Late Eocene Early Oligocene Research Articles

Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada

Research Article| June 01, 1999 Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada Elizabeth L. Miller; Elizabeth L. Miller 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Trevor A. Dumitru; Trevor A. Dumitru 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Roderick W. Brown; Roderick W. Brown 2Victorian Institute of Earth and Planetary Sciences, La Trobe University, Bundoora, Victoria 3083, Australia Search for other works by this author on: GSW Google Scholar Phillip B. Gans Phillip B. Gans 3Department of Geological Sciences, University of California, Santa Barbara, California 93106 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1999) 111 (6): 886–905. https://doi.org/10.1130/0016-7606(1999)111<0886:RMSOTS>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Elizabeth L. Miller, Trevor A. Dumitru, Roderick W. Brown, Phillip B. Gans; Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada. GSA Bulletin 1999;; 111 (6): 886–905. doi: https://doi.org/10.1130/0016-7606(1999)111<0886:RMSOTS>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract New fission-track data together with 1:24 000-scale geologic mapping and analysis of Tertiary sedimentary deposits provide better constraints on the time and nature of motion along the Snake Range décollement, a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada. Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not available. These new data suggest that the Snake Range décollement forms part of a more extensive, 150-km-long north-south–trending fault system, the Snake Range–Deep Creek Range fault system. This fault system extends along the eastern flank of the northern and southern Snake Range, Kern Mountains, and Deep Creek Range, and accommodated at least 12–15 km of rapid slip in the Miocene, ca. 17 Ma. This component of motion is distinctly younger (by about 15–20 m.y.) than an earlier episode of slip and extension across the region bracketed stratigraphically and geochronologically as late Eocene–early Oligocene age.Apatite fission-track ages (n = 57) in most parts of the Snake Range and adjacent ranges cluster at 17 Ma, indicating rapid cooling from >125 to <50 °C during exhumation at that time. In the northern Snake Range, zircon fission-track ages (n = 3) are essentially concordant with the apatite ages, indicating very rapid cooling from >310 to <50 °C. Formation of at least part of the pervasive mylonitic fabrics in the northern Snake Range may have occurred during this Miocene time interval, very late rather than early in the extensional history of the region. Coarse fanglomerate and rock-avalanche deposits in flanking Tertiary basins provide additional evidence for major tectonism at this time. Comparison of the timing of events in the northern Snake Range to that along strike of the fault system indicates that Miocene slip along the low-angle northern Snake Range décollement and exhumation of extensive footwall mylonites were coeval with more typical Basin and Range high-angle rotational faulting in the Deep Creek Range and Kern Mountains to the north and in the southern Snake Range to the south. This suggests that the two styles of faulting (low-angle detachment and high-angle rotational) can occur simultaneously along the length of a single normal fault system. Data from the northern Snake Range also underscore the importance of a vertical component of uplift of the range in Miocene time, leading to the present domal geometry of the northern Snake Range décollement.When considered together with footwall deformational fabrics, the new data are most simply explained as the consequence of higher local geothermal gradients and a shallower brittle-ductile transition zone along the northern Snake Range part of the fault system. It can be speculated that the Snake Range metamorphic core complex represents the top of a stretching welt of hotter, deeper level crust that rose during extension. This rising welt may have been localized by the presence of previously thickened crust beneath the region and could have been triggered by increased regional magmatism and heating accompanying rapid extension in Miocene time. This content is PDF only. Please click on the PDF icon to access. 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Brittle tectonics and major dextral strike-slip zone in the Buda karst (Budapest, Hungary)

Three large (kilometric-scale) caves were studied in the Buda hills and the main directions of cave corridors, fault planes and mineralized veins were measured. Different stages of mineralizations are recognised: calcite scaleno-hedrons, baryte, silica, gypsum. New investigations of fluid inclusions in the baryte suggest a crystallization temperature of 50 °C and a freshwater fluid source. Microtectonic analysis allows the reconstruction of the successive tectonic events: (1) a NE-SW extensional phase at the Late Eocene-Early Oligocene limit (phase I), (2) a strike-slip phase with NW-SE compression and NE-SW extension during the Late Oligocene-Early Miocene (phase II), (3) a NW-SE transtensional phase (phase III) and finally (4) a NE-SW extensional phase of Quaternary age (phase IV). The major phase is the strike-slip one, characterized by an important dextral strikeslip zone: the Ferenc-hegy zone.

Brittle tectonics and major dextral strike-slip zone in the Buda karst (Budapest, Hungary)

Three large (kilometric-scale) caves were studied in the Buda hills and the main directions of cave corridors, fault planes and mineralized veins were measured. Different stages of mineralizations are recognised: calcite scaleno- hedrons, baryte, silica, gypsum. New investigations of fluid inclusions in the baryte suggest a crystallization temperature of 50 °C and a freshwater fluid source. Microtectonic analysis allows the reconstruction of the successive tectonic events: (1) a NE-SW extensional phase at the Late Eocene-Early Oligocene limit (phase I), (2) a strike-slip phase with NW-SE compression and NE-SW extension during the Late Oligocene-Early Miocene (phase II), (3) a NW-SE transten- sional phase (phase III) and finally (4) a NE-SW extensional phase of Quaternary age (phase IV). The major phase is the strike-slip one, characterized by an important dextral strike- slip zone: the Ferenc-hegy zone. © Elsevier, Paris

Lithostratigraphy and planktonic foraminiferal biostratigraphy of the late Eocene-Middle Miocene sequence in the area between Wadi Al Zeitun and Wadi Al Rahib, Al Bardia area, northeast Libya

The present study deals with the lithostratigraphy and planktonic foraminiferal biostratigraphy of the Late Eocene-Middle Miocene sequence in the Al Bardia area, northeast Libya. The lithostratigraphical studies carried out on three stratigraphical surface sections, namely Wade Al Rahib, Wadi Al Hash and Wadi Al Zeitun, led to the recognition of three rock units from base to top: (1) the Al Khowaymat Formation (Late Eocene-Early Oligocene); (2) the Al Faidiyah Formation (Late Oligocene-Early Miocene); and (3) the Al Jaghboub Formation (Early-Middle Miocene). The planktonic foraminiferal biostratigraphical analysis led also to the recognition of nine planktonic foraminiferal zones ranged in age from Late Eocene to Early Miocene with one larger foraminiferal zone of Middle Miocene age. These are, from base to top, as follows: Truncorotaloides rohri Zone (Late-Middle Eocene, Lutetian), Globigerinatheka semiinvoluta and Turborotalia cerroazulensis s.l. Zones (Late Eocene, Priaborian), Cassigerinella chipolensis/Pseudohasitgerina micra Zone (Early Oligocene, Rupelian), Globigerina ciperoensis ciperoensis, Globorotalia kugleri Zones (Late Oligocene, Chattian), Globigerinoides primordius Zone (Early Miocene, Aquitanian), Globigerinoides altiaperturus/Catapsydrax dissimilis and Globigerinoides trilobus Zones (Early Miocene, Burdigalian), and the larger benthonic foraminiferal zone, Borelis melo melo Zone (Middle Miocene, Langhian to Serravallian). The study of planktonic foraminifera proved the existence of a regional unconformity between the Early and Late Oligocene, with the Middle Oligocene deposits being absent (absence of Globigerina ampliapertura and Globorotalia opima opima Zones), and another, smaller unconformity located between the Late Eocene and Early Oligocene, in which the uppermost part of the Late Eocene is missing.

Extensional forced folding and décollement of the pre-rift series along the Rhine graben and their influence on the geometry of the syn-rift sequences

Abstract On the western margin of the Rhine graben, forced folding and décollement of competent Dogger strata occurred during the latest Eocene-Early Oligocene (Priabonian) as a result of normal faulting in the basement. The Dogger series are represented by NNE-striking thick layers of oolitic limestones strongly disrupted by extensional structures and incipient boudinage. To the east, the overlying Priabonian (syn-rift) sequences exhibit divergent onlaps, steeply dipping intraformational unconformities and a general drag syncline geometry. These syn-depositional structures attest to a close control by the progressive tilting and normal dip-slip sliding of the Dogger strata. Interpretation of a shallow seismic reflection profile reveals that this tilting is related to an underlying master normal fault which dips 60–70° to the east. This fault offsets the Variscan crystalline basement and the gently dipping Triassic cover by about 2 km. To be balanced, the structural arrangement requires the presence of a décollement layer located between the top of the Triassic series and the Dogger series, corresponding to the Late Triassic incompetent gypsiferous clays and marls. The response of the competent Dogger to the imposed forced folding is through the development of a large-scale extensional syncline in the hangingwall of the master detachment, compensated by the development of a piggy-back half-graben in the footwall. In the light of this new interpretation, a mechanical hypothesis is proposed including a two-stage evolution model of extension and subsidence for the Rhine graben.

Timing and genesis of early marine caymanites in the hydrothermal palaeokarst system of Buda Hills, Hungary

Sedimentological, biostratigraphic and magnetostratigraphic studies were carried out on five Late Eocene–Early Oligocene profiles in the Buda Hills, Hungary. The Szépvölgy Limestone Formation accumulated on the pre-Eocene basement. Basal strata of the limestone consist of a thin conglomerate followed by a coralgal limestone. The overlying limestone contains abundant Nummulites, Discocyclina . The contact between these two members is sharp. The Szépvölgy Limestone body, covering 60–65 km 2 , is considered a carbonate bank. Above the limestone, the siliciclastic slope deposits of the pelagic and turbiditic Buda Marl, and the euxinic Tard Clay accumulated. A huge multiphase hydrothermal cave system developed in the Szépvölgy Limestone resulting in a long-term composite palaeokarstic evolution lasting from the Late Eocene to the Quaternary. The first palaeokarst phase during the Late Eocene is represented by two generations of early marine cavity filling sediments of caymanite-type, deposited at sea-level. The earlier, carbonate infilling is conformable while the younger, siliciclastic one is disconformable with the bedding of the host rock. Bio- and magnetostratigraphic studies indicate that deposition of the caymanites-bearing palaeokarst host sequence started in the Late Eocene, during Chron C15r (35.3 Ma) and terminated in the Early Oligocene during Chron C13n (33 Ma). Two marker horizons are present, the first is between the Szépvölgy Limestone and Buda Marl at ∼34.6 Ma, and the second horizon is between the Buda Marl and the Tard Clay at 33.5 Ma. The Szépvölgy Limestone, deposited on a mobile shelf, represents a deepening upward sequence, interrupted by two lowstand events. They can be correlated with the PHd event of Keller et al., 1987 (Global distribution of late Palaeogene hiatuses. Geology 15, 199–203) and resulted in marine palaeokarstification. The carbonate shelf with the infillings drowned at 34.6 Ma. The caymanites accumulated at about 35.2–35.0 Ma and represent short-term palaeokarstic events. Their deposition record was estimated in several thousand years.

Tectonic evolution of the Tertiary basin at Batu Arang, Selangor Darul Ehsan, Peninsular Malaysia

In the Batu Arang area are found Tertiary sedimentary rocks that outcrop within an approximately elliptical basin surrounded by undulating to hilly terrain developed over Permian meta­ sedimentary rocks consisting of quartzites, schistose quartzites, phyllites, indurated shales and rare cherts. The Tertiary strata cover an area of some 15 km2 and rest unconformably over the meta­ sedimentary rocks that generally strike NNW-SSE with variable, steep to vertical dips. The Tertiary strata can be separated into two sequences: an upper one termed the Boulder Beds that consists of semi-consolidated, sandy to gravelly and bouldery sediments that unconformably overlie the lower sequence known as the Coal Measures or Batu Arang Beds comprising interbedded shales, structureless clays, fine grained to pebbly sandstones and some lignite seams. The Boulder Beds consist mainly of sub-angular to rounded, pebble to boulder sized clasts of quartzite, sandstone and vein quartz as well as minor chert, schist, phyllite and shale fragments, in a matrix of gravel, sand or clayey sand, though a few beds of inter-stratified sand and sandy clay are also seen. Differences in colour, as well as differences in composition and roundness of clasts, has allowed for differentiation of three separate sequences within the Boulder Beds. The Beds are up to 300 m thick at the centre of the basin where their bedding is horizontal, though along the basin edges, they show inward dips of 20° to 45°. A number of small, syn-depositional, normal faults, and post-depositional, wrench faults, cut these beds whose age is not know, though a late Middle Miocene to Pliocene age is proposed by correlation with regional tectonic events. The Boulder Beds are alluvial fan deposits that have developed through erosion of steep, fault induced slopes. The Coal Measures or Batu Arang Beds consist of several facies arranged in the following stratigraphical order: i) Sandstone with shale and clay intercalations, ii) Shale, iii) Coal beds, comprising two main coal seams, shale and sandstone, and iv) Sandstone with shale and clay and minor conglomerates including a basal pebbly sandstone. The strata show a synclinal structure plunging gently southeastwards, with the strata having an average dip of 14° towards the centre of the basin, though local variations are common especially towards the basin edges. The fine grained sediments range from stiff, structureless clays to well laminated and fissile shales containing abundant carbonaceous materiaL The shales are also often silty and even sandy and grade into clayey sandstone in places. The inter-bedded sandstones are mostly fine grained, though ranging up to coarse grained, and even pebbly. The coal occurs mostly as thin laminae or streaks in shales, but also builds thicker layers of more than 30 cm thick. In the eastern part of the basin, two thick coal seams are found as well as some thin beds of coal and coaly shale. The two seams are about 65 m stratigraphically apart; the Upper Seam attaining a thickness of up to 15 m, and the Lower Seam averaging 8 m in thickness. The coal appears to be intermediate between high-grade lignite and sub-bituminous coal and is a hard, black rock with a resinous lustre. Palynomorph assemblages indicate an Eocene to Oligocene age for the coals that are likely to have been peat-swamp deposits in an alluvial plain setting, whilst the inter-bedded shales were deposited in a freshwater lacustrine setting. Correlation with known stratigraphic successions in Sumatra shows that the Coal Measures are time-equivalent (Late Eocene-Early Oligocene) with, and have a somewhat similar depositional history as, the Parapat Formation of the North Sumatra Basin, the Permatang Group and Kelesa Formation of the Central Sumatra Basin, and the Lahat Formation of the South Sumatra Basin. Differences in lithology, however, occur and reflect differences in geographical locations; the sediments at Batu Arang having been deposited at a higher altitude, and in a more inland location, as compared with the Sumatran Basins that formed at lower altitudes, close to the then Sundaland coastline. Differences in geographical locations have also influenced the subsequent geological histories of the individual basins. No lithological equivalent of the Boulder Beds is found in the Tertiary strata of the Sumatran Basins, though its deposition is considered to be time-equivalent (Middle Miocene-Pliocene) with the Baong Formation of the North Sumatra Basin, the Minas Group and Binio Formation of the Central Sumatra Basin, and the Air Benakat Formation of the South Sumatra Basin. Correlation with regional tectonic events indicates that a pull-apart basin (of half-graben shape)

Origen y evolución de los Platyrrhini (Primates, Anthropoidea), con especial referencia a los registros de Argentina

The are new light about the Platyrrhini origin provided by the fossil record of the Old World, which demonstrates that catarrhines and platyrrhines are certainly a monophyletic group. Although the exact provenance of the Platyrrhini is uncertain at this time, some characters of the latter are shared with primitive anthropoid primates from the Fayum Depression, in the Egyptian late Eocene-early Oligocene, and then we can accept an African origin for platyrrhines. Symplesiomorphies exhibited by these two groups imply that the New World monkeys could be recognised morphologically more conservative. The evolutionary history of the Infraorder Platyrrhini has been a source of discussion, especially in recent years when the fossil record has increased dramatically. These advances contributed to a better understanding of their diversity and, on the other hand, the fossil diversity demonstrates that the living platyrrhines are a result of several evolutionary trends since the late Oligocene of South America. Fossil evidences come from several localities of South America and the Caribbean islands. The Patagonian fossil record includes specimens that show many particular characters not found among other fossil or living platyrrhines. Some living groups demonstrate close affinities with fossil relatives, such as those from La Venta, Colombia, and with a few specimens of Patagonia, while others are considered as part of a broad range of adaptive diversity whose extinct relatives are still unknown.

The structural evolution of central Crete: insight into the tectonic evolution of the south Aegean (Greece)

The recognition of crustal extension as a major mechanism of deformation of Crete and south Aegean region during Late Alpine times greatly clarifies the tectonic evolution of the area. Detailed structural analyses of the nappes of central Crete have enabled the tectonic evolution of the island, during Late Alpine times, to be reconstructed. Structural analyses of the upper nappes of Crete, which are similar to those of the Cyclades islands and were metamorphosed in pre-Oligocene time, also allows the tectonic evolution of Crete to be correlated with the better-known tectonic evolution of the Cyclades area. This history can be summarised as follows: In the Palaeocene–Eocene, ophiolites and the previously metamorphosed and deformed in the Late Cretaceous (D 1u) Asteroussia nappe constituted the hangingwall plate during subduction and the formation of HP rocks in the Cyclades. During the Late Eocene–Early Oligocene, possibly during the early extension of the Cyclades, crustal masses extruded toward the outer area of Crete probably accelerated the westward stacking (D 2u) of the developing nappes. In the Late Oligocene–Early Miocene, compression in the area of Crete became southward-directed (D 3), causing the subduction of the lower nappes and the southward stacking of the upper nappes. In the Cyclades, extension and southward escape of crustal masses continued. In the Miocene, crustal extension in Crete (D 4) caused the exhumation and uplift of the lower HP–LT metamorphosed nappes, while compression migrated southward to an external area, probably forming the Active Hellenic Subduction Zone (AHSZ). Finally, a Late Miocene–Early Pliocene (D 5) compression affected Crete, probably because of the lateral convergence of the European and African plates and initiation of the escape of the Turkish plate. Thus, the intensity and duration of deformation in the Cyclades area controlled and governed the structural evolution of Crete.

Geodynamic evolution of the Central Dinarides

The central part of the Dinarides, unlike the northwestern and southeastern Dinarides, shows a consistent pattern in the distribution of the large tectonostratigraphic units. From southwest (Apulia) to northeast, the following five main units originating in different parts of the Mesozoic Tethys, can be distinguished: (1) Adriatic–Dinaridic carbonate platform formations; (2) carbonate–clastic units (`flysch bosniaque') of the passive continental margin; (3) ophiolite-bearing formations of the Dinaridic Ophiolite Zone, also containing radiolarites, greywackes and shales, including mélange formations, and overstepped Late Jurassic to Cretaceous cover sequences; (4) active continental margin units represented by Upper Cretaceous–Palaeogene trench sediments with blueschists, tectonized ophiolite mélange, Alpine medium-pressure metamorphic rocks originated from Upper Cretaceous–Palaeogene sediments, and Alpine synkinematic granitoids; (5) a nappe consisting of allochthonous Palaeozoic–Triassic formations which is thrust onto the ophiolites and genetically related sedimentary formations. The frontal parts of this nappe directly overlie the northeastern margin of the Adriatic–Dinaridic carbonate platform, i.e. the External Dinarides. The evolution of the Central Dinarides was controlled by a sequence of tectonic events which took place within the Alpine cycle: (1) Rifting processes lasting some 40–50 Ma which started in the Late Permian and ended in the Norian. (2) Opening of the oceanic Dinaridic Tethys commenced in the Late Triassic and persisted for some 70–80 Ma until the end of the Jurassic. (3) Subduction processes and emplacement of ophiolites started in Late Jurassic–Early Cretaceous times; during the closing phase of the `Dinaridic Ocean' (or Dinaridic Tethys) the Palaeozoic–Triassic nappe was emplaced on the ophiolites. The thrusting was accompanied by Alpine metamorphism (120–100 Ma). (4) The main Alpine deformational event and medium-pressure metamorphism with synkinematic granite plutonism took place in Late Eocene–Early Oligocene times when the ophiolite and Palaeozoic–Triassic nappes were emplaced on the Adriatic–Dinaridic carbonate platform which, at the same time, was deformed resulting in the development of the External Dinarides fold-and-thrust belt.

Geology of the Saros graben and its implications for the evolution of the North Anatolian fault in the Ganos–Saros region, northwestern Turkey

Palaeo- and neo-tectonic evolutions of the Gulf of Saros, northwestern Turkey, were investigated based on geological mapping, geomorphology, seismicity and GPS measurements. In this area three overlapping basins were differentiated: the Thrace, Enez and Saros basins. The Thrace basin opened during the middle Eocene on the continental Strandja Massif as a post-collisional, fault-controlled extensional basin after the closing of the Intra-Pontide Ocean to the south. This basin reached its greatest extent during the Late Eocene–Early Oligocene and then, turned into an intramontane terrestrial basin from the middle Miocene onwards. The Enez basin opened along the southern margin of the former Thrace basin as an E–W-trending half graben during the middle Miocene. The age and geometry of this basin corresponds to the extensional basins of the Aegean graben system to the south. Stratigraphy and structures related to the North Anatolian fault indicate that the fault started to be active since Pliocene and modified older structures. The fault zone evolved in two stages in and around the Gulf of Saros. During the initial stage the fault was trending as a single segment with a transpressional nature and without any stepping, and caused thrusting and folding subparallel to the main trace of the fault. Since the Late Pliocene the fault was left-stepping in the Saros area, giving rise to the Saros pull-apart basin. We modelled these two stages by using boundary element method, and found that there is a good correlation between the modelling results and geology and geomorphology.

Late Eocene–early Oligocene tectonism, volcanism, and floristic change near Gray Butte, central Oregon

Mid-Tertiary strata near Gray Butte, central Oregon, record volcanism and sedimentation on the margin of a west-tilted basin that was subsequently disrupted by a northeaststriking fault system. Compositional characteristics of the volcanic rocks support correlation of the section to the upper Eocene‐Oligocene part of the John Day Formation. The ~1.2-kmthick section contains five fossil floras documenting climatic change in late Eocene‐early Oligocene time and a progression between better known Eocene and Oligocene floras in the region. The presence of the transitional floras is a consequence of the subsidence of the Gray Butte basin to accommodate a section that is nearly four times thicker than better studied correlative strata ~50 km to the northeast that consist almost entirely of volcanic rocks. The lowest flora is within flood-plain facies, whereas the other four are hosted in lacustrine strata. Although alteration of volcanic rocks most closely associated with the floras precludes development of a precise isotopic-age chronology, regional correlations and several accurate isotopic-age determinations indicate that the principal interval of climatic cooling may have been in late Eocene time (ca. 38‐39 Ma) rather than at the Eocene-Oligocene boundary. The paleoclimate interpretation is tempered, however, by the low diversity of the floras (12‐24 species) and possible taphonomic biases in comparing flood-plain and lacustrine environments. Mapping established the presence of the Cyrus Springs fault zone, a large (1.2 km vertical displacement, possibly >7 km dextral offset) shear zone that is possibly a surface expression of the Klamath‐Blue Mountain gravity-anomaly lineament. The orientations of this fault zone, subsidiary sinistral structures, dikes, and fold axes suggest that the presumed Mesozoic structure marked by the lineament was reactivated as a dextralnormal fault by east-northeast‐west-southwest‐oriented compressive stress. This stress is consistent with early Oligocene North America‐Farallon convergence but is inconsistent with northwest-southeast to northsouth compression suggested by structures farther east. Stress either varied temporally or was partitioned into complex local strain domains. The Cyrus Springs fault zone may have become active at about 28‐30 Ma,resulting in uplift and southward tilting of part of the Gray Butte basin fill, and ceased activity before deposition of the horizontal upper Miocene‐Pliocene Deschutes Formation. The Gray Butte area was also an eruptive center for rhyolitic and alkaline-mafic lava and tuff both before and after initiation of movement on the Cyrus Springs fault. Mid-Tertiary volcanism and sedimentation near the western end of the Blue Mountains, heretofore not clearly related to active structures, may have taken place within a regional transtensional regime associated with stress orientations different from those of Neogene time. Local basins with higher subsidence rates accumulated relatively thick sequences of lacustrine tuffaceous strata, and their fossil floras show progressive climate change through the Eocene-Oligocene boundary interval.

Laser probe 40 Ar/ 39 Ar ages of early- and late-stage alteration assemblages, Rosario porphyry copper-molybdenum deposit, Collahuasi District, I Region, Chile

Sensibly identical laser probe 40 Ar/ 39 Ar dates of 32.9 + or - 0.5 (2Sigma ) and 32.9 + or - 0.6 Ma are determined for, respectively, early-stage hydrothermal biotite and late-stage hydrothermal muscovite from the giant Rosario porphyry copper deposit in the Collahuasi district of northern Chile. The age determinations were carried out on ca. 1 cm 2 , 1- to 0.5-mm-thick wafers of altered rock or on smaller chips broken therefrom, obviating the requirement for mineral separation and permitting direct petrographic control of the analytical procedure. The age data, although not unambiguous, imply that this extremely large hydrothermal center was emplaced over a short interval. Hydrothermal activity in this central Andean transect migrated eastward over a period of ca. 3 to 4 m.y., from Copaquire, through Quebrada Blanca to Rosario, the intensity of mineralization increasing with time. Rosario, Chuquicamata, and La Escondida, the largest of the West Fissure (Domeyko fault system) array, were apparently emplaced coevally, in the terminal stage of the late Eocene-early Oligocene metallogenic episode.

Pre-Priabonian Palaeogene formations in southwestern Bulgaria and northern Greece: stratigraphy and tectonic implications

The Paril Formation (South Pirin and Slavyanka Mountains, southwestern Bulgaria) and the Prodromos Formation (Orvilos and Menikion Mountains, northern Greece) consist of breccia and olistostrome built up predominantly of marble fragments from the Precambrian Dobrostan Marble Formation (Bulgaria) and its equivalent Bos-Dag Marble Formation (Greece). The breccia and olistostrome are interbedded with thin layers of calcarenites (with occasional marble pebbles), siltstones, sandstones and limestones. The Paril and Prodromos formations unconformably cover the Precambrian marbles, and are themselves covered unconformably by Miocene and Pliocene sediments (Nevrokop Formation). The rocks of the Paril Formation are intruded by the Palaeogene (Late Eocene–Early Oligocene) Teshovo granitoid pluton, and are deformed and preserved in the two limbs of a Palaeogene anticline cored by the Teshovo pluton (Teshovo anticline). The Palaeocene–Middle Eocene age of the formations is based on these contact relations, and on occasional finds of Tertiary pollen, as well as on correlations with similar formations of the Laki (Kroumovgrad) Group throughout the Rhodope region.The presence of Palaeogene sediments within the pre-Palaeogene Pirin–Pangaion structural zone invalidates the concept of a ‘Rhodope metamorphic core complex’ that supposedly has undergone Palaeogene amphibolite-facies regional metamorphism, and afterwards has been exhumed by rapid crustal extension in Late Oligocene–Miocene times along a regional detachment surface. Other Palaeogene formations of pre-Priabonian (Middle Eocene and/or Bartonian) or earliest Priabonian age occur at the base of the Palaeogene sections in the Mesta graben complex (Dobrinishka Formation) and the Padesh basin (Souhostrel and Komatinitsa formations). The deposition of coarse continental sediments grading into marine formations (Laki or Kroumovgrad Group) in the Rhodope region at the beginning of the Palaeogene Period marks the first intense fragmentation of the mid- to late Cretaceous orogen, in particular, of the thickened body of the Morava-Rhodope structural zone situated to the south of the Srednogorie zone. The Srednogorie zone itself was folded and uplifted in Late Cretaceous time, thus dividing Palaeocene–Middle Eocene flysch of the Louda Kamchiya trough to the north, from the newly formed East Rhodope–West Thrace depression to the south.

Magnetobiostratigraphic chronology of the Eocene–Oligocene transition in the CIROS-1 core, Victoria Land margin, Antarctica: Implications for Antarctic glacial history

Research Article| January 01, 1998 Magnetobiostratigraphic chronology of the Eocene—Oligocene transition in the CIROS-1 core, Victoria Land margin, Antarctica: Implications for Antarctic glacial history Gary S. Wilson; Gary S. Wilson 1Byrd Polar Research Center, Ohio State University, 1090 Carmack Road, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar Andrew P. Roberts; Andrew P. Roberts 2Department of Oceanography, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom Search for other works by this author on: GSW Google Scholar Kenneth L. Verosub; Kenneth L. Verosub 3Department of Geology, University of California, Davis, California 95616 Search for other works by this author on: GSW Google Scholar Fabio Florindo; Fabio Florindo 4Istituto Nazionale di Geofisica, Via di Vigna Murata, 605, I-00143 Rome, Italy Search for other works by this author on: GSW Google Scholar Leonardo Sagnotti Leonardo Sagnotti 4Istituto Nazionale di Geofisica, Via di Vigna Murata, 605, I-00143 Rome, Italy Search for other works by this author on: GSW Google Scholar Author and Article Information Gary S. Wilson 1Byrd Polar Research Center, Ohio State University, 1090 Carmack Road, Columbus, Ohio 43210 Andrew P. Roberts 2Department of Oceanography, University of Southampton, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom Kenneth L. Verosub 3Department of Geology, University of California, Davis, California 95616 Fabio Florindo 4Istituto Nazionale di Geofisica, Via di Vigna Murata, 605, I-00143 Rome, Italy Leonardo Sagnotti 4Istituto Nazionale di Geofisica, Via di Vigna Murata, 605, I-00143 Rome, Italy Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1998) 110 (1): 35–47. https://doi.org/10.1130/0016-7606(1998)110<0035:MCOTEO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gary S. Wilson, Andrew P. Roberts, Kenneth L. Verosub, Fabio Florindo, Leonardo Sagnotti; Magnetobiostratigraphic chronology of the Eocene—Oligocene transition in the CIROS-1 core, Victoria Land margin, Antarctica: Implications for Antarctic glacial history. GSA Bulletin 1998;; 110 (1): 35–47. doi: https://doi.org/10.1130/0016-7606(1998)110<0035:MCOTEO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In 1986, cores were obtained to a depth of 702 m (with 98% recovery) from the CIROS-1 drill hole beneath the Ross Sea on the Victoria Land margin. Glaciogene sediments identified near the base of the hole mark the earliest known record of Antarctic glaciation. Initial biostratigraphic analysis indicated that the lower 336 m of the core is early Oligocene in age, and that the upper 366 m is of late Oligocene–early Miocene age. Recently, the chronology of the CIROS-1 core has been questioned. We developed a magnetostratigraphy for the lower 400 m of the CIROS-1 core to clarify the chronology. Our magnetobiostratigraphic results indicate that the base of the CIROS-1 core is early-late Eocene in age (corresponding to Chron C16r; ca. 36.5 Ma). We identify the Eocene-Oligocene boundary at about 410–420 m, within a 20-m-thick, poorly stratified, bioturbated sandy mudstone. This makes the CIROS-1 core the highest latitude site (77.1°S) from which this datum event has been recognized. At 366 m, a 4 m.y. hiatus, which lies immediately beneath fluvial sediments, accounts for most of Chrons C11 and C12. We recognize three major climatic episodes in the CIROS-1 core: (1) the late Eocene (34.5–36.5 Ma, 430–702 m), when relatively warm conditions dominated and there were high sedimentation rates and some glacial activity; (2) the late Eocene-early Oligocene boundary interval (28.5–34.5 Ma, 340–430 m), which was a transition from relatively warm to cooler conditions that coincided with glacial intensification, sea-level fall, and subaerial erosion of the shelf; and; (3) the late Oligocene–early Miocene (22–28.5 Ma, 50–340 m), when large-scale glaciation dominated the region and glaciers grounded across the continental shelf. From correlation with global oxygen isotope and sea-level records, we infer that the Antarctic climate and surrounding oceans cooled after separation of Australia and Antarctica and development of deep-water circulation between them. This marked the onset of the Eocene–Oligocene transition at ca. 34.5 Ma. A major East Antarctic ice sheet did not develop until the early-late Oligocene boundary, toward the end of the Eocene-Oligocene transition (ca. 28.5 Ma). Outlet glaciers did not breach the Transantarctic Mountains and ground across the Ross Sea Shelf until 0.5 m.y. later (ca. 28 Ma). This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The kinematics of extension and inversion in the Malay Basin, offshore Peninsular Malaysia

The evolution of the Malay Basin, a Tertiary extensional basin offshore Peninsular Malaysia, is explained in terms of a simple kinematic model that accounts for the following key observations: (1) Major basement faults along the basin axis are E-trending rather than NW-trending, (2) Through-going strike-slip faults are absent from the basin axis and margins, and (3) En echelon fold pattern in postrift strata seems to have been influenced by the geometry of underlying extensional half-grabens. Basin development during late Eocene-early Oligocene began with sinistral transtensional shear of a broad NW-trending shear zone (axial shear zone) which contains pre-existing E-trending basement faults. The shearing caused crustal blocks that are bounded by the faults to rotate anticlockwise and form E­ trending half-grabens between them. Reversal of shear during the early to middle Miocene, from sinistral to dextral, caused transpressive deformation and inversion of the half-grabens. The intensity of deformation increases southeastwards towards the West Natuna Basin as a result of the buttressing effect of the N atuna basement ridge which resisted the dextral motion along the axial shear zone.

The post-Pan-African thermal and extensional history of crystalline basement rocks in eastern Tanzania

Apatite fission track thermochronology from basement rocks in eastern Tanzania reveals a protracted history of denudation, uplift and extensional tectonism for the region after Carboniferous time. The data show that post-Pan-African development of eastern Tanzania was characterised by long periods of slow cooling punctuated by at least three relatively rapid cooling events, during the Early Cretaceous, Late Cretaceous-Early Palaeogene and Late Eocene-Early Oligocene. The relatively rapid cooling resulted from episodes of increased denudation rates related to the formation and reactivation of high-angle block faults that moved in response to intraplate stress. The episodes of denudation are manifested as packages of clastic sedimentary rocks in East African basins. Abrupt offsets in the trends of the apatite fission track data at major faults is consistent with development of a nascent rift valley within the broad region of previously block faulted and mildly extended crust, during Neogene time. The nascent rift valley was focused within a zone of weakness, associated with reactivation of favourably orientated regional-scale Proterozoic structures. This is particularly true of the subsurface boundary between the thick lithosphere of the Tanzanian Craton and the relatively thinner lithosphere of the Proterozoic Mozambique Belt.

Abstract: Magnetic Stratigraphy of the Type Refugian Stage (late Eocene-early Oligocene), Western Santa Barbara County, California&amp;nbsp;

Abstract: Magnetic Stratigraphy of the Type Refugian Stage (late Eocene-early Oligocene), Western Santa Barbara County, California&amp;nbsp;

Mid-crustal flow during Tertiary extension in the Ruby Mountains core complex, Nevada

Structural analysis and geochronologic data indicate a nearly orthogonal, late Eocene–Oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer-thick, extensional, mylonitic shear zone of the Ruby Mountains metamorphic core complex, Nevada. New U-Pb radiometric dating indicates that the development of a northward-trending lineation in the infrastructure is partly coeval with the development of a pervasive, west-northwest–trending lineation in the mylonitic shear zone. U-Pb monazite data from the leucogranite orthogneiss of Thorpe Creek indicate a crystallization age of ca. 36–39 Ma. Zircon fractions from a biotite monzogranite dike yield an age of ca. 29 Ma. The three dated samples from these units exhibit a penetrative, approximately north-south–trending elongation lineation. This lineation is commonly defined by oriented bundles of sillimanite and/or elongated aggregates of quartz and feldspar, indicating a synmetamorphic and syndeformational origin. The elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic, nonmylonitic infrastructural core of the northern Ruby Mountains. A portion of this midcrustal flow is coeval with the well-documented, west-northwest sense of slip in the structurally overlying kilometer-thick, mid-Tertiary mylonitic shear zone. Lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure, suggesting fundamental plastic decoupling between structural levels in this core complex. Furthermore, the infrastructure is characterized by overlapping, oppositely verging fold nappes, which are rooted to the east and west. One of the nappes may be synkinematic with the intrusion of the late Eocene orthogneiss of Thorpe Creek. In addition, the penetrative, elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes, suggesting a kinematically related evolution. The area is evaluated in terms of a whole-crust extension model. Magmatic underplating in the lower crust stimulated the production of late Eocene–early Oligocene granitic magmas, which invaded metasedimentary and Mesozoic granitic rocks of the middle crust. The midcrustal rocks, weakened by the magmatic heat influx, acted as a low-viscosity compensating material, decoupled from an extending upper crust. The fold nappes and lineation trends suggest large-scale flow of the weakened crust into the study area. The inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks. At present the inferred Eocene–early Oligocene phase of upper-crust extension remains unknown, but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large-scale upper-crustal extension.

Plume tectonics as a cause of mass wasting on the southeast Greenland continental margin

Deposition of sediments on the southeast Greenland shelf is limited by its narrow width, low degrees of tectonic subsidence, and lack of a block faulted basement topography. A marked hiatus on the continental slope, removing the Eocene and Oligocene, together with a late Eocene-early Oligocene hiatus on the continental rise, can be related to mass wasting along the continental slope. Erosion correlates with a phase of rapid tectonic subsidence, caused by relative retreat of the Iceland Plume from the area, affecting the oceanic crust more than the continental shelf. Relative deepening of > 1 km over 10 my steepened the continental slope and triggered erosion. Following the cessation of rapid subsidence, a submarine fan progressively onlapped the continental slope. The lack of a major mid Oligocene erosion surface on the shelf, despite shallow water depths, indicates that the magnitude of the mid Oligocene eustatic sea level fall is much less than the 200m predicted by the Exxon sea level curve.