Late Oligocene Time Research Articles

The palaeogeographic and palaeoenvironmental evolution of a Palaeogene mixed carbonate–siliciclastic cool-water succession in the Otway Basin, Southeast Australia

The Otway Basin in southeast Australia contains a thick sequence of Cenozoic shelfal carbonates and siliciclastics that preserve signals relating to the progressive opening of the Southern Ocean since the Paleogene. This multidisciplinary study integrates outcrop and subsurface well data from over 100 wells and bores throughout the Otway Basin with micropalaeontological analyses to constrain the age and palaeoenvironments of the Nirranda Group (Late Eocene to Middle Oligocene) and the Heytesbury Group (Late Oligocene to mid Miocene). These data were used to deduce the Late Eocene to Late Oligocene palaeogeographical evolution of the area. During the Late Eocene paralic high energy siliciclastic shoreline to shelf facies dominated the region, deepening southwards where mid to outer shelf conditions preserved high energy sandy carbonate facies. Above the Eocene–Oligocene boundary low energy inner to mid shelfal silt and muddy sand persisted to the north, deepening southward to carbonate-dominated low energy outer shelf to bathyal marls. The change from siliciclastics to carbonates at the Eocene–Oligocene boundary in the Otway Basin may relate to regional tectonics. In the Early Oligocene, high energy inner to outer shelf sand bodies formed in front of marine to inner shelf mudstone facies; the sand units are likely to have been influenced by strong local longshore drift and ocean swells that increased as the Southern Ocean widened to create a larger fetch. In the eastern half of the basin, later in the Early Oligocene, mixed paralic to inner shelf siliciclastic and carbonate facies were deposited passing to inner to mid shelf marl and mudstone and outer shelf to bathyal marls basinward. During this time, low to high energy shelfal calcarenite, chalk and marl dominated the westerly edge of the basin. The contrast in facies from west and east in the basin is inferred to be due to contrasting terrigenous input, environmental energy and ramp/shelf geometry. By Late Oligocene times (the Clifton Formation) the Otway Basin was dominated by high energy carbonate facies deposited in mid to outer shelf palaeoenvironments. The base of the Clifton Formation preserves a shift in facies and foraminiferal faunas that correlates to the major sea level fall at the Early–Late Oligocene boundary. This sea level fall is related to a major ice advance in Antarctica that corresponds to mid-Oligocene unconformities globally. The switch from low to high energy facies across the Early–Late Oligocene boundary in the Otway Basin suggests that the Southern Ocean swells experienced by the modern Otway coast were well established by Middle Oligocene time, evidence of the strengthening ‘proto‘ Antarctic swell regime. By Early Miocene times, with final deepening of the Drake Passage, the Antarctic Circum-Polar Current formed and predominantly inner to outer shelf marls and limestones were deposited in the Otway Basin.

Climatic oscillations versus environmental changes in the interpretation of Tertiary plant assemblages

Abstract Zonal plant communities are the best climatic indicators because they thrive under specific climatic conditions. However, palaeoclimatic estimates based on mere comparisons of local fossil plant assemblages, even if assessed by multivariate methods, may be misleading, if the taphonomical bias is neglected. The fossil record expresses only limited parts of the vegetation of a given period. The example of the Lower Miocene section at Bílina, North Bohemian Basin, shows how quickly the composition of the plant megafossil record changes as a result of different environmental factors and sedimentary settings. These changes can imitate climatic oscillation because swamp, fluvial and upland facies represented within this site have different vegetation-palaeoclimatic aspects. On the other hand, if one compares two assemblages originating from the same sedimentary and environmental setting (preferably mesophytic conditions), the resulting palaeoclimatic changes can be estimated more safely. Two local assemblages from the Oligocene volcanic area of the České středohoří Mts in North Bohemia are of mesophytic character. Both are embedded in diatomites deposited in crater-maar lakes. The assemblage of Suletice (20–29 Ma), with prevailing thermophilous elements, indicates more warm-temperate conditions, whereas the assemblage of Bechlejovice (27 Ma), composed mostly of deciduous broad-leaved trees and shrubs, corresponds to a temperate climate. Thus, together, these two floral assemblages suggest that a climatic oscillation occurred within Late Oligocene time in Central Europe.

Tectonic development of Central Thailand: new evidences from airborne geophysical data

The N-S oriented central floodplain of Thailand is situated between the NE-SW-trending strike-slip Uttaradit Fault Zone (UTFZ) to the north, the northern shoreline of the Gulf of Thailand to the south, and irregular paths of normal faults to the east and west. Major structures of the area include the NW-SE-trending sinistral strike-slip Mae Ping and Three Pagoda Fault Zones (MPFZ and TPFZ) located to the west and a N-S-oriented Central Volcanic Belt (CVE) located to the east of the plain. Tertiary basins located within the Central Plain have geometries indicating that they were formed due to the reversed movement from sinistral to dextral ofthe major NW-SE-trending strike-slip fault zones during Late Oligocene-Early Miocene times. Geophysical data interpretation results (magnetic, gamma-radiometric, digital terrain data and regional gravity data) show the N-S-trending CVE, outline the known major fault zones, discover the Chao Phraya Fault Zone (CPFZ) and other subsurface lineaments and magnetic bodies within the Central Plain, as well as suggest a new fault path for the TPFZ. The findings indicate that the tectonic evolution of the Central Plain was controlled by the movement along the CVE, which may represent an ancient suture zone, and strike-slip faults bounding small sub­ continental blocks. These movements include: extension (oblique) along the old sutures and volcanic arcs, reversed sinistral-to-dextral movement on the NW-SE-trending MPFZ, TPFZ and CPFZ, and reversed dextral-to-sinistral UTFZ. Indochina's southeast extrusion tectonic setting associated with the collision of the Indian and Eurasian plates may have reactivated these lithospheric weak zones and caused the opening of the Central Plain and the Gulf of Thailand since Late Oligocene time.

Sequence stratigraphy and sea-level history of Oligocene strata of the northern Aral Sea region (Kazakhstan): Implications for glacioeustatic reconstructions

We have applied sequence stratigraphic techniques to the upper Eocene to Oligocene strata of the region north of the Aral Sea and northern Usturt Plateau (western Kazakhstan) and compared the inferred sea-level history with other records derived from different parts of the world. During late Eocene to early Miocene time, the entire region of study was within the northeastern part of the Paratethys sea. Clastic and carbonate deposition occurred in a range of environments from fluvial and lacustrine in coastal plains (northern Priaral) to relatively deep marine in the north Usturt trough. Stratal geometries and sedimentation rates indicate that the deposition of the northern Priaral sections occurred under tectonically quiet conditions, whereas noticeable syndepositional subsidence took place in the central part of the north Usturt trough. The same Oligocene sequences have been identified in these areas despite significant spatial variations in tectonic subsidence and sediment supply rates. We therefore conclude that eustasy played a major role in controlling sedimentation in the region of study, especially in the northern Priaral. Through sequence stratigraphic analysis, we identified four sequence boundaries resulting from regional sea-level falls. Two major sea-level falls are identified in the earliest Oligocene and at the early–late Oligocene boundary, and two smaller sea-level falls took place in late early Oligocene and latest Oligocene time. Each of these events can be correlated to peaks in the global δ 18 O record. In addition, paleoclimatic data from the region indicate that regional periods of cooling are coincident with the intervals of inferred sea-level falls. We therefore conclude that global glacioeustasy represents the main controlling force for the sea-level record we observe in the northern Priaral and north Usturt trough. This makes the region of study very useful for reconstruction of eustatic history for late Eocene–Oligocene time.

Constraints on Moho depth and crustal thickness in the Liguro-Provençal basin from a 3D gravity inversion: geodynamic implications

Abstract 3D gravity modelling is combined with seismic refraction and reflection data to constrain a new Moho depth map in the Liguro-Provençal basin (Western Mediterranean Sea). At seismically controlled points, the misfit between the gravimetric solution and the seismic data is about 2 km for a range of Moho depths between 12 km (deep basin) and 30 km (mainland). The oceanic crust thickness in the deep basin (5 km) is smaller than the average oceanic crust thickness reported in open oceans (7 km), pointing to a potential mantle temperature 30–50°C below normal and/or very slow oceanic spreading rate. Oceanic crust thickness is decreasing towards the Ligurian Sea and towards the continent-ocean boundary to values as small as 2 km. Poor magma supply is a result of low potential mantle temperature at depth, lateral thermal conduction towards an unextended continental margin, and decrease of the oceanic spreading rate close to the pole of opening in the Ligurian Sea. Re-examination of magnetic data (palaeomagnetic data and magnetic lineations) indicates that opening of the Liguro-Provençal Basin may have ceased as late as Late Burdigalian time (16.5 Ma) or even later. The absence of a significant time gap between cessation of opening in the Liguro-Provençal Basin and rifting of the Tyrrhenian domain favours a continuous extension mechanism since late Oligocene time driven by the African trench retreat.

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.

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.

Mid-Tertiary rifting and magmatism in the Traill Ø region, East Greenland

A number of planar normal faults displace Tertiary basaltic intrusions in the Traill Ø region of East Greenland. This 'post-magmatic' extension occurs over an along-strike length of >350 km and over an across-strike width of >100 km with fault displacements and fault-block rotations indicating that (3 increases in magnitude eastwards from near unity to β ≈ 1.1. The stretching direction was approximately E-W, orthogonal to the strike of major faults in the region, most of which are reactivated Mesozoic structures. Two periods of Tertiary basaltic magmatism are recognized in the area. 40 Ar– 39 Ar step heating suggests that the first event, which gave rise to tholeiitic sills with a composite exposed thickness of c. 1 km, occurred at c. 54 Ma, and the second, which formed smaller volumes of alkaline dykes and two large syenite complexes, occurred at c. 36 Ma. Field observations indicate that 'post-magmatic' extension occurred after 54 Ma and possibly continued after 36 Ma. The tholeiitic intrusions are probably related to a stretching event (β ≤ 1.1) prior to the onset of sea-floor spreading in the Norway Basin, whilst the alkaline basalts and 'post-magmatic' rifting are associated with the separation of the Jan Mayen continental block from the East Greenland margin during the Eocene-Oligocene. Both tholeiitic and alkaline basalts formed by the partial melting of garnet-spinel peridotite with greater degrees of partial melting (8–16%) for the earlier tholeiitic group. Rifting in East Greenland ceased with the development of an oceanic ridge between Jan Mayen and East Greenland in late Oligocene time. Subsequent minor compression, probably the result of ridge-push, is suggested by the development of broad wavelength, low amplitude folds in the Traill Ø region.

The significance of the fracture pattern of the Late-Eocene Montserrat fan-delta, Catalan Coastal Ranges (NE Spain)

Sedimentation of the Montserrat fan-delta occurred during Bartonian/Priabonian times as a response to basement uplift due to transpressive faulting in the Catalan Coastal Ranges. The sediments are heavily fractured as a result of post-Middle Eocene tectonism associated with subsequent movements along the Catalan Coastal Ranges. By use of cross-cutting relations, mineralisation, fracture morphologies and kinematic indicators, relative dating of the fracture sets, and correlation to regional tectonic stresses have been performed. NNE-SSW-oriented fractures (Set A) are suggested to initiate during Rupelian time, as a system of sub-vertical (Mode I) and conjugate (Mode II) fractures, which indicate vertical S Z = σ 1 and WNW-ESE-oriented S h = σ 3. During Middle to Late Oligocene times several episodes of fracturing occurred within the Montserrat fan-delta. Reactivation of Set A (NNE-SSW) fractures by sinistral oblique displacement, was associated with shift in principal stress from vertical S Z = σ 1 to a horizontal N-S-oriented stress S H = σ 1. Conjugate fractures (Set B NE and Set B NW oriented, respectively, NE-SW and NW-SE) developed as a response to a sub-horizontal approximately N-S-oriented S H = σ 1. Set C fractures (oriented WNW-ESE) characterize orthogonal fracture patterns with Set A fractures, suggesting that Set C represents the younger generation. Observations of Set C fractures in the southeasternmost part of the Montserrat fan-delta prove reverse displacement, and transport towards the north-northeast, indicating a horizontal NNE-SSW-oriented S H = σ 1, east-westerly-oriented S h = σ 2, and vertical S Z = σ 3. Changing stress situations in the Montserrat fan-delta during Oligocene times correspond with successive change in the regional stress. A second phase of reactivation of Set A occurred during latest Oligocene-Early Miocene times. Hanging-wall blocks faulted down-to-the-east propose vertical S Z = σ 1 and WNW-ESE-oriented S h = σ 3 during this phase, which is most likely to be correlated with the regional latest Oligocene-Early Miocene extension in NE Spain.

Structural Synthesis and Tectonic Evolution of the Maracaibo and Barinas-Apure Basins, Western Venezuela: ABSTRACT

Seismic profiles have imaged the complex and multiphase geometry of major structures in the Maracaibo and Barinas-Apure basins and foothills of the adjacent orogens. Pre-Mesozoic structuring can be documented in subsurface as a peneplaned south-southeast vergent fold-thrust-belt of Hercynian or Caledonian age. In Jurassic times, rifting developed in connection with the opening of western Tethys and thick continental red beds were deposited in the rift grabens. In Late Cretaceous times, postrift thermal subsidence was subsequently recorded. From Maastrichtian through Eocene times, the obduction of the Tethyan ophiolites and the coeval tectonic accretion of the Caribbean allocthon loaded the South American foreland lithosphere, inducing the development of a flexural basin in the northeastern part of the Maracaibo basin. In Late Eocene and Oligocene times, a major extensional event took place in the East Zulia and Falcon area soon after the Caribbean compressional deformation ceased. Since the Neogene, structuring was related to the Andean orogeny, and the deformation was partially superimposed onto older Paleogene and Jurassic structures. These results, integrated with geochemical and chronostratigraphic models, have been used to understand the evolution of the petroleum system and to improve the play fairway risk assessment for exploration.

Multiple episodes of Cenozoic denudation in the northeastern Brooks Range: fission-track data from the Okpilak batholith, Alaska

The northeastern Brooks Range of Alaska is a complex Mesozoic to Cenozoic northward-verging fold and thrust belt. In response to regional compression, shortening in the upper crust has occurred through the duplexing of thrust sheets and formation of associated fault-bend folds. Apatite and zircon fission-track data from the Okpilak batholith and adjacent sedimentary rocks exposed within the northeastern Brooks Range provide new constraints on the timing, magnitude, and rate of cooling of these thrust sheets as they were rapidly denuded in response to uplift during Cenozoic time. Fission-track results indicate that a previously recognized episode of Paleocene cooling was followed by at least two younger episodes of rapid cooling during Middle Eocene and Late Oligocene time. The two younger episodes of rapid cooling are interpreted to reflect denudation in response to uplift resulting from Cenozoic thrusting and related folding. As a result of structural thickening, up to 8 km of material was eroded from the top of the batholith between ~41–45 Ma (Middle Eocene). Renewed shortening and emplacement of an underlying thrust sheet at ~25 Ma (Late Oligocene) resulted in at least 2 km of uplift and erosion of sedimentary rocks immediately north of the batholith. These results suggest that, even though Paleocene uplift and erosion may have occurred across the northeastern Brooks Range, the major episode of thrust faulting, responsible for structural emplacement of the batholith into its present position and kilometre-scale denudation, most likely occurred during Middle Eocene time.

Geochemistry and tectonic significance of augen gneisses from the southern Menderes Massif (West Turkey)

AbstractThe protoliths of mylonitized augen gneisses exposed in the southern sector of the Menderes Massif (West Turkey) are calc-alkaline, peraluminous, S-type, late- to post-tectonic tourmaline- and garnet-bearing, two-mica leucogranites. They cut and post-date the fabrics of the ‘main Menderes metamorphism’ which took place between the early Eocene and early Oligocene and intrude metamorphic basement rocks comprising the so-called ‘Palaeozoic schist envelope’ of the massif. They are themselves cut by an extensive network of tourmaline-rich dykes. Chemical, mineralogical, isotopic and field relations suggest that the granitic protolith crystallized from a boron-rich, water-saturated melt, derived from partial melting of metagreywacke in the lower crust during peak Barrovian-type metamorphism. The protolith was probably emplaced during lateorogenic extensional collapse of the thickened crust in west Turkey during late Oligocene time.

A time-slice approach to the history of Cenozoic sedimentation in the Indian Ocean

Results from drilling at over 100 sites have been combined with a new plate tectonic model to produce a series of palinspastic maps which represent “snap-shots” of the changing patterns of sediment accumulation in the Indian Ocean through the Cenozoic. The maps clearly delineate the main sedimentary depocenters and show how the sedimentary record has responded on an oceanwide scale to major changes in paleogeography (tectonics), paleocirculation and/or climate. Paleogene sediment accumulation rates were generally low, with significant accumulation in only limited areas, suggesting weak ocean circulation and stable, well-stratified conditions. By contrast, the vigorous thermohaline circulation of the Neogene resulted in substantial widespread sedimentation. Accumulation rates were generally high and displayed a wide range of values. Following rapid growth of the East Antarctic ice sheet in the Mid-Miocene, carbonate sedimentation has been progressively restricted to the shallow ridges and plateaus of the western Indian Ocean, but accumulates there with locally high rates. The link between tectonics and terrigenous sedimentation is exemplified by the major deep-sea fans off East Africa (Zambezi fan) and India (Indus and Bengal fans) which developed in response to uplift and erosion beginning in Late Oligocene time in the East African Rift System and the Himalayas, respectively. Although maps from only five widely spaced times are shown, the methodology is such that a continuous series of maps, at intervals as close as 1 m.y. can readily be generated. Ultimately the combination of such time-slice maps with the results of detailed time series studies will provide a powerful tool for understanding past oceanic environments.

Classification and distribution of Oligocene Aetiocetidae (Mammalia; Cetacea; Mysticeti) from western North America and Japan

Abstract Fossil whales in the very rare, primitive, extinct cetacean family Aetiocetidae are small, relict, toothed mysticetes that persisted into Late Oligocene time after more highly derived baleen‐bearing mysticetes had already evolved. No known aetiocetid could be ancestral to baleen‐bearing mysticetes, but aetiocetid morphology is in many ways intermediate between archaeocetes and baleen‐bearing mysticetes, demonstrating the probable transitional steps passed through in the evolution of baleen‐bearing mysticetes. Their discovery indicates that mysticetes evolved from Archaeocetes, and supports theories of the monophyly of Cetacea. Late Oligocene aetiocetids have been found on both sides of the North Pacific Ocean: on Vancouver Island, British Columbia, Canada; in Oregon and Washington, USA; in Baja California Sur, México; and the islands of Kyushu and Hokkaido, Japan.The most primitive North American aetiocetid, Chonecetus sookensis Russell, 1968, is from the early Late Oligocene Hesquiat Formation on Vancouver Island, British Columbia, Canada. A more derived, Late Oligocene species, Chonecetus goedertorum Barnes and Furusawa, new species, from the Late Oligocene Pysht Formation, Olympic Peninsula, Washington, has the primitive placental mammalian tooth count of 11/11. The type genus of the family, Aetiocetus Emlong, 1966, has as its type species, A. cotylalveus Emlong, 1966, known only from the Late Oligocene Yaquina Formation on the coast of Oregon. It has 11 upper teeth on each side of the rostrum. A more derived species, Aetiocetus weltoni Barnes and Kimura, new species, from a higher stratigraphic level in the Yaquina Formation, has a more posteriorly positioned cranial vertex and a tooth count of 11/12.We describe four new species of aetiocetids in three genera from the Late Oligocene Morawan Formation near Ashoro, Hokkaido, Japan. The most primitive, Ashorocetus eguchii Barnes and Kimura, new genus and species, has a primitive stage of cranial telescoping, and is closely related to Chonecetus Russell, 1968. Another, Morawanocetus yabukii Kimura and Barnes, new genus and species, in some ways intermediate between Chonecetus and Aetiocetus, has a suite of unique derived characters, including a much foreshortened brain case. The third, Aetiocetus tomitai Kimura and Barnes, new species, is the most primitive species of Aetiocetus yet discovered. The fourth, Aetiocetus polydentatus Sawamura, new species, the most derived species of Aetiocetus known, has a highly telescoped cranium, homodonty, polydonty and a dental count of 13–14/14–15.The fossil record now indicates considerable diversity in the family, with several different contemporaneous lineages in three new subfamilies: Chonecetinae, Morawanocetinae and Aetiocetinae. Aetiocetids are not known outside the North Pacific. Many Recent mysticetes are essentially cosmopolitan, and aetiocetids might have also been relatively widely dispersed. We suspect that with time their remains will be found around other ocean basins also. If so, then they may be potentially useful in trans‐oceanic geological correlations.

Subsidence and uplift of the Late Cretaceous-Cenozoic margin of California: New evidence from the Gualala and Point Arena basins

Cretaceous-Paleocene deposits of the Gualala basin and overlying deposits of the Neogene Point Arena basin record the complex tectonic and depositional history of the California continental margin as it changed from a convergent tectonic regime to a translational regime during the evolution of the San Andreas transform fault system. The Gualala and Point Arena deposits are among the northernmost onshore exposures of upper Cretaceous and Cenozoic marine strata west of the San Andreas fault and, as such, constitute a critically important but not intensely studied succession of rocks. In this paper, lithologic, stratigraphic, and paleobathymetric data from the Gualala and Point Arena stratigraphic successions are used to produce quantitative geohistory analyses that illustrate the subsidence and uplift of a portion of the California continental margin over the past 80 million yr. The history of subsidence and uplift revealed through geohistory analysis of the Gualala and Point Arena basin-fill deposits clearly reflects two episodes of basin development directly associated with the convergent and transform phases in the evolution of the California margin. Beginning in Late Cretaceous time (∼80 Ma), the Gualala basin underwent slow rates of tectonic subsidence (∼30 m/m.y.). In late Paleocene time (∼57 Ma), tectonic subsidence increased to ∼140 m/m.y. and was accompanied by rapid rates of sediment accumulation (∼440 m/m.y.) that continued through the Eocene until terminated by basin filling, cessation of subduction tectonics, and flexing and uplift of the continental margin. Initial development of the Neogene Point Arena basin occurred in latest Oligocene time (∼24 Ma), some 2 to 4 m.y. after the birth of the San Andreas transform system. The earliest history of the Point Arena basin was marked by volcanism and extremely high rates of tectonic subsidence (∼1,060 m/m.y.) characteristic of a transtensional origin. Based on our analysis, the tectonic and depositional histories of the Gualala and Point Arena basins can be summarized as follows: (1) during Late Cretaceous to middle Eocene, submarine fans of the Gualala and German Rancho Formations were deposited in a >2,000-m-deep subsiding basin that probably formed as a fore-arc basin in a setting characterized by oblique convergence and subduction along the California margin; (2) flexing and erosion of Cretaceous-Paleocene bathyal deposits of the Gualala basin occurred in the Oligocene epoch as the Pacific-Farallon spreading ridge approached the California margin, and as the accretionary prism outboard of the Gualala fore-arc basin migrated landward; (3) initial development of the Point Arena basin in earliest Miocene time (∼24 Ma) was accompanied by extrusion of the Iversen Basalt during the transitional period between convergent and transform tectonic regimes; (4) rapid subsidence of the Neogene Point Arena basin during Miocene time was related to transtension associated with the San Andreas fault, a pattern common to many of the small pull-apart basins formed along the California margin at this time; (5) early deposition in the Point Arena basin was dominated by submarine-fan deposits of the lower Miocene Skooner Gulch and Gallaway Formations, whereas siliceous biogenic sediments of the overlying Point Arena Formation signalled reduced deposition of coarse-grained terrigenous sediment during middle to late Miocene time; and (6) margin flexure and uplift of the Point Arena basin occurred in middle Pliocene (∼3.5 Ma) and early Pleistocene (∼1.6 Ma) time as a function of transpressional deformation of the crust in response to Pacific-North American plate interaction.

Tertiary arc rifting in northern Luzon, Philippines

The North Luzon terrane (NLT), comprising the section of Luzon north of the Philippine Fault, is one of the largest arc terranes in the Philippine Archipelago. Numerous features suggest that the NLT is a late Oligocene to early Miocene analogue for the processes in the modern intra‐arc rift zone at the northern end of the Mariana Trough. First, the NLT has bifurcating magmatic arcs sharing similar magmatic histories. These include the Northern Sierra Madre (NSM) and Cordillera Central (CC) magmatic arcs, which are separated by the Cagayan basin but which are linked in the Caraballo Range to the south. The rock record indicates that the NSM, CC, and Caraballo Ranges were active arcs in late Eocene to late Oligocene time. Second, seismic reflection and well data indicate that the Cagayan basin formed by extensional faulting in late Oligocene to early Miocene time. Third, alkalic arc magmatism, recognized to be a precursor of intra‐arc rifting in modern settings, occurred at the juncture of the NSM and CC arcs in late Oligocene to early Miocene time. Fourth, oceanic crust, represented by the Itogon ophiolite, formed at the southwestern end of the Cagayan basin in late Oligocene to early Miocene time. Major and trace element chemistry show that the Itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities. The rock record and geophysical offshore data suggest that the NLT was developing in an island arc system above the subducting West Philippine plate in late Eocene time. Rifting occurred in the island arc from late Oligocene to early Miocene time but did not mature into backarc spreading, most likely because of the collision of the Benham Rise, a basaltic rise in the West Philippine basin, with the NLT. The arc rifting in the NLT may be another manifestation of the extensional tectonism that affected most of Southeast Asia in late Oligocene to early Miocene time, during which the South China and Southeast Sulu basins formed. Subsequent to arc rifting, the history of the NLT has been linked to the subduction of the South China plate along the Manila Trench. The structural history of the Cagayan basin and magmatic history of the southern CC suggest that the subduction in the Manila Trench at the latitude of the NLT began about 15 Ma.

Thermobarometric and40Ar/39Ar geochronologic constraints on Eohimalayan metamorphism in the Dinggy� area, southern Tibet

Mineral assemblages in the Dinggye area of southern Tibet (28°N; 88°E) provide new insights regarding the poorly understood “Eohimalayan” metamorphic event in the eastern Himalayan orogen. Major element partitioning thermobarometry of pelitic rocks indicates temperatures of 750–830 K at depths of 14±3 km, consistent with the presence of kyanite, sillimanite, and andalusite schists in the area. Laser and resistance furnace40Ar/39Ar analyses of hornblendes from intercalated amphibolites yield closure ages of 25 Ma. Overlap between the probable range of Ar closure temperatures for these hornblendes and the metamorphic temperatures estimated through thermobarometry suggests that Eohimalayan metamorphism in the Dinggye area occurred in Late Oligocene time, no more than about 10 million years before the main or “Neohimalayan” phase of metamorphism in Early to Middle Miocene time. Muscovite, biotite, and K-feldspar40Ar/39Ar ages indicate an important episode of rapid cooling between 16 and 13 Ma, which is interpreted as a signature of tectonic denudation related to movement on N-dipping extensional structures of the South Tibetan detachment system.

Palaeomagnetic rotations in the eastern Betic Cordillera, southern Spain

Palaeomagnetic declinations from the External Zones of the eastern Betic Cordillera (southern Spain) and an adjacent area of the Internal Zone indicate variable and locally very large clockwise rotations. The rotations occurred after latest Oligocene times, and probably before the late Miocene. This overlaps a period of dextrally oblique convergence during the early to middle Miocene along the Internal/External Zone Boundary and within the External Zone. Rotations in the External Zone are recorded by palaeomagnetic results from 27 sites in Upper Jurassic ( Ammonitico rosso facies) limestones from the Subbetic Zone, and one site of a similar age from the Prebetic. The sites show either two- or three-component behaviour. The low-temperature component is coincident with the present-field direction. The intermediate-temperature component was probably acquired during Miocene folding. The high-temperature component passes fold tests (Miocene age) and a conglomerate test (Eocene-early Oligocene age). Palaeomagnetic declinations from individual tectonic blocks in the Subbetic are consistent, but indicate large differential rotations between blocks. These blocks are underlain by low-angle thrust faults, which probably accommodated much of the rotation. The largest rotations occur on relatively small isolated blocks of Jurassic carbonates in a highly deformed Triassic evaporite sequence. The single site in the Prebetic has not rotated significantly relative to stable Iberia. In Malaguide rocks of the Sierra Espun˜a, in the adjacent Internal Zone, stable palaeomagnetic components were measured at one site in upper Miocene sedimentary rocks, at three sites in upper Oligocene-lower Miocene red marls, at one site in Oligocene marls, at one site in Jurassic limestones, and at two sites in Permo-Triassic red beds. The palaeomagnetic results suggest that about 60° of clockwise rotation occurred in the latest Oligocene-earliest Miocene, and a further 140° of clockwise rotation subsequently. One site in late Miocene sedimentary rocks yields unrotated declinations which, if not representing an overprinted direction, indicate that the rotation was complete by the end of the Miocene.

Deformation and age of the Red Mountain intrusive system (Urad-Henderson molybdenum deposits), Colorado: Evidence from paleomagnetic and 40Ar/39Ar data

Paleomagnetic and 40Ar/39Ar age-spectrum data from most stocks of the Red Mountain intrusive system, in the northwest Colorado mineral belt, provide an improved understanding of the structural and cooling history of the suite of intrusions host to a world-class molybdenum deposit. Paleomagnetic data from five stocks at the surface and eight younger stocks exposed in the subsurface Henderson Mine support field observations (for example, dike and vein orientations, stock geometries, and distribution of zones of mineralization) that imply moderate tilting (15°-25° down to the east-southeast) since latest Oligocene time after cooling and mineralization. Surface stocks contain magnetizations carried by both magnetite and hematite. The Red Mountain stock is the youngest surface intrusion and contains mostly normal polarity magnetizations (for example, D = 321°, I = 59°, α95 = 19°, k = 9, N = 6 samples, site RM9), whereas older East Knob and Rubble Rock breccia intrusions contain a nearly antipodal, well-characterized magnetization (East Knob stock: declination = 161°, inclination = -47°, α95 = 13°, k = 23, average of five site means). Polarity changed from reverse to normal during emplacement and cooling of the Red Mountain intrusions exposed at the surface. 40Ar/39Ar age-spectrum data on biotite and orthoclase from the Red Mountain stock and stocks of the Henderson Mine indicate the reversal to be older than 30 Ma. All Henderson Mine stocks have normal polarity magnetizations (Primos stock: D = 333°, I = 51°, α95 = 5°, k = 44, average of six site means) which, on the basis of 40Ar/39Ar age spectra from orthoclase and biotite, were blocked between 28.7 and 27.6 Ma. Magnetite and maghemite are the major carriers of magnetization in these rocks. On the basis of an 40Ar/39Ar thermochronologic study of the Red Mountain intrusive system, thermal activity started at or just before 29.9 ± 0.3 Ma and ended at 26.95 ± 0.08 Ma. The age-spectrum data are interpreted to indicate that the porphyry of Red Mountain, one of the oldest stocks, was emplaced before 29.9 ± 0.3 Ma (possibly before 30.38 ± 0.09 Ma). Nearby lamprophyre dikes were emplaced at 29.8 ± 0.1 Ma; rhyolite dikes intruded at 29.4 ± 0.2 Ma. The Urad and Seriate stocks intruded after 29.8 Ma but before emplacement of the Vasquez stock at 28.71 ± 0.08 Ma. The system core cooled below 280 ± 40 °C (the argon closure temperature of biotite) at 27.59 ± 0.03 Ma. The last period of thermal activity involved pulses of magnetite-sericite alteration around the Seriate stock between 27.51 ± 0.03 and 26.95 ± 0.08 Ma; this activity did not thermally overprint unaltered parts of the intrusive system. Tilting of the Red Mountain area is implied by a comparison between a grand mean (on the basis of 10 stock means, D = 333°, I= 49°, α95 = 5°, k = 78) and a mid-Tertiary reference field. The Red Mountain intrusive system and host Precambrian rocks probably were deformed along a nearly north-south horizontal axis in response to northwest-side down, strike-slip faulting with displacement largely along the Woods Creek fault zone. Late Tertiary deformation of Precambrian-cored parts of the Front Range, host to numerous mineral deposits, was more complicated than simple, near-vertical uplift of the crust.

Compositional gradients in large reservoirs of silicic magma as evidenced by ignimbrites versus Taylor Creek Rhyolite lava domes

The Taylor Creek Rhyolite of southwest New Mexico consists of 20 lava domes and flows that were emplaced during a period of a few thousand years or less in late Oligocene time. Including genetically associated pyroclastic deposits, which are about as voluminous as the lava domes and flows, the Taylor Creek Rhyolite represents roughly 100 km3 of magma erupted from vents distributed throughout an area of several hundred square kilometers. Major-element composition is metaluminous to weakly peraluminous high-silica rhyolite and is nearly constant throughout the lava field. The magma reservoir for the Taylor Creek Rhyolite was vertically zoned in trace elements, 87Sr/86Sr, and phenocryst abundance and size. Mean trace-element concentrations, ranges in concentrations, and element-pair correlations are similar to many subalkaline silicic ignimbrites. However, the polarity of the zonation was opposite that in reservoirs for ignimbrites, for most constituents. For example, compared to the Bishop Tuff, only 87Sr/86Sr and Sc increased upward in both reservoirs. Quite likely, a dominant but nonerupted volume of the magma reservoir for the Taylor Creek Rhyolite was zoned like that for the Bishop Tuff, whereas an erupted, few-hundred-meter-thick cap on the magma body was variably contaminated by roof rocks whose contribution to this part of the magma system moderated relatively extreme trace-element concentrations of uncontaminated Taylor Creek Rhyolite but did not change the sense of correlation for most element pairs. The contaminant probably was a Precambrian rock of broadly granitic composition and with very high 87Sr/86Sr. Although examples apparently are not yet reported in the literature, evidence for a similar thin contaminated cap on reservoirs for large-volume silicic ignimbrites may exist in the bottom few meters of ignimbrites or perhaps only in the pumice fallout that normally immediately precedes ignimbrite emplacement. 87Sr/86Sr in sanidine phenocrysts of the Taylor Creek Rhyolite is higher than that of their host whole rocks. Covariation of this isotope ratio with sanidine abundance and size indicates positive correlations for all three features with decreasing distance to the roof of the magma reservoir. The sanidine probably is more radiogenic than host whole rock because growing phenocrysts partly incorporated Sr from the first partial melt of roof rocks, which contained the highly radiogenic Sr of Precambrian biotite ± hornblende, whereas diffusion was too slow for sanidine to incorporate much of the Sr from subsequently produced less radiogenic partial melt of roof rocks, before eruption quenched the magma system. Disequilibrium between feldspar phenocrysts and host groundmass is fairly common for ignimbrites, and a process of contamination similar to that for the Taylor Creek Rhyolite may help explain some of these situations.