Intermittent Uplift Research Articles

Which contributes more to the relict flora distribution pattern in East Asia, geographical processes or climate change? New evidence from the phylogeography of Rehderodendron kwangtungense

BackgroundRelict species are important for enhancing the understanding of modern biogeographic distribution patterns. Although both geological and climatic changes since the Cenozoic have affected the relict flora in East Asia, the contributions of geographical processes remain unclear. In this study, we employed restriction-site associated DNA sequencing (RAD-seq) and shallow genome sequencing data, in conjunction with ecological niche modeling (ENM), to investigate the spatial genetic patterns and population differentiation history of the relict species Rehderodendron kwangtungense Chun.ResultsA total of 138 individuals from 16 populations were collected, largely covering the natural distribution of R. kwangtungense. The genetic diversity within the R. kwangtungense populations was extremely low (HO = 0.048 ± 0.019; HE = 0.033 ± 0.011). Mantel tests revealed isolation-by-distance pattern (R2 = 0.38, P < 0.001), and AMOVA analysis showed that the genetic variation of R. kwangtungense occurs mainly between populations (86.88%, K = 7). Between 23 and 21 Ma, R. kwangtungense underwent a period of rapid differentiation that coincided with the rise of the Himalayas and the establishment of the East Asian monsoon. According to ENM and population demographic history, the suitable area and effective population size of R. kwangtungense decreased sharply during the glacial period and expanded after the last glacial maximum (LGM).ConclusionOur study shows that the distribution pattern of southern China mountain relict flora may have developed during the panplain stage between the middle Oligocene and the early Miocene. Then, the flora later fragmented under the force of orogenesis, including intermittent uplift during the Cenozoic Himalayan orogeny and the formation of abundant rainfall associated with the East Asian monsoon. The findings emphasized the predominant role of geographical processes in shaping relict plant distribution patterns.

Phreatic eruptions and deformation of Ioto Island (Iwo-jima), Japan, triggered by deep magma injection

On Ioto Island (Iwo-jima), 44 phreatic eruptions have been recorded since 1889, when people began to settle there. Four of these eruptions, after the beginning of continuous observation by seismometers in 1976, were accompanied by intense seismic activity and rapid crustal deformation beforehand. Other eruptions on Ioto were without obvious crustal activities. In this paper, we discuss the mechanisms of phreatic eruptions on Ioto. Regular geodetic surveys and continuous GNSS observations show that Ioto intermittently uplifts at an abnormally high rate. All of the four eruptions accompanied by the precursors took place during intermittent uplifts. The crustal deformation before and after one of these eruptions revealed that a sill-like deformation source in the shallow part of Motoyama rapidly inflated before and deflated after the beginning of the eruption. From the results of a seismic array and a borehole survey, it is estimated that there is a layer of lava at a depth of about 100–200 m, and there is a tuff layer about 200–500 m beneath it. The eruptions accompanied by the precursors probably occurred due to abrupt boiling of hot water in hydrothermal reservoirs in the tuff layer, sealed by the lava layer and triggered by intermittent uplift. For the eruptions without precursors, the hydrothermal systems are weakly sealed by clay or probably occurred on the same principle as a geyser because phreatic eruptions had occurred beforehand and hydrostatic pressure is applied to the hydrothermal reservoirs.

The development and evolution of landform based on neotectonic movement: The Sancha river catchment in the southwestern China

Neotectonics has changed the coupled process of endogenic and exogenic geological dynamics, which mold the modern landform. Geomorphologic analysis is essential for identifying and understanding the tectonic activity and indicates the responsive mechanism of the landform to tectonic activity. At first, this research reconstructed the twisted Shanpen period planation surface, computed the valley floor width-to-height ratio of Sancha river and extracted the cross sections marking the river terraces to analyze the characteristics of the neotectonics. And then, the relation between neotectonic movement and landform development was analyzed by dividing the landform types. At last, the spatial variation of landform evolution was analyzed by extracting the Hypsometric Integral of sub-catchments. The Sancha river catchment’s neotectonic movement presents the tilt-lift of earth’s crust from NW to SE, which is characterized by the posthumous activity of Yanshan tectonic deformation. The spatial distribution of river terraces indicates that Sancha river catchment has experienced at least four intermittent uplifts and the fault blocks at both the sides of Liuzhi–Zhijin basement fault have differentially uplifted since the late Pleistocene. As the resurgence of Liuzhi–Zhijin basement fault, the Sancha river catchment was broken into two relative independent landform units. The spatial variations of the landform types near the Sancha river and the sub-catchments’ landform evolution are characterized by periodic replacement. The styles of geological structure have controlled the development of landform far away from the Sancha River and influenced the landform evolution. The posthumous activities of the secondary structure have resulted in the spatial variation of sub-catchments’ landform evolution, which presents periodic replacement with local exceptions. The present study suggests that spatial variations of the development and evolution of modern landform of Sancha River catchment owe their genesis to the interplay between the hydrodynamic force and tectonic activity in the neotectonic period. Likewise, the application of geomorphic indicators also provides a new way to assess the regional crustal stability.

Canyon incision chronology based on ignimbrite stratigraphy and cut-and-fill sediment sequences in SW Peru documents intermittent uplift of the western Central Andes

Based on an 40Ar/39Ar- and U/Pb-based chronostratigraphy of ignimbrite sheets and the geomorphological features of watersheds, river profiles and slope deposits in the Ocoña–Cotahuasi–Marán (OCM) and Colca valleys of southwest Peru, we reconstruct the valley incision history of the western Central Andes over the last c. 25Myr. We further document the Pleistocene and Holocene evolution of deep valleys on the basis of 14 10Be surface-exposure ages obtained on debris-avalanche deposits and river straths.The data suggest that uplift was gradual over the past 25Myr, but accelerated after c. 9Ma. Valley incision started around 11–9Ma and accelerated between 5 and 4Ma. Incision was followed by several pulses of valley cut-and-fill after 2.3Ma. Evidence presented suggest that the post-5Ma sequence of accelerated canyon incision probably resulted from a combination of drainage piracy from the Cordilleran drainage divide towards the Altiplano, accentuated flexural tilting of the Western Cordillera towards the SE, and increased rainfall on the Altiplano after late Miocene uplift of the Eastern Cordillera. The valley deepening and slope steepening driven by tectonic uplift gave rise to large occurrences of rockslope failure. The collapsed rock masses periodically obstructed the canyons, thus causing abrupt changes in local base levels and interfering with the steadiness of fluvial incision. As a result, channel aggradation has prevailed in the lower-gradient, U-shaped Pacific-rim canyons, whereas re-incision through landslide deposits has occurred more rapidly across the steeper V-shaped, upper valleys. Existing canyon knickpoints are currently arrested at the boundary between the plutonic bedrock and widespread outcrops of middle Miocene ignimbritic caprock, where groundwater sapping favouring rock collapse may be the dominant process driving headward erosion.

The polycyclic Lausche Volcano (Lausitz Volcanic Field) and its message concerning landscape evolution in the Lausitz Mountains (northern Bohemian Massif, Central Europe)

The Tertiary Lausitz Volcanic Field covers a broad area encompassing parts of Eastern Saxony (Germany), Lower Silesia (Poland) and North Bohemia (Czech Republic). Volcanism was predominantly controlled by the volcano-tectonic evolution of the Ohře Rift and culminated in the Lower Oligocene. This paper deals with the highest volcano of this area, the Lausche Hill (792.6m a.s.l.) situated in the Lausitz Mountains. We offer a reconstruction of the volcanic edifice and its eruptive history. Its complex genesis is reflected by six different eruption styles and an associated petrographic variety. Furthermore, the Lausche Volcano provides valuable information concerning the morphological evolution of its broader environs. The remnant of an alluvial fan marking a Middle Paleocene–Lower Eocene (62–50Ma) palaeo-surface is preserved at the base of the volcano. The deposition of this fan can be attributed to a period of erosion of its nearby source area, the Lausitz Block that has undergone intermittent uplift at the Lausitz Overthrust since the Upper Cretaceous. The Lausche Hill is one of at least six volcanoes in the Lausitz Mountains which show an eminent low level of erosion despite their Oligocene age and position on elevated terrain. These volcanoes are exposed in their superficial level which clearly contradicts their former interpretation as subvolcanoes. Among further indications, this implies that the final morphotectonic uplift of the Lausitz Mountains started in the upper Lower Pleistocene (~1.3Ma) due to revived subsidence of the nearby Zittau Basin. It is likely that this neotectonic activity culminated between the Elsterian and Saalian Glaciation (~320ka). The formation of the low mountain range was substantially controlled by the intersection of the Lausitz Overthrust and the Ohře Rift.

Early to Middle Jurassic tectonic evolution of the Bogda Mountains, Northwest China: Evidence from sedimentology and detrital zircon geochronology

The Bogda Mountains, as an important intracontinental orogenic belt, are situated in the southern part of the Central Asian Orogenic Belt (CAOB), and are a key area for understanding the Mesozoic evolution of the CAOB. However, the tectonic evolution of the Bogda Mountains remains controversial during the Mesozoic Era, especially the Early to Middle Jurassic Periods. The successive Lower to Middle Jurassic strata are well preserved and exposed along the northern flank of the Western Bogda Mountains and record the uplift processes of the Bogda Mountains. In this study, we analysed sedimentary facies combined with detrital zircon U-Pb geochronology at five sections of Lower to Middle Jurassic strata to detect the tectonic evolution and changes of provenance in the Bogda area. During Early to Middle Jurassic times, the fluvial, deltaic and lacustrine environments dominated in the western section of the Bogda area. The existence of Early Triassic peak age indicates that the Bogda Mountains did not experience uplift during the period of early Badaowan Formation deposition. The Early Triassic to Late Permian granitoid plutons and Carboniferous volcanic rocks from the Barkol and Santanghu areas were the main provenances. The significant change in the U-Pb age spectrum implies that the Eastern Bogda Mountains initiated uplift in the period of late Badaowan Formation deposition, and the Eastern Junggar Basin and the Turpan-Hami Basin were partially partitioned. The Eastern Bogda Mountains gradually became the major provenance. From the period of early Sangonghe to early Toutunhe Formations deposition, the provenance of the sediments and basin-range frame were similar to that of late Badaowan. However, the Eastern Bogda Mountains suffered intermittent uplift three times, and successive denudation. The uplifts respectively happened in early Sangonghe, late Sangonghe to early Xishanyao, and late Xishanyao to early Toutunhe. During the deposition stage of Toutunhe Formation, a relatively strong tectonic reactivation took place along the Late Palaeozoic Bogda rift belt accompanied by relatively large-scale magmatism. The distinct basement structure between the eastern and western Bogda rift could be the structure basis of difference uplift in the Bogda area during the Mesozoic Era. The Early to Middle Jurassic episodic uplift of Eastern Bogda Mountains perhaps was related to the post-collisional convergence of the Qiangtang Block from late Badaowan to early Sangonghe, the closure of the western Mongol-Okhotsk Ocean at the Early-Middle Jurassic boundary and the tectonic accretion at the south Asian margin of Pamir Block during late Middle Jurassic times.

Insoluble Residue Analysis of Limestone in Kolhan Group: Tectonic Implications

The data from the insoluble residue analysis of nearly ninety eight samples collected from different horizons fully corroborate the petrogenetic evidences obtained from the petrographic and field features of the Kolhan Limestone. High grade limestones containing nearly 10% of insolubles constitute about half of the limestone samples. Only in ten samples, the silt-clay portion dominates over the sand portion and such very high grade pockets (containing at times about 95% CaCO3) on chemical analysis should show almost equal distribution of SiO3 and Al2O3, both together totaling to 5 to 10%. In the remaining forty samples the sand fraction clearly outweighs the silt-clay fraction and these varieties of equally high grade pockets should analyze SiO2 content distinctly greater than the percentage of Al2O3 [1]. The sedimentation history in the Kolhans indicates a change from braided fluvial-ephemeral pattern to a fan delta lacustrine type. Repeated fault-controlled uplift of the source, followed by subsidence, generated multiple fining-upward cycles and a retrograding fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on fan-delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced closure of the land locked basin, repeatedly disrupting the fining upward pattern [2]. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Such a retrograding fan delta are rarely reported and may be a stratigraphic response of connected rift basins at the early stage of extension.

The Landslides Developed with the Evolution of the River Terraces in the Loess Plateau, China

Landslide investigation in the center part of loess plateau reflects that most of large-scale old and ancient landslides lie on river terraces at different heights, but the small ones usually occur along the sides of V-shaped loess valleys, which implies that there is an inherent connection between the landslides and the erosion action. Since river terraces were formed by intermittent uplift of the crust, which controls the features of river erosion and the thickness of loess on the terraces. The analysis of typical loess landslides in this region shows that during relative static period, lateral erosion was dominated, so river valley was widened, which induced some large-scale landslides because the toe of slope was strongly eroded. While during the uplift period, vertical erosion was dominated, and the loess began to deposit on the terraces above water level, with the downcutting of the river, a narrow V-shaped valley formed, and then small loess landsides occurred on both sides of the rivers. Therefore, intermittent crustal uplifts not only led to the formation of multi-degree of terraces, but also induce alternative occurrence of large-scale and small loess landslides.

A Study on the Relationship between Debris Flow Deposit and Environmental Evolution in Quaternary

Based on field test,Typical debris flow profile in Quaternary was analyzed on the eastern edge of Qinghai-Tibet Plateau. According to relationship between debris flow deposition profile and geological age,debris flow deposition and neotectonics history,deep-sea oxygen isotope curve,loess-paleosol series were compared. Like other sediments, debris flow sedimentary body is constructed with action of neotectonics and climate.Debris flow deposits within the body surported by the absolute age date was a regular change in the vertical,which also reflected the environmental evolution. The relationship between environmental evolution and debris flow deposits was analyzed and the following conclusions were drawn: Fluctuations of debris flow deposits and intermittent uplift of neotectonics was basically a coincidence since the Quaternary in Xiaojiang River Basin whose debris flow deposits may correspond to climate of cold (cool) wet or warm (hot) wet.Fluctuations of debris flow deposits and climate cycles was basically a coincidence in Bailongjiang River Basin whose debris flow deposits only corresponds to the warm (hot) wet climate.

Transient convective uplift of an ancient buried landscape

Sedimentary basins in the North Atlantic Ocean preserve a record of intermittent uplift during Cenozoic times1. These variations in elevation are thought to result from temperature changes within the underlying Icelandic mantle plume2. When parts of the European continental shelf were episodically lifted above sea level, new landscapes were carved by erosion, but these landscapes then subsided and were buried beneath marine sediments3. Here, we use three-dimensional seismic data to reconstruct one of these ancient landscapes that formed off the northwest coast of Europe during the Palaeocene–Eocene Thermal Maximum. We identify a drainage network within the landscape and, by modelling the profiles of individual rivers within this network, we reconstruct the history of surface uplift. We show that the landscape was lifted above sea level in a series of three discrete steps of 200–400 m each. After about 1 million years of subaerial exposure, this landscape was reburied. We use the magnitude and duration of uplift to constrain the temperature and velocity of a mantle-plume anomaly that drove landscape formation. We conclude that pulses of hot, chemically depleted, mantle material spread out radially beneath the lithospheric plate at velocities of ~35 cm yr−1.

Evaluating the coastal deformation mechanisms of the Raukumara Peninsula, northern Hikurangi subduction margin, New Zealand and insights into forearc uplift processes

Coastlines that trend both parallel and normal to the Hikurangi subduction margin along the Raukumara Peninsula, New Zealand, provide a globally unique situation that enables study of the coastal deformation and its spatial variance across and along the forearc. The data suggest there is margin-parallel zoning of the forearc deformation processes across the Raukumara segment of the Hikurangi margin. On forearc promontories closest to the Hikurangi trench (within 80 km), Holocene marine and fluvial terraces record localised zones of intermittent uplift driven by large coseismic events on landward-dipping nearshore reverse faults. At greater distances from the trench (>100 km), Holocene and Pleistocene coastal stratigraphy and geomorphology suggest that surface uplift occurs by a gradual mechanism: either by frequent small earthquakes or by an aseismic process. In concurrence with geophysical models, we suggest the gradual uplift of this area is most likely driven by the buoyancy of underplated sediment with a contribution from subduction of the over-thickened crust of the Hikurangi Plateau. There is an intermediate zone at c. 80–100 km from the trench that is uplifted at low to moderate rates, probably by the effects of Hikurangi Plateau subduction with some contributions from outboard upper plate faulting and inboard sediment underplating.

Gamburtsev Subglacial Mountains provenance of Permian–Triassic sandstones in the Prince Charles Mountains and offshore Prydz Bay: Integrated U–Pb and T DM ages and host-rock affinity from detrital zircons

Permian–Triassic drainage radiates from the Gamburtsev Subglacial Mountains (GSM) in central Antarctica. Proximal to the GSM are Permian–Triassic fluvial sandstones in the Prince Charles Mountains (PCM), and neighbouring ?Triassic red beds in Prydz Bay (PB) ODP740A. We analysed detrital zircons for U–Pb ages, Hf-isotope compositions, and trace elements to determine the age, rock-type and source of the host magma, and “crustal” model age (T DM C). Populations of detrital zircons are (1) 700 to 500 Ma, host magmas granitoid and alkaline rock, T DM C ranges from 2.5 to 1.1 Ga, and (2) 1200–800 Ma, host magmas mafic granitoid and alkaline rock, T DM C 2.1 to 1.5 Ga. The bedrock of the PCM-PB region is a potential provenance of the detrital zircons, but the same populations in Permian siltstone south of the PCM and in sediment inclusions in ice at Lake Vostok indicate that the GSM–Vostok Subglacial Highlands (VSH) are the main provenance. Similar detrital zircons in other sandstones in Gondwanaland downslope from a wider central Antarctic reflect an upslope provenance including the GSM–VSH as a complex of 1200–800 Ma (Grenville) and older cratons with mafic granitoids embedded in 700–500 Ma fold belts with granitoids and alkaline rocks. During the past 1000 Ma, the GSM has undergone intermittent uplift on a scale resembling that of the present uplands of Central Asia.

Reconstructing Late Cenozoic Stream Gradients from High-Level Chert Gravels in Central Eastern Kansas

Interpreting the evolution of Kansas' landscape east of the Flint Hills provides major challenges. In the Neogene (late Tertiary) and perhaps part of the Pleistocene, streams transported a variety of sedimentary materials, including chert gravels derived from the Flint Hills. Gentle intermittent uplift stimulated the stream system to cut down, locally removing and reworking the gravels to create stream-terrace deposits that consist mostly of chert pebbles, which now lie well above the floodplains of modern streams. By correlating the elevations of these gravels, the gradients of the trunk streams that deposited them can be reconstructed. Interestingly, these ancient streams flowed southeast at a little more than a foot per mile (0.2 m/km), roughly the same as the gradient of the trunk streams in the region today.&#x0D; The evolving landscape in eastern Kansas also has been strongly influenced by an extensive network of fractures that is widespread in the midcontinent region and may be worldwide in extent. In northeastern Kansas, glaciation during the Pleistocene disrupted the southeasterly drainage and established the present location of the Kansas River. South of the Kansas River and its immediate tributaries, however, the general southeasterly drainage has been preserved.&#x0D; We have made use of the wealth of topographic-elevation data now available in digital form known as DEMs or digital elevation models. Coupled with GIS procedures, the DEMs helped link the mapped distribution of chert gravels with hypothetical fitted surfaces that represent ancient stream gradients. Furthermore, DEM data placed in shaded-relief map form emphasize the influence of fractures in evolution of the drainage system.&#x0D;

A retreating fan-delta system in the Neoproterozoic Chattisgarh rift basin, central India: Major controls on its evolution

A thick wedge of immature siliciclastics characterizes the basal part of the basin-fill succession in the eastern part of the Neoproterozoic Chattisgarh Basin. It comprises a mosaic of mass-flow–deposited conglomerate and pebbly sandstone, coarse-grained sandy braided stream deposits, wave-lag pebble sheets, high-energy, wave-dominated foreshore and upper shoreface facies, and wave- and tide-dominated lower shoreface facies. The succession is characterized by strong lateral and vertical facies variation. The facies associations and stacking pattern point to the development of an alluvial-fan to fan-delta complex, with outbuilding of delta lobes onto a high-energy shallow-marine shelf via a high-gradient, narrow delta plain. The delta-front deposits of coarse-grained sand interfinger and grade into prodelta mud in the seaward direction. The dominance of coarse-grained sandy detritus with a high content of fresh feldspar grains points to mechanical weathering of granites and gneisses of the cratonic basement and a semiarid climate. The sediment caliber and the climate exerted a major control on the facies and on processes of sediment transfer from the fan to the shelf. A critical balance between progradation, aggradation, and retrogradation controlled the evolution of delta successions within an overall transgressive regime related to the extension of a cratonic rift basin. Repeated fault-controlled uplift of the source, followed by subsidence and transgression, generated multiple fining-upward cycles and a retreating fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced regressions, repeatedly disrupting the fining-upward pattern. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Retreating fan deltas are rarely reported and may be a stratigraphic response of marine-connected rift basins at the early stage of extension. The delta stratigraphy is analogous to the submarine fan stratigraphy. In the case where basin shales act as hydrocarbon sources, the fan deltas are potential hydrocarbon reservoirs. The coarse-grained lobes can be excellent reservoirs, with the enclosing prodelta muds acting as seals to vertical migration of hydrocarbon. The coarse-grained deposits of the delta plain and proximal delta front, occurring as laterally coalesced, large sand bodies blanketed by transgressive prodelta mud, might also form good reservoirs.

Tectonic interpretations of Holocene marine terraces, east coast of Coastal Range, Taiwan

The east coast of the Coastal Range in Taiwan is well known for its multiple Holocene marine terraces and their very rapid uplift. The study area extends from Hualien to Taitung over ca 150 km of coastline. Here, we present some new data on the age and height of Holocene marine terraces, and we discuss their significance for Holocene coastal tectonics. The Holocene marine terrace sequence is subdivided into a maximum of 10 steps. The highest and oldest terrace, mostly underlain by transgressive deposits, represents the sea-level position of the culmination of Holocene sea-level rise and reaches up to ca 80 m above sea level. Transgressive deposits are radiocarbon dated at several sites, back to ca 13,000 yr BP. The lower terraces are mostly erosional ones, overlain by thin coral beds in situ, less than 1 m thick, and indicate successive lowering of relative sea level during the late Holocene. The uplift rate of the coast is very high, ranging from 5 to 15 m/ka. Three tectonic subregions, A–C, are identified, based on the pattern of marine terrace features and uplift rate. The presence of many steps during Holocene time implies that intermittent uplifts, associated with large earthquakes at intervals of one thousand to several hundred years, have repeatedly occurred in this coastal area, but with different time and amount at each subregion. The uplift of subregion A resulted from an onshore active fault (Milun fault). Causative faults for the uplift of the subregions B and C are not identified. Offshore active faults, striking obliquely to the coastline, appear to be causative faults.

Downcutting and uplifting in the middle part of Qinling oro-genic belt during the late Quaternary

Layered cave passages formed on the walls of the Qianyou River valley in the middle part of the Qinling Mountains since the Pleistocene due to the intermittent uplifts. 12 speleothem samples near ruins of palaeowater tables in 3 cave passages are dated by using the 230Th method. The results show that the 3 caves began to uplift (358±38) ka, (247±28) ka, (118±19) ka ago respectively. Given the differences of elevation between the caves, we could obtain the downcutting rates of the valley: (0.23±0.02) mm/a during 358-247 ka, (0.19±0.03) mm/a during 247–118 ka, and (0.51(0.08) mm/a since 118 ka. This implies that more and more strong uplifting sustained in the middle part of Qinling since 358 k

Analysis of TAB inner lead fatigue in thermal cycle environments : KEVIN D. CLUFF. IEEE Transcations on Components, Packaging, and Manufacturing Technology, Part A, 18(1), 101 (March 1995)

Palaeomagnetic poles derived from Precambrian formations can be valuable for determining relative, and sometimes absolute, ages of the formations. In this paper palaeomagnetic results are presented from a variety of these formations in Tanzania and Zambia. The Ikorongo Group sediments of Tanzania give a pole at 80° E, 25° S commensurate with an age of 900–1000 m.y. The lower Buanji Series of southern Tanzania yields a pole at 263°E,87°N indicating an age of either Late Precambrian (c. 650 m.y.) or Early Cambrian. The Plateau Series outcrop at the southern end of Lake Tanganyika gives several poles falling on the Late Precambrian to Ordovician apparent polar wander loop recognized by McElhinny et al. (1974), and a small amount of evidence from the Abercorn Sandstone and southern part of the Plateau Series outcrop suggests an age of c. 900 m.y. for these rocks. Dating of formations at the southern end of the Lake Tanganyika depression gives an estimate of 1500 m for the minimum amount of downthrow at this end of the rift system. Five sites from the Mbozi gabbro—syenite complex of southern Tanzania give a pole at 68° E, 72° N and two sites from Mbala dolerites of Zambia yield a pole close to one from the Bukoban dolerites of Tanzania and a similar age (c. 806 m.y.) is suggested.Some palaeomagnetic information is now available from all the Proterozoic platform sediments margining the Tanganyika craton and a correlation scheme is given which incorporates this information together with geochronological data. These formations postdate geosynclinal sequences involved in the Kibaran (c. 1300 m.y.) and Irumide (c. 1100 m.y.) mobile belts, and geological environment and situation demonstrate that the Tanganyika craton was subject to intermittent uplift between about 1000 m.y. and Cambrian times.

パプアニューギニア北東部のサンゴ礁段丘の総合研究 パプアニューギニア，ヒュンオン半島のサンゴ礁段丘に記録された第四紀後期（５２ｋａ以降）の地震隆起

A flight of coral terraces, up to ca. 1, 000 m above sea level, is well preserved on the northern coast of the Huon Peninsula, Papua New Guinea, and records the sea level and tectonic history since ca. 300 ka. The region is located near the boundary between the Western Pacific and Australian plates. The uplift rate since the last interglacial maximum (oxygen isotope stage 5e) ranges from 0.7 m/ka at the northwest end of the coast, and up to 3.5 m/ka towards the southeast end. Late Quaternary paleoseismicity is estimated from subdivided Holocene and late Pleistocene terraces, each of which records meter-scale coseismic uplift events.The study area covers about 40 km of coast. Holocene coral terraces, up to 25 m above sea level, are divided into seven levels at maximum, four to five at most sites. The highest Holocene terrace is a reef crest representing former sea level of the culmination of the sea level rise at ca. 6-6.6 ka BP. The lower terraces are regressive terraces, which record successive intermittent uplift events, probably caused by great earthquakes. Timing of the Holocene uplift events is ca. 2.5, 3.8, 4.4-5.1, and 5.4 ka BP at Kwambu-Kilasairo area on the northwestern coast, and at ca. 0.8, 1.4, 1.8, 2.5, 3.9 and 5.4 ka BP at Kanomi-Nanda area to Hubegong on the central and southern coast. Difference in the timing of the uplift events implies that there are at least two tectonic suregions on this coast.Similar small regressive terraces are also recognized from the detailed profiles of late Pleistocene terraces. For example, in the central and southern part of the study area, 15-7 extra steps occur between transgressive terrace IIIa (ca. 52 ka) and terrace II (ca. 38 ka). Thus, we infer that repeated meter-scale uplift events have occurred at least since ca. 52 ka. The apparent interval of uplift events, about 1-1.5 ka, resembles to that for Holocene.Such a meter-scale uplift with ka-scale recurrence intervals records a different earthquake deformation cycle from the centimeter-scale uplift associated with the 1992 May earthquake (M= 7.2 : Pandolfi et al., 1994). Such a small event with shorter recurrence interval is apparently not expressed geomorphologically as in the form of regressive terrace.

銚子半島高神低地における完新世海水準変化

This paper discusses the relative Holocene sea level change in a small lowland of Takagami, located on the central part of the Choshi Peninsula, easternmost of Kanto district. Samples obtained by drilling machine and peat-corer were used for radiocarbon dating, sedimentary facies observation and analysis of molluscan, diatom and pollen assemblages. Our data, combined with our first report (OTA et al., 1985), reveal that the Postglacial transgression has started at c. 10, 000y.B.P. at the latest. A rapid sea level rise has continued till c. 7, 000y.B.P., followed by a decrease of water depth. The upper limit of marine deposits is about 4.0m above mean sea level. The culmination of relative sea level rise has occurred at about 5, 500y.B.P., judged by clustering of radiocarbon ages of the lowest terrestrial peat bed in c. 5, 000-5, 500y.B.P. Since then, sea level probably has lowered to the present position without any intermittent uplift.

Pleistocene deformation of a portion of the Southern Cascadia Forearc: Prairie Creek Formation, northern California

The Prairie Creek Formation, a sedimentary unit of limited outcrop extent (150 km2) on the southern Cascadia forearc in northern California, provides geologic data that constrain the style of tectonic activity in this portion of the forearc during the time interval 4(?) Ma to about 0.5 Ma. The formation records deposition of approximately 550 m of mostly fluvial sediment during a period of subsidence of the forearc. The formation was deposited by a large braided river, most likely an ancestral Klamath River, that prograded into the forearc. Sedimentation mainly consisted of sand and well‐rounded conglomeratic alluvium. However, within the lower half of the section, there occur several thin horizons of coarse angular conglomerate of a distinctive lithology. The only likely provenance for this anomalous conglomerate is fault scarps intermittently exposed near the area of deposition. The tectonic setting of this portion of the southern Cascadia forearc in late Pliocene or early Pleistocene through mid‐Pleistocene was therefore one of general forearc subsidence with intermittent uplift due to faulting. Intercalation of sand and gravel braidplain deposits in the fluvial section suggests periodic changes in the net subsidence rate. Subsidence culminated in a brief period of submergence followed shortly by uplift to as much as 500 m in the late Pleistocene. The transition to uplift records a significant change in tectonic regime. This change may be due to the deformational effects of either or both of the following tectonic processes: northward encroachment of the Mendocino triple junction during the late Pleistocene, or a late Pleistocene change in the direction and/or rate of convergence of the Juan de Fuca plate relative to the overriding North American plate.