川北广元须家河组一段沉积相与沉积环境演化分析

四川盆地北部三叠系—侏罗纪发育,出露良好,是三叠纪—侏罗纪转换时期古气候与古环境变化的良好物质记录.其中,晚三叠世诺利期是四川盆地海陆转换的重要过渡期,尤其是对陆地生态系统的影响更加显著.以川北广元剖面为研究对象,对晚三叠世诺利期末须家河组须一段进行沉积相及沉积环境演化的分析,结果表明:研究剖面发育潟湖沼泽—三角洲平原...     

西藏措勤盆地的上古生界-下中生界：潜在的油气沉积建造

新的地层和古生物学研究结果表明,措勤盆地在晚古生代一早中生代不存在长达75Ma以上的沉积间断．其中,晚二叠世-晚三叠世诺利期都是海相碳酸盐岩地层,晚三叠世瑞替期-早中侏罗世为陆缘碎屑岩地层．两者之间为角度不整合接触．措勤盆地在晚二叠世-晚三叠世诺利期一直处于海相碳酸盐岩盆地中．晚三叠世瑞替期-早中侏罗世仍然是接受巨厚沉积的低洼地区。从宏观的油气勘探的战略评价角度看．措勤盆地在中二叠世栖霞期-晚三叠世诺利期的海相碳酸盐岩地层具有生油层的性质,上三叠统瑞替阶-中下侏罗统具有盖层的性质,两者之间的角度不整合具有储集层的性质。措勤盆地中二叠统-下侏罗统构成一个油气的有利勘探层系．称为古格层系。     

西藏措勤盆地的上古生界—下中生界：潜在的油气沉积建造

新的地层和古生物学研究结果表明,措勤盆地在晚古生代一早中生代不存在长达75Ma以上的沉积间断．其中,晚二叠世-晚三叠世诺利期都是海相碳酸盐岩地层,晚三叠世瑞替期-早中侏罗世为陆缘碎屑岩地层．两者之间为角度不整合接触．措勤盆地在晚二叠世-晚三叠世诺利期一直处于海相碳酸盐岩盆地中．晚三叠世瑞替期-早中侏罗世仍然是接受巨厚沉积的低洼地区。从宏观的油气勘探的战略评价角度看．措勤盆地在中二叠世栖霞期-晚三叠世诺利期的海相碳酸盐岩地层具有生油层的性质,上三叠统瑞替阶-中下侏罗统具有盖层的性质,两者之间的角度不整合具有储集层的性质。措勤盆地中二叠统-下侏罗统构成一个油气的有利勘探层系．称为古格层系。     

Paleo-environments and tectonic setting of the Mesozoic Thung Yai Group in Peninsular Thailand,with a new record of Parvamussium donaiense Mansuy, 1914

The Thung Yai Group extends over a large area of peninsular Thailand, along the eastern margin of the Shan Thai block. Bound by angular unconformities 300 m thick dominantly detritic brackish to non-marine deposits with few intercalated limestone beds between Triassic marine and Tertiary non-marine sediments, represent the Thung Yai Group that comprises four formations: Khlong Min, Lam Thap, Sam Chom, and Phun Phin Formations. In the Ao Luk–Plai Phraya (ALPP) area, the Khlong Min and Lam Thap formations yield marine, brackish-water and non-marine fossil assemblages. These include trace fossils and for the first time in peninsular southern Thailand, the bivalve Parvamussium donaiense Mansuy, 1914. Based on fossil determinations, the Thung Yai Group has a late Early Jurassic to Early Cretaceous age.Our new observations help unravel the tectonic history of Mesozoic Peninsular Thailand. After the complete closure of the Paleotethys in the Late Triassic, renewed inundation, from the late Early Jurassic to the early Middle Jurassic, brought a regime of shallow to open marine and lagoon sedimentation over northwestern, western and southern peninsular Thailand, in the eastern part of Sundaland bordering the Mesotethys to the west.     

Depositional and palaeoenvironmental variation of lower Turonian nearshore facies in the Bohemian Cretaceous Basin,Czech Republic

Dark grey strata belonging to the basal horizons of the Bílá Hora Formation (lower Turonian) were exposed during quarrying at the locality of Plaňany (Bohemian Cretaceous Basin). Based mainly on quarry maps, the early Turonian rocky bottom was reconstructed in the area of about 14,800 m². Two sedimentologic and palaeoecological settings were recognized in the area. Dark grey deposits form part of the first setting, representing a fill of large and deep depressions on the northern foot of the Plaňany elevation. The second setting with a phosphatic lag is located on the elevated part of the area. Dark grey sedimentation belongs to the UC6a and particularly to the UC6b nannoplankton zones. During the latter zone the dark sedimentation passed upwards into light siltstones. The enrichment of C_org and S, clay minerals with an important kaolinite peak, formation of framboidal pyrite and the enrichment of macrofauna and phosphatic particles are characteristic of the basal portions of the dark deposits. The sulphate reduction zone is suggested for this sedimentary environment. In the associations of phosphatic particles, shark coprolites, faecal pellets and sponge fragments prevail. No phosphatic lag is developed. On the other hand, the phosphatic lag directly overlying the Cenomanian relics is most characteristic of the second setting. This lag is a product of sedimentary condensation, characterized by a long-lasting concentration of phosphatic particles and phosphogenesis, accompanied by encrustation of closely adjacent free rock surfaces by a faunal community with Terebella. Additional biostratigraphic data presently contributed to a proposed correlation of both settings. Micropalaeontological data (foraminifera, palynomorphs, nannoplankton) indicate that the phosphatic lag and basal dark grey deposits may be approximately coeval. The stagnant depositional conditions with only very slow sea-level rise are thought to have lasted for a relatively long period that includes a significant part of the Whiteinella archaeocretacea Zone (lowermost Turonian). In elevated parts, condensation could proceed under conditions of prevailingly weak currents and strong oxidation of organic matter, while decomposition of organic matter was probably very slow and incomplete in depressions below the elevation. The sedimentary condensation in both settings is highlighted by the remarkable formation of abundant glauconite in local deposits.     

The age of mid-proterozoic phosphatic metasediments in Finland as indicated by radiometric UPb dates

Two uranium-bearing phosphatic metasedimentary occurrences at Lampinsaari, Vihanti and Temo, Nilsiä, in Finland have been dated by isotopic UPb analyses of whole rock samples to be 1876±2 Ma old. The dates reflect the time of the Svecokarelian regional metamorphism in high amphibolite facies. An analysis of a uranium thucolite from the deposit at Nuottijärvi, Paltamo, that was metamorphosed in intermediate greenschist facies gives a date of 1897±7 Ma, suggesting that the phosphatic sedimentation had occurred before that time. An upper time limit for the sedimentation is set by a previous date of 2080±45 Ma, which was obtained from a banded iron one formation. The absence of thorium in the deposits suggests a marine environment of deposition.     

非海相双壳类江西蛤（Jiangxiella）首次发现于日本

首次报道日本西南部发现的晚三叠世非海相双壳类江西蛤（Jiangxiella)，并讨论这一发现的意义。认为原产于湘、赣、粤地区的江西蛤是地理分布极有限的土著属，它在日本西南部的发现表明当时该区与华南东部的古地理关系极为密切，可能原属同一地块。     

Biostratigraphy and depositional environments of subsurface sediments in well Arani-A,Palar basin,Tamil Nadu

The first exploratory well Arani–A was drilled in the Palar basin to a depth of 2400m and terminated within the granitic basement.This well offered the first ever opportunity to understand biostratigraphy, sedimentation history and depositional environment of the entire sedimentary column based on arenaceous foraminifera, spores, pollen and dinoflagellate cyst assemblages. Previous studies on few scattered outcrops around Sriperumbudur, Chengalpattu and Sathyavedu areas have documented palynofossil assemblage of Neocomian–Aptian age. The present study reveals the presence of middle Jurassic (Bajocian-Callovian) sediments (2360-1725 m) resting on the granitic basement. The sediments are interpreted to have deposited under lacustrine/estuarine conditions with high tides providing occasional marine influence. The middle Jurassic sediments are conformably overlain by late Jurassic (Oxfordian–Tithonian) sediments (1725 - 950 m). The late Jurassic sediments have been inferred to have got deposited under fluctuating near shoremarginal marine conditions. There is a 55m thick boulder bed (950 - 895 m) separating the overlying Valanginian sediments. Early Cretaceous (Valanginian-Early Albian) sediments are developed in the interval from 895-50m. The boulder bed possibly corresponds to the missing Berriasian stage of the earliest Cretaceous representing an unconformity of the order of ~5 Ma across Jurassic-Cretaceous boundary. These sediments are inferred to have deposited under shallow inner neritic conditions. The sediments from 50m to surface consist mainly of lateritic sandstone and alluvium. The sedimentary history of Palar basin began in Bajocian stage of middle Jurassic (170-168 Ma) and ended in early Albian stage of early Cretaceous (113-105 Ma). The late Albian marine transgression which facilitated huge sedimentation in Cauvery and Krishna-Godavari basins has bypassed the Palar basin thus adversely affecting the hydrocarbon potential.     

Microbial–serpulid build-ups in the Norian–Rhaetian of the Western Mediterranean area: ecological response of shelf margin communities to stressed environments

Microbial–serpulid communities are the main reef-builders of most Norian–Rhaetian carbonate platforms of Italy. They represent minor, yet significant communities in the shallow western Tethys, in contrast with the highly diversified sponge- and coral-dominated Dachstein reefs widespread from Eastern Alps to Australia. The microbial–serpulid build-ups are systematically associated with narrow intraplatform troughs and developed on the outer margin–upper slope area under marine conditions unsuitable for the development of coralgal reefs. The development of these settings can be related to transtensional tectonics affecting an elongate belt which was roughly parallel to the Piedmont–Ligurian Ocean spreading axis during the Jurassic. Dysoxic and mesosaline waters are the main cause of the success of the Norian microbial–serpulid build-ups; whereas, from the late Norian to the Rhaetian, eutrophication and climate change control their distribution. These environments acted during Norian–Rhaetian times as refuges, where opportunitistic organisms could survive and build frameworks during a period otherwise dominated by coralgal reefs.     

Sand,hearths, lithics and a bit of bioturbation: Site formation processes at Umhlatuzana rockshelter,South Africa

Umhlatuzana rockshelter is known for its continuous record of Middle and Later Stone Age lithic assemblages. This study presents multiproxy geoarchaeological data (micromorphology, X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy) to reconstruct the depositional and post-depositional history of the site. Although the Stone Age deposits macroscopically appear homogeneous, micromorphological analysis reveals the existence of primary, unaltered depositional microlayering throughout the sequence. Sediments related to combustion activities on-site are observed in both the Holocene and Pleistocene deposits. Post-depositional geochemical alterations result in the formation of several phosphatic minerals that significantly affect the site's preservation conditions. One of those is vashegyite, a rare magnesium phosphate mineral related to acidic and moist sedimentary environments. Bioturbation features are prominent at the microscale, but sediment mixing does not seem to affect the vertical distribution of the artifacts. The observation of horizontal microlayering in both the Pleistocene and Holocene illuminates the dominant mechanism of sedimentation throughout the site's 70,000-year occupational history. It moreover shows that the lithics can be analysed as coherent assemblages.     

Strontium isotope evolution of Late Permian and Triassic seawater

The ⁸⁷Sr/⁸⁶Sr values based on brachiopods and conodonts define a nearly continuous record for the Late Permian and Triassic intervals. Minor gaps in measurements exist only for the uppermost Brahmanian, lower part of the Upper Olenekian, and Middle Norian, and only sparse data are available for the Late Permian. These 219 measurements include 67 brachiopods and 114 conodont samples from the Tethyan realm as well as 37 brachiopods and one conodont sample from the mid-European Middle Triassic Muschelkalk Sea. The Late Permian/Lower Triassic interval is characterized by a steep 1.3 × 10⁻³ rise, from 0.7070 at the base of the Dzhulfian to 0.7082 in the late Olenekian, a rate of change comparable to that in the Cenozoic. In the mid-Triassic (Anisian and Ladinian), the isotope values fall to 0.7075, followed again by a rise to 0.7081 in the Middle/Late Norian. The ⁸⁷Sr/⁸⁶Sr values decline again in the Late Norian (Sevatian) and Rhaetian to 0.7076.The sharp rise in the ⁸⁷Sr/⁸⁶Sr values during the Late Permian/Early Triassic was coincident with widespread clastic sedimentation. Because of the paucity of tectonic uplifts, the enhanced erosion may have been due to intermittent humid phases, during mainly an arid interval, coupled with the absence of a dense protective land plant cover following the mass extinction during the latest Permian. The apex of the ⁸⁷Sr/⁸⁶Sr curve at the Olenekian/Anisian boundary coincides with cessation of the large-scale clastic sedimentation and also marks the final recovery of land vegetation, as indicated by the renewed onset of coal formation in the Middle Triassic. The rising ⁸⁷Sr/⁸⁶Sr values from the Middle Carnian to the Late Norian coincide with the uplift and erosion of the Cimmeride-Indosinian orogens marking the closure of the Palaeotethys. The subsequent Rhaetian decline that continues into Jurassic (Pliensbachian/Toarcian boundary), on the other hand, coincides with the opening of the Vardar Ocean and its eastern continuation in the Izmir-Ankara Ophiolitic Belt.Samples from the Upper Muschelkalk are more radiogenic than the global trend. This may reflect separation of the basin from the open ocean. Due to strong meteoric influx from a large land mass in the north, the Germanic Basin became increasing brackish up section in the north and east, but because of the high evaporation rates, the salt content was not much reduced in the southern and central basin where a rich, but increasingly endemic, marine fauna survived.     

Rifts,rivers and climate recovery: A new model for the Triassic of England

Triassic basins of England developed under a regime of largely W–E extension and progressed from non-marine fluvial and aeolian sedimentation (Sherwood Sandstone Group), through marine-influenced playa lacustrine deposits (Mercia Mudstone Group) to marine environments (Penarth Group). A new tectono-stratigraphic model for the Sherwood Sandstone Group is proposed in which two major long-distance river systems developed under conditions of relative fault inactivity in the Early Triassic (Budleigh Salterton Pebble Beds and equivalent) and Middle Triassic (Otter Sandstone and equivalent). These are separated by a late Early Triassic syn-rift succession of fluvio–aeolian sandstones (Wildmoor Sandstone and Wilmslow Sandstone formations) and playa lacustrine muds (Nettlecombe Formation) which show major thickness variation and localisation with hanging wall basins. The partitioning of syn-rift deposits into mudstones within upstream basins (close to the source of water and sediment) and clean aeolian or fluvio–aeolian sandstones in downstream basins is similar to the pattern observed in the underlying late Permian. Under conditions of rapid tectonic subsidence chains of extensional basins may become disconnected with upstream basins (Wessex Basin) acting as traps for fines and water permitting more aeolian activity in temporarily unlinked downstream basins (Worcester and Cheshire basins). In addition to tectonic controls, fluctuating climate, relief related to limestone resilience in arid settings, the smoothing effect of fill and spill sedimentation and Tethyan sea-level change all contributed toward the observed Triassic stratigraphy in England.     

长江源各拉丹冬一带晚三叠世那底岗日组

长江源各拉丹冬一带那底岗日组,为一套基-中酸性的火山岩或火山碎屑岩-沉积岩组合,其上与中侏罗统雀莫错组不整合、与下晚三叠世巴贡组整合接触,火山岩u—Pb同位素年龄为212±1．7Ma,时代为晚三叠世诺利晚期-瑞替期,为晚三叠世拉竹龙-金沙江洋盆？向南消减形成的火山岩浆弧。     

Stratigraphy and allogenic controls of the western Portugal Cretaceous: an updated synthesis

The stratigraphy of the western Portugal on-shore Cretaceous record (western Iberian margin, Lusitanian Basin) is described, including formal units and a selection of informal units prevailing in the geological literature. This paper is a synthesis based on a review of previous works, but with an innovative emphasis on the interpretation of eustatic and tectonic controls. The sedimentary record is dominated by siliciclastics and comprises fluvial and deltaic coastal marine siliciclastic systems, as well as extensive deposits of shallow marine carbonate platforms, both open and rimmed. Several regional unconformities and transgressive/regressive cycles are identified and the allogenic controls interpreted, namely the geodynamic events along the boundaries of the Iberian plate. Above the Berriasian deposits belonging to the Upper Jurassic cycle, the five main unconformity-bounded units are: (1) upper Berriasian–lower Barremian, (2) upper Barremian–lower Aptian, (3) upper Aptian–uppermost Cenomanian, (4) mid lower Turonian–lower Campanian and (5) middle Campanian–Maastrichtian. These units show transgressive peaks in the lower Hauterivian, lower Aptian, base of the upper Cenomanian and mid lower Turonian. The general trend of the Lower Cretaceous reflects the transition from late rifting to passive margin, with the last break-up unconformity dated as late Aptian. The Lusitanian Basin achieved full infill by the Cenomanian, when a large carbonate platform extended far inland. The later deposits were preserved only in the northern sector and the accompanying unconformities reflect transpressive intraplate stresses generated in boundaries of the plate with Africa and Eurasia. With very low accommodation being created throughout the Late Cretaceous, fluvial deposits were dominant, including a few marine levels related with eustatic rises in the early Turonian, the Coniacian, the early Campanian and the Maastrichtian.     

Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe

Santonian-Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment-ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high- and low-productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well-oxygenated, current-swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate-fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment-seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post-oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo-Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.     

Phosphate-rich deposits associated with the Mio-Pliocene unconformity in south-east Australia

Phosphates are present on the surface of the Mio-Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south-east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south-east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south-eastern Australia.     

中国侏罗纪年代地层学研究的现状

“国际地层表”依据菊石带建立起来的侏罗纪年代地层系统在全球海相侏罗系的划分和对比中有着广泛的应用,但却很难直接应用于非海相侏罗纪地层系统中。中国的侏罗系多属非海相,近年来我国地质工作者们不但将中国的海相侏罗系与全球侏罗纪年代地层系统进行了较合理的对比,发现了穿越海相三叠系-侏罗系界线的连续沉积的剖面,而且建立了非海相侏罗系的阶。但是中国非海相侏罗系区域性阶的时代和不同阶之间的界线有待海相化石和地层测年来确定或检验     

唐古拉山侏罗系沉积的若干特征及沉积盆地背景条件的探讨

对于唐古拉山侏罗系沉积类型的不同认识由来已久,然而无论是将其视为地槽型沉积,还是地台型沉积,均难以令人满意地解释该区侏罗纪沉积的特点和性质。 作者在野外工作和室内研究的基础上,从地球化学因素、古生态分析、沉积岩沉积构造和矿物结构成熟度等综合考虑,对该区侏罗纪沉积的特点和性质提出下列证据: （1）含盐度值一般均低于广海的平均盐度值。 （2）发育了海相一半咸水相一陆相的双壳类动物群序列,并以半咸水双壳类属占优势。 （3）沉积构造多为浅水成因的小型波痕,交错层理沉积组合。 （4）沉积岩粒度分析表现了近岸环境的概率曲线特征。 （5）沉积岩矿物结构,成熟度较高,未见海相成因的粒级层理。 结论是唐古拉山区侏罗系应为近岸局限海环境下的沉积产物,在海退时期还发育了滨岸湖相沉积。 根据羌塘地区侏罗纪沉积相空间展布的特点,可以认为唐古拉山区侏罗系仅仅是大西洋型大陆边缘沉积的一个组成部分,是陆架大幅度坳陷所承受的巨厚陆源碎屑沉积,整个羌塘地区侏罗纪沉积在空间上的演替关系及其巨大厚度可借助于大西洋型大陆边缘特有的大陆堤前展沉积模式得到较好的解释。     

Restricted shallow-water sedimentation of Early Archean stromatolitic and evaporitic strata of the Strelley Pool Chert,Pilbara Block,Western Australia

The 3.4 Ga-old Strelley Pool Chert is a 25-m thick sedimentary unit near the top of the predominantly volcanic Warrawoona Group in greenstone belts of the eastern Pilbara Block, Western Australia. It is here subdivided into 5 members containing 13 lithofacies. The basal Member, I, is composed of quartzose sandstone deposited in a high-energy wave- or tide-dominated shallow-water system. Overlying this are Members II and III, which make up the bulk of the formation and were deposited in a low-energy, partially restricted hypersaline basin. They record a predominantly regressive succesion of deposits including subaqueous laminite, stromatolite and evaporite; stromatolite, carbonaceous laminite, black-and-white banded chert, evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions; and subaerially deposited windblown sand, evaporite and evaporite-solution layers. Members IV and V record the progradation of a volcaniclastic alluvial fringe.The Strelley Pool Chert represents an association of sedimentary environments directly comparable to that observed in modern, low-energy, shallow-marine carbonate-evaporite systems, such as along the Trucial Coast of the Persian Gulf, and abundantly developed in Phanerozoic carbonate platform deposits. There is no evidence, however, that uniquely identifies the environment as having been marine. Deposition may have taken place in either a large hypersaline lake or a restricted marine basin. Evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes. Silicified evaporites, including coarsely crystalline layers resembling Messinian selenite, are widespread and similar to younger evaporite deposits. They clearly indicate that evaporites were common within shallow-water Archean sequences. The presence of an assemblage of biogenic deposits, including organic laminite, stromatolites, encrusting carbonaceous mats, carbonaceous granules and oncolites, deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic, points to the existence of an ecologically and probably biologically diverse microbial community 3.4 Ga ago.     

On the origin of fore-arc basins: new evidence of formation by rifting from the Jurassic of Alexander Island, Antarctica

The Middle Jurassic–Lower Cretaceous Fossil Bluff Group of Alexander Island, Antarctica represents the fill of a fore-arc basin unconformably overlying an accretionary complex. Like most fore-arc basins, this example had been considered to have a passive origin, as a topographic hollow between the arc and the trench-slope break. Recent discoveries of igneous rock coeval with sedimentation have altered this view. Oxfordian–Kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin, within a single formation. Chemically, most basalts are high-Nb types, which cannot have originated in a supra-subduction zone setting. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic arc, it is concluded that a late Jurassic pause in subduction led to active rifting to form the fore-arc basin.