Late Miocene Time Research Articles

Evolution of the Lycian Allochthon, western Turkey, as a north-facing Late Palaeozoic to Mesozoic rift and passive continental margin

Regional tectono-stratigraphic analysis allows widely distributed outcrops of mainly Mesozoic sedimentary rocks within the Lycian Allochthon, SW Turkey, to be correlated and placed within four regionally developed thrust sheets, the Karadağ Thrust Sheet (lowest), the Teke Dere Thrust Sheet, the Köyceğiz Thrust Sheet (highest), and the Yavuz Thrust Sheet. The Karadağ Thrust Sheet records Late Carboniferous, Lower and Upper Permian continental shelf/lagoonal deposition. The overlying Teke Dere Thrust Sheet includes a rift succession of Late Permian age that was subaerially exposed during much of the Triassic; a marine transgression followed in the Early Jurassic succeeded by subsidence that formed a continental slope from Middle Jurassic to Palaeocene times. The overlying Köyceğiz Thrust Sheet records Upper Triassic oceanic crust (along a rifted margin), overlain by a Lower Jurassic carbonate platform; this then subsided to form a continental slope that survived until Late Cretaceous times. The Lycian Allochthon is restored as a north-facing Mesozoic rift and passive margin taking account of structural evidence indicating southward thrust emplacement and comparisons of sedimentary successions. Mainly deep-water sediments of Triassic to Late Cretaceous age, preserved as blocks within mélange units above the Lycian Thrust Sheets (Layered Tectonic Mélange and Ophiolitic Mélange), are interpreted as deep-water sediments deposited on Mesozoic (Neotethyan) oceanic crust. Subduction of the ocean basin proceeded from north to south, beginning with accretion of oceanic-derived mélange and disrupted thrust sheets. Debris was shed into a continentward-migrating flexural foredeep, initially located along the distal edge of the continental margin in Campanian–Maastrichtian times; this foredeep then propagated southwards in stages over more proximal continental crust (including an intra-platform basin). The first main stage of southeastward propagation was in Palaeocene and Eocene times followed by a second stage in Oligocene–Miocene times. The Lycian Allochthon was finally emplaced over the most proximal (southeasterly) foredeep (i.e. the Kaş basin) in Late Miocene time. Copyright © 1999 John Wiley & Sons, Ltd.

Scientists study deep geological structure between New Hebrides Arc and Eastern Australian Margin

French and Australian scientists hope that deep seismic profiling performed earlier this year in the southwest Pacific will improve understanding of the geological framework, crustal characteristics, and evolution of basins and ridges between the eastern Australian margin and the New Hebrides Trench system. The data already have indicated that the basements of several basins are thinned and oceanic, that the basements of several ridges are continental, and that, contrary to previous thinking, the Loyalty Basin was formed in late Cretaceous time.Submerged continental fragments separated by deep ocean basins dominate the region in the west, and the Australian Plate is subducting beneath the New Hebrides Arc in the east. Subduction started in Late Miocene time [MacFarlane et al., 1988] at a rate of 12 cm/ year with a convergence direction of WSW‐ENE [Dubois et al., 1977].

Neogene–Quaternary evolution of the central Apennine orogenic system (Italy): a structural and palaeomagnetic approach in the Molise region

We report on new palaeomagnetic and magnetic fabric analyses of mainly Upper Miocene sedimentary sequences from the external central Apennine fold and thrust belt (Molise area), where the principal compressive structures are clearly non-coaxial. The sampling was carried out on the E–W-oriented Matese–Frosolone thrust sheet, that for its geographical position and structural setting (superposition of thrusting, strike-slip and extensional tectonics since Late Miocene to present-day) represents a key structure for the comprehension of the Neogene–Quaternary evolution of the entire Molise area. Palaeomagnetic results suggest that the Matese–Frosolone thrust sheet counterclockwise rotated at least 35° after Messinian times. These data confirm that the present-day trend variability observed in the main compressional structures in the Apennine chain can be related to rotations about vertical axes rather than to changes in the stress field orientation, at least since Late Miocene times. Magnetic fabric analyses indicate that the studied sediments were subjected to very mild deformation, suggesting that the surface emergence of the thrust front of the Matese–Frosolone unit is located farther north, far from the studied area. Well-defined magnetic lineations of tectonic origin were only observed in sites close to localised belts of strike-slip deformation.

Palinspastic reconstruction and topographic evolution of the Eastern Alps during late Tertiary tectonic extrusion

This paper presents a new palinspastic restoration of the Eastern Alps for Neogene time and an attempt to reconstruct the Neogene palaeogeology, palaeotopography and palaeohydrography in connection with the structural evolution. The Eastern Alps underwent radical horizontal displacement during the Neogene due to large strike-slip systems and formation of structural windows. Our palinspastic reconstruction considers: (a) the rearrangement of tectonic units dismembered during tectonic extrusion, (b) the tectonic denudation driven by displacement of the crystalline blocks, (c) geochronological arguments, and (d) the sedimentary record of the syn-extrusion basins. The rearrangement of tectonic blocks results in a remarkably good fit of highly dismembered zones both in crystalline and sedimentary areas and shows the pre-Miocene unstretched pattern of the Eastern Alps, reduced to 65% of its present-day E–W elongation. Using this structural frame and considering the sedimentary record, a set of palaeogeologic and palaeotopographic sketch maps with the palaeo-river systems is presented for three time slices (pre-, syn- and post-extension situation). In Late Oligocene and Early Miocene times, the western Eastern Alps were already mountainous, whereas the eastern part of the orogen formed lowlands or hilly areas. Enhanced block movement in the course of the extrusion process around the Early/Middle Miocene boundary led to the formation of intramontane sedimentary basins and a fault-induced reorientation of the drainage pattern, which forms the basis of the modern river system in the area east of the Tauern window. This region, where pre-Miocene land surfaces are preserved, probably became a mountainous area not before Late Miocene time and never reached the elevations of the areas further west.

Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia

Research Article| September 01, 1998 Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia Brian K. Horton Brian K. Horton 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (9): 1174–1192. https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>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 Brian K. Horton; Sediment accumulation on top of the Andean orogenic wedge: Oligocene to late Miocene basins of the Eastern Cordillera, southern Bolivia. GSA Bulletin 1998;; 110 (9): 1174–1192. doi: https://doi.org/10.1130/0016-7606(1998)110<1174:SAOTOT>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 A large volume of Tertiary synorogenic sediment accumulated on top of the Eastern Cordillera of southern Bolivia as the Andean orogenic wedge was shortened, thickened, and uplifted. Oligocene to upper Miocene strata were deposited in five basins that were separated by active, north-trending, fold-thrust structures of the then-frontal part of the orogenic wedge. These coarse-grained deposits recorded accumulation in the most proximal sector of the Andean foreland basin system, the wedge-top depozone.Analyses of depositional systems, sediment dispersal patterns, and clast provenance of 0.6–2.3-km-thick, Oligocene to upper Miocene wedge-top successions demonstrate that fault-propagation and fault-bend folds commonly isolated individual basins while serving as primary sediment sources. Growth strata which formed by progressive fold-limb rotation indicate thrust-fault displacement and related folding concurrent with deposition. Alluvial fans defined most basin margins, whereas braided streams or small lakes occupied basin axes. Diagnostic stratigraphic units confined to individual basins suggest that streams were rarely able to cut across growing folds to connect adjacent basins.Growth strata and crosscutting and onlapping relationships between contractional structures and wedge-top strata delineate the chronology of fold-thrust deformation in the Eastern Cordillera. Five new 40Ar/39Ar dates and previously published K-Ar dates, ranging from 30 to 8 Ma, define an Oligocene phase of west-vergent backthrusting followed by primarily east-vergent thrusting during Miocene time. Timing of displacement on two east-vergent thrusts is determined by 40Ar/39Ar ages of tuffs within adjacent growth strata sequences of the Tupiza Formation conglomerate (16.14 ± 0.06 Ma) and Oploca Formation (13.33 ± 0.15 Ma, 15.7 ± 2.4 Ma). These ages, combined with basin depositional histories, demonstrate synchronous and out-of-sequence thrust displacement during Miocene shortening.Upper-crustal contractional deformation and wedge-top deposition terminated in the Eastern Cordillera during late Miocene time as the thrust front propagated eastward into the Subandean Zone. Continued thrust-front migration produced the present-day configuration in which Eastern Cordillera wedge-top basins, originally developed above the toe of the orogenic wedge at relatively low elevations, are now >250 km west of the active thrust front and at ∼3 km elevation. The modern wedge-top depozone overlies the active frontal part of the orogenic wedge and consists of strata in thrust-bounded, Subandean Zone basins and sediment overlying blind structures beneath the westernmost Chaco Plain.In general, wedge-top deposits become highly susceptible to erosional recycling as the orogenic wedge propagates forward and the wedge surface is uplifted. Nevertheless, Eastern Cordillera wedge-top deposits have been preserved for ∼10–30 m.y. in southern Bolivia. Such long-term preservation may reflect the inability of drainage systems to remove sediment mass from this low-precipitation region of the central Andes. Retention of sediment mass within the orogenic belt may promote critical thrust-wedge conditions in which propagation of the thrust front is favored. 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.

Processes of Late Cretaceous to Late Miocene episodic thrust-sheet translation in the Lycian Taurides, SW Turkey

The Lycian Taurides of SW Turkey consist of an allochthonous Mesozoic passive margin succession that was episodically detached from its autochthon and translated southeastwards between latest Cretaceous to Late Miocene times. A combination of structural analysis, sedimentology, palaeontology and geochemistry allows subdivision of this orogenic event into: (1) latest Cretaceous trench–passive margin collision causing ophiolite obduction and detachment of the Köyceđiz Thrust Sheet; (2) Mid–Late Eocene continent–continent collision renewing southeastwards thrusting and causing detachment of the Teke Dere Thrust Sheet and Yavus Thrust Sheets; (3) Miocene extensional collapse of the orogen with southeastwards translation of the allochthon coeval with rift-basin formation in the hinterland.

Late Miocene to Recent transtensional tectonics in the Sierra San Fermı́n, northeastern Baja California, Mexico

Basins and ranges within part of the Gulf of California Extensional Province (Mexico) have experienced complex distributed deformation, including normal and strike-slip faulting and block rotations, linked to dextral shear at the Pacific–North America plate boundary. In the Sierra San Fermı́n and southern Sierra San Felipe (northeastern Baja California), normal faulting began between 12.5 and 6 Ma, although most extension occurred between about 6 and 3 Ma, strongly influencing thickness and distribution of ash-flow tuffs and sedimentary deposits. Extension is generally <10% in 6 Ma rocks and somewhat more in 12.5 Ma rocks. Inversion of kinematic data, interpreted together with published paleomagnetic data, suggests that the axis of least principal stress was oriented between W–E and SW–NE in late Miocene time. Our data indicate an important change in the amount of dextral shear, but not necessarily the least principal stress direction (WSW–ENE), at about 3 Ma. Structural constraints limit significant sinistral strike-slip faulting, conjugate to the dextral plate boundary, to the last ∼3 My. Progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the Pacific–North America plate boundary zone, and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector α changes to lower and lower values.

Late Cenozoic exhumation of the Cascadia accretionary wedge in the Olympic Mountains, northwest Washington State

The apatite fission-track method is used to determine the exhumation history of the Olympic subduction complex, an uplifted part of the modern Cascadia accretionary wedge. Fission-track ages are reported for 35 sandstones from the Olympic subduction complex, and 7 sandstones and 1 diabase from the Coast Range terrane, which structurally overlies the Olympic subduction complex. Most sandstone samples give discordant results, which means that the variance in grains ages is much g reater than would be expected for radioactive decay alone. Discordance in an unreset sample is caused by a mix of detrital ages, and in a reset sample is caused by a mix of annealing properties among the detrital apatites and perhaps by U loss from some apatites. Discordant grainage distributions can be successfully interpreted by using the minimum age, which is the pooled age of the youngest group of concordant fission-track grain ages in a dated sample. The inference is that this fraction of apatites has the lowest thermal stability, and will be the first to reset on heating and the last to close on cooling. Comparison of the minimum age with depositional age provides a simple distinction between reset samples (minimum age younger than deposition) and unreset samples (minimum age older than deposition). The success of the minimum-age approach is demonstrated by its ability to resolve a well-defined age-elevation trend for reset samples from the Olympic subduction complex. Microprobe data suggest that the apatites that make up the minimum-age fraction are mostly fluorapatite, which has the lowest thermal stability for fission tracks among the common apatites. Reset minimum ages are all younger than 1 5Ma, and show a concentric age pattern; the youngest ages are centered on the central massif of the Olympic Mountains and progressively older ages in the surrounding lowlands. Unreset localities are generally found in coastal areas, indicating relatively little exhumation there. Using a stratigraphically coordinated suite of apatite fission-track ages, we estimate that prior to the start of exhumation, the base of the fluorapatite partial annealing zone was located at ~10 0°C and ~4.7 km depth. The temperature gradient at that time was 19.6 ± 4. 4°C/km, similar to the modern gradient in adjacent parts of the Cascadia forearc high. Apatite and previously published zircon fission-track data are used to determine the exhumation history of the central massif. Sedimentary rocks exposed there were initially accreted during late Oligocene and early Miocene time at depths of 12.1‐14.5 km and temperatures of ~242‐28 9°C. Exhumation began at ca. 18 Ma .A rock currently at the local mean elevation of the central massif (1204 m) would have moved through the α-damaged zircon closure temperature at about 13.7 Ma and ~10.0 km depth, and through the fluorapatite closure temperature at about 6.7 Ma and ~4. 4km depth. On the basis of age-elevation trends and paired cooling ages, we find that the exhumation rate in the central massif has remained fairly constant, ~0.75 km/m.y., since at least 14 Ma. Apatite fission-track data a re used to construct a contour map of long-term exhumation rates for the Olympic Peninsula. The average rate for the entire peninsula is ~0.28 km/m.y., which is comparable with modern erosion rates (0.18 to 0.32 km/m.y.) estimated from sediment yield data for two major rivers of the Olympic Mountains. We show that exhumation of this part of the Cascadia forearc high has been dominated by erosion and not by extensional faulting. Topography and erosion appear to have been sustained by continued accretion and thickening within the underlying Cascadia accretionary wedge. The rivers that drain the modern Olympic Mountains indicate that most of the eroded sediment is transported into the Pacific Ocean, where it is recycled back into the accretionary wedge, either by tectonic accretion or by sedimentary accumulation in shelf and slope basins. The influx of accreted sediments is shown to be similar to the outflux of eroded sediment, indicating that the Olympic segment of the Cascadia margin is currently close to a topographic steady state. The record provided by our fissiontrack data, of a steady exhumation rate for the central massif area since 14 Ma, suggests that this topographic steady state developed within several million years after initial emergence of the forearc high.

Late Miocene–Pleistocene tectono-sedimentary evolution and subsidence history of the central Betic Cordillera (Spain): a case study in the Guadix intramontane basin

The Guadix Basin became established as an intramontane basin in the central sector of the Betic Cordillera at the beginning of Late Miocene time. Its geodynamic evolution starts with a unit of Tortonian marine sediments and is completed by a unit of Late Turolian–Pleistocene continental sediments. In the two units, six depositional sequences have been differentiated whose boundaries, in most cases, coincide with tectonic events. Geohistorical diagrams show the results of quantitative analyses of subsidence in the northern sector of the basin and permit correlation of the main events with significant changes in the history of subsidence and uplift. A period of strong subsidence occurred at the beginning of Tortonian time causing the formation of a marine basin 800 m deep. The mid- and end-Tortonian tectonic events involved periods of uplift leading to shallowing in the basin. After continentalization at the end of the Tortonian, the basin was uplifted continuously from Late Turolian to Late Pleistocene times, finally accumulating sediments at a height of 1000 m.

Precise dating of the Holmatindur cooling event in eastern Iceland: Evidence for mid‐Miocene bipolar glaciation

A succession of basaltic lavas, volcaniclastic sediments, and lignite beds in the Holmatindur region of eastern Iceland provides the means for tying magnetic reversal stratigraphy and a record of major North Atlantic cooling to a precise radiometric timescale. A prominent part of the section is the Holmatindur clastic bed, which is up to 66 m thick and can be traced 80 km along strike. The geological significance of this unit lies in its composition and associated thin lignite seams. The volcaniclastic composition is basaltic (in contrast to the common rhyolitic/dacitic composition of tuff units in the area), and hyaloclastic material is dominant. Hydroexplosive volcanism most likely produced the hyaloclastite, through subglacial eruptions within the paleovolcanic zone. Pollen assemblages from the lignite beds indicate a dramatic climatic deterioration, from subtropical to cool temperate conditions, through these sediments. Feldspar crystals separated from the Holmatindur clastic bed were determined to be from 10.72 (± 0.16) Ma, from 40Ar‐39Ar incremental heating experiments. We correlate the Holmatindur cooling event with a relative maximum in benthic foraminiferal δ18O (Zone Mi6 of Miller et al. [1991]), which has been interpreted as an episode of ice accumulation within the mid‐Miocene cooling period, as well as with pulses in ice‐rafted debris supplied to deep water sites in the North Atlantic Ocean, a global sea level drop of 70 m, and submarine canyon cutting, all of which occurred within the early part of magnetic anomaly 5 normal (C5n). While strong evidence exists for mid‐Miocene ice sheets in Antarctica, northern hemisphere glaciation is thought to have started much later. The Iceland sites studied here demonstrate that significant ice accumulation occurred, albeit intermittently, as early as late middle Miocene time in the North Atlantic Ocean. From radiometric dating of Tertiary lavas that lie below and above the well‐established lower boundary C5n, we conclude that the onset of this long normal polarity period occurred at 10.94 (± 0.16) Ma. The new data are compatible with the Cande and Kent [1995] interpolated estimate for this boundary (10.95 Ma) but are significantly older, or more precise, than previous direct dating by conventional K‐Ar methods.

Updated azca (Farallon)—South America relative motions during the last 40 My: implications for mountain building in the central Andean region

Recently published seafloor data around the Antarctica plate boundaries, as well as calibration of the Cenozoic Magnetic Polarity Time Scale, allow a reevaluation of the Nazca (Farallon)–South America relative convergence kinematics since late Middle Eocene time. The new reconstruction parameters confirm the basic characteristics determined in previous studies. However, two features are notable in the present data set: a strong increase in convergence rate in Late Oligocene time, and a slowdown during Late Miocene time. The former is coeval with the early development of important tectonic characteristics of the present Central Andes, such as compressional failure in wide areas of the region, and the establishment of Late Cenozoic magmatism. This supports the idea that a relationship exists between strong acceleration of convergence and mountain building in the Central Andean region.

Pretreating zinciferous surfaces

Between Late Cretaceous and Miocene time the central part of Tunisia emerged. During this period a karstic system developed in Senonian limestones of the Ain Khamouda area. It is known that three facies occur in the resulting caves: (1) red mineralised crust containing goethite, hemimorphite and smithsonite; (2) white clay containing 0.1–8.5% ZnO; (3) Miocene sand and sandstone. The white clay has been studied using STEM (scanning transmission electron microscopy) in order to determine with which minerals Zn is associated. Two zinciferous phases were found, the first being a swelling clay of sauconite type, the second a nearly amorphous Zn-hydroxide gel. Halloysite which is the main component of the white clay seems totally devoid of Zn. The difference between Zn minerals contained in the crust and in the clay can be related to pH gradients. Minute particles (3–10 nm) of pure Ag, were found in the clay, indicating that the present mineralisation is probably related to the alteration of sulphides.

Late Miocene-Pleistocene Extensional Faulting, Northern Gulf of California, Mexico and Salton Trough, California

A belt of low-angle normal (or detachment) faults ∼250 km long extends from the northern end of the Salton Trough, California to southern Laguna Salada, Baja California, Mexico. The detachment system is divided into two principal segments. The northern segment, here termed the “west Salton detachment system,” comprises top-to-the-east detachment faults along the eastern Peninsular Ranges that root under the Salton Trough. The southern segment, here termed the Laguna Salada detachment system, comprises top-to-the-west detachment faults in northeastern Baja California and the Yuha Desert region of the southwesternmost Salton Trough. Detachments of that system root under Laguna Salada and the Peninsular Ranges of northern Baja California. Both of these systems experienced a major episode of activity in late Miocene to Pleistocene time, synchronous with deposition of the Imperial and Palm Spring formations, and the Laguna Salada detachment system may still be active. Thus, their activity temporally overlapped, partly or completely, with activity on dextral faults of the San Andreas boundary between the Pacific and North American plates, and with accretion of new transitional crust. Some of the detachment faults in the northern segment may have had mid-Miocene normal slip and/or Cretaceous thrust or normal slip as well, although compelling evidence for either is lacking. These detachment faults are distinctly younger than detachments east of the San Andreas fault, which generally ceased activity by middle or late Miocene time and are overlapped by marine or lacustrine rocks (Bouse Formation); these units are equivalent in age to the syntectonic strata of the Salton Trough but are much thinner and essentially undeformed.

Development and exhumation of a Neogene sedimentary basin during extension, east-central Nevada

Research Article| February 01, 1998 Development and exhumation of a Neogene sedimentary basin during extension, east-central Nevada Brian K. Horton; Brian K. Horton 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar James G. Schmitt James G. Schmitt 2Department of Earth Sciences, Montana State University, Bozeman, Montana 59717 Search for other works by this author on: GSW Google Scholar Author and Article Information Brian K. Horton 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 James G. Schmitt 2Department of Earth Sciences, Montana State University, Bozeman, Montana 59717 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 (2): 163–172. https://doi.org/10.1130/0016-7606(1998)110<0163:DAEOAN>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 Brian K. Horton, James G. Schmitt; Development and exhumation of a Neogene sedimentary basin during extension, east-central Nevada. GSA Bulletin 1998;; 110 (2): 163–172. doi: https://doi.org/10.1130/0016-7606(1998)110<0163:DAEOAN>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 An analysis of the Miocene Horse Camp basin in the central Basin and Range province demonstrates that individual components of an extensional basin system may undergo initial subsidence and subsequent uplift and exhumation during extension. The Horse Camp basin developed during Miocene time above a west-dipping detachment fault and between two east-striking accommodation faults. Growth strata indicate that the west-dipping detachment was active throughout basin filling. Lateral facies variations in alluvial-fan, fan-delta, and lacustrine deposits indicate that the east-striking accommodation faults along the northern and southern basin margins bounded primary sediment source areas. During late Miocene time, the hanging-wall block of the detachment system was broken by the west-dipping Railroad Valley normal fault, thereby inducing development of an extensional basin, Railroad Valley, in the newly formed hanging wall and uplift and exhumation of the older Horse Camp basin in the footwall.The total amount of exhumation during Neogene extension is calculated for a single range block, the northern Grant Range. Early exhumation is recorded by the detrital compositions of middle-late Miocene strata in the southern part of the Horse Camp basin. Provenance modeling indicates that approximately 1600 m of Mississippian to Silurian section was exhumed in the range during middle-late Miocene Horse Camp deposition. From late Miocene to Holocene time, both the northern Grant Range and Horse Camp basin were exhumed in the footwall of the Railroad Valley fault. A comparison between the currently exposed bedrock geology and calculated late Miocene exposure levels from the provenance model indicates that 1000–2000 m of Cambrian–Ordovician section must have been exposed in the range after late Miocene deposition in the Horse Camp basin.Modern alluvial fans in eastern Railroad Valley have drainage networks that contain exposures of the fill of the Horse Camp basin, indicating that older basin fill is being recycled into the modern extensional basin. Extensional basin systems having similar basinward shifts of the major fault system are particularly susceptible to uplift, exhumation, and erosional recycling, not unlike the structurally dismembered margins of foreland basin systems. 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 Yerer-Tullu Wellel volcanotectonic lineament: a transtensional structure in central Ethiopia and the associated magmatic activity

An east-west trending structure in central Ethiopia, the Yerer Tullu-Wellel volcanotectonic lineament, intersects the Main Ethiopian Rift and lies between latitudes 8°20′N and 9°05′N. Interpretation of Landsat images, calibrated by field study, indicates the relative chronology of the observed fracture systems (northeast-southwest, east-west, north-south and northwest-southeast). Along this structure there are several central volcanoes concentrated along the latitude of 8°45′N. Most of these volcanoes have basic lava flows at their base with acid dome, plug, and/or pyroclastic deposits as their evolved end members. The chemistry of these volcanic rocks shows a progressive decrease in alkalinity and silica undersaturation from western areas towards the rift margin. Radiometric ages of these volcanoes range from 12 Ma to Recent and indicate that the volcanic activity shifted eastward in time. The central volcanoes are usually located at the intersections of the above described fracture systems. The role of the transtensional Yerer Tullu-Wellel volcano-tectonic lineament since Late Miocene times is discussed within the geodynamic evolution of the Main Ethiopian Rift.

Abstract: Fold and Thrust Belt Along the Western Flank of the Eastern Cordillera of Colombia: Style, Kinematics and Timing Constraints Derived &gt;From Seismic Data and Detailed Surface Mapping&amp;nbsp;

The Eastern Cordillera of Colombia is considered to be the result of compression and uplift during the late Miocene–Pliocene Andean orogeny. Nevertheless, detailed mapping carried out on a portion of the fold belt exposed along the western flank of the Eastern Cordillera suggests that deformation began in late Paleocene–early Eocene time, in the form of a west-vergent, forward-propagating fold-and-thrust belt. Several structures related to this event remain concealed beneath the late Paleocene–late Eocene unconformity, and synkinematic sediments are preserved locally within some of these thrust sheets. The relief generated during this deformational episode constrained the depositional axis of late Eocene fluvial sediments to run along the present-day crestal zone of the Eastern Cordillera. Onlapping basal Miocene molassic sediments on tilted middle Miocene fluvial deposits suggest that a series of seismically delineated intercutaneous wedges located at the thrust front formed in late Miocene time. These structures mark the onset of the Andean deformational episode. Thrusting polarity reversed during the latter event, that is, toward the hinterland, and Paleogene structures were reactivated. Additionally, the late Paleocene–early Eocene unconformity surface was folded and incorporated into north-plunging folds, and Cretaceous extensional faults were inverted. As a result, the Eastern Cordillera developed its present pop-up, doubly vergent fold-belt geometry, flanked by thick molassic sequences that document the Miocene–Pliocene Andean orogeny. Regionally, the latest Campanian–Maastrichtian uplift of the Central Cordillera, Paleocene–early Eocene thrusting in the Middle Magdalena Valley and along the western flank of the Eastern Cordillera, and ultimately inversion and major uplift of the entire Eastern Cordillera in late Miocene–Pliocene time, attests to the eastward propagation of deformation from the Central toward the Eastern Cordillera. Kinematically, the driving element of deformation would have been the obliquely accreting western terranes of the Central and Western Cordilleras and the Panama arc. These transferred their easterly component of convergence to the Eastern Cordillera through a basal intracrustal detachment.

Buffing apparatus suitable for thin plate work

Between Late Cretaceous and Miocene time the central part of Tunisia emerged. During this period a karstic system developed in Senonian limestones of the Ain Khamouda area. It is known that three facies occur in the resulting caves: (1) red mineralised crust containing goethite, hemimorphite and smithsonite; (2) white clay containing 0.1–8.5% ZnO; (3) Miocene sand and sandstone. The white clay has been studied using STEM (scanning transmission electron microscopy) in order to determine with which minerals Zn is associated. Two zinciferous phases were found, the first being a swelling clay of sauconite type, the second a nearly amorphous Zn-hydroxide gel. Halloysite which is the main component of the white clay seems totally devoid of Zn. The difference between Zn minerals contained in the crust and in the clay can be related to pH gradients. Minute particles (3–10 nm) of pure Ag, were found in the clay, indicating that the present mineralisation is probably related to the alteration of sulphides.

The structural response to evolving plate kinematics during transpression: evolution of the Lebanese restraining bend of the Dead Sea Transform

Abstract Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region’s structural history, up to late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Ma the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon-Jabel Barouk structure, has remained broadly fixed. However, the rates of amplification of this structure and the coastal flexure appear to have varied. Continuing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.

Age and geochemistry of an ‘anorogenic’ crustal melt and implications for I-type granite petrogenesis

The Capoas intrusion is a metaluminous, high-K calc-alkaline, I-type biotite granite emplaced within Permian-Jurassic sedimentary rocks of the North Palawan Continental Terrane (NPCT) in the western Philippines. The NPCT is a fragment of the Mesozoic Andean-type margin of southeast China that was separated from the mainland during the late Oligocene-early Miocene opening of the South China Sea. Zircons from the granite have xenocrystic cores, and form a discordant array with a lower intercept age of 15 (+ 3/ −4) Ma. Monazites have concordant 207Pb/235U ages with a mean age of 13.4 (± 0.4) Ma. The late middle Miocene age and the location of the pluton in the NPCT uniquely constrain the formation of the Capoas granite in a post-rifting, non-collisional tectonic setting unrelated to any subduction zone. The major and trace element geochemistry of the granite and the presence of apparently Proterozoic xenocrystic zircon indicate that the pluton is composed largely, if not entirely, of older continental crust. The only viable heat source for crustal melting and/or assimilation was widespread basaltic magmatism that occurred in the area following cessation of seafloor spreading in the South China Sea in early Miocene time. The geochemical affinity of the Capoas granite with calc-alkaline magmatic arc and collisional granites is therefore a function of the source rocks that were melted to produce the granite rather than the specific tectonic setting in which the granite was generated. The calc-alkaline source rocks most likely formed in the Mesozoic Andean-type margin of south China and subsequently underwent partial melting in late middle Miocene time in an ‘anorogenic’ setting.

The tectonic evolution and associated sedimentation history of Sarawak Basin, eastern Malaysia: a guide for future hydrocarbon exploration

Abstract A seismic-stratigraphic study of the reprocessed regional lines for the offshore Sarawak area was undertaken with the aim of reviewing the present understanding of the tectonics and the palaeo-depositional environments of the Sarawak Basin. The study was integrated with biostratigraphy and wireline-log data from the wells drilled throughout the basin. Seven unconformities were identified within the Tertiary sediments and these were used as the markers for the seismic correlations. Where the unconformities become conformable, well data were used to guide the correlation of the conformities. Palaeo-environment maps were generated which document the interaction of tectonics and sediments throughout the basin history. The development of the Sarawak Basin commenced in late Oligocene times with deposition along a coastline running in a NW-SE direction, which is almost perpendicular to the present-day coastline. The coastline was oriented to the present-day NE-SW during late Miocene times. Geochemistry of Sarawak Basin oils shows generation of hydrocarbons from land plant dominated source rocks. The distribution of these types of source rocks are favoured in the Coastal Fluvial, Coastal and Lower Coastal Plain environments. The quantity and quality of the source rocks deteriorates with the distance from the coastline. The palaeo-environment maps illustrate the likely distribution of Sarawak Basin source rocks which will help in effectively planning for future exploration in Sarawak. The study also revealed that the Sarawak Basin was formed as a result of NW-SE-trending right-lateral fault movement during late Oligo-Miocene times. This dextral movement was responsible for creating the NW-SE coastline and divided the offshore Sarawak area into two sub-basins. Deposition and preservation of coastal plain and shallow marine sediments continued in the eastern area while the western area remained as a ‘high’ until late Miocene times. The dextral strike-slip movement which controlled the evolution of the Sarawak Basin is sub-parallel to a number of lineaments elsewhere in Sarawak. The timing of movement of these suggests progressive younging in an eastward direction. It is also believed the late structuration of the sediments and the formation of structural traps is linked to these tectonic movements.