Eocene Volcanism Research Articles

Auriferous sulfides from Hired gold mineralization, South Birjand, Lut block, Iran

The Hired gold mineralization is located in the Lut Block, eastern Iran. Reduced intrusion of subvolcanic granodiorite to quartz diorite stock with porphyry texture of the post Eocene age intruded into the Eocene volcanics and pyroclastic units led to alteration and gold mineralization. Mineralization occurred as three separate parts: intrusion-hosted, proximal and distal. The Hired gold mineralization can be considered as gold mineralization related to reduced granitoid intrusions. Free gold has not been observed in the form of native gold during microscopic studies. Therefore, it appears that gold probably occurs as invisible or submicrometer-sized inclusions in sulfide minerals. Electron probe microanalysis (EPMA) confirmed the existence of sulfide minerals such as pyrrhotite, galena, pyrite and arsenopyrite having the highest gold content; however, because of the relative abundance of pyrite and arsenopyrite, a high gold concentration corresponds to the presence of these sulfides. Considering the paragenetic sequences, gold mineralization occurred as invisible gold in the early and late hydrothermal stages.

Regional clockwise rotation of the Armutlu Peninsula, Western Turkey, resolved from palaeomagnetic study of Eocene volcanics

The North Anatolian Fault (NAF), one of the major tectonic entities of Turkey, is a dextral fault system lying within an intracontinental transform zone up to 100 km in width. This shear zone is composed of continental blocks delimited by strike–slip faults with the Armutlu Peninsula being one of the largest of these blocks and surrounded by two main branches of the NAF. Such fault-bounded blocks typically display rotations and a variety of different kinematic models have been proposed to explain these. In this study Eocene volcanic rocks of the Armutlu Peninsula have been sampled from 50 palaeomagnetic sites. Following alternating field and thermal demagnetisation, and component analysis, a dual polarity axis is resolved which shows that the peninsula has rotated 16° ± 6.8° clockwise relative to the predicted field direction at this location in Eocene times. This clockwise block rotation is interpreted in terms of dextral motion of a tectonic block bounded by two strands of the NAF. The palaeomagnetic data also indicate that the Eocene rocks of the Armutlu Peninsula were emplaced at a palaeolatitude statistically identical to the present and we conclude that northward movement of this sector of the Anatolides since Eocene times has been below the limits of palaeomagnetic detection.

Subduction-related Eocene shoshonites from the Cenozoic Urumieh-Dokhrat magmatic arc (Qaleh-Khargooshi area, West of the Yazd province, Iran)

The fifth phase of Eocene volcanism in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), comprises lavas with shoshonitic characteristics, ranging from trachybasalt to trachydacite. In shoshonites of the Qaleh-Khargooshi area, leucite changes to analcime, analcime to albite, sanidine to albite and anorthoclase, and clinopyroxene to albite. These reactions, together with other field, petrographic and geochemical features, are evidence of an original absarokitic magma which was changing to generate the different rock units. The parental magma of these shoshonites was produced by a low degree of partial melting from a metasomatized enriched mantle source. During the ascent of shoshonitic magma through the continental crust, crustal contamination, chemical changes and mineralogical transformations took place. The geochemistry of these shoshonitic rocks is similar to potassic volcanic rocks of continental arcs or convergent margins.

Geochemical features of the Okhotsk Sea Cenozoic volcanism

Cenozoic volcanic rocks were discovered on major rises of the inner Okhotsk Sea. In this paper, these rocks are geochemically typified, and the geodynamic conditions of their formation reconstructed. For this purpose, mineralogical and geochemical analyses as well as radioisotope age determinations were carried out. The radioisotope age determinations show that the Cenozoic volcanic rocks were formed during the Paleogene–Pleistocene. Within this period, Eocene and Plio-Pleistocene volcanic complexes are particularly prominent. Mineralogical and geochemical analyses show that the rocks belong to the calc-alkaline volcanic series. The Eocene volcanic rocks were formed under subaerial conditions, whereas the Plio-Pleistocene volcanics were formed under submarine conditions. The results of the study suggest that the Okhotsk Sea Basin was formed during the destruction of the Asian continental margin, the Eocene volcanism reflecting subaerial, the Plio-Pleistocene volcanism submarine stages of the Okhotsk Sea evolution.

On age of Nazimovskaya and Khasan formations in the Paleogene succession of southwestern Primor’e

Materials of previous studies and new original geological and paleobotanic data are used to substantiate synchronism (late Eocene) of the Khasan and Nazimovskaya formations, the key stratigraphic units of the Paleogene in Primor’e. Coal-bearing sediments of the units rest upon early-middle Eocene volcanics (Kraskino rhyolite tuffs, Zaisanovka basaltic andesites) being overlain by the lower Fatash Subformation of the lower Oligocene with flora of the Kraskino type. The Ust’-Davydovskaya Formation of the Rechnoi Peninsula (outskirts of Vladivostok) is the most probable analogue of the Khasan (=Nazimovskaya) Formation.

Palaeomagnetically defined rotations of fault-bounded continental blocks in the North Anatolian Shear Zone, North Central Anatolia

The 1200 km-long North Anatolian Transform Fault connects the East Anatolian post-collisional compressional regime in the east with the Aegean back-arc extensional regime to the west. This active dextral fault system lies within a shear zone reaching up to 100 km in width, and consists of southward splining branches. These branches, which have less frequent and smaller magnitude earthquake activity compare to the major transform, cut and divide the shear zone into fault delimited blocks. Comparison of palaeomagnetic data from 46 sites in the Eocene volcanics from different blocks indicate that each fault-bounded block has been affected by vertical block rotations. Although clockwise rotations are dominant as expected from dextral fault-bounded blocks, anticlockwise rotations have also been documented. These anticlockwise rotations are interpreted as due to anticlockwise rotation of the Anatolian Block, as indicated by GPS measurements, and the effects of unmapped faults or pre-North Anatolian Fault tectonic events.

Cenozoic uplift of Nuussuaq and Disko, West Greenland—elevated erosion surfaces as uplift markers of a passive margin

Remnants of a high plateau have been identified on Nuussuaq and Disko, central West Greenland. We interpret the plateau as an erosion surface (the summit erosion surface) formed mainly by a fluvial system and graded close to its former base level and subsequently uplifted to its present elevation. It extends over 150 km east–west, being of low relative relief, broken along faults, tilted westwards in the west and eastwards in the east, and having a maximum elevation of ca. 2 km in central Nuussuaq and Disko. The summit erosion surface cuts across Precambrian basement rocks and Paleocene–Eocene lavas, constraining its age to being substantially younger than the last rift event in the Nuussuaq Basin, which took place during the late Maastrichtian and Danian. The geological record shows that the Nuussuaq Basin was subjected to subsidence of several kilometres during Paleocene–Eocene volcanism and was transgressed by the sea later during the Eocene. By comparing with results from apatite fission track analysis and vitrinite reflectance maturity data, it is suggested that formation of the erosion surface was probably triggered by an uplift and erosion event starting between 40 and 30 Ma. Surface formation was completed prior to an uplift event that started between 11 and 10 Ma and caused valley incision. This generation of valleys graded to the new base level and formed a lower erosion surface, at most 1 km below the summit erosion surface, thus indicating the magnitude of its uplift. Formation of this generation of valleys was interrupted by a third uplift event also with a magnitude of 1 km that lifted the landscape to near its present position. Correlation with the fission-track record suggests that this uplift event started between 7 and 2 Ma. Uplift must have been caused initially by tectonism. Isostatic compensation due to erosion and loading and unloading of ice sheets has added to the magnitude of uplift but have not significantly altered the configuration of the surface. It is concluded that the elevations of palaeosurfaces (surfaces not in accordance with present climate or tectonic conditions) on West Greenland's passive margin can be used to define the magnitude and lateral variations of Neogene uplift events. The striking similarity between the landforms in West Greenland and those on many other passive margins is also noted.

Eocene volcanism above a depleted mantle slab window in southern Alaska

The Caribou Creek volcanic fi eld lies along the continent-side edge of forearc basin rocks in south-central Alaska and consists of over 1000 m of shallow-dipping basalt and andesite lavas with minor mafi c pyroclastic deposits. Dacite and rhyolite lavas along with shallow intrusions form dome complexes with associated pyroclastic deposits that overlie and crosscut the basalt and intermediate lavas. The basalts are tholeiitic and strongly depleted in the light rare earth elements (La/ Yb = 0.18–1.5), with concentrations of high fi eld strength elements (e.g., Zr, Hf, Ti, Y) similar to mid-ocean-ridge basalt and with variable enrichment in fl uid-mobile elements (e.g., Cs, Ba, and Pb). Intermediate and felsic rocks show enrichment in the rare earth elements and fl uid-mobile elements plus Rb and K, but retain low La/Yb ratios (0.48–3.6). A few andesite and dacite samples are strongly depleted in the heavy rare earth elements and are geochemically similar to adakites (e.g., Sr/Y up to 52). Ten 40 Ar/ 39 Ar ages for the Caribou Creek volcanic rocks range from 49.4 ± 2.2 to 35.6 ± 0.2 Ma. An adakite-like tuff beneath the other volcanic rocks yields an age of 59.0 ± 0.4 Ma. Caribou Creek basalts were derived from mid-ocean-ridge–like depleted mantle that was emplaced beneath the southern margin of Alaska through a slab window following spreading ridge subduction. Caribou Creek volcanism was coeval with oblique subduction, oroclinal bending, and right-lateral strike-slip faulting in south-central Alaska, all of which could have induced crustal extension to allow adiabatic melting of the depleted mantle reservoir to form basaltic magmas. The basalts then evolved by fractional crystallization with moderate to high degrees of assimilation of Jurassic arc basement rocks to form the intermediate and felsic magmas. Enrichment of the basaltic parent magmas in fl uid-mobile elements occurred by contamination from the Jurassic arc rocks and/or by contamination with metasomatic mantle remnant from preceding subduction. High heat fl ow through the slab window induced partial melting of garnet-bearing mafi c parts of the Jurassic arc basement to form the adakite-like rocks. The Caribou Creek volcanic rocks demonstrate that slab windows can directly infl uence magmatism inboard of accretionary prism and forearc basin settings given a suitable deformation regime (e.g., crustal extension) and that the infl uence of a slab window on continental margin magmatism can be long-lived (>20 m.y.).

Episodic Volcanism in the Buck Creek Complex (Central British Columbia, Canada): A History of Magmatism and Mantle Evolution from the Jurassic to the Early Tertiary

The Buck Creek volcanic complex of the Intermontane Superterrane of the Canadian Cordillera records a long history of volcanic activity from the Cretaceous through to the Eocene, when magmatic activity peaked, to the Miocene. Its basement includes primitive continental arc volcanic rocks of the Jurassic Hazelton Group (∼200 Ma; mainly basalt and andesites) emplaced prior to the accretion of Stikinia during the Mesozoic. In contrast, in the post-accretionary volcanic complex, the major pulse of volcanism started with the extrusion of continental margin calc-alkaline rocks (basaltic andesites to rhyolites) of the Cretaceous Tip Top Hill Formation (∼85 Ma). Their Nd-Sr isotopic compositions resemble those of the pre-accretionary Hazelton Group. However, this post-accretionary Cretaceous arc was more evolved and its crust was significantly thicker than the Hazelton. Mafic rocks of both the Hazelton and Tip Top Hill suites were generated from a spinel-bearing mantle source. The Eocene volcanics (∼50 Ma) evolved from typical high-K calc-alkaline mafic/intermediate rocks to flows that resemble intraplate tholeiitic basalts. Collectively, the Eocene rocks had a common (and comparatively deep) source, likely garnet-bearing subcontinental lithosphere. These compositional variations are probably related, in part, to an increase in the degree of partial melting. They record a gradual change from a compressional to an extensional tectonic environment, and overall represent an arc setting that matured and thickened over ca. 35 m.y., culminating in Eocene extension coincident with the cessation of compression in the foreland fold-and-thrust belt. Overlying intraplate alkali basaltic rocks of Miocene age appear to represent the final stage of this transition, as subduction-modified lithospheric mantle was replaced by a new asthenospheric mantle source. The association of Cretaceous, Eocene, and Miocene volcanic rocks in the Buck Creek complex is widespread in the region, suggesting that this model of tectono-magmatic evolution can be applied elsewhere in central British Columbia.

Eocene paleomagnetism of the Caucasus (southwest Georgia): oroclinal bending in the Arabian syntaxis

The Caucasus is very important for our understanding of tectonic evolution of the Alpine belt, but only a few reliable paleomagnetic results were reported from this region so far. We studied a collection of more than 300 samples of middle Eocene volcanics and volcano-sedimentary rocks from 10 localities in the Adjaro–Trialet tectonic zone (ATZ) in the western part of the Caucasus. Stepwise thermal demagnetization isolates a characteristic remanent magnetization (ChRM) in 19 sites out of 31 studied. ChRM reversed directions prevail, and a few vectors of normal polarity are antipodal to the reversed ones after tilt correction. The fold test is positive too, and we consider the ChRM primary. Analysis of Tertiary declinations and strikes of Alpine folds in the Adjaro–Trialet zone and the Pontides in Northern Turkey shows a large data scatter; Late Cretaceous data from the same region, however, reveal good correlation between paleomagnetic and structural data. Combining Late Cretaceous and Tertiary data indicates oroclinal bending of the Alpine structures which are locally complicated with different deformation. The overall mean Tertiary inclination is slightly shallower than the reference Eurasian inclination recalculated from one apparent polar wander path (APWP), but agrees with other. This finding is in accord with geological evidence on moderate post-Eocene shortening across the Caucasus. We did not find any indication of long-lived paleomagnetic anomalies, such as to Cenozoic anomalously shallow inclinations further to the east in Central Asia.

Cretaceous — Eocene Volcanism Along Eastern Coast, India — Its Implications

Cretaceous — Eocene Volcanism Along Eastern Coast, India — Its Implications

Eocene volcanism in the Buck Creek basin, central British Columbia (Canada): transition from arc to extensional volcanism

The Eocene Buck Creek volcanic complex (central British Columbia, Canada) is a part of the Challis–Kamloops volcanic province that extends from Yukon across British Columbia to the northern United States, running approximately parallel to the margin of the North American continent. The complex is associated with the formation of a fault-bounded basin controlled by strike–slip faults. The volcanic rocks were emplaced during two short-lived (<1–2My) periods. The first 52Ma old period produced high-K calc-alkaline mafic and intermediate lavas and related subvolcanic intrusions of the Goosly Lake Formation. Their age coincides with the age of two silver deposits, which are related to the intrusions. The second period which took place 50±1Ma ago, produced high-K calc-alkaline mafic and intermediate lavas (Buck Creek Formation), and overlying intraplate tholeiitic to high-K calc-alkaline basalts (Swans Lake unit). The rocks of the three units have similar Sr and Nd isotopic ratios and mantle-normalized trace element patterns characterized by a depletion of Nb, Ta and Ti suggesting a similar mantle source, garnet-bearing subcontinental lithosphere, for the lavas of all three units. The average contents of SiO2 and strongly incompatible trace elements in the lavas decrease from the Goosly Lake rocks towards the top of the complex (Swans Lake unit). The compositional changes could be, in part, related to an increase of the degree of partial melting towards the top. Volcanic rocks of similar composition and age are widespread in this part of the Eocene Challis–Kamloops belt in central British Columbia. The magmatic activities are related to subduction of the Farallon/Kula plate under the North American continent in a regime characterized by strike–slip faulting and oblique plate movement along the continental margin of western Canada. It is inferred that oblique plate convergence decoupled into a component of convergence normal to the plate margin, and a component of strike–slip faulting approximately parallel to the margin within the overriding North American plate. These events triggered a melting in the mantle wedge overlying the subduction zone and generated the parent magmas for the rocks of this part of the Challis–Kamloops belt.

A Unified Alteration Index (UAI) for Mafic Rocks

The geomechanical properties of diabase in the ophiolitic melange and basalt in the Lower and Middle Eocene volcanics of Ankara, Turkey, were determined as part of an investigation of potential dam sites. A classification is proposed based on a concept called the “unified alteration index (UAI),” which expresses a percentage decrease in uniaxial strength and compressional wave velocity of the mafic rocks determined in the laboratory. The UAI classification consists of five groups—unaltered, slightly, moderately, highly, and extremely altered—for this proposed modified classification system. A statistical analysis was used in examining the relationships between the UAI and geomechanical properties of the mafic rocks, and revealed that the UAI = exp (−0.0495 – 0.001σ c ) (r 2 = 0.64) and UAI = 1.0058 – 7.663e-5C p - 8.227e-9C p 2 (r 2 = 0.93).

Paleomagnetism of Tertiary rocks on the Korean Peninsula: tectonic implications for the opening of the East Sea (Sea of Japan)

Two different distributions of declinations with dual polarities are found in the Tertiary rocks of the Yeonil Basin and Yangnam Basin along the Yangsan Fault in the southeastern part of the Korean Peninsula. These are northward declinations in the Middle Miocene Yeonil Group and strong eastward declinations in the Early Miocene Yangbuk Group and in the Eocene Volcanic Rocks. This is interpreted by invoking (1) the Yangsan Fault as a dextral wrench fault system by spreading in the Uleung Basin of the East Sea (Sea of Japan), and (2) a clockwise vertical rotation caused by the movement of the fault at around 17.3 Ma just prior to the sedimentation of the Yeonil Group. The timing of the deflection found in Korea is distinguished from that in southeastern and northwestern Japan at around 15 Ma. Two stages of spreading are thus constrained in the East Sea area: parallel at 23–15 Ma and pivotal at around 15 Ma. Crustal thinning also occurred. A reliable pole position (84.9°N, 292.6°E, d p/ d m=3.3°/4.8°) for the Miocene (about 15 Ma) in Korea is obtained from the Yeonil Group.

Cenozoic magmatism in Kalimantan and its related geodynamic evolution

The NE–SW Tertiary magmatic belt of central Kalimantan is related to two separate periods of subduction; during the Eocene–Oligocene and Late Oligocene–Miocene. The younger magmatic belt is superimposed upon the earlier belt. This magmatic belt is characterized chiefly by Late Oligocene–Miocene volcanic products, among which limited exposures of the Eocene volcanics have also been mapped by previous investigators. This calc-alkaline magmatic belt has become known as the ‘gold belt’ of Central West Kalimantan on account of a number of discoveries of Neogene epithermal gold mineralization. This mineralization is found in central to proximal volcanic settings and occurred at relatively shallow depths. The earliest known subduction-related magmatism took place in the Eocene–Early Oligocene with the emplacement of calc-alkaline silicic pyroclastics, followed by a period of continental collision. Subsequent subduction-related magmatism continued from Late Oligocene–Pleistocene, during which time the magma evolved from calc-alkaline to potassic calc-alkaline. Plio-Pleistocene magmatism resulted in the formation of basalt flows. The present available K–Ar ages of the Cenozoic volcanics range from 51 to 1 Ma.

Regional Significance of Neotectonic Counterclockwise Rotation in Central Turkey

Counterclockwise rotation is a characteristic feature of the results of most paleomagnetic studies of the Pontides and Anatolides of central Turkey, applicable to regions both north and south of the North Anatolian fault zone. In this paper, we report new data from Eocene volcanics and assess existing data from the calc-alkaline volcanic suites of this age. Although there are regional variations, probably resulting from rotations of individual fault blocks, an average counterclockwise rotation of ∼33° is identified across a region extending from 34° to 38° E Long. A mean Eocene paleolatitude of 27° N is compatible with ongoing northward movement and residual closure of a few degrees across the Pontide orogen during the latter part of its paleotectonic history. It seems probable that this rotated domain extends as far west as the Aegean graben system of western Turkey and as far south as the Taurides. Paleomagnetic evidence from younger volcanics suggests that the bulk of the rotation occurred during Quate...

CORRELAÇÃO DO MAGMATISMO DAS BACIAS DA MARGEM CONTINENTAL BRASILEIRA COM O DAS ÁREAS EMERSAS ADJACENTES

The available information on the Cretaceous-Tertiary magmatism in Brazilian marginal basins is synthesized, aiming at a comparison with similar events that affected the emerged adjacent areas. The Eocretaceous pre-Aptian basaltic volcanism is more common in the basins from Espirito Santo to the Sduth, where it can be correlated with dikes and lava flows of the Serra Geral Formation. It is also present in Northeast Brazil, at the southern part of the submerged Potiguar Basin. At the extreme south of Brazil, the presence of acid magmatism in the Parana Basin allows one to suppose the existence of similar compositions in the poorly known Pelotas Basin. The quiescence of magmatism during Aptian-Albian time is a common characteristic of almost all Brazilian marginal basins, as well as in the emerged continental area, but rocks belonging to this age are present in the Ceara Basin and in the Cabo area (PE). Neocretaceous-Eocene magmatism of alkaline trend is present in the emerged region of the Cabo Frio Magmatic Lineament. Possibly it can be extended to the submerged margin. Eocene volcanism is very important at the Cabo Frio Platform and south of Campos Basin. The islands situated along the Santos BasirTs margin are composed by feldspathoid-bearing alkaline rocks, the emplacement of which is related to the Rio de Janeiro Lineament. The Tertiary volcanism in Northeast Brazil lasted until Miocene, in the Potiguar Basin and emerged region, but in South-Southeast Brazil it seems to have ceased after Eocene. The Fernando de Noronha Fracture Zone extended up to the coast, allowing the onset of Oligocene alkaline volcanism. The Remanche Fracture Zone extends up to the Ceara Basin and the Sao Luis Craton, and controls the acid magmatism observed in the Atlântico and Ceara highs. The Vitoria-Trindade Fracture Zone also seems to extend to the continent, as suggested by the structures and basic dikes of the Fundao Suite. The basins situated between the Ceara State and the Marajo Island are amagmatic, as well as the Reconcavo Basin, in part because they have been installed on rigid crust of Sao Luis and Sao Francisco cratons. It is suggested that the faster continent withdrawal in the South as compared to the Northeast Brazil associated to different rift processes could have been the cause for cessation of volcanism only after the Miocene in the last region.

A Revised Interpretation of Eocene Volcanic Stratigraphy in the Lower North and South Fork Shoshone River Valleys: ABSTRACT

A Revised Interpretation of Eocene Volcanic Stratigraphy in the Lower North and South Fork Shoshone River Valleys: ABSTRACT

The ‘Rajang accretionary prism’ and ‘Lupar Line’ problem of Borneo

Abstract The Rajang Group, composed of the Belaga and Lupar Formations of Sarawak and the Embaluh Group and Selangkai Formation (in part) in Kalimantan, had a turbidite sedimentation history from Early Cretaceous to Late Eocene. These rocks generally young northwards. Inliers within the eastern Miri Zone have been mapped as the Kelalan and Mulu Formations. The Rajang Group was compressed into a steeply dipping quartz-veined phyllite-quartzite complex by the Sarawak orogeny and unconformably overlain locally in the north by the Upper Eocene continental to neritic Tatau Formation, extensively in the south by Middle to Upper Eocene basal continental sequences of the Ketungau, Mandai and Melawi basins, and widely in the north by the Upper Oligocene coastal to marine Nyalau Formation. In Sabah and East Kalimantan, the Upper Cretaceous to Upper Eocene Mentarang, Sapulut, Trusmadi, and possibly the East Crocker Formations, also belong to the Rajang Group. The West Crocker Formation demonstrates rapid facies changes into the more shaly Temburong Formation, and was deposited as sandy turbidites throughout the Oligocene. To the south their equivalents are the nearshore Kelabit and Long Bawang Formations. The West Crocker Formation was folded and uplifted in several Miocene pulses, resulting in regional unconformities and igneous events at Mount Kinabalu. The West Crocker Formation has not been metamorphosed, and dips are shallower than in the Belaga and Trusmadi Formations. Its provenance probably was from the uplifted Upper Cretaceous to Eocene Lurah and Kelalan formations of NE Kalimantan and East Sarawak. It is therefore proposed that the West Crocker and Temburong formations be excluded from the Rajang Group. Middle to Late Miocene Crocker Formation uplift and deformation, herein called the Sabah orogeny, was synchronous with spectacular basin inversion throughout the South China Sea and in the Meratus Mountains of Kalimantan. Uplift ceased in the Late Miocene and undeformed post-inversion formations unconformably overlie inverted folded structures. The Rajang Group flysch-belt may be interpreted as a north-facing accretionary prism. The Schwaner Mountains represent a subduction-related Lower to Upper Cretaceous volcanoplutonic arc. Scattered Eocene volcanism, Miocene Sintang intrusives and Pliocene Metalung volcanics in Central Kalimantan and Sarawak post-date subduction. The Rajang Basin (Danau Sea) rapidly narrowed and by Eocene time subduction was transformed to collision as the Rajang Group was compressed between the Schwaner Mountain Zone and the Luconia-Balingian-Miri block. The Ketungau Basin is in sharp contact with the Rajang Group along the bounding Lupar Fault, which can be traced northwards into the East Natuna region. Palaeocurrents show that the Upper Eocene basal sandstones have a provenance in the metamorphosed Sibu Zone. The Melawi and Mandai basins of Kalimantan also unconformably overlie the flysch-belt. The basins are not forearc and were formed after transformation of the accretionary prism to a landmass formed of a collisional orogenic complex.

Timing and Episodicity of Middle Eocene Volcanism and Onset of Conglomerate Deposition, Idaho

Seventeen $^{40}Ar/^{39}Ar$ incremental release mineral ages from lava flows and tuffs in the Lost River and Lemhi Ranges, Idaho, (1) demonstrate deposition of thick sequences of syntectonic conglomerate in middle Eocene and Oligocene time and (2) reveal two temporally and compositionally distinct phases of the Challis Volcanic Group. Thick sections of coarse conglomerates are preserved in two NNW-trending half graben. These were previously assigned to the Pliocene ( ? ) but are reassigned to the Eocene and Oligocene based on ages from four tuffs interbedded with conglomerates. Andesitic and dacitic lava flows and tuffs of the Challis Volcanic Group, up to 2.5 km thick, were deposited between about 49 and 48 Ma, whereas rhyolite tuffs, only tens of meters thick, accumulated between about 46 and 45.5 Ma. The rhyolite tuffs are intercalated within or underlie the syntectonic conglomerates. Lithologic, stratigraphic, and geochronologic evidence correlate these tuffs with the tuff of Challis Creek, which erup...