic processes and a change in sediment source and structural style. Most importantly, the model has a broad potential for reconstructing and predicting the evolution of arc-continent collision through space and time.

Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya

Abstract: The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time. Primary components of the thrust sheet include Upper Proterozoic metasedimentary rocks of the Bhimphedi Group, Cambrian-Ordovician granite bodies, and Ordovician (through Devonian?) conglomerate, sandstone, shale, and limestone of the Phulchauki Group. Deformation, metamorphism, uplift, and erosion accompanied Tertiary emplacement of the thrust sheet, but there is also evidence of widespread early Paleozoic tectonism, such as: (1) Rocks of the Bhimphedi Group were metamorphosed at ca. 490 Ma, as indicated by Th-Pb ages of monazite inclusions in garnet crystals. (2) Bhimphedi rocks are interpreted to have been imbricated along a south-vergent(?) thrust system during early Paleozoic time, with U-Pb ages recording ductile deformation through ca. 484 Ma but ending (at least locally) by ca. 473 Ma. (3) Cambrian-Ordovician granite bodies may have been generated by crustal melting during thrust loading, and at least some are interpreted to have been emplaced as syntectonic sills along the early Paleozoic thrust faults. (4) Ordovician conglomeratic strata were shed from the uplifted Bhimphedi Group and Cambrian-Ordovician granite bodies and are interpreted to have accumulated in a foreland basin setting with respect to the early Paleozoic thrust system. Our findings, together with evidence for early Paleozoic ductile deformation and metamorphism in adjoining regions, indicate that early Paleozoic tectonism has played an important role in shaping the Himalayan orogen. This early Paleozoic tectonism has been overlooked in most current models for the formation of the Himalayan orogen.

Plate tectonics and basin subsidence history

Abstract: Tectonic setting exerts fi rst-order control on basin formation as refl ected in basin subsidence history. While our approach ignores the effects of fl exural loading and eustatic sea-level change, consistency of backstripped subsidence histories (i.e., with local loading effects of sediment removed) suggests consistent tectonic driving mechanisms in each tectonic setting, with the possible exception of forearc basins. Based on published subsidence curves and open-fi le stratigraphic data, we show the subsidence characteristics of passive margins, strike-slip basins, intracontinental basins, foreland basins, and forearc basins. Passive margin subsidence is characterized by two stages, rapid initial, synrift subsidence and slow post-rift thermal subsidence, with increasing subsidence rates toward the adjacent ocean basin. Subsidence of intracontinental basins is similar in magnitude to that seen in passive margin settings, but the former is generally slower, longer lived, and lacks initial subsidence. Long-lived subsidence for many intracontinental basins is consistent with cooling following thermal perturbation of thick lithosphere found beneath old parts of continents. Basins associated with strike-slip faults are usually short lived with very rapid subsidence. Changes in local stress regimes as strike-slip faults evolve, and migrate over time, coupled with three-dimensional heat loss in these small basins likely explain this subsidence pattern. Foreland basin subsidence rates refl ect the fl exural response to episodic thrust loading. Resultant subsidence curves are punctuated by convex-up (accelerating) segments. Forearc basins have the least consistent subsidence patterns. Subsidence histories of these basins are complex and may refl ect multiple driving mechanisms of subsidence in forearc settings. Second-order deviations in subsidence suggest reactivation or superimposed tectonic events in many basin settings. The effects of eustatic sea-level change may also explain some deviations in curves. For many of these settings, subsidence histories are suffi ciently distinctive to be used to help determine tectonic setting of ancient basin deposits.

Middle-Late Permian mass extinction on land

Abstract: The end-Permian mass extinction has been envisaged as the nadir of biodiversity decline due to increasing volcanic gas emissions over some 9 million years. We propose a different tempo and mechanism of extinction because we recognize two separate but geologically abrupt mass extinctions on land, one terminating the Middle Permian (Guadalupian) at 260.4 Ma and a later one ending the Permian Period at 251 Ma. Our evidence comes from new paleobotanical, paleopedological, and carbon isotopic studies of Portal Mountain, Antarctica, and comparable studies in the Karoo Basin, South Africa. Extinctions have long been apparent among marine invertebrates at both the end of the Guadalupian and end of the Permian, which were also times of warm-wet greenhouse climatic transients, marked soil erosion, transition from high- to low-sinuosity and braided streams, soil stagnation in wetlands, and profound negative carbon isotope anomalies. Both mass extinctions may have resulted from catastrophic methane outbursts to the atmosphere from coal intruded by feeder dikes to flood basalts, such as the end-Guadalupian Emeishan Basalt and end-Permian Siberian Traps.

An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation

Abstract: Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly > 40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe(OH)_3 binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from Fe2+ and respiration-influenced CO2. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleo-proterozoic iron formation.

Cretaceous and Triassic subduction-accretion, high-pressure- low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Abstract: Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

Eastward migration of the Qaidam basin and its implications for Cenozoic evolution of the Altyn Tagh fault and associated river systems

Abstract: The Qaidam basin is the largest Cenozoic intermontane basin within the Tibetan plateau, bounded by the Qilian Shan range to the north, the East Kunlun range to the south, the Altyn Tagh range to the west, and the Ela Shan range to the east Its deposits consist of nonmarine sedimentary rocks, with a maximum thickness exceeding 13,400 m Modern sedimentation mainly takes place along the marginal areas of the basin, where alluvial fans are formed along a series of rivers that originate from the surrounding mountains Nevertheless, drilling, sedimentary, and seismic data reveal that the depocenter of the basin during much of Cenozoic time was not developed as foreland basins of the Qilian Shan belt to the north and the East Kunlun belt to the south, but instead the depocenter was in the center of the western part of the basin and had a northwest-southeast trend that shifted eastward ~380 km during Oligocene to Quaternary time This indicates that most of the basin sediments were not derived from the surrounding mountain ranges The results of the fi ssion-track age dating reveal that the rapid uplift of the Yousha Shan anticline bounding the depocenter to the southwest initiated at ca 31 Ma, indicating that the southeastward shift of the depocenter began at that time, yielding a migration rate for the depocenter of 102 mm/yr The northwestern margin of the depocenter of the Qaidam basin lies to the east of the westward prolongation of the Qaidam basin, the Tula trough, a 300-km-long narrow trough fl anked by the Altyn Tagh range along the Altyn Tagh strikeslip fault on the north and the East Kunlun thrust belt on the south The sedimentary and deformational evidence from this region indicates that the Tula trough was formed as a syncline, along which a paleoriver fl owed to the east into the Qaidam basin To the southwest, this river valley merges with the trace of the Altyn Tagh fault The latter and its westward continuation are interpreted to have extended to the northern margin of the Pamir folded belt north of the western syntaxis of the Himalaya We propose that most of the sediments within the depocenter of the Qaidam basin were transported from the northern margin of the Pamir folded belt along a progressively lengthening 2000-kmlong longitudinal river that developed along the left-lateral Altyn Tagh fault and its western continuation, as the eastward extrusion of the Tibetan plateau took place in Oligocene time Both the crustal uplift and continuous southeastward aggradations of the sediments carried by the paleo‐Kunlun River are interpreted to be the cause for southeastward migration of the depocenter of the Qaidam basin The longitudinal river disappeared in late Cenozoic time (2‐4 Ma) due to stream capture and climate change

Eruptive history and geochronology of Mount Mazama and the Crater Lake region, Oregon

Abstract: Geologic mapping, K-Ar, and 40 Ar/ 39 Ar age determinations, supplemented by paleomagnetic measurements and geochemical data, are used to quantify the Quaternary volcanic history of the Crater Lake region in order to define processes and conditions that led to voluminous explosive eruptions. The Cascade arc volcano known as Mount Mazama collapsed during its climactic eruption of ∼50 km 3 of mainly rhyodacitic magma ∼7700 yr ago to form Crater Lake caldera. The Mazama edifice was constructed on a Pleistocene silicic lava field, amidst monogenetic and shield volcanoes ranging from basalt to andesite similar to parental magmas for Mount Mazama. Between 420 ka and 35 ka, Mazama produced medium-K andesite and dacite in 2:1 proportion. The edifice was built in many episodes; some of the more voluminous occurred approximately coeval with volcanic pulses in the surrounding region, and some were possibly related to deglaciation following marine oxygen isotope stages (MIS) 12, 10, 8, 6, 5.2, and 2. Magmas as evolved as dacite erupted many times, commonly associated with or following voluminous andesite effusion. Establishment of the climactic magma chamber was under way when the first preclimactic rhyodacites vented ca. 27 ka. The silicic melt volume then grew incrementally at an average rate of 2.5 km 3 k.y. −1 for nearly 20 k.y. The climactic eruption exhausted the rhyodacitic magma and brought up crystal-rich andesitic magma, mafic cumulate mush, and wall-rock granodiorite. Postcaldera volcanism produced 4 km 3 of andesite during the first 200–500 yr after collapse, followed at ca. 4800 yr B.P. by 0.07 km 3 of rhyodacite. The average eruption rate for all Mazama products was ∼0.4 km 3 k.y. −1 , but major edifice construction episodes had rates of ∼0.8 km 3 k.y. −1 . The long-term eruption rate for regional monogenetic and shield volcanoes was d∼0.07 km 3 k.y. −1 , but only ∼0.02 km 3 k.y. −1 when the two major shields are excluded. Plutonic xenoliths and evidence for crystallization differentiation imply that the amount of magma intruded beneath Mount Mazama is several times that which has been erupted. The eruptive and intrusive history reflects competition between (1) crystallization driven by degassing and hydrothermal cooling and (2) thermal input from a regional magma flux focused at Mazama. Before ca. 30 ka, relatively small volumes of nonerupted derivative magma crystallized to form a composite pluton because the upper crust had not been heated sufficiently to sustain voluminous convecting crystal-poor melt. Subsequently, and perhaps not coincidentally, during MIS 2, a large volume of eruptible silicic magma accumulated in the climactic chamber, probably because of heating associated with mantle input to the roots of the system as suggested by eruption of unusually primitive magnesian basaltic andesite and tholeiite west of Mazama.

Terrestrial cosmogenic nuclide surface exposure dating of the oldest glacial successions in the Himalayan orogen: Ladakh Range, northern India

Abstract: Terrestrial cosmogenic nuclide surface exposure dating of moraine boulders and alluvial fan sediments define the timing of five glacial advances over at least the last five glacial cycles in the Ladakh Range of the Transhimalaya. The glacial stages that have been identified are: the Indus Valley glacial stage, dated at older than 430 ka; the Leh glacial stage occurring in the penultimate glacial cycle or older; the Kar glacial stage, occurring during the early part of the last glacial cycle; the Bazgo glacial stage, at its maximum during the middle of the last glacial cycle; and the early Holocene Khalling glacial stage. The exposure ages of the Indus Valley moraines are the oldest observed to date throughout the Himalayan orogen. We observe a pattern of progressively more restricted glaciation during the last five glacial cycles, likely indicating a progressive reduction in the moisture supply necessary to sustain glaciation. A possible explanation is that uplift of Himalayan ranges to the south and/or of the Karakoram Mountains to the west of the region may have effectively blocked moisture supply by the south Asian summer monsoon and mid-latitude westerlies, respectively. Alternatively, this pattern of glaciation may reflect a trend of progressively less extensive glaciation in mountain regions that has been observed globally throughout the Pleistocene.

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

Abstract: The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵ Hf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO 2 and FeO T compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt. In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns.

History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

Abstract: The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.

Thermal histories from the central Alborz Mountains, northern Iran: Implications for the spatial and temporal distribution of deformation in northern Iran

Abstract: We integrate new and existing thermochronological, geochronological, and geologic data from the western and central Alborz Mountains of Iran to better constrain the late Cenozoic tectonic evolution of northern Iran in the context of the Arabia-Eurasia collision. New data are presented for two granitic plutons north of the Alborz Range crest. Additional new apatite (U-Th)-He data are also presented for volcanic, intrusive, and detrital apatite grains from two transects south of the range crest. Our most definitive results include zircon and apatite (U-Th)-He and limited K-feldspar ^(40)Ar/^(39)Ar thermal history data from the Cretaceous (ca. 98 Ma) Nusha pluton that reveal that the Alborz basement underwent generally slow denudation (∼0.1 km/m.y.) as late as 12 Ma with more accelerated exhumation (∼0.45 km/m.y.) that likely began shortly after 12 Ma. The Lahijan pluton, a late Neoproterozoic–Cambrian basement exposure near the Caspian shore, records apatite (U-Th)-He closure at 17–13 Ma. Additional (U-Th)-He results from detrital apatites sampled along two separate horizontal transects all consistently yielded latest Miocene to Pliocene apparent ages that imply that even supracrustal cover rocks within the Alborz have undergone significant, regionally extensive exhumation. Overall, our data are consistent with ∼5 km of regionally extensive denudation since ca. 12 Ma. The onset of rapid exhumation in the Alborz at ca. 12 Ma appears to be consistent with other timing estimates that place the onset of the Arabia-Eurasia collision between 14 and 10 Ma.

Cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets

Abstract: The interpretation of differentially weathered mountainous areas along the fringes of Pleistocene ice sheets is fundamental for determining ice-sheet behavior and thickness during the last glaciation. Two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice, but that it was differentially erosive as a function of its spatially variable basal thermal regime. Cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern Baffi n Island shed light on Laurentide Ice Sheet (LIS) dynamics and thickness. Tors on weathered upland surfaces have minimum 10 Be ages between ca. 50 and ca. 170 ka (n = 12), whereas the majority of erratics perched in the uplands range from ca. 10 to ca. 13 ka (n = 14), indicating that the whole landscape was glaciated during the Last Glacial Maximum (LGM). Glacially sculpted bedrock (n = 7) near sea level refl ects the age of deglaciation, and increasing amounts of isotopic inheritance in highelevation sculpted bedrock (n = 3), higher elevation intermediately weathered bedrock (n = 11), and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus. Cold-based glaciation of uplands was contemporaneous with warm-based glaciation in fi ords, suggesting the presence of ice streams. 26 Al and 10 Be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure, burial, and limited glacial erosion. The paired isotope data reveal a minimum duration of burial of 80‐500 k.y., presumably by cold-based ice, and indicate that the tors have an age of at least 150‐580 ka. These data, together with other reconstructions of ice streams in the eastern Canadian Arctic, suggest that ice streams were a large infl uence on northeastern LIS dynamics throughout the late Quaternary. Thus, the northeastern LIS was sensitively tied with North Atlantic thermohaline circulation and abrupt climate change.

Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway

Abstract: We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5‐6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in ! 13 C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fi elds, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent’s changing position relative to Earth’s spin axis.

The paradox of minibasin subsidence into salt: Clues to the evolution of crustal basins

Abstract: Why do salt-floored minibasins subside? An almost universal explanation is that salt is forced from beneath the sinking basin by the weight of its sedimentary fill. This explanation is valid if the average density of the basin fill exceeds that of salt, which in the Gulf of Mexico needs at least 2300 m of siliciclastic fill to ensure enough compaction. However, most minibasins start sinking when they are much thinner than this. Some mechanism other than density inversion must explain the early history of these minibasins. Conventional understanding of minibasin subsidence is thus incomplete. Here, we identify five alternatives to density-driven subsidence of minibasins. (1) During diapir shortening, the squeezed diapirs inflate, leaving the intervening minibasins as bathymetric depressions. (2) In extensional diapir fall, stretching of a diapir causes it to sag, producing a minibasin above its subsiding crest. (3) During decay of salt topography, a dynamic salt bulge subsides as upward flow of salt slows, which lowers the salt surface below the regional sediment surface. (4) During sedimentary topographic loading, sediments accumulate as a bathymetric high above salt. (5) Finally, subsalt deformation affecting the base of salt may produce relief at the top of salt. Each mechanism (including density-driven subsidence) produces a different bathymetry, which interacts with sediment transport to produce different facies patterns in each type of minibasin. The particular mechanism for minibasin subsidence depends on the tectonic environment, regional bathymetry, and sedimentation rate. Their spatial variation on a continental margin creates provinces in which a given minibasin style is dominant. An appreciation of subsidence mechanisms should thus improve our understanding of minibasin fill patterns and allow genetic comparisons between minibasins. The mechanics of a minibasin sinking into fluid salt is in many ways analogous to a crustal basin sinking into a fluid asthenosphere. However, minibasins lack the complex rheologies, thermal histories, and compositional variations that make study of crustal basins so challenging. Minibasins are thus natural analogs and have the potential to elucidate fundamentals of subsidence mechanics.

The timing of glacier advances in the northern European Alps based on surface exposure dating with cosmogenic 10Be, 26Al, 36Cl, and 21Ne

Abstract: Exposure dating of boulder and bedrock surfaces with 10Be, 21Ne, 26Al, and 36Cl allows us to constrain periods of glacier expansion in the European Alps. The age of 155 ka from a boulder of Alpine lithology located in the Jura Mountains (Switzerland) provides a minimum age for pre-LGM (Last Glacial Maximum), more extensive Alpine glaciations. During the LGM, glaciers expanded onto the foreland after 30 ka. By 21.1 ± 0.9 ka deglaciation had begun, and the Rhône Glacier abandoned the E-mail: ivy@phys.ethz.ch. Ivy-Ochs, S., Kerschner, H., Reuther, A., Maisch, M., Sailer, R., Schaefer, J., Kubik, P.W., Synal, H., and Schlüchter, C., 2006, The timing of glacier advances in the northern European Alps based on surface exposure dating with cosmogenic Be, Al, Cl, and Ne, in Siame, L.L., Bourlès, D.L., and Brown, E.T., eds., In Situ–Produced Cosmogenic Nuclides and Quantifi cation of Geological Processes: Geological Society of America Special Paper 415, p. 43–60, doi: 10.1130/2006.2415(04). For permission to copy, contact editing@geosociety.org. © 2006 Geological Society of America. All rights reserved.

Quaternary slip rate and geomorphology of the Alpine fault: Implications for kinematics and seismic hazard in southwest New Zealand

Abstract: Glacial landforms at 12 localities in 9 river valleys are offset by the southern end of the onshore Alpine fault Offsets cluster at ∼435, 1240, and 1850 m, consistent with evidence for glacial retreat at 18, 58, and 79 calendar ka The peak of an offset fluvial aggradation surface is correlated with the Last Glacial Maximum at 22 ka Displacement rates derived from features aged 18, 22, 58, and 79 cal ka are 242 ± 22, 232 ± 49, 214 ± 26, and 235 ± 27 mm/yr, respectively, with uncertainties at the 95% confidence level The joint probability, weighted mean, and arithmetic mean of all observations pooled by rank are 231 ± 15, 232 ± 14, and 231 ± 17 mm/yr, respectively We conclude that the mean surface displacement rate for this section of the Alpine fault is 231 mm/yr, with standard error in the range of 07–09 mm/yr The reduction in estimated long-term slip rate from 26 ± 6 mm/yr to 23 ± 2 mm/yr results in an increase in estimated hazard associated with faulting distributed across the rest of the plate boundary Model-dependent probabilities of Alpine fault rupture within the next 50 yr are in the range 14%–29% The 36 ± 3 mm/yr of total plate motion (NUVEL-1A) is partitioned into 23 ± 2 mm/yr of Alpine fault dextral strike slip, 12 ± 4 mm/yr of horizontal motion by clockwise block rotations and oblique dextral-reverse faulting up to 80 km southeast of the Alpine fault, and 5 ± 3 mm/yr of heave on reverse faults at the peripheries of the plate boundary

Miocene to present activity along the Red River fault, China, in the context of continental extrusion, upper-crustal rotation, and lower-crustal flow

Abstract: Detailed mapping of field relationships along the Red River fault reveals information about the distribution and magnitude of slip along the fault, about its interactions with other regional fault systems, and the relationship between river incision and growth of the eastern margin of the Tibetan Plateau. The Red River fault is complex, consisting of up to four strands, and is dominated by right-lateral strike-slip displacement. Evidence for an extensional component of displacement is strongest along the northern part of the fault, and decreases to the southeast, to zero southeast of a major bend in the fault. Results of this study indicate dextral displacement on the Red River fault is probably at least ∼40 km, 15–16 km of which predates incision of the Red River in Pliocene time or later, and probably also predates plateau growth and development of other regional fault systems. Long-term average slip rate on the Red River fault is a minimum of ∼5 mm/yr. However, regional active tectonics are characterized by rotation of the upper crust around the eastern Himalayan syntaxis, bounded to the east by the Xianshuihe-Xiaojiang fault system, which deflects but does not cut the Red River fault. The distributed nature of the Xianshuihe-Xiaojiang fault system as it approaches the Red River fault is important for accommodating shearing across the strong crustal anisotropy formed by the Red River fault and Ailao Shan shear zone. The Red River fault appears to terminate in the extending Dali fault system in northwest Yunnan. Pliocene surface uplift, river incision, and rotation around the eastern Himalayan syntaxis are inconsistent with the pre-Pliocene displacement on the Red River fault and lateral extrusion along the Ailao Shan shear zone advocated by the “extrusion model.” This suggests a significant change in crustal conditions on the southeast margin of the Tibetan Plateau in Pliocene time, possibly the result of lower-crustal flow.

Geophysical and geochemical signatures associated with gas hydrate–related venting in the northern Cascadia margin

Abstract: This paper presents a comprehensive, multidisciplinary study of cold vents associated with near-seafloor gas hydrate. Several cold vents characterized by seismic blank zones have been identified on the northern Cascadia margin near Ocean Drilling Program (ODP) Site 889/890. The most prominent vent site (Bullseye vent) has been the subject of intense geophysical and geochemical studies, including two- and three-dimensional (2D/3D) seismic imaging, heat flow measurements, piston coring with measurements of sediment physical properties and pore-fluid geochemistry, seafloor video observation, and sampling with the unmanned submersible ROPOS. The main seismically derived constraining observations are: (1) blanking increases with seismic frequency, (2) at low frequencies, layers can be traced through the zones, (3) blank zones widen with depth, (4) blank zones are underlain by a bottom simulating reflector (BSR), and (5) no velocity anomalies were detected across the vents. Constraints from piston core and thermal probe analyses are: (1) massive hydrate was recovered just below the seafloor at Bullseye vent, and (2) chemical alteration of sediments was observed by reduced magnetic susceptibility, increased thermal conductivity, and an elevated sulfate/methane interface. Additional constraints are: (1) no thermal anomaly was observed, (2) widespread carbonates and active chemosynthetic communities were found, and (3) elevated levels of methane were detected in the water column above Bullseye vent. We present a model for the seismic blanking at Bullseye vent that honors the constraints from all observations. The cold vents represent channels or networks of filamentous fractures containing hydrate and/or free gas. Free gas can be present within the hydrate stability field only in fractures, which may be coated with hydrate that prevents the inflow of water. The overall concentration of hydrate or gas within the vent must be small, because there was no observable velocity anomaly.

Sinkhole “swarms” along the Dead Sea coast: Reflection of disturbance of lake and adjacent groundwater systems

Abstract: More than a thousand sinkholes have developed along the western coast of the Dead Sea since the early 1980s, more than 75% of them since 1997, all occurring within a narrow strip 60 km long and <1 km wide. This highly dynamic sinkhole development has accelerated in recent years to a rate of ∼150–200 sinkholes per year. The sinkholes cluster mostly over specific sites up to 1000 m long and 200 m wide, which spread parallel to the general direction of the fault system associated with the Dead Sea Transform. Research employing borehole and geophysical tools reveals that the sinkhole formation results from the dissolution of an ∼10,000-yr-old salt layer buried at a depth of 20–70 m below the surface. The salt dissolution by groundwater is evidenced by direct observations in test boreholes; these observations include large cavities within the salt layer and groundwater within the confined subaquifer beneath the salt layer that is undersaturated with respect to halite. Moreover, the groundwater brine within the salt layer exhibits geochemical evidence for actual salt dissolution (Na/Cl = 0.5–0.6 compared to Na/Cl = 0.25 in the Dead Sea brine). The groundwater heads below the salt layer have the potential for upward cross-layer flow, and the water is actually invading the salt layer, apparently along cracks and active faults. The abrupt appearance of the sinkholes, and their accelerated expansion thereafter, reflects a change in the groundwater regime around the shrinking lake and the extreme solubility of halite in water. The eastward retreat of the shoreline and the declining sea level cause an eastward migration of the fresh–saline water interface. As a result the salt layer, which originally was saturated with Dead Sea water over its entire spread, is gradually being invaded by fresh groundwater at its western boundary, which mixes and displaces the original Dead Sea brine. Accordingly, the location of the western boundary of the salt layer, which dates back to the shrinkage of the former Lake Lisan and its transition to the current Dead Sea, constrains the sinkhole distribution to a narrow strip along the Dead Sea coast. The entire phenomenon can be described as a hydrological chain reaction; it starts by intensive extraction of fresh water upstream of the Dead Sea, continues with the eastward retreat of the lake shoreline, which in turn modifies the groundwater regime, finally triggering the formation of sinkholes.