mid-Pliocene Time Research Articles

Sensitivity of mid-Pliocene climate to changes in orbital forcing and PlioMIP's boundary conditions

Abstract. We compare results obtained from modeling the mid-Pliocene warm period using the Community Earth System Models (COSMOS, version: COSMOS-landveg r2413, 2009) with the two different modeling methodologies and sets of boundary conditions prescribed for the two phases of the Pliocene Model Intercomparison Project (PlioMIP), tagged PlioMIP1 and PlioMIP2. Here, we bridge the gap between our contributions to PlioMIP1 (Stepanek and Lohmann, 2012) and PlioMIP2 (Stepanek et al., 2020). We highlight some of the effects that differences in the chosen mid-Pliocene model setup (PlioMIP2 vs. PlioMIP1) have on the climate state as derived with COSMOS, as this information will be valuable in the framework of the model–model and model–data comparison within PlioMIP2. We evaluate the model sensitivity to improved mid-Pliocene boundary conditions using PlioMIP's core mid-Pliocene experiments for PlioMIP1 and PlioMIP2 and present further simulations in which we test model sensitivity to variations in paleogeography, orbit, and the concentration of CO2. Firstly, we highlight major changes in boundary conditions from PlioMIP1 to PlioMIP2 and also the challenges recorded from the initial effort. The results derived from our simulations show that COSMOS simulates a mid-Pliocene climate state that is 0.29 ∘C colder in PlioMIP2 if compared to PlioMIP1 (17.82 ∘C in PlioMIP1, 17.53 ∘C in PlioMIP2; values based on simulated surface skin temperature). On the one hand, high-latitude warming, which is supported by proxy evidence of the mid-Pliocene, is underestimated in simulations of both PlioMIP1 and PlioMIP2. On the other hand, spatial variations in surface air temperature (SAT), sea surface temperature (SST), and the distribution of sea ice suggest improvement of simulated SAT and SST in PlioMIP2 if employing the updated paleogeography. Our PlioMIP2 mid-Pliocene simulation produces warmer SSTs in the Arctic and North Atlantic Ocean than those derived from the respective PlioMIP1 climate state. The difference in prescribed CO2 accounts for 0.5 ∘C of temperature difference in the Arctic, leading to an ice-free summer in the PlioMIP1 simulation, and a quasi ice-free summer in PlioMIP2. Beyond the official set of PlioMIP2 simulations, we present further simulations and analyses that sample the phase space of potential alternative orbital forcings that have acted during the Pliocene and may have impacted geological records. Employing orbital forcing, which differs from that proposed for PlioMIP2 (i.e., corresponding to pre-industrial conditions) but falls into the mid-Pliocene time period targeted in PlioMIP, leads to pronounced annual and seasonal temperature variations. Our result identifies the changes in mid-Pliocene paleogeography from PRISM3 to PRISM4 as the major driver of the mid-Pliocene warmth within PlioMIP and not the minor differences in forcings.

Geologic constraints and speleogenesis of Cova des Pas de Vallgornera, a complex coastal cave from Mallorca Island (Western Mediterranean)

The flat areas of eastern and southern Mallorca host a remarkable coastal karst, where Cova des Pas de Vallgornera stands out due to its length (more than 74 km) and its special morphological suite. The pattern of the cave is quite heterogeneous showing sharp differences produced by the architecture of the Upper Miocene reef: spongework mazes and collapse chambers dominate in the reef front facies, whereas joint-guided conduits are the rule in the back reef carbonates. Regarding the speleogenesis of the system, a complex situation is envisaged involving three main agents: coastal mixing dissolution, drainage of meteoric diffuse recharge, and hypogene basal recharge related to local geothermal phenomena. The cave system is disposed in two main tiers of passages, of which geomorphologic interpretations are derived from their elevation data. The evolutionary trends as well as the chronology of the different cave sections are difficult to establish owing to the frequent shifting of the coastal base level during the Plio-Quaternary. In this respect, the genesis and evolution of the cave were fully controlled by sea-level fluctuations in the Western Mediterranean basin, with the main phases of cave formation, based on vertebrate paleontological data, going back to mid-Pliocene times.

Tectonosedimentary evolution of the Karacasu and Bozdoğan basins in the Central Menderes Massif, W Anatolia

The Karacasu and Bozdogan basins, which trend obliquely to modern grabens in the Central Menderes Massif, are investigated in terms of morphology, basinfill architecture, and structure. Evaluation of the previous geophysical and new structural data indicates that the basins are symmetrical grabens mostly running in a N-S direction. Analysis of sedimentary facies of the basins' infill supports the simple graben model by revealing lateral alluvial/colluvial fans in basin margins and axial fluvial/lacustrine environments in a central trough. The long-term changes between fluvial and lacustrine conditions in the basins are attributed to paleoclimatic origin. Paleostress analysis of slickensides substantiates that the basins were deformed under E-W extension and related N-S compression in the Late Miocene times, and this stress field would have been started in early Miocene times parallel to the Buyuk Menderes Graben (BMG). Following a basin-wide unconformity related to a short-lived compression in mid-Pliocene times, the stress field changed to a N-S extension as a result of the onset of the Buyuk Menderes Detachment in the north in the Late Pliocene-Early Quaternary. Emergence of the modern BMG bounded by high-angle normal faults in the north caused a progressive drop of base level and resulted in southward-developing fluvial incision since the Late Quaternary in the studied basins.

Active extensional faults in the central-eastern Iberian Chain, Spain

Among the conspicuous extensional structures that accommodate the onshore deformation of the Valencia Trough at the centraleastern Iberian Chain, a number of large faults show evidence of activity during Pleistocene times. At the eastern boundary of the Jiloca graben, the Concud fault has moved since mid Pliocene times at an average rate of 0.07-0.08 mm/y, while rates from 0.08 to 0.33 mm/y have been calculated using distinct stratigraphic markers of Middle to Late Pleistocene age. A total of nine paleoseisms associated to this fault have been identified between 74.5 and 15 ka BP, with interseismic periods ranging from 4 to 11 ka, estimated coseismic displacements from 0.6 to 2.7 m, and potential magnitudes close to 6.8. The other master faults of the Jiloca graben (Calamocha and Sierra Palomera faults) have also evidence of Pliocene to Late Pleistocene displacement, with average slip rates of 0.06 and 0.11-0.15 mm/y, respectively. At the eastern boundary of the Teruel graben, the Sierra del Pobo fault has been active since Late Miocene times, at slip rates of 0.06-0.11 mm/y. Quaternary activity its better constrained for the Teruel fault, which offsets two fluvial terraces, with an estimated slip rate of 0.12 mm/y since 76 ka BP. A widespread, NNE-SSW trending fault system extends over the easternmost Iberian Chain (Maestrat sector), with abundant proofs of activity during Early to Middle Pleistocene s.l. times. Nevertheless, such proofs are mainly geomorphologic, while dated stratigraphic markers allowing precise assessment of slip rates are absent.

The late Cretaceous to Holocene sequence stratigraphy of a sheared passive upper continental margin, northern KwaZulu-Natal, South Africa

High resolution single-channel seismic data reveal seven seismic units (A–G) from the narrow and steep upper portions of the sheared passive continental margin of northern KwaZulu-Natal. Several characteristic internal geometries are evident: onlapping sheet-like (Unit C), aggradational/progradational (Unit A and Facies D3), and progradational (Unit B, Facies D1/2 and Facies G1). On the basis of internal geometry and bounding unconformities, lowstand (Units B, Facies D3 and lowermost Unit G), possible transgressive (Unit C), highstand (upper Unit G) and falling stage (Facies D1 and D2) systems tracts are delineated. Units E and F are not reconcilable due to acoustic opacity. Overall, five incompletely preserved sequences are observed. Sequence one comprises Unit A of suspected Middle Maastrichtian age, separated from the overlying sequence by a subaerial unconformity surface SB1. Units B and C form sequence two and span the late Cretaceous (Late Maastrichtian) to mid–late Palaeocene. Two major hiatuses ensued (Angus and Jimmy) until mid Pliocene times when Unit D was deposited during shelf margin advance linked to hinterland uplift. These falling stage systems tract deposits are separated from latest Pliocene/early Pleistocene lowstand shelf edge deltaic deposits by a subaerial unconformity marking the deposition of sequence four (Facies D3). The age of Unit E is uncertain. Unit F represents a series of shorelines formed during Oxygen Isotope Stage (OIS) 5a to 2 on the regressive and transgressive limbs preceding and following the Last Glacial Maximum (LGM). The final partial sequence preserved is comprised of Unit G formed during the OIS 2 transgression to present mean sea level and subsequent development of the contemporary highstand wedge. In terms of stratigraphic architecture, this study shows an evolution from a ramp-style margin with a poorly defined shelf break to that of a shelf break profile with a narrow and steep shelf. In this respect, the narrowness of this shelf cannot be accounted for by the inheritance of shearing effects. Rather, the stratigraphic architecture reflects a period of slow and protracted subsidence with strong sediment dispersal, later punctuated by hinterland uplift. This formed the late Pliocene shelf margin clinoforms of Unit D and established the shelf break position at the offlap break of this wedge. The resulting narrow shelf may play a significant role in the preservation of later sequences, particularly in light of the continued uplift of the margin causing off-shelf sediment bypass where TST and HST sequences may not be preserved. This is reflected in the isolated preservation of shoreline and shallow marine deposits of the Pleistocene Units E and F.

Morphotectonics of the Concud Fault (Iberian Chain, Spain): Comparing Geomorphic and Geologic Indices of Activity of an Intraplate Extensional Fault

The results of geomorphic analysis of the Concud fault-generated mountain front (central Iberian Chain, Spain) are introduced into classifications of fault activity proposed by previous authors, and compared with slip rates calculated from geologic markers. The Concud fault is an extensional structure active since the mid Pliocene times. It gives rise to a 60 to 120 m high mountain front, where footwall rocks belonging to the Triassic and Jurassic (north-western sector) and Miocene (south-eastern sector) crop out. Conspicuous triangular facets are preserved on Jurassic rocks of the central sector, while short, generally non-incised alluvial fans make the piedmont. The value of the Mountain-front sinuosity index is Smf = 1.24 for the whole mountain front (1.17 and 1.32, respectively, for both segments showing distinct footwall lithology), as obtained by the most conservative procedure. Average valley floor width/height ratios calculated for seventeen gullies crossing the fault are Vf = 0.30 (250 m upstream from the fault trace) and Vf = 0.22 (500 m upstream). These geomorphic indices, together with qualitative features of the escarpment and piedmont landscape, indicate ‘moderate’ to ‘rapid’ fault activity. The range of slip rates estimated from such morphotectonic classification (0.03 to 0.5 mm/y) encloses the range calculated from offset Late Pliocene and Pleistocene stratigraphic markers (0.07 to 0.33 mm/y). Nevertheless, the highest potential slip rate (0.5 mm/y) clearly represents an overestimate: the mountain front could give the impression of an anomalously high level of activity owing to episodic rejuvenation caused by base level drop.

The Lake El’gygytgyn Scientific Drilling Project – Conquering Arctic Challenges through Continental Drilling

Between October 2008 and May 2009, the International Continental Scientific Drilling Program (ICDP) co-sponsored a campaign at Lake Elgygytgyn, located in a 3.6-Ma-old meteorite impact crater in northeastern Siberia. Drilling targets included three holes in the center of the 170-m-deep lake, utilizing the lake ice cover as a drilling platform, plus one hole close to the shore in the western lake catchment. At the lake's center. the entire 315-m-thick lake sediment succession was penetrated. The sediments lack any hiatuses (i.e., no evidence of basin glaciation or desicca- tion), and their composition reflects the regional climatic and environmental history with great sensitivity. Hence, the record provides the first comprehensive and widely time- continuous insights into the evolution of the terrestrial Arctic since mid-Pliocene times. This is particularly true for the lowermost 40 meters and uppermost 150 meters of the sequence, which were drilled with almost 100% recovery and likely reflect the initial lake stage during the Pliocene and the last ~2.9 Ma, respectively. Nearly 200 meters of under- lying rock were also recovered; these cores consist of an almost complete section of the various types of impact brec- cias including broken and fractured volcanic basement rocks and associated melt clasts. The investigation of this core sequence promises new information concerning the Elgygytgyn impact event, including the composition and nature of the meteorite, the energy released, and the shock behavior of the volcanic basement rocks. Complementary information on the regional environmental history, including the permafrost history and lake-level fluctuations, is being developed from a 142-m-long drill core recovered from the permafrost deposits in the lake catchment. This core con- sists of gravelly and sandy alluvial fan deposits in ice-rich permafrost, presumably comprising a discontinuous record of both Quaternary and Pliocene deposits.

The Lake El'gygytgyn Scientific Drilling Project – Conquering Arctic Challenges through Continental Drilling

Between October 2008 and May 2009, the International Continental Scientific Drilling Program (ICDP) co-sponsored a campaign at Lake El´gygytgyn, located in a 3.6-Ma-old meteorite impact crater in northeastern Siberia. Drilling targets included three holes in the center of the 170-m-deep lake, utilizing the lake ice cover as a drilling platform, plus one hole close to the shore in the western lake catchment. At the lake’s center. the entire 315-m-thick lake sediment succession was penetrated. The sediments lack any hiatuses (i.e., no evidence of basin glaciation or desiccation), and their composition reflects the regional climatic and environmental history with great sensitivity. Hence, the record provides the first comprehensive and widely timecontinuous insights into the evolution of the terrestrial Arctic since mid-Pliocene times. This is particularly true for the lowermost 40 meters and uppermost 150 meters of the sequence, which were drilled with almost 100% recovery and likely reflect the initial lake stage during the Pliocene and the last ~2.9 Ma, respectively. Nearly 200 meters of underlying rock were also recovered; these cores consist of an almost complete section of the various types of impact breccias including broken and fractured volcanic basement rocks and associated melt clasts. The investigation of this core sequence promises new information concerning the El´gygytgyn impact event, including the composition and nature of the meteorite, the energy released, and the shock behavior of the volcanic basement rocks. Complementary information on the regional environmental history, including the permafrost history and lake-level fluctuations, is being developed from a 142-m-long drill core recovered from the permafrost deposits in the lake catchment. This core consists of gravelly and sandy alluvial fan deposits in ice-rich permafrost, presumably comprising a discontinuous record of both Quaternary and Pliocene deposits. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.11.03.2011" target="_blank">10.2204/iodp.sd.11.03.2011</a>

Increasing long-wavelength relief across the southeastern flank of the Sierra Nevada, California

A high degree of correlation between present-day relative rock uplift measured using continuous GPS geodesy and spatially averaged surface elevations suggests that long-wavelength topographic relief is presently increasing along the southeastern flank of the Sierra Nevada range and within an adjacent portion of the northern Basin and Range province. Current estimates for erosion rate are an order of magnitude smaller than the relative rates determined by geodesy. Thus, although the uplift serves to enhance long-wavelength relief, it cannot be explained entirely as an isostatic response to erosion. If uplift rates have been constant through time, the duration over which the uplift could have been active (<1.6 Myrs) is shorter than expected based on our current understanding of the geodynamic history of the region. One possible explanation for the observed present-day uplift pattern is that it represents an ongoing isostatic response to gradual reduction of the equivalent elastic thickness (EET) of the lithosphere under temporally constant loads. Another explanation is that the loads currently supporting the high elevations of the southern Sierra are evolving with time. According to either hypothesis, vertical surface motions may have slowly accelerated since ~mid-Pliocene time. Several possible mechanisms for progressive reduction of EET may be attributable to thermo-mechanical disequilibrium that began with the removal of an ultramafic root from the Sierran batholith during late Miocene or early Pliocene time. Specific mechanisms for ongoing enhancement of loads are less obvious. Based on these results, we suggest that dense networks of long-running continuous GPS stations around the world currently represent an underutilized resource for studies of orogenesis and upper mantle processes.

Pliocene seasonality across the North Atlantic inferred from cheilostome bryozoans

Previous studies have shown an inverse correlation between zooid size in cheilostome bryozoans and ambient water temperature. This relationship underlies the MART technique which uses intracolonial variation in zooid size to predict mean annual range in temperature experienced by bryozoan colonies during their life. Here we apply the MART technique to study Early and Mid Pliocene bryozoans from Central America (Panama, Costa Rica), the USA (Florida, South Carolina, North Carolina, Virginia) and the UK (Suffolk) to reconstruct palaeoseasonality across a range of latitudes for the North Atlantic during the Pliocene Epoch. Compared to the present-day, our analyses suggest greater seasonality (ca 4.5 °C) in the southern Caribbean at the time of Cayo Agua Formation deposition (ca 4.25 Ma), in keeping with inferred upwelling prior to the closure of the isthmian barrier at 2.7 Ma. Bryozoans also indicate seasonal upwelling on the Gulf Coast of Florida in a similar manner to the present-day. Because upwelling can be highly localised and prone to spatial and temporal variation in the Gulf of Mexico today, it contributes little to a broad understanding of Pliocene North Atlantic waters. However, MART estimates for the coastal plain region indicate a general reduction in the annual range in temperature relative to the present, suggesting that the colder surface waters that today reach south to Cape Hatteras had less influence in Early to Mid Pliocene times. These results, along with evidence from other proxies, strongly support reduced seasonality and warmer conditions along the eastern seaboard of the USA in the Early to Mid Pliocene. Finally, the MART estimates amongst Coralline Crag localities provide evidence for an increased annual range in temperature in the southern North Sea than at present. Our study shows that bryozoan MART estimates provide a powerful, independent proxy for palaeoseasonality and is the first to demonstrate that the MART technique can be applied to infer palaeoclimates across a wide range of latitudes focusing on a variety of geological formations and geographical regions.

Fluctuation history of the interior East Antarctic Ice Sheet since mid-Pliocene

AbstractCosmogenic 10Be and 26Al measurements from bedrock exposures in East Antarctica provide indications of how long the rock surface has been free from glacial cover. Samples from the crests of Zakharoff Ridge and Mount Harding, two typical nunataks in the Grove Mountains, show minimum 10Be ages of 2.00 ± 0.22 and 2.30 ± 0.26 Ma, respectively. These ages suggest that the crests were above the ice sheet at least since the Plio–Pleistocene boundary. Adopting a ‘reasonable’ erosion rate of 5–10 cm Ma-1 increases the exposure ages of these two samples to extend into the mid-Pliocene. The bedrock exposure ages steadily decrease with decreasing elevation on the two nunataks, which indicates ~200 m decrease of the ice sheet in the Grove Mountains since mid-Pliocene time. Seven higher elevation samples exhibit a simple exposure history, which indicates that the ice sheet in the Grove Mountains decreased only ~100 m over a period as long as 1–2 Ma. This suggests that the East Antarctic Ice Sheet (EAIS) was relatively stable during the Pliocene warm interval. Five lower elevation samples suggest a complex exposure history, and indicate that the maximum subsequent increase of the EAIS was only 100 m higher than the present ice surface. Considering the uncertainties, their total initial exposure and subsequent burial time could be later than mid-Pliocene, which may not conflict with the stable mid-Pliocene scenario.

Neotectonic development of the Çameli Basin, southwestern Anatolia, Turkey

Abstract This study of the Çameli Basin presents a detailed basin evolution combined with structural analysis and provides the first detailed time-stratigraphic framework for the neotectonic development of Neogene grabens along the Fethiye-Burdur Fault Zone in southwestern Anatolia. During the Early Tortonian, the Çameli Basin was established as a broad fault-bounded fluviolacustrine basin that experienced NW-SE extension. By Mid-Pliocene time, continued NW-SE extension resulted in the formation of a new intrabasinal fault zone that split the basin longitudinally into two compartments. The development of a new generation of normal faults further split the basin into four narrow half-graben compartments at the end of the Late Pliocene. Structural analysis of basin-bounding and intrabasinal faults related to this three-stage basin development shows that NW-SE extension apparently persisted from Late Miocene to early Quaternary time. The youngest (i.e. Holocene), deformation is characterized by dextral shear along NE-SW-trending strikeslip faults and continuing NW-SE extension. The Late Miocene foundering of the basin was related to extension in the northerly hinterland zone of the still-emplacing Lycian nappes, whereas outward growth of the Hellenic Arc in response to the westward Anatolian extrusion is the main cause for NW-SE extension from the Pliocene onward. Dextral strike-slip faulting is localized and is associated with the activity of NW-SE-trending faults that accommodated NE-SW extension. The simultaneous activity of these faults suggests the existence of biaxial extensional tectonics, as initially proposed for the Burdur-Dinar area. Sinistral strike-slip faulting, continuing along the eastern Hellenic Arc, penetrated the southernmost part of Turkey but has not yet reached the Çameli Basin area. Our biostratigraphically well-constrained tectonosedimentary model for the evolution of the Çameli Basin provides a reliable time-stratigraphic framework for NE-SW extension in the ‘Fethiye-Burdur Fault Zone’ of SW Anatolia. We believe that this fault zone represents a broad zone of isolated or interconnected NE-SW-trending basins that formed under prevailing NW-SE extension, rather than being a significant strike-slip fault zone.

A kinematic model for the development of the Afar Depression and its paleogeographic implications

The Afar Depression is a highly extended region of continental to transitional oceanic crust lying at the junction of the Red Sea, the Gulf of Aden and the Ethiopian rifts. We analyze the evolution of the Afar crust using plate kinematics and published crustal models to constrain the temporal and volumetric evolution of the rift basin. Our reconstruction constrains the regional-scale initial 3D geometry and subsequent extension and is well calibrated at the onset of rifting (∼20 Ma) and from the time of earliest documented sea-floor spreading anomalies (∼6 Ma Red Sea; ∼10 Ma Gulf of Aden). It also suggests the Danakil block is a highly extended body, having undergone between ∼200% and ∼400% stretch. Syn-rift sedimentary and magmatic additions to the crust are taken from the literature. Our analysis reveals a discrepancy: either the base of the crust has not been properly imaged, or a (plume-related?) process has somehow caused bulk removal of crustal material since extension began. Inferring subsidence history from thermal modeling and flexural considerations, we conclude subsidence in Afar was virtually complete by Mid Pliocene time. Our analysis contradicts interpretations of late (post 3 Ma) large (∼2 km) subsidence of the Hadar area near the Ethiopian Plateau, suggesting paleoclimatic data record regional, not local, climate change. Tectonic reconstruction (supported by paleontologic and isotopic data) suggests that a land bridge connected Africa and Arabia, via Danakil, up to the Early to Middle Pliocene. The temporal constraints on land bridge and escarpment morphology constrain Afar paleogeography, climate, and faunal migration routes. These constraints (particularly the development of geographic isolation) are fundamentally important for models evaluating and interpreting biologic evolution in the Afar, including speciation and human origins.

Global sea-level and the (pen-)insularity of late Cenozoic Britain

Abstract During mid-Pliocene times, Britain was surrounded by warm temperate seas. In the south, these received relatively little clastic input, accumulated mainly biogenic sediments, and appear to have allowed free circulation of marine waters between the North Sea and the Atlantic Ocean, either around or across southern Britain. In late Pliocene times, commencing at about 2.5 Ma bp , the first major global sea-level falls associated with northern hemisphere glaciation occurred. At about the same time, climatic changes appear to have accelerated the rate of headwater extension and bedload transport of the principal rivers flowing towards the southern North Sea. The combination of sea-level fall and progressive deltaic progradation into the southern North Sea converted Britain into a European peninsula. By the mid-Quaternary Cromerian stage, the Great European (Ur-Frisia) Delta top was sufficiently moribund, and subject to subsidence, to allow interglacial sea-levels to encroach southwards across it. During the following Anglian Glaciation, impounded glacio-marine or glacio-lacustrine waters appear to have extended southwards, across both the former delta top and the bounding Weald-Artois chalk ridge, into the English Channel. Subsequently a marine connection between the North Sea and the Atlantic Ocean, via the English Channel, was alternately established and broken, as global sea-level rose and fell (synchronously with interglacial and glacial periods), cyclically converting Britain from island to peninsula and back again.

Limited Pliocene Glacier Extent and Surface Uplift in Middle Taylor Valley, Antarctica

ABSTRACTLimited surface uplift, glacial extent, and climatic change since mid-Pliocene time are inferred from surficial deposits in middle Taylor Valley, Antarctica (77°30'-77°50'S., 160°0'-163°40'...

Pliocene Paleoclimate and East Antarctic Ice-Sheet History from Surficial Ash Deposits

The preservation, age, and stratigraphic relation of an in situ ashfall layer with an underlying desert pavement in Arena Valley, southern Victoria Land, indicate that a cold-desert climate has persisted in Arena Valley during the past 4.3 million years. These data indicate that the present East Antarctic Ice Sheet has endured for this time and that average temperatures during the Pliocene in Arena Valley were no greater than 3 degrees C above present values. One implication is that the collapse of the East Antarctic Ice Sheet due to greenhouse warming is unlikely, even if global atmospheric temperatures rise to levels last experienced during mid-Pliocene times.

Evidence of rapid Cenozoic uplift of the shoulder escarpment of the Cenozoic West Antarctic rift system and a speculation on possible climate forcing

The Cenzoic West Antarctic rift system, characterized by Cenozoic bimodal alkalic volcanic rocks, extends over a largely ice-covered area, from the Ross Sea nearly to the Bellingshausen Sea. It is bounded on one side by a spectacular 4-to 5-km-high rift-shoulder scarp (maximum bedrock relief 5 to 7 km) from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. Jurassic tholeiites crop out with the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed discontinuously along the lower elevation (1-2 km) section of the Transantarctic Mountains to the Weddell Sea. Various lines of evidence, no one of which is independently conclusive, lead the authors (as others have also suggested) to interpret the following. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been rising since about 60 Ma, at episodic rates of {approximately}1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100m/m.y. Uplift rates vary along the scarp, which is cut by transverse faults. The authors speculate that this uplift may have climatically forcedmore » the advance of the Antarctic ice sheet since the most recent warm period. They suggest a possible synergistic relation between episodic tectonism, mountain uplift, and volcanism in the Cenozoic West Antarctic rift system and waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time.« less

The Northwest Atlantic Mid-Ocean Channel of the Labrador Sea. V. Sedimentology of a giant deep-sea channel

The Northwest Atlantic Mid-Ocean Channel (NAMOC) is one of the largest deep-sea channels of the world's oceans. During the late Cenozoic glacial period, the channel played a major role in the depositional history of the Labrador Sea and northwest Atlantic in controlling sedimentation of a broad (approx. 500 m thick and 200 km wide) lens of turbidites. This sediment sequence interfingers laterally with the acoustically transparent pelagic and contourite facies found in the Labrador Basin. The meandering channel is a depositional–erosional feature formed by submarine mass flows, predominantly turbidity currents.The channel contains a meandering talweg that appears to be associated with a sequence of submarine point bars containing thick-bedded, coarse-grained turbidites and gravel layers (channel-fill facies). Old channel positions on seismic profiles indicate that the channel has migrated laterally up to 30 km both to the west and to the east.Natural levees flank the channel for its entire length, extending laterally into turbidite plains 60–100 km wide. The spill-over facies comprises thin-bedded, fine-grained turbidites dominated by thinly laminated muds. Individual units of parallel-laminated mud, which result from single turbidity currents overtopping the channel banks, average 3 cm in thickness. A layer by layer correlation of a sequence of spill-over turbidites is possible between two adjacent cores 70 km apart. Coarse-grained off-channel sediments recently discovered on both levees at distances up to 55 km from the NAMOC occur in tributary channels.Turbidity current activity in the channel probably started with the onset of glaciation at about mid-Pliocene time and ceased at about 7000 years BP, when deglaciation proceeded rapidly. The sedimentation rate for the last episode of overbank deposition on the levees, which probably occurred between 11 000 and 7000 years BP, is 13 cm/1000 years. Towards the end of glacial episodes the northwestern Labrador Sea was probably covered with sea ice.

The role of the Kyrenia Range Lineament, Cyprus, in the geological evolution of the eastern Mediterranean area

Rewarding insights into major crustal lineaments come from the integrated study of well exposed examples. One is the Kyrenia Range, a narrow arcuate lineament of several hundred kilometres in length comprising northern Cyprus and its offshore extension. The Kyrenia Range consists mostly of Mesozoic and Tertiary sedimentary and subordinate volcanic and metamorphic rocks, disposed in four rock groups separated by unconformities recording deformation events. The lineament is dominated by a steeply dipping composite thrust pile located partly along, and partly straddling, the abrupt northward termination of crust similar to the Troodos Igneous Complex at depth. The 200 Ma history of the lineament involved episodic rift, passive-margin, active-margin, strike-slip and uplift phases. The area was rifted off Gondwana in the late Triassic to form a southerly Turkish microcontinent capped by a gently subsiding carbonate platform. After formation of a small ocean basin to the south during the Cretaceous (Troodos ocean), northward subduction began (?Santonian). The first major deformation (D1) is attributed to pervasive (?dextral) strike-slip, which removed the Mesozoic passive margin and brecciated and metamorphosed the remaining platform. In the Maastrichtian and early Tertiary the area subsided and scree breccias were shed from scarps into pelagic carbonate-depositing seas, while bimodal within-plate-type lavas were erupted in an extensional setting influenced by strike-slip. By mid Eocene time, shortening, first evidence by flysch and olistostrome deposition, culminated in strong southward thrusting (D2) and localized metamorphism. Northward subduction south of Cyprus ensued and the range lay in an extensional fore-arc setting in late Eocene and Miocene time. The area then subsided dramatically and accumulated thick turbidite sequences derived from eroding Tauride Mountain areas to the northeast. Faulting and general uplift in the late Miocene was followed by renewed compressional deformation climaxing in mid Pliocene time (D3) with large-scale thrusting and tilting. Pulsed vertical uplift continued through the Quaternary. Similar volcanic and metamorphic rocks formed along the Kyrenia Lineament at intervals. Sedimentary rocks emerge as the most sensitive tectonic setting indicators. Long-lived lineaments like the Kyrenia Range are inherently very complicated, and perceived simple solutions in other cases should be viewed with some scepticism.

Post-collision isostatic readjustment in the Southern Banda Arc

Summary The collision between the Australian continent and island arcs marginal to SE Asia is currently taking place in the Banda Arc region, with some of the most intense deformation on, and around, the island of Timor. From Late Miocene to Mid-Pliocene times the area was dominated by a compressional tectonic regime which resulted in the emplacement from the N of large thrust sheets on the deformed Australian continental margin. During the last 3 Ma, however, compression seems to have been unimportant in the islands and vertical movements, both positive and negative; have been commonplace and rapid. Considerable departures from isostatic equilibrium are suggested by gravity surveys in the region and in some cases the sign of the isostatic anomalies accords well with the observed sense of vertical movement. In other areas, such as Northern Timor and the inner (volcanic) arc, uplift is occurring where the the gravity data suggest that there should be subsidence. A possible explanation is that a contribution to the high gravity values is made by a cold, and therefore dense, subducted slab which is now sinking independently after rupture near the continental margin. Because of the rupture, the sinking slab no longer exerts a downward pull on the overlying lithosphere which has thus been freed to rebound isostatically.