Late Oligocene Time Research Articles

Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies

Research Article| July 01, 2006 Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies Sergio Rocchi; Sergio Rocchi 1Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I56126 Pisa, Italy Search for other works by this author on: GSW Google Scholar Wesley E. LeMasurier; Wesley E. LeMasurier 2Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, Colorado 80309, USA Search for other works by this author on: GSW Google Scholar Gianfranco Di Vincenzo Gianfranco Di Vincenzo 3Istituto di Geoscienze e Georisorse (IGG), Consiglio Nazionale delle Ricerche (CNR), Via Moruzzi 1, I56124 Pisa, Italy Search for other works by this author on: GSW Google Scholar GSA Bulletin (2006) 118 (7-8): 991–1005. https://doi.org/10.1130/B25675.1 Article history received: 14 Jul 2004 rev-recd: 27 Jan 2006 accepted: 24 Mar 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sergio Rocchi, Wesley E. LeMasurier, Gianfranco Di Vincenzo; Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies. GSA Bulletin 2006;; 118 (7-8): 991–1005. doi: https://doi.org/10.1130/B25675.1 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 The Dorrel Rock intrusive complex in Marie Byrd Land, West Antarctica, consists of a coarse-grained gabbro cut by fine-grained benmoreite and trachyte dikes, all exposed in a single nunatak. It is the only exposed plutonic body related to late Cenozoic volcanism in this part of the West Antarctic rift system. Our 40Ar-39Ar age determinations indicate emplacement of the gabbro took place ca. 34 Ma, followed by dike injection at ca. 33.5 Ma. Marie Byrd Land volcanoes are all younger than ca. 27–29 Ma, and lie on a low-relief Late Cretaceous erosion surface that has been disrupted by block faulting and dome uplift since late Oligocene time. The erosion surface and overlying volcanoes are well preserved, but in contrast, we estimate that at least 3 km of overburden has been eroded away to expose the gabbro. This anomaly is most easily explained if most of the exhumation took place during a period of rapid erosion between ca. 34 Ma and 27–29 Ma and was followed by a pronounced decrease in erosion rate in the late Oligocene.Temporal anomalies in the degree of dissection of volcanic edifices, together with evidence from hydrovolcanic deposits, suggest there was an ice cap in Marie Byrd Land in the late Oligocene and that inland (200+ km) volcanoes were being actively eroded by glaciers until ca. 15 Ma. This is consistent with seismic and stratigraphic work in the Ross Sea, which documents at least two expansions of the West Antarctic Ice Sheet in the early mid-Miocene. We find that rates of glacial erosion in Marie Byrd Land increase significantly with nearness to the coast, and in nonresistant rock. Thus, the observation that inland volcanoes younger than ca. 15 Ma show no effects of glacial erosion, except for one with a basal section of weak tuffs, suggests that a transition from warm-based to cold-based glaciers took place around 15 Ma. These findings are similar to many of those reported from well-studied McMurdo Sound and Ross Sea localities, so they provide a wider regional picture of middle to late Cenozoic climatic and surficialgeologic events in Antarctica. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Exhumation of the Ailao Shan shear zone recorded by Cenozoic sedimentary rocks, Yunnan Province, China

The role of strike‐slip faults, such as the Ailao Shan shear zone in southwestern China, in accommodating the India‐Eurasia collision remains controversial. Cenozoic sedimentary rocks preserved along the northeast side of the Ailao Shan shear zone, however, record the nature and timing of exhumation of shear zone rocks. Field mapping shows that the lower strata consist of early Oligocene lacustrine red mudstone and gypsum and the upper of late Oligocene to Miocene fluvial conglomerate with rare, fine‐grained, fossil leaf‐bearing intervals. Sediment derived from shear zone gneiss is present only in the upper part of the section. Our observations indicate deformation began in Oligocene time, with the onset of rapid exhumation by early Miocene time, consistent with existing geochronologic results. Exhumation must have ended before the formation of a regional low‐relief erosion surface and the onset of brittle deformation along the Red River fault, probably in early Pliocene time. Rapid exhumation of the Ailao Shan shear zone has been interpreted to have occurred in transtension, accommodated by a normal fault on the northeast margin of the shear zone. However, we observe consistent and pervasive shortening structures within the Cenozoic strata, including overthrusting and folding of the basin along faults defined by thick gouge zones within weak Cenozoic lithologies. Deformation is likely syndepositional. These structures are distinct from subvertical deformational features associated with the active Red River fault. Our field data thus support mid‐Cenozoic transpressional exhumation of the Ailao Shan shear zone. This is consistent with regional evidence for widespread left‐lateral and compressional deformation in the region to the east of the eastern Himalayan syntaxis during this time. Focused erosion along the Red River, established by late Oligocene time, may have contributed to deeper exhumation of the northeast part of the shear zone.

Late Oligocene climatic changes: Evidence from calcareous nannofossils at Kerguelen Plateau Site 748 (Southern Ocean)

Palaeoecological changes in nannofossil assemblages in the Southern Ocean during Oligocene times are examined through high-resolution, quantitative analyses of samples from Ocean Drilling Program (ODP) Site 748 (Kerguelen Plateau). We quantitatively characterized the palaeoecological preference of groups of species and compared their general trends with those determined at Maud Rise (ODP Sites 690 and 689) (Persico, D., Villa, G., 2004. Eocene–Oligocene calcareous nannofossils from Maud Rise and Kerguelen Plateau (Antarctica): palaeoecological and palaeoceanographic implications. Marine Micropaleontology 52, 153–179). We then attempt a correlation between the main assemblage variation and sea-surface temperature (SST) changes in the Southern Ocean at this time. Relatively stable, cool conditions are interpreted to have persisted from earliest to late Oligocene times, when an increase in abundance of temperate-water taxa is recorded, both at Maud Rise and Kerguelen Plateau, before the Mi-1 event. This reveals a climatic event that probably involved both sites, and which is comparable to that indicated by the global oxygen isotope curve ([Miller, K.G., Wright, J.D., Fairbanks, R.G., 1991. Unlocking the Ice House: Oligocene–Miocene oxygen isotopes, eustasy, and margin erosion. Journal of Geophysical Research, 96, 6829–6848; Zachos, J., Pagani, M., Sloan, L., Thomas, E., Billups, K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292, 686–693.]). In the uppermost Oligocene at Site 689, the temperate-water taxa index ([temperate / temperate + cool] * 100) increases from 25.2 Ma. Similarly, at Site 748, the temperate-water taxa index indicates an increase in SST in the late Oligocene, from about 26.5 Ma, at the base of Chron C8n.2n, which is offset by about 1 m.y. between the two areas. The reason for this time difference can probably be found in the location of Site 748 with respect to Site 689, and in the palaeoceanographic setting, taking into account that Site 748 lies north of a deep water passage that separates Kerguelen Plateau from Antarctica, and north of the present day front of the Antarctic Circumpolar Current. The presence of warm-water taxa, exclusively at Site 748, corroborates this hypothesis.

Miocene sedimentary evolution of the NE–SW-trending Selendi and Gördes Basins, W Turkey: implications for extensional processes

Western Turkey is one of the classic regions for the study of extensional processes. In a recent interpretation, the regional metamorphic basement, the Menderes Massif, was exhumed related to large-scale extensional processes, possibly during latest Oligocene time. Regional-scale NE–SW-trending basins, including the Gördes and Selendi basins, were created, associated with this exhumation. The NE–SW-trending basins are seen as regional-scale “corrugations” (10–15 km across) in the surface of an extensional detachment orientated parallel to the direction of extension. The detached upper plate is represented by erosional remnants of unmetamorphosed Tethyan ophiolitic melange. We describe and interpret the Gördes and Selendi basins separately and then show that their sedimentary facies and evolution are very similar. Basal coarse alluvial fan deposits accumulated after extensional removal of the upper plate. These basal sediments, rarely exposed, were rotated by up to 50°, probably as a result of local extension or transtension. Later during the early Miocene, the Gördes and Selendi basins were infilled by a generally fining-upward succession derived from the south, based on palaeocurrent evidence. Coarse alluvial fan deposition was followed by the deposition of finer-grained alluvial deposits from braided streams flowing northwards over a topographically subdued flood plain. Later, deposition was within shallow ephemeral lakes after a possible change to a moister climate. Calc–alkaline magmatism was initiated with the fall-out of fine-grained silicic ash, followed by coarser-grained tuff derived from local magmatic centres. During early Pliocene time, the basins were dissected by E–W extensional faults, associated with the establishment of the modern pattern of westward drainage to the Aegean Sea. We use the assembled sedimentary evidence to test alternative tectonic models for Miocene basin formation in western Turkey. Our preferred tectonic model emphasises the importance of deposition in regional basins created by exhumation of metamorphic basement that was previously assembled during early Tertiary closure of the Tethys ocean.

Pre-Neogene controls on present-day fault activity in the West Netherlands Basin and Roer Valley Rift System (southern Netherlands): role of variations in fault orientation in a uniform low-stress regime

The West Netherlands Basin (WNB) and the neighbouring Roer Valley Rift System (RVRS) form the most prominent tectonic features of the onshore Netherlands. The two basins have a common tectonic origin and similar Mesozoic evolution, their Neogene–Quaternary evolution, however, is markedly different. While the WNB is tectonically/seismically inactive and is characterised by uniform Neogene–Quaternary subsidence, in the RVRS fault controlled intra-plate deformation has taken place since Late Oligocene times with pronounced seismic activity. Considering the present-day NE–SW regional extension as well as the similar basin orientation and Mesozoic evolutions one would not expect strikingly different neotectonic activity in the two basins. Detailed analysis of Mesozoic, Tertiary and Quaternary fault patterns revealed that (1) the Palaeozoic–Mesozoic tectonic fabric has great influence on the Cenozoic deformation style of the basin system; (2) present-day faulting in the RVRS is related to the reactivation of pre-existing faults and (3) the Mesozoic fault pattern is slightly different in the two basins. Using a novel technique (three-dimensional (3-D) slip tendency analysis) we aim to determine whether these differences in fault orientation are substantial enough to produce significantly different resolved stresses along the faults. This modelling not only tests a common geological phenomena but indirectly also delivers important constraints regarding the origin of faulting in the study area. During the analysis 3-D geometric models of 84 mapped faults were investigated. The results of this recently developed modelling technique reveal that the faults in both basins should behave similarly under the condition of a laterally homogeneous regional stress field, which is in disagreement with the present-day tectonic activity of the region. On the other hand the modelling also reveals that the predicted tendency of slip is very sensitive to the ratio of the minimum horizontal and the vertical stress ( S). As an explanation for the pronounced fault activity in the RVRS and the tectonic quietness of the WNB it is proposed that either the regional extension in the WNB is “weaker” (in term of higher S ratio) than that in the RVRS or the slip thresholds in the two basins are different. The study emphasised that differentiation in the tectonic evolution has already started in the Tertiary, therefore, it is reasonable to assume that the different tectonic stress field and/or slip threshold between the two areas is directly related to the origin of the Cenozoic rifting in the RVRS. Several hypotheses are discussed to account for these two possibilities, taking into account the lithospheric structure and the Cenozoic tectonic evolution of the two basins as well as surrounding areas.

Cenozoic tectonics of the China continental margin: insights from Taiwan

Abstract The continental margin to the east and south of China comprises an active margin in the East China Sea, a collision mountain belt in Taiwan, and a passive margin in the South China Sea. These three segments were generally regarded as separate tectonic entities and their interrelations have long been the subject of debate. Here we synthesize available information to outline the tectonic and geological background of the China margin, examine the link between Taiwan and the neighbouring China margins, and thereby establish a Cenozoic evolutionary model. The China margin is floored with a pre-Cenozoic continental basement covered with an up to 10-km-thick pile of Cenozoic sedimentary strata. The continental basement has been invariably stretched and moulded into a series of northeast-trending horsts and grabens. Except in the Okinawa Trough of the East China Sea, the Cenozoic sedimentary cover typically exhibits a two-tier tectonostratigraphic structure, with narrow Palaeogene rift basins draped by a blanket-like Neogene-Quaternary sequence. The two-tier structure prevails in the entire inner part of the China margin, including the Taiwan Strait off western Taiwan. In the outer China margin, however, the two-tier structure persists only in the South China Sea, and is in stark contrast with the collisional orogen of Taiwan and the Ryukyu arc of the East China Sea. By untangling the contractional deformation of the northern Taiwan mountain belt, it has been possible to reconstruct a precollisional tectonostratigraphic section with a distinctive two-tier structure shown by a Palaeogene half-graben covered with a Miocene drape sequence. When put together with Palaeogene rift basins of the Taiwan Strait, it becomes clear that the precollisional continental margin of Taiwan resembles that of the South China Sea, characterized by two lines of Palaeogene rift basins. Hence before the collision started in Late Miocene times, Taiwan was part of the passive South China margin that extended northward to the southern Ryukyu area. Ever since the end of the Cretaceous, the China continental margin has been dominated by extensional tectonics, regardless of the presence or absence of subduction zones. In the Early Cenozoic, extensive crustal attenuation resulted in region-wide subsidence and formation of rift basins. Extension in the South China Sea culminated in Late Oligocene times, when part of the outer margin was drifted away by the opening ocean basin. In the East China Sea, the margin remained intact and became separated from the South China Sea margin by a transform fault. From the Miocene onwards, the South China Sea margin has been passively subsiding, sporadically punctuated with basaltic volcanism. In the East China Sea margin, the Okinawa Trough has opened and the Ryukyu volcanic arc thrived. The NE edge of the South China Sea margin was deformed as the Taiwan orogen.

Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet

Research Article| November 01, 2003 Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet Sun-Lin Chung; Sun-Lin Chung 1Department of Geosciences, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Dunyi Liu; Dunyi Liu 2Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China Search for other works by this author on: GSW Google Scholar Jianqing Ji; Jianqing Ji 3School of Earth and Space Sciences, Peking University, Beijing, China Search for other works by this author on: GSW Google Scholar Mei-Fei Chu; Mei-Fei Chu 4Department of Geosciences, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Hao-Yang Lee; Hao-Yang Lee 4Department of Geosciences, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Da-Jen Wen; Da-Jen Wen 4Department of Geosciences, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Ching-Hua Lo; Ching-Hua Lo 4Department of Geosciences, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Tung-Yi Lee; Tung-Yi Lee 5Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Qing Qian; Qing Qian 6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Search for other works by this author on: GSW Google Scholar Qi Zhang Qi Zhang 6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (11): 1021–1024. https://doi.org/10.1130/G19796.1 Article history received: 09 May 2003 rev-recd: 31 Jul 2003 accepted: 01 Aug 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sun-Lin Chung, Dunyi Liu, Jianqing Ji, Mei-Fei Chu, Hao-Yang Lee, Da-Jen Wen, Ching-Hua Lo, Tung-Yi Lee, Qing Qian, Qi Zhang; Adakites from continental collision zones: Melting of thickened lower crust beneath southern Tibet. Geology 2003;; 31 (11): 1021–1024. doi: https://doi.org/10.1130/G19796.1 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 SocietyGeology Search Advanced Search Abstract Adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones. Here we report the first example of such magmas from southern Tibet in an active continental collision environment. The Tibetan adakites were emplaced from ca. 26 to 10 Ma, and their overall geochemical characteristics suggest an origin by melting of eclogites and/or garnet amphibolites in the lower part (≥50 km) of thickened Tibetan crust. This lower-crustal melting required a significantly elevated geotherm, which we attribute to removal of the tectonically thickened lithospheric mantle in late Oligocene time. The identification of collision-type adakites from southern Tibet lends new constraints to not only the Himalayan-Tibetan orogenesis—how and when the Indian lithosphere started underthrusting Asia can be depicted—but also the growth of the early continental crust on Earth that consists dominantly of the tonalite-trondhjemite-granodiorite suites marked by adakitic geochemical affinities. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Out-of-sequence, basement-involved structures in the Sadlerochit Mountains region of the Arctic National Wildlife Refuge, Alaska: Evidence and implications from fission-track thermochronology

Fission-track thermochronology and structural analysis set limits on the timing and nature of structural development of the Sadlerochit Mountains, along the southern edge of the coastal plain in the Arctic National Wildlife Refuge (ANWR) of northeastern Alaska. The Sadlerochit Mountains are the northernmost part of the north-vergent Brooks Range fold-and-thrust belt and lie close to the Arctic continental margin. Thermochronology results indicate that sedimentary rocks exposed within Ignek Valley, south of the Sadlerochit Mountains, were subjected to two episodes of rapid cooling from elevated paleotemperatures at ca. 45 Ma and at some time since ca. 31 Ma, whereas similar-aged rocks exposed along the northern flank of the Sadlerochit Mountains cooled rapidly at ca. 45 Ma and ca. 27 Ma. Combined with five additional analyses from the Beli Unit #1 well, located northwest of the Sadlerochit Mountains, the thermochronology results indicate that the Sadlerochit Mountains region was progressively heated during Late Cretaceous through middle Eocene time, after which two major episodes of rapid cooling occurred in the middle Eocene at ca. 45 ± 3 Ma (±2σ) and in the late Oligocene at ca. 27 ± 2 Ma (±2σ). These episodes of rapid cooling are interpreted to have occurred in response to kilometer-scale erosional denudation resulting from rapid (≤5 m.y.) uplift due to structural thickening during the emplacement of thrust sheets in a basement-involved duplex. Initially, at least one thrust sheet was probably emplaced to the north of the Sadlerochit Mountains at ca. 45 Ma. Subsequently, at ca. 27 Ma, (1) the Sadlerochit Mountains thrust sheet was probably emplaced out of sequence behind the earlier-emplaced thrust sheet(s), and (2) basement-involved deformation formed structures beneath the coastal plain to the north. Both of these events occurred far within the continent, >1200 km from the southern Alaska convergent plate boundary. These results indicate that maximum burial, and hence peak hydrocarbon generation, occurred prior to middle Eocene time, before the formation of potential traps in and immediately north of the Sadlerochit Mountains. However, (1) hydrocarbons generated from these rocks could have migrated updip into existing stratigraphic traps prior to structural deformation, and (2) hydrocarbons generated later in more distal parts of the basin could have migrated updip into subsurface structures formed in middle Eocene and late Oligocene time north of the Sadlerochit Mountains and along strike to the east and west.

Tertiary Provenance History of the Northern and Central Altiplano (Central Andes, Bolivia): A Detrital Record of Plateau-Margin Tectonics

ABSTRACT The internally drained Altiplano plateau in the high-elevation hinterland of the central Andes contains an extremely thick (up to 12 km) succession of Tertiary nonmarine strata that recorded the topographic evolution of eastern and western plateau margins. In this study, temporal and spatial variations in detrital composition (based on 113 sandstone thin sections and 31 conglomerate clast counts), combined with sediment dispersal patterns and regional stratigraphic correlations, help delineate the Tertiary history of Altiplano basin development in relationship to thrust deformation and arc magmatism along plateau margins. Stratigraphic and provenance data suggest limited subsidence and west-directed sediment dispersal during the early Tertiary history of the Altiplano region. By late Eocene time, development of a rapidly subsiding basin (probable foredeep) was underway, as recorded by extensive east-directed fluvial systems of the 3-6.5-km-thick Potoco Formation. Potoco compositional data are compatible with sediment sources in the incipient Western Cordillera and regions farther west, including (1) a probable thrust-belt source terrane composed of Paleozoic-Mesozoic strata and Precambrian granitic/gneissic basement and (2) a subordinate magmatic-arc source terrane composed of Tertiary volcanic rocks. Increased abundances of plagioclase grains and volcanic lithic fragments upsection indicate a progressively greater arc influence, due to either enhanced volcanism or eastward arc migration. By late Oligocene time, west-directed fluvial and alluvial-fan systems of the upper Potoco Formation and the 3-km-thick Coniri Formation reached the Altiplano basin, providing a minimum uplift age for the backthrust belt along the western flank of the Eastern Cordillera. Paleozoic-clast conglomerate and relatively quartz-rich sandstone containing sedimentary and metasedimentary lithic fragments show unroofing of predominantly Paleozoic strata in the backthrust belt. Continued late Oligocene to Quaternary accumulation in the Altiplano reveals that the basin persisted as a depositional region during a well-documented period of Neogene shortening and uplift in the Andean thrust belt farther east. This intra-orogenic (hinterland) deposition between eastern and western highs during ongoing shortening suggests that the Altiplano basin existed as a crustal-scale piggyback basin, possibly in an internally drained plateau-like geomorphic setting, since 25 Ma. Provenance data and independent structural and thermochronological evidence for spatially and temporally varying tectonic histories along the eastern and western plateau margins imply that several modes of basin development were active during growth of the Altiplano. Although flexural subsidence due to thrust loading along both plateau margins, initially in the west and subsequently in the east, may explain most Tertiary subsidence, possible closed-basin conditions due to structural damming could have promoted additional sediment accumulation from late Oligocene to Quaternary time.

Scotland’s denudational history: an integrated view of erosion and sedimentation at an uplifted passive margin

Abstract Denudational history is commonly reconstructed from basin sediments derived from the denuded source area, and less frequently from the source area itself. Northern Britain is an important source area for the surrounding sedimentary basins and this paper reviews the erosional history of Scotland from Devonian time to the present using evidence both from onshore geology and geomorphology and from patterns of sedimentation in surrounding basins. Cover rocks were extensive in Scotland during late Palaeozoic time but the persistence of sediment source areas within the upland areas of Scotland makes it unlikely that basement highs were ever completely buried, and depths of post-Devonian erosion of basement have been correspondingly modest (< 1–2 km). During Mesozoic time, Scotland experienced several major erosional cycles, beginning with uplift, reactivation of relief and stripping of cover rocks, followed by progressive reduction of relief through etchplanation and culminating in extensive marine transgressions in Late Triassic, Late Jurassic and Late Cretaceous time. Mid-Paleocene pulses of coarse sediment to the Moray Firth Basin coincided with major uplift. This uplift was associated with major differential tectonics within the Highlands, with warping and faulting along the margins of the Minch and the inner Moray Firth Basins. Tectonic activity was renewed on a lesser scale in late Oligocene time and continued into Late Neogene time. Differential weathering and erosion under the warm to temperature humid climates of Neogene time created the major elements of the preglacial relief, with formation of valleys, basins, scarps and inselbergs, features often closely adjusted to lithostructural controls and, in some cases, with precursors that can be traced back to Devonian time. The history that can be ‘read’ from the onshore region complements the source area history interpreted from sedimentary basins derived from these areas.

Paleogene synorogenic sedimentation in the Altiplano plateau and implications for initial mountain building in the central Andes

Sedimentologic data and palynological ages from Paleogene clastic deposits of the northern and central Altiplano plateau suggest foreland basin development in the central Andes by mid-Paleocene time. The nonmarine Potoco Formation (3000‐6500 m thick) constitutes the majority of Cenozoic basin fill. The Potoco overlies the Santa Lucia Formation (50‐300 m thick), previously dated as mid-Paleocene by mammal fossils and magnetostratigraphy. New geochronologic data for the Potoco Formation include late Eocene to Oligocene palynomorph assemblages recovered throughout lower to upper stratigraphic levels. These ages and published 40 Ar/ 39 Ar and K-Ar ages from overlying upper Oligocene‐lower Miocene volcaniclastic rocks indicate (1) nondeposition or greatly reduced deposition (average sediment-accumulation rates ,10 m/m.y.) from mid-Paleocene to middle Eocene time (top Santa Lucia to lowermost Potoco Formation), followed by (2) rapid deposition (average sediment-accumulation rates up to 500 m/ m.y.) throughout late Eocene and Oligocene time (majority of Potoco Formation). Lithofacies and paleocurrent data confirm both depositional phases. A 20‐100-mthick interval of superimposed paleosols in the basal Potoco supports reduced sediment accumulation and low rates of subsidence during mid-Paleocene to middle Eocene time. The overlying main body of the Potoco contains facies assemblages and stratigraphic architecture (dominantly nonero

Fault strength and transpressional tectonics along the Castle Mountain strike-slip fault, southern Alaska

Analyses of structures along the Castle Mountain fault reveal the mechanical relations and relative timing of orogen-parallel strike-slip faulting to distributed wrenching and shortening and to plate motion in the southern Alaskan transpressive margin. The Castle Mountain fault is a >200-km- long, orogen-parallel, right-lateral seismogenic fault in the southern Alaskan plate margin. Exposures of the fault in the study area have been exhumed from 3−4 km depth and ∼80 °C. Moment tensor summations and stress inversions of slip data from fault networks within 400 m of the Castle Mountain fault and analyses of structures in forearc deposits 2−4 km from the fault yield maximum incremental shortening and maximum compressive stress (σ 1 ) axes that are subhorizontal and trend ∼325°. This trend is close to the 340° North American–Pacific plate convergence direction. The inferred σ 1 makes a 70°−80° angle with the ∼070°-striking Castle Mountain fault, indicating that the fault slips at a lower shear stress than predicted by laboratory rock friction experiments if hydrostatic pore pressure exists in the fault (i.e., coefficient of friction ∼0.85, Byerlee9s Law). The forearc structures record coaxial strain and apparently remain in their formative orientations, showing that previously documented distributed shearing in the form of vertical-axis block rotations in the forearc ceased prior to formation of the structures in late Oligocene time. The mechanical weakness of the Castle Mountain fault probably results in part from its clay- rich gouge, which averages about 43 wt% clay phases. However, the gouge composition does not account for slip in response to σ 1 at 70°−80° to the fault. Another mechanism, such as elevated pore pressure, further weakens the fault and enables it to slip right laterally in response to the small component of oblique motion across the plate boundary. Upon its inception in early Tertiary time, the fault could not have formed a >200-km-long planar zone of low-friction clay-rich gouge or elevated pore pressure; thus, the mechanisms that weaken it must have developed over time. The distributed shearing in the forearc may have ended in response to the weakening and localization of dextral slip onto the Castle Mountain fault.

Tectonic evolution of the NW Red Sea-Gulf of Suez rift system

Abstract The NW Red Sea-Gulf of Suez rift system was initiated during Late Oligocene time and underwent extension in a N65°E direction, almost orthogonal to pre-existing WNW-trending Pan African shear-zone fabrics in the crystalline basement of the Sinai-African plate. Earliest syn-rift sediments are Upper Oligocene continental clastic deposits with minor syn-rift basalts. Early Miocene sedimentation was dominated by shallow marine clastic deposits, which developed variable stratigraphic architectures as a response to the interaction of extensional faulting, sea-level changes, sediment supply and dispersal. Analysis of fault geometries, fault kinematics and sedimentation patterns indicates that rift-normal extension predominated throughout the Late Oligocene-Early Mid-Miocene evolution of the rift. Reactivation of the Precambrian basement fabrics was the main factor controlling the fault architecture, fault linkage and evolution of the NW Red Sea-Gulf of Suez rift. Individual faults were initially strongly segmented and offset across ‘soft-linked’ relay structures. With increased extension these faults became linked by breaking down relay structures with the development of local ‘hard-linked’ transfer faults, thus giving rise to the rhomboidal fault pattern of the rift system. In Mid-Miocene time, the Levant-Gulf of Aqaba transform boundary was established, linking the Red Sea rift plate boundary to the convergent Bitlis-Zagros plate boundary. This resulted in a dramatic decrease in extension rates within the Gulf of Suez whereas the northern Red Sea continued to extend, with significant syn-rift sediments deposited in Late Miocene-Pliocene time in offshore fault-bounded basins.

Siliciclastic Facies Belt Formation and the Late Oligocene to Middle Miocene Partial Drowning of the Southern Florida Platform

ABSTRACT A series of siliciclastic shoreline deposits began to prograde onto the southern Florida Platform during deposition of the early Oligocene-age Suwannee Limestone. These sediments were fine to very fine quartz sands deposited in belts parallel to the margin of the platform. The influx and mixing of these siliciclastic sediments did not affect the continued deposition of carbonate sediments on the platform because the sediments did not contain significant quantities of mud. Major siliciclastic sediment influxes occurred during minor regressive, eustatic sea level events affecting primarily the platform margins. Beginning in late Oligocene time, the frequency and volume of siliciclastic sediment influx onto the southern Florida Platform began to cause the drowning of the eastern margin of the platform. The eastern margin of the platform receded from the current geographic margin position westward a distance of about 65 miles. In the central part of the platform about 600 ft of mixed carbonate and siliciclastic sediment was deposited from the late Oligocene to the middle Miocene (Chattian to Serravallian) compared to a maximum of about 65 ft on the eastern margin. Subsequent influx of predominantly siliciclastic sediment with some carbonates in the late Miocene and early Pliocene terminated the growth of the platform and infilled the drowned part of the platform to produce the approximate geometry observed today.

Evolutionary transformations of hystricognath Rodentia and the climatic change in the Eocene to Late Oligocene time interval

Evolutionary transformations of hystricognath Rodentia and the climatic change in the Eocene to Late Oligocene time interval

Thrust dissection control of deep-water clastic dispersal patterns in the Klematia–Paramythia foreland basin, western Greece

The Klematia–Paramythia basin is an internal part of the middle Ionian zone of the Hellenide orogen in western Greece. It consists of Middle Eocene to Late Miocene turbidites, up to 3300 m thick, which were deposited in a series of submarine fans. Field studies suggest that the configuration and the depositional environments of the basin were affected by two tectonic phases. During the first tectonic phase, in Middle Eocene to Late Oligocene times, a foreland basin was formed west of the Pindos Thrust front. During the second tectonic phase, in the Early Miocene, the Ionian zone (a part of the foreland basin) was subdivided by internal thrusting into three sub-basins (internal, middle and external) and changed to a complex type foreland basin. Comparison of the type and facies associations of the turbidite deposits that accumulated within the basin suggests that these two tectonic phases had a significant effect on sedimentary dispersal patterns. During the first tectonic phase in the Klematia–Paramythia basin (when it was part of the foreland basin), fine-grained turbidites, up to 1050 m thick, accumulated on the distal part of a submarine fan. The lower part (900 m thick) of these deposits consists of thin to thick interbedded sandstone/mudstone beds which are interpreted as lobes and lobe-fringe (outer-fan) deposits. The upper parts (150 m thick) of these deposits are composed of very thin to thin siltstone/mudstone beds, representing a basin plain environment. During the second tectonic phase, sediments up to 2260 m thick were deposited in the Klematia–Paramythia basin. These deposits are interpreted as lobes and lobe-fringe (outer-fan) fine-grained turbidites in the central part of the basin, channel and interchannel deposits (inner-fan) in some areas of the periphery of the basin, and shelf deposits in the northern and southern terminations of the basin.

A cross section of the convergent Pacific margin of Nicaragua

Prestack depth migration of multichannel seismic reflection lines across the Pacific margin of Nicaragua has yielded an accurate depth image to about a 9‐km depth from the deep ocean basin to the coast. The margin contains the Sandino forearc basin, probably underlain by oceanic igneous basement and fronted by a small prism accreted at the seaward end of the continental basement. Seismic stratigraphy and drill hole information indicate that sediment has been accumulating since Late Cretaceous. The margin configuration formed between late Cretaceous and Paleocene time and has endured since that time. Uplift of the outer high and slope was probably coeval with subsidence of a deep basin beneath the shelf. From middle‐late Eocene time to Oligocene time, the outer high was a barrier to sediment transport. A similar Late Cretaceous to Oligocene tectonic history has been described for the Guatemalan and Costa Rican segments of the Pacific margin. We speculate that the structure of the Pacific forearc basin formed by subduction initiation at the edge of the Caribbean igneous province. Since late Oligocene time, margin‐wide subsidence occurs in the Nicaraguan margin, perhaps related to subduction erosion of the upper plate. Coeval steep reverse and normal faulting along local structures in the forearc basin might occur by transpression along margin‐parallel strike‐slip faults. These faults have been active since the early development of the basin, but the greatest rate of vertical displacement along them was in early ‐ middle Miocene time, probably related to a plate kinematic reorganization involving the collision of Central and South America.

Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolution of north-central Sonora (Arizpe and Bacanuchi Quadrangles), northwest Mexico

The Arizpe and Bacanuchi Quadrangles provide a geologic history representative of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapping, 40Ar/39Ar dating, and geochemical analyses have been combined to provide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcropping rocks are Lower Cretaceous strata of the Bisbee Group, which along with the Picacho conglomerate record a middle Cretaceous compressive tectonic event and associated sedimentation. Laramide igneous activity is widespread and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichtian age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodiorite was emplaced at ca. 68 Ma on the basis of a 40Ar/39Ar biotite age (67.97 ± 0.19 Ma) and cooled relatively rapidly according to less precise 40Ar/39Ar hornblende and K-feldspar ages from the same sample (64.8 ± 1.0 Ma and 62.8 ± 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger (40Ar/39Ar biotite age of 58.67 ± 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma (40Ar/39Ar biotite age of 56.73 ± 0.14 Ma and a less precise 40Ar/39Ar hornblende age of 55.0 ± 0.7 Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred between middle and late Eocene time and probably until the early Oligocene. Subsequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which 40Ar/39Ar hornblende ages of 27.25 ± 0.09 and 27.32 ± 0.06 Ma and a biotite age of 26.97 ± 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that occurred during the late Oligocene, as indicated by a sanidine 40Ar/39Ar age of 25.48 ± 0.05 Ma. This late Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is characterized by northwest-striking normal faults and folds, which expose the deepest structural levels of the area, and by the related basin fill, the Bacanuchi conglomerate. The second phase is represented by north-striking normal faults and by the syntectonic basin fill, the Arizpe conglomerate. Basaltic andesite volcanic flows at the base of the Arizpe conglomerate yielded 40Ar/39Ar (whole-rock) ages of 23.52 ± 0.17 and 21 ± 0.20 Ma. The extensional deformation (27 to 23 Ma) in the study area is coeval with the development of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora. The mafic volcanic rocks and clastic sedimentary units associated with this extension resemble the basin fills that in other parts of Sonora are assigned to the Baucarit Formation. Geochemical information from samples representing each of the igneous events displayed high-K calc-alkalic and mostly metaluminous compositions. The older units including the Arroyo Alcaparros andesitic rocks, the Cerro Las Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Rancho Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu anomalies, suggesting garnet and plagioclase removal in the source. The younger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volcanic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyroxene or amphibole fractionation.

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area, southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO 2 with increasing SiO 2 content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10 −13.4 to 10 −10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.

Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa

Research Article| February 01, 2000 Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa C. J. Ebinger; C. J. Ebinger 1Department of Geology, Royal Holloway, University of London, Egham TW20 0EX, United Kingdom Search for other works by this author on: GSW Google Scholar T. Yemane; T. Yemane 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D. J. Harding; D. J. Harding 4NASA Goddard Space Flight Center, Greenbelt, Maryland 20771 Search for other works by this author on: GSW Google Scholar S. Tesfaye; S. Tesfaye 2Ethiopian Institute of Geological Surveys, Addis Ababa, Ethiopia5Department of Geological Sciences, University of Colorado, Boulder, Colorado 80305 Search for other works by this author on: GSW Google Scholar S. Kelley; S. Kelley 3Department of Earth Sciences, Open University, Milton Keynes MK7 6AA, United Kingdom Search for other works by this author on: GSW Google Scholar D.C. Rex D.C. Rex 6Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (2): 163–176. https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.CO;2 Article history received: 22 Oct 1997 rev-recd: 28 Feb 1998 accepted: 01 Apr 1999 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 C. J. Ebinger, T. Yemane, D. J. Harding, S. Tesfaye, S. Kelley, D.C. Rex; Rift deflection, migration, and propagation: Linkage of the Ethiopian and Eastern rifts, Africa. GSA Bulletin 2000;; 112 (2): 163–176. doi: https://doi.org/10.1130/0016-7606(2000)112<163:RDMAPL>2.0.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 The Main Ethiopian and Eastern (Gregory) rifts, sectors of the East African rift system, overlap in a 300-km-wide system of extensional basins that is more than three times the breadth of either rift away from the zone of overlap. The oldest volcanic rocks (Eocene) and possibly the oldest rift basins (Oligocene) of the East African rift system occur in this zone of overlap. The objectives of field, remote sensing, and geochronology (K-Ar and 40Ar/39Ar) studies in southwestern Ethiopia were to establish a chronology of rifting and volcanism in the zone of overlap, and to correlate stratigraphic sequences with those in the Kenya rift to the south and in the Main Ethiopian rift to the north. Field observations and cross sections show that basins are bounded by steeply dipping faults, stratal dips are <30°, and that extension accommodated by the intrusion of dikes is volumetrically insignificant. Thus, the style of faulting is similar to that elsewhere in East Africa south of the Afar rift. Initial volcanism between ca. 45 and 33 Ma preceded faulting and uplift, except for reactivation of some Mesozoic rift structures near the Sudan-Ethiopia border and in northern Kenya. Extensional basins began to form in late Oligocene time in the Eastern rift, and in early Miocene time in the Main Ethiopian rift. Small degrees of extension and associated volcanism in the broadly rifted zone may have been triggered by extension in the Red Sea, as well as by lithospheric heating above a mantle plume. The anomalous breadth of the zone is a consequence of rift propagation and migration, rather than basin-and-range–style extension; both the Main Ethiopian rift and Eastern rifts have propagated along north-south lines, and the Eastern rift has migrated ∼200 km eastward since late Oligocene time. The distribution of seismicity and Quaternary volcanism suggest that the Eastern and Main Ethiopian rifts are currently linked across a 200-km-wide zone between the Omo and Segen basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.