Middle Eocene To Early Oligocene Research Articles

PORPHYRY COPPER RECURRENCE IN THE ANDES OF CHILE AND ARGENTINA

Abstract Porphyry Cu deposits in the Chilean and Argentinian central Andes occur in a series of orogen-parallel magmatic arcs, which migrated episodically eastward since the Early Cretaceous. The three Cenozoic belts, corresponding to Paleocene-early Eocene, middle Eocene-early Oligocene, and Miocene-early Pliocene epochs, cut obliquely across a composite belt of subeconomic porphyry Cu mineralization formed at several times during the Late Carboniferous to Triassic interval. Based mainly on U-Pb zircon and Re-Os molybdenite ages, 10 Cenozoic porphyry Cu centers, including major deposits in all three of the Cenozoic belts, occupy the same sites as late Paleozoic-Triassic porphyry Cu mineralization where their respective magmatic arcs cross one another. The sites of recurrent porphyry Cu mineralization are believed to be underlain by long-lived dike conduits that were utilized at least twice—first in the late Paleozoic-Triassic and then again in the Cenozoic—to rapidly transmit hydrous magma from deep to shallow levels of the crust. Contenders for preferential dike localization include arc-oblique fault and subjacent ductile shear zones—transcrustal discontinuities—long hypothesized to be present in the region, particularly where they intersect magmatic arcs and associated porphyry Cu belts. Regardless of the controls on porphyry Cu recurrence, alteration zones hosted by late Paleozoic-Triassic volcanic and plutonic rocks in the central Andes of Chile and Argentina must be considered prime exploration targets for potentially large, high-grade Cenozoic porphyry Cu deposits.

Outcrop to petrography scale and new age constraint of the Jatisamit Mélange in the Luk Ulo area, Central Java, Indonesia

To resolve the formation of the Luk Ulo Mélange in the Karangsambung area of Central Java, the matrix-dominated Jatisamit Mélange, which is a member of the Luk Ulo Mélange, was examined. Outcrop and petrographic observations of sandstone boudins and their shale matrix from the Jatisamit Mélange were used to assess deformational mechanisms and formation. We suggest the Jatisamit Mélange underwent three deformation phases: (1) formation of a web structure in the sandstone boudins through a cataclasis mechanism; (2) formation of boudins and cleavage through a ductile deformation; and (3) formation of open fractures and Riedel shears through pressure increasing. These deformations occurred at relatively low to medium temperatures during progressively increasing differential stress. Using illite crystallinity, the matrix of the Jatisamit Mélange has a maximum metamorphic temperature of 150–200 °C (diagenesis zone) at a 4–5 km depth. In addition, calcareous nannofossil assemblages of the Jatisamit Mélange and other related rock units in Karangsambung suggest a depositional matrix age of middle Eocene–early Oligocene. Therefore, the mixing of the Luk Ulo Mélange occurred not earlier than the middle Eocene–early Oligocene at a shallow structural level in the suture zone during the collision of the southeastern margin of Sundaland with the Gondwana micro-continent.

First Pyrotheria (Mammalia, Meridiungulata) from the Quebrada de Los Colorados Formation (middle Eocene–early Oligocene) at Los Cardones National Park, northwestern Argentina

First Pyrotheria (Mammalia, Meridiungulata) from the Quebrada de Los Colorados Formation (middle Eocene–early Oligocene) at Los Cardones National Park, northwestern Argentina

Petrography and Provenance of the Sub-Himalayan Kuldana Formation: Implications for Tectonic Setting and Palaeoclimatic Conditions

In this paper, the depositional environment, age, and tectonic context of the Sub-Himalayan Kuldana Formation are discussed in detail. To determine the Kuldana Formation’s depositional environment, age, and tectonic setting, sedimentological, palaeontological, and petrographic investigations have been conducted accordingly. The Kuldana Formation lithologically consists of both siliciclastic and carbonate rocks. Petrographically, the Kuldana Formation’s sandstone is divided into litharenite and feldspathic litharenite petrofacies. The sandstone plots on the QtFL and QmFLt suggest that the sandstone of the Kuldana Formation derived from a recycled orogen provenance field that developed during the collision of the Indian and Eurasian plates in the Lesser and Higher Himalayas. The plots in the diamond diagram further demonstrate that the detritus of the Kuldana Formation was derived from low and middle-to-upper rank metamorphic rocks of the Himalayas. Throughout the deposition of sandstone, paleo-climate conditions were semi-humid to semiarid. Dolostone and limestone are the two main types of carbonate rocks found in the Kuldana Formation. According to Dunham’s Classification, the Kuldana Formation limestone is classified as mudstone, wackstone, and packstone. These petrofacies suggest that the limestone was deposited in an inner-outer ramp setting. The bioclasts include bivalves, brachiopods, crinoid, gastropods, Globigerinoides spp., Lockhartia pustulosa, miliolids, Nummulites atacicus, Nummulites discorbina, Nummulites mamillatus, Nummulites djodjokartae, Nummulites vascus, and ostracods suggesting that the age of Kuldana Formation is Middle Eocene-early Oligocene. The Kuldana Formation was deposited during the initial stages of the Himalayan Orogeny as a result of the Ceno-Tethys Ocean’s regression and transgression, as revealed by a succession of siliciclastic and non-clastic rocks.

Pluton Exhumation in the Precordillera of Northern Chile (17.8°–24.2°S): Implications for the Formation, Enrichment, and Preservation of Porphyry Copper Deposits

Abstract Hypogene mineralization in porphyry Cu deposits is typically associated with crustal thickening and rapid exhumation, whereas supergene enrichment requires slow exhumation to allow sufficient time for leaching and downward transport of Cu before it is lost to surface erosion. Therefore, spatial and temporal patterns of exhumation within a metallogenic belt can highlight favorable locations for hypogene mineralization, supergene enrichment, and preservation. Here, we determine average pluton exhumation rates along an ~730-km segment of the middle Eocene-early Oligocene metallogenic belt in northern Chile (17.8°–24.2°S). By combining zircon U-Pb geochronology with Al-in-hornblende geobarometry, we pinpoint the time and depth at which each pluton was emplaced and use the age of overlying cover units or supergene minerals to date its arrival at the surface (or near-surface) environment. Uranium-lead zircon ages for 49 samples from plutons and porphyries range from Carboniferous to Eocene (~314–35 Ma). Al-in-hornblende emplacement depths for 19 plutons are ~4–7 km, with one Carboniferous pluton emplaced at ~12 km. Two phases of net exhumation are identified: early Permian-Middle Triassic and middle Eocene-late Oligocene, with an intervening period of net burial. The highest exhumation rates (>0.30 km/m.y.) derive from the second phase, coeval with the Incaic orogeny and the main phase of hypogene mineralization. Present-day preservation of plutons and porphyry Cu deposits required low post-Oligocene average exhumation rates of <~0.01 km/m.y.—favorable for the development of many world-class supergene blankets. However, spatial variability in exhumation and burial across the belt led to poor conditions for supergene development locally: enrichment was hampered in some places by rapid exhumation after hypogene mineralization (e.g., ≥~4 km at El Abra), by burial beneath significant cover (e.g., Ministro Hales, Queen Elizabeth), or, in the Inti region of northernmost Chile, by a combination of the two.

Late Eocene-Oligocene granulite facies garnet-sillimanite migmatites from the Mogok Metamorphic belt, Myanmar, and implications for timing of slip along the Sagaing Fault

The Mogok Metamorphic Belt (MMB) in Myanmar is a polymetamorphic, mainly Paleogene granulite-uppermost amphibolite facies terrane consisting mainly of marbles and calc-silicates hosting spinel, ruby and sapphire. Jurassic charnockite-syenite intrusions, as well as Eocene-Miocene leucogranite intrusions are also present. Pelitic rocks are uncommon, and where present, have sillimanite, both as primary inclusions in garnet and as secondary Bt + Sil coronas around garnet. Core samples from the Kyi-Tauk-Pauk gold mine at Thabeikkiyin, north of Mandalay, are mostly Grt + Bt + Sill gneisses and migmatites with uncommon interbanded Opx + Grt + Bt gneisses. Pseudosection modelling suggests prograde garnet growth occurred by biotite-dehydration melting that reached peak P–T conditions of 870–970 °C and ~ 0.9 GPa, and was followed by garnet breakdown forming coarse retrograde Bt + Sil coronas. UPb monazite data show an early high-grade granulite event at 43–32 Ma, and a later upper amphibolite sillimanite-grade event peaking at 23–20 Ma, with a change in monazite chemistry after c. 22 Ma that is consistent with fluid/(melt) interaction and garnet breakdown. Elevated Th/U ratios from ~35 to 22 Ma, and at ~18 Ma are compatible with melt influx at that time, timing that is similar to the age of the regional Kabaing leucogranite in the Mogok valley area. Our data show that peak granulite facies metamorphism along the Mogok Metamorphic belt was mainly Middle Eocene-Early Oligocene, with upper amphibolite facies metamorphism extending to earliest Miocene. The MMB is cut abruptly by the Sagaing fault, a large-scale dextral fault that extends from the Andaman Sea north to the East Himalayan syntaxis. Our new UPb monazite data constrain the oldest age of initiation of the eastern branch of the cross-cutting Sagaing dextral strike-slip fault at <20–18 Ma.

Oil-oil correlation, geochemical characteristics, and origin of hydrocarbons from Mansourabad oilfield, SW Iran

Biological markers and carbon isotope data were used to delineate characteristics, maturity, and source of the crude oils from Asmari and Bangestan reservoirs of the Mansourabad oilfield, SW Iran. In spite of extremely similar source-related parameters reflecting an identical source, non-biodegraded Asmari and Bangestan oils have different gravities. Based on not-equilibrated maturity parameters, Bangestan oils are slightly more mature than Asmari oils resulting in higher API gravity. Geochemical indicators propose a carbonate-marl source rock deposited in a marine environment under anoxic-suboxic conditions as the source of the oils. Strong marine organic matter signature of predominantly algal origin along with robust signals of contribution from land plants were detected. Based on geochemical evidence, the Mansourabad field Asmari and Bangestan oils being sourced from a mix of Kazhdumi and Pabdeh organic matters. Carbon isotope composition and V/Ni ratio of the oils delineate Kazhdumi Formation of Albian age as the main source rock. Nevertheless, partial contribution of Pabdeh source rocks of Middle Eocene-Early Oligocene were also deduced based on medium concentration of oleanane, low sulfur content, and less than unity values of C29/C30 hopanes ratio. It seems that the degree of contribution from Pabdeh-derived oils were insufficient to impact a typical mixed signature over the geochemical fingerprint of the reservoired oils. According to the stratigraphic levels, the latest more mature Kazhdumi-derived oils only charged the adjacent reservoir, resulting in higher overall maturity of the Bangestan oils. Since maturity and bulk geochemical properties of the Asmari and Bangestan oils are different, it can be concluded that fluids within the reservoirs are completely separated from each other by effective static barriers of dense Pabdeh and Gurpi beds.

Hydrocarbon reserves of Gachsaran oilfield, SW Iran: Geochemical characteristics and origin

Abstract Biomarkers and carbon isotope data were used to depict geochemical properties and to delineate source of the crude oils from Asmari and Bangestan reservoirs of the giant Gachsaran oilfield, SW Iran. Despite the distinct horizons of the reservoirs and severe petrophysical heterogeneity inside each reservoir, all oil samples show an identical level of thermal maturity. Furthermore, similar source-related parameters reflect identical source rock for both reservoirs. They indicate carbonate-marl source deposited in a marine environment under anoxic-suboxic conditions. Strong marine organic matter signatures of predominantly algal origin along with evidence of a terrigenous angiosperm plant contribution were detected. Geochemical evidence suggests that the Gachsaran field Asmari and Bangestan oils being sourced from a mix of organic-rich Kazhdumi and Pabdeh source rocks of Albian and Middle Eocene-Early Oligocene ages, respectively. Carbon isotope composition and V/Ni ratio signatures indicate Kazhdumi Formation as the main source rock. Meanwhile, significant presence of oleanane and less than unity values of C 29 /C 30 hopanes ratio demonstrate a partial contribution of Pabdeh Formation. This contradiction could be explained by two scenarios; 1) incorporating very immature non-expelled Pabdeh oils into the mature Kazhdumi oils during migration or 2) partial charge form low maturity Pabdeh oils that is insufficient to impact a typical mixed signature over the geochemical fingerprint of the reservoired oils. Additionally, geochemical similarities give rise to an assumption of reservoir and trap inter-connection for this giant field.

The phylogeny and biogeography of Hakea (Proteaceae) reveals the role of biome shifts in a continental plant radiation.

The frequency of evolutionary biome shifts during diversification has important implications for our ability to explain geographic patterns of plant diversity. Recent studies present several examples of biome shifts, but whether frequencies of biome shifts closely reflect geographic proximity or environmental similarity of biomes remains poorly known. We explore this question by using phylogenomic methods to estimate the phylogeny of Hakea, a diverse Australian genus occupying a wide range of biomes. Model-based estimation of ancestral regions indicates that Hakea began diversifying in the Mediterranean biome of southern Australia in the Middle Eocene-Early Oligocene, and dispersed repeatedly into other biomes across the continent. We infer around 47 shifts between biomes. Frequencies of shifts between pairs of biomes are usually similar to those expected from their geographic connectedness or climatic similarity, but in some cases are substantially higher or lower than expected, perhaps reflecting how readily key physiological traits can be modified to adapt lineages to new environments. The history of frequent biome-shifting is reflected in the structure of present-day assemblages, which tend to be more phylogenetically diverse than null-model expectations. The case of Hakea demonstrates that the radiation of large plant clades across wide geographic areas need not be constrained by dispersal limitation or conserved adaptations to particular environments.

Main radiation events in Pan-Octodontoidea (Rodentia, Caviomorpha)

Fil: Arnal, Michelle. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Departamento Cientifico de Paleontologia de Vertebrados; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

Wide rift model in Bohai Bay Basin: insight into the destruction of the North China Craton

The Bohai Bay Basin is a Cenozoic extensional basin along the eastern aspect of Asia. Whether the Bohai Bay Basin is a pull-apart or rift basin is controversial. The Bohai Bay Basin exhibits a high density of extensional faults and records destruction of the North China Craton. Many structural analyses have been performed on the Bohai Bay Basin, especially the Tan-L and Taihang Mountain fault systems which control its boundary. The initial deposition of Kongdian Formation was mainly distributed along the boundary of Bohai Bay Basin during the Palaeocene–early Eocene. Subsequently, tectonic activity migrated toward the interior of the basin during deposition of Shahejie Formation in the middle Eocene–early Oligocene. Bohai Bay Basin crust was thickened in early Mesozoic time and has thinned since late Mesozoic time. The crustal strength profile of Bohai Bay Basin is characterized by very weak lower crust, which differs from that of adjacent crust. In regard to the crustal structure, lithospheric thickness, and extensional style, an alternative rift model is proposed. Initial Bohai Bay Basin rifts were characterized by metamorphic core complexes affecting the North China Craton, which reflects collapse of parts of the early Mesozoic intra-plate orogen. Furthermore, westward subduction of the Palaeo-Pacific Plate led to upwelling of asthenosphere mantle. Persistent upwelling of mantle decreased the strength of lower crust and led to the warm heat-flow regime and generation of a lower crustal fluid layer and wide rifting. Outward flow of ductile lower crust following late Cretaceous extension thinned the lower crust and generated the overall sag appearance of the basin in early Cenozoic time. The model supports a model whereby a wide rift narrows with time. For the Bohai Bay Basin, extension and strike-slip faulting were two independent deformation systems superimposed on each other.

Inicio del enfriamiento global del eoceno medio y expansión de ambientes con vegetación abierta en la patagonia central.

Climate-driven changes in terrestrial environments and biomes after the Early Eocene Climatic Optimum are poorly documented from southern continents. Particularly, Middle Eocene-Early Oligocene leaf and pollen data from Central Patagonia (46oS, Argentina) are not sufficient to characterize floristic paleocommunities. Paleosols of the Ca&ntilde;ad&oacute;n Vaca (~45-42 Ma) and Gran Barranca (42-38.5 Ma) members (Sarmiento Formation), studied at Ca&ntilde;ad&oacute;n Vaca, solve such deficiency and help to reconstruct Middle Eocene landscapes in the beginning of the Cenozoic cooling-drying trend. Vitric Entisols, mollic Andisols and andic Alfisols, showing granular structure and diverse micropeds, are cyclically arranged mainly in response to variation in fine volcaniclastic eolian supply, which in turn governed ecosystem stability and maturity. Soils formed in loessic plains crossed by minor ephemeral rivers, supported open herbaceous-arboreal communities which grew in seasonal, subhumid and warmtemperate conditions. Phytoliths produced by Arecaceae, megathermic graminoids, sedges and dicots, from the upper part of the studied unit, represent subtropical savannas with grasses and variable number of palms and other trees. Considering the abundant paleobotanical and paleopedological antecedents of Late Paleocene-Early Eocene warm and humid forested environments in the same region, the lower Sarmiento Formation records the initial expansion of open herbaceous communities and the appearance of grassy habitats during the greenhouse to icehouse transition in the Middle Eocene.

A palaeo Tibet–Myanmar connection? Reconstructing the Late Eocene drainage system of central Myanmar using a multi-proxy approach

Strain resulting from the collision of India with Asia has caused fundamental changes to Asian drainage patterns, but the timing and nature of these changes are poorly understood. One frequently proposed hypothesis involves the connection of the palaeo Tsangpo drainage to a precursor to the Irrawaddy River of central Myanmar in the Palaeogene. To test this hypothesis, we studied the provenance of Palaeogene fluvio-clastic sedimentary rocks that crop out in central Myanmar, namely the Late Middle Eocene–Early Oligocene Pondaung and Yaw Formations. Isotopic analysis on bulk-rock and petrographic data indicate a primary magmatic arc source, and a secondary source composed of recycled, metamorphosed basement material. Although the exact location of both sources is hardly distinguishable because Burmese and Tibetan provinces share common lithological features, the presence of low-grade metamorphic fragments, the heterogeneity in Sr–Nd isotopic values of bulk sediments and westward-directed palaeoflow orientations indicate a proximal source area located on the eastern Asian margin. Central Myanmar was the locus of westward-prograding deltas opening into the Indian Ocean, supplied by the unroofing of an Andean-type cordillera that extended along the Burmese margin. We found no evidence to support a palaeo Tsangpo–Irrawaddy River, at least during the Late Eocene. Supplementary material: Data locations, and isotopic and petrographic results are available at www.geolsoc.org.uk/SUP18655 .

New Palaeogene cingulates (Mammalia, Xenarthra) from Santa Rosa, Perú and their importance in the context of South American faunas

The record of Palaeogene cingulate xenarthrans in low latitudes is very poor. The cingulate fauna from the Yurúa River near Santa Rosa in eastern Perú is important because it is one of the oldest known from the Palaeogene of Perú and because of its tropical latitudinal position. Although remains are scarce, we recognize three new taxa: two species of Astegotheriini (Dasypodidae); Parastegosimpsonia peruana gen. et sp. nov., of small size, related to Eocene species from Patagonia; and another new species, ?Parastegosimpsonia sp. nov., which consists of an incomplete osteoderm representing the largest species of this tribe. A third species, Yuruatherium tropicalis gen. et sp. nov., of indeterminate suprageneric rank, shares features with Machlydotherium Ameghino (Casamayoran-Tinguirirican SALMA – middle Eocene-early Oligocene of Patagonia) and is similar to Eocoleophorus Oliveira et al. (Deseadan SALMA – late Oligocene of Brazil). We assign ?Machlydotherium intortum Ameghino (from the late Eocene of Patagonia) to Yuruatherium. Sediments bearing these cingulates also yielded rodents, marsupials and notoungulates, among the most frequent mammals. The absolute age of the sediments is unknown but an estimated age is inferred from studies of the mammalian assemblages. The age of the Santa Rosa local fauna is still controversial and given the groups taken into account, could be from Early Eocene to Late Oligocene. According to sequences of southern cingulate faunas (especially those of Dasypodidae), the cingulates from Santa Rosa also suggest an age between the Late Eocene and Early Oligocene. Nevertheless, the very low latitude of the Santa Rosa local fauna should be taken into account because in lower latitudes it is not uncommon to find taxa with a more generalized set of characters than those present in contemporary taxa from higher latitudes. http://zoobank.org/urn:lsid:zoobank.org:pub:37CAEF7C-3CCC-4C3C-9869-D584FD73323C

Paleogeographic and tectonic controls on the evolution of Cenozoic basins in the Altiplano and Western Cordillera of southern Peru

Integrated studies of stratigraphy, sedimentology, paleogeography and tectonic controls on Cenozoic basins provide the basis for a series of time-slice reconstructions of basin evolution in the Andes of southern Peru. The Altiplano and adjacent margin of the Western Cordillera are characterized by several Paleocene–Miocene synorogenic continental basins with thicknesses locally exceeding 10km. The evolution of these basins has been controlled by NW-trending tectonic features that mark the Altiplano–Western Cordillera and Altiplano–Eastern Cordillera boundaries and the Condoroma structural high. Sedimentary deposits of Paleocene age preserved in the Altiplano are the result of nonmarine sedimentation in a distal foreland basin. During the early Eocene, predominantly dextral strike-slip movements in the Altiplano between the Cusco–Lagunillas and Urcos–Ayaviri fault systems created the transpressional Kayra basin. The Soncco and Anta basins (middle Eocene–early Oligocene) are related to NE shortening (43–30Ma) and represent proximal, wedge-top and foredeep basin environments preserved on the Altiplano. At ~29–28Ma, a change to predominantly E–W shortening produced sinistral strike-slip motion along NW-striking faults, resulting in intermontane, transpressional basins. In the Altiplano, the Tinajani and Punacancha (29–5Ma), and Paruro (12–6Ma) basins were controlled by the Cusco–Lagunillas and the Urcos–Ayaviri fault systems. The Maure, Tincopalca–Huacochullo and Condoroma basins (22–5Ma) of the Western Cordillera developed between the Condoroma high and the Cusco–Lagunillas fault system. Oligocene–Miocene sedimentation commonly evolved from proximal (alluvial) facies along the borders to distal (lacustrine) facies. These basins were linked to sinistral strike-slip faults that evolved into reverse-sinistral structures. Plate kinematics may play a role in Andean basin evolution, with deformation influenced by major preexisting faults that dictated paleogeographic trends, but orientations of regional compression that appear to coincide with the plate convergence direction. However, the processes of slab flattening and steepening exerted a primary control on regional crustal shortening and filling of synorogenic basins.

Middle Eocene to early Oligocene calcareous nannofossil biostratigraphy at IODP Site U1333 (equatorial Pacific)

We present a biostratigraphic and biochronologic study of calcareous nannofossils of middle Eocene - early Oligocene age recovered during IODP Expedition 320, at Hole U1333C in the equatorial Pacific Ocean. The study succession encompasses nannofossil Zones NP16–NP21 (equivalent to CP13–CP16) and Chrons C20r–C12r (middle Eocene-early Oligocene). The distribution patterns of calcareous nannofossil taxa are studied by means of relative abundance and semiquantitative counts with the final aim to test the reliability of biohorizons used in the Paleogene standard biozonations (Martini 1971; Okada and Bukry 1980) and check alternative bioevents included in a more recent mid-latitudes biostratigraphic scheme (Fornaciari et al. 2010). Calibration ages are estimated based on the ranges of the biozones relative to a detailed magnetostratigraphy constructed for the site. Of particular biostratigraphic significance, our study shows that the Top of Sphenolithus furcatolithoides, the Base of common and continuous occurrence (Bc) of Dictyococcites bisectus and the total range of Sphenolithus obtusus can be used to better constrain the middle Eocene interval. The studied sediments cover the crucial time period that followed maximum Cenozoic warmth and led up to the initial major glaciation on Antarctica, including two important climatic events, the Middle Eocene Climatic Optimum (MECO), a transient episode of global warming during a long-term cooling trend, and the Oi-1 event. The peculiar regime in sedimentation observed in the equatorial Pacific, which roughly consists of alternating phases of Carbonate Accumulation Events (CAE) and crashes in carbonate content, are correlated with increases and decreases in calcareous nannofossil abundances. A more detailed comparison indicates that the MECO corresponds to an interval with very low carbonate in between CAE3 and CAE4. This event is correlative with the Top of S. furcatolithoides, the Bc of D. bisectus and a prominent increase in the relative abundance of heavy calcified nannofossils (e.g., discoasters).

Oligocene regional denudation of the northern Afar dome: Pre- and syn-breakup stages of the Afro-Arabian plate

Research Article| November 01, 2012 Oligocene regional denudation of the northern Afar dome: Pre- and syn-breakup stages of the Afro-Arabian plate Yoav Avni; Yoav Avni † 1Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel †E-mail: yoav.avni1@gmail.com Search for other works by this author on: GSW Google Scholar Amit Segev; Amit Segev 1Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Search for other works by this author on: GSW Google Scholar Hanan Ginat Hanan Ginat 2The Dead Sea and the Arava Science Center, Kibbutz Qetura, Israel Search for other works by this author on: GSW Google Scholar Author and Article Information Yoav Avni † 1Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Amit Segev 1Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel Hanan Ginat 2The Dead Sea and the Arava Science Center, Kibbutz Qetura, Israel †E-mail: yoav.avni1@gmail.com Publisher: Geological Society of America Received: 01 Nov 2011 Revision Received: 19 Apr 2012 Accepted: 14 May 2012 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (11-12): 1871–1897. https://doi.org/10.1130/B30634.1 Article history Received: 01 Nov 2011 Revision Received: 19 Apr 2012 Accepted: 14 May 2012 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yoav Avni, Amit Segev, Hanan Ginat; Oligocene regional denudation of the northern Afar dome: Pre- and syn-breakup stages of the Afro-Arabian plate. GSA Bulletin 2012;; 124 (11-12): 1871–1897. doi: https://doi.org/10.1130/B30634.1 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 A regional truncation surface developed during the Oligocene in the northern Red Sea and southern Levant as a result of the combined effects: (1) the formation of an ∼3000 km × 1500 km crustal dome affecting the northeastern sector of the Afro-Arabian continent above the Afar plume, and (2) widespread fluvial erosion that truncated the northern edge of this dome. The regional truncation surface separates two main stratigraphic divisions: (1) middle Eocene–early Oligocene prerift clastics preserved below the regional truncation surface and (2) late Oligocene–Holocene Red Sea Supergroup deposited after the regional truncation surface developed. The prerifting sequence is composed of two major units: (1) Eocene marine intraformational debris flows (Themed clastics), and (2) fluviolacustrine continental conglomerates, most of them monomictic (Mahatardi Conglomerate). The Red Sea Supergroup is composed of polymictic conglomerates, which include fragments of older rock units, down to the Precambrian. The reconstructed regional truncation surface and its subcrop map shed new light on the initial stages of the Afro-Arabian breakup, indicating two principal phases: (1) an Oligocene slow crustal doming, with reactivation of preexisting faults and the development of the regional truncation surface, all triggered by the upwelling of the Afar plume; and (2) late Oligocene–early Miocene rifting of the Red Sea and Suez rift, which refaulted the region, including the initiation of the Dead Sea Transform along a preexisting suture line. This two-stage model indicates the mutual genetic relationship between the early Oligocene phase of tectonic development following the penetration of the Afar plume and the post-Oligocene rifting and continental breakup of the Afro-Arabian continent. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Kinematic evolution of Andean fold‐thrust structures along the boundary between the Eastern Cordillera and Middle Magdalena Valley basin, Colombia

Surface and subsurface data support a kinematic reconstruction of Cenozoic fold‐thrust deformation along the Eastern Cordillera‐Magdalena Valley transition in Colombia. The La Salina fault (LSF) marks the boundary between west‐vergent Eastern Cordillera structures and hinterland deposits of the Middle Magdalena Valley basin. Apatite fission track and (U‐Th)/He thermochronological results for the west‐directed LSF reveal initial hanging wall exhumation during middle Eocene–early Oligocene (45–30 Ma) shortening, renewed exhumation in the early middle Miocene (18–12 Ma), and accelerated late Miocene‐Pliocene (12–3 Ma) exhumation. Vitrinite reflectance data suggest maximum burial of 4–6 km, helping constrain Cenozoic basin architecture. Mapping of the LSF reveals hanging wall Cretaceous–Eocene rocks in a broad anticline‐syncline pair with limited faulting and footwall Eocene–Quaternary basin fill in a complex series of tight thrust‐related folds. Limited displacement along the westernmost (frontal) thrust suggests that shortening is largely accommodated by east‐directed thrusting within a broader triangle zone of a passive‐roof duplex (and probable minor strike‐slip deformation). In the preferred kinematic restoration, the most recent phase of shortening to transpressional deformation represents out‐of‐sequence reactivation of the LSF consistent with irregular crosscutting relationships among footwall structures. Earliest exhumation by 45–30 Ma in the Eastern Cordillera fold‐thrust belt is correlated with increased sedimentary lithic fragments and high compositional maturity in sandstones of the adjacent Magdalena Valley basin. Exhumation since ∼15 Ma coincided with decreased compositional maturity and elevated accumulation rates for the Real Group. The compositional provenance shifts are attributed to westward advance of fold‐thrust deformation into the proximal (eastern) segments of the Magdalena Valley basin.

A refined calcareous nannofossil biostratigraphy for the middle Eocene–early Oligocene Southern Ocean ODP sites

This work presents the stratigraphic distribution of several species of calcareous nannofossil in the middle Eocene early–Oligocene from four Ocean Drilling Program (ODP) sites located between 60° and 65°S paleolatitude in the Southern Atlantic and Indian Oceans. Useful nannofossil datums that should facilitate construction of age-models and contribute to an integrated chronology for the upper Paleogene Southern Ocean sediments from ~42 to 33Ma are summarized. The distribution patterns of calcareous nannofossils, studied by means of quantitative and semiquantitative methods, provide an improvement of the classical Southern Ocean biozonations, introducing new biostratigraphically useful biohorizons, and testing their reproducibility within and outside the region.

Paleostress analysis of Cenozoic faulting in the Kraishte area, SW Bulgaria

Paleostress calculations from fault planes in Paleogene sediments and in the underlying basement were used to determine the orientation and the chronology of the principal stresses during the Cenozoic tectonic evolution of the Kraishte area (southwest Bulgaria). The results show that middle Eocene–early Oligocene WSW–ENE extension led to the formation of grabens and half grabens filled with thick continental to marine deposits in the hanging wall of extensional detachments.The Paleogene sediments and their basement were then intruded by subvolcanic bodies and dykes during SW–NE extension between 32 and 29 Ma. These WSW–ENE and SW–NE extensional stages are related to the general rollback of the Hellenic slab.From the late Oligocene to the earliest Miocene, SSE–NNW transtension generated coal-bearing sedimentary basins. The anticlockwise rotation of the main tensile axis by almost 50° with respect to the previous tectonic stage was probably related to a switch from the Aegean back-arc extension to crustal stretching and extrusion of continental fragments around the Moesian platform.Since the middle Miocene extension in the Kraishte area, accommodated by faults with relatively small displacement, led to the formation of restricted basins filled with alluvial to lacustrine deposits.