Changing Tectonics, Cooling Climates and the Dawn of Crustal Extension: Late Eocene to Early Miocene (ca. 35–20 Ma)

The paleotopographic history of the North American Cordilleran orogen holds the key to understanding mechanisms of orogenesis and subsequent orogenic collapse. It has been suggested that the orogenic front in western Montana (USA) and Alberta (Canada) was more than 4 km high during Late Cretaceous−early Eocene contractional deformation and during the initial phase of extension in the middle Eocene; however, the late Eocene−Oligocene topographic evolution during continued extensional collapse remains poorly constrained. Here we extend the paleotopographic record in the Kishenehn Basin in northwestern Montana and southeastern British Columbia (Canada) to the late Oligocene by studying δ18O values of fossil mollusks and cement and paleosol carbonates. The molluscan taxa changed from three sympatric groups with preferred habitats ranging from tropical wet, semi-arid subtropical, and temperate during the middle and late Eocene, to mainly a single group associated with temperate environment during the Oligocene, reflecting a decline in molluscan biodiversity induced by climate cooling across the Eocene−Oligocene transition. Reconstructed δ18O values of alpine snowmelt and basinal precipitation decreased by 1.4‰ and 3.8‰, respectively, from the middle to late Eocene, reflecting climate cooling and ∼1 km surface uplift of the basin floor. The reconstructed alpine snowmelt δ18O values then increased by 2.9‰ in the Oligocene suggesting a ∼0.5 km drop in elevation of the orogenic front. Collectively, the results of our new and previously published δ18O data chronicle the paleotopographic response to the change from flat-slab subduction to slab rollback over a 45 m.y. period. These data suggest that the orogenic front was characterized by high elevation (>4 km) in the ancestral Lewis-Clark-Livingston ranges during latest Cretaceous−early Eocene (ca. 75−52 Ma) contraction. The initial phase of extension related to the Kishenehn Basin created a lowland basin with a surface elevation of only ∼1.5 km during the early middle Eocene (ca. 46−44 Ma) whereas the ranges remained >4 km high. The high range elevations were sustained for at least 12 m.y. in the middle to late Eocene concurrent with extension, while the basin floor elevation was uplifted to ∼2.5 km by the latest Eocene (ca. 36−34 Ma). Basin aggradation can explain at most half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km) and sustained high range elevation suggest that range denudation and crustal extension was compensated by the isostatic and thermal effects of slab rollback and/or passage of a slab window and infusion of hot asthenosphere beneath the continent. The range elevation in the orogenic front decreased ∼0.5 km by the late Oligocene (ca. 28 Ma), associated with a decrease in rock uplift rate associated with extension. A post-Oligocene elevation drop of ∼1 km resulted in both the ranges and basin floor reaching modern topography in the Kishenehn Basin drainage, likely due to the regional effect of Neogene Basin and Range extension. This study, along with the previous investigation of the Kishenehn Basin by Fan et al. (2017), are the first studies that systematically investigate paleorelief of the orogenic belt by reconstructing paleoelevations of the mountains and the basin at the same time. The results highlight that the Cordilleran orogenic front of northern Montana and southern British Columbia sustained its high elevation edifice for at least 12 m.y. after the start of extension. We suggest that initial crustal extension did not result in orogenic demise because of concurrent thermal and isostatic uplift.

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The Late Eocene-Early Miocene Unconformities of the NW Indian Intraplate Basins and Himalayan Foreland: A Record of Tectonics or Mantle Dynamics?

Tectonics ◽

10.1029/2018tc005286 ◽

2018 ◽

Vol 37 (10) ◽

pp. 3970-3985 ◽

Cited By ~ 1

Author(s):

Yani Najman ◽

Stuart D. Burley ◽

Alex Copley ◽

Michael J. Kelly ◽

Kaushal Pander ◽

...

Keyword(s):

Late Eocene ◽

Early Miocene ◽

Mantle Dynamics

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Synchronous Eocene deformation recorded on either side of a major Miocene thrust bounding the Almyropotamos tectonic window in Evia, Greece

10.5194/egusphere-egu21-3275 ◽

2021 ◽

Author(s):

Taylor Ducharme ◽

Iwona Klonowska ◽

David Schneider ◽

Bernhard Grasemann ◽

Kostantinos Soukis

Keyword(s):

Regional Scale ◽

Late Eocene ◽

White Mica ◽

Early Miocene ◽

Lower Grade ◽

Metamorphic History ◽

Tectonic Window ◽

Platform Carbonates ◽

C 30 ◽

Type 3

Southern Evia in Greece exposes an inverted high pressure-low temperature (HP-LT) metamorphic sequence that has been loosely correlated with the Cycladic Blueschist Unit (CBU). On the island, the CBU is divided into the metavolcanic and ophiolitic Ochi Nappe and predominantly metacarbonate Styra Nappe. A lower-grade unit, the Almyropotamos Nappe, is exposed in the core of a N-S trending antiform and comprises Eocene platform carbonates overlain by metaflysch. The Almyropotamos Nappe occupies a tectonic window defined by the Evia Thrust, a brittle-ductile fault zone that emplaced the Ochi and Styra nappes atop the Almyropotamos Nappe. New multiple single-grain white mica total fusion 40Ar/39Ar ages indicate that deformation occurred along the Evia Thrust at 25-23 Ma. White mica 40Ar/39Ar data on either side of the tectonic window record Eocene dates between 40 and 32 Ma, consistent with previously published 40Ar/39Ar dates and a single Rb-Sr age of c. 30 Ma. These ages broadly coincide with estimates for the timing of NE-directed thrusting of the Ochi Nappe over the Styra Nappe. Strain associated with thrusting localized as cylindrical folds in Styra marbles, with fold axes parallel to the stretching lineation and a clear strain gradient increasing toward the upper contact with the Ochi Nappe. The most prominent structures in the Ochi Nappe are a strong L-S fabric defined by acicular blue amphibole and type-3 refold structures with fold axes trending parallel to the NE-SW oriented stretching lineation. Whereas the Ochi Nappe and Styra Nappe locally preserve peak blueschist facies mineral assemblages, all three units commonly display evidence only for retrogressed initial HP-LT assemblages in the form of ferroglaucophane inclusions in albite porphyroblasts. Isochemical phase diagrams calculated in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2&#177;O2 system support minimum peak metamorphic conditions of 12.5 &#177; 1.5 kbar and 465 &#177; 75 &#176;C for an Ochi Nappe blueschist, and 6.0 &#177; 0.5 kbar and 315 &#177; 15 &#176;C for an albite mica schist from the Evia Thrust. Peak P-T conditions for the Ochi Nappe support a metamorphic history more closely resembling that of the Lower Cycladic Blueschist Nappe, indicating that the entire section of the CBU exposed on Evia lies below the Trans-Cycladic Thrust. The Early Miocene ages from the Evia Thrust overlap with the proposed timing for the initiation of bivergent greenschist facies extension in the Cyclades. The remainder of the region, including high-strain corridors within individual nappes such as the Almyropotamos Thrust, uniformly records Eocene deformation ages. The similarity in 40Ar/39Ar ages across the tectonic window contrasts with age relationships observed in similar tectonic packages on Lavrion, and suggests that regional scale deformation persisted until the Late Eocene before strain became localized in brittle-ductile corridors by the Early Miocene.&#160;

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Peonza: new gastropod genus from the middle Tertiary of Patagonia and Chile

Journal of Paleontology ◽

10.1017/s0022336000022873 ◽

1994 ◽

Vol 68 (2) ◽

pp. 279-286 ◽

Cited By ~ 7

Author(s):

Amalia M. Olivera ◽

William J. Zinsmeister ◽

S. Graciela Parma

Keyword(s):

New Species ◽

Temporal Distribution ◽

Late Eocene ◽

Tierra Del Fuego ◽

Early Miocene ◽

Late Oligocene ◽

Southern Chile ◽

Santa Cruz ◽

Two New Species

A new Tertiary gastropod genus, Peonza n. gen., is described, along with two new species, P. torquata from southern Argentina and P. benjamina from southern Chile. These muricacean gastropods, of uncertain familial status, occur in the late Eocene San Julián Formation and in the late Oligocene to early Miocene? Monte León Formation, Santa Cruz Province, Argentina. They also were recorded in the (probably) Oligocene Magellanian beds in Tierra del Fuego, Argentina, and in early Miocene deposits of the Tres Montes region in the Chilean Canals. In spite of the small number of specimens, Peonza n. gen. seems to have had a wide geographic and temporal distribution.

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Patterns of terrestrial plant carbon: late Mesozoic and Cenozoic

The Paleontological Society Special Publications ◽

10.1017/s2475262200007851 ◽

1992 ◽

Vol 6 ◽

pp. 225-225

Author(s):

Jerry Olson ◽

Garland R. Upchurch

Keyword(s):

Global Scale ◽

Late Eocene ◽

Early Miocene ◽

Strongly Correlated ◽

Carbon Density ◽

Terrestrial Plant ◽

Late Mesozoic ◽

Major Increase ◽

Temperate Rainforests ◽

Global Carbon

Variation in terrestrial productivity and biomass impacts evolution through linkages between productivity and biodiversity and through the types of resources available for consumption by herbivores. Geographic variation in terrestrial plant carbon is known on a global scale for extant biomes and is strongly correlated with precipitation, temperature, and the area of wetlands. Although estimates of extant terrestrial plant carbon density are still somewhat uncertain, the highest densities clearly occur in tropical and temperate rainforests, and the lowest occur in deserts, semideserts, and arctic/alpine tundra. Patterns of variation in ancient terrestrial plant carbon can be estimated through the correlation between biome/climate and carbon density, provided individual biomes show little change through time in primary productivity or density of plant carbon.Density of terrestrial plant carbon has been estimated on a global scale for the latest Cretaceous, late Paleocene/Eocene, middle-late Eocene, early Miocene, and Holocene/Recent using the biomal reconstructions of Wolfe (1984), Upchurch (this symposium), and others. Latest Cretaceous (Maastrichtian) estimates indicate a relatively low value of 700-800 gigatons, which may underestimate carbon due to the presence of extensive latest Cretaceous coastal wetlands. However, much of this figure is readliy explainable by extensive deserts in Asia and little evidence for areally extensive tropical rainforest.Major increase in terrestrial plant carbon occurred during the Paleocene/earliest Eocene in conjunction with a major areal increase in rainforest. During the early Miocene terrestrial global carbon was approximately 1200-1300 gigatons. This figure decreased by about half between the early Miocene and Holocene/Recent. The decrease in terrestrial carbon density resulted from a decrease in area of tropical and subtropical forests and increase in area of deserts, grasslands, and mediterranean woodlands/chapparal.

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Where are the proto-South China Sea slabs? SE Asia plate tectonic and mantle flow insights from TERRA global mantle convection models

10.5194/egusphere-egu2020-12407 ◽

2020 ◽

Author(s):

Yi-An Lin ◽

Lorenzo Colli ◽

Jonny Wu

Keyword(s):

South China Sea ◽

South China ◽

Late Eocene ◽

Early Miocene ◽

Philippine Sea Plate ◽

Plate Tectonic ◽

China Sea ◽

Se Asia ◽

Plate Models ◽

Geodynamic Models

In this study we explored the contrasted plate tectonic reconstructions proposed for the proto-South China Sea and SE Asia. We implemented four different end-member plate models into global geodynamic models to test their predicted mantle structure against tomography. All models reproduced the Sunda slabs beneath Peninsular Malaysia, Sumatra and Java and the proto-South China Sea (PSCS) slabs beneath present Palawan, northern Borneo, and offshore Palawan; some models also predicted slabs under the southern South China Sea. PSCS slabs generated from double-sided PSCS subduction and earlier Borneo rotation generated a slightly better fit to tomography but pure southward PSCS subduction was also viable. A smaller Philippine Sea plate (PSP) with a short ~1000 km restored northern slab (i.e. Ryukyu slab) was clearly superior to a very long >3000 km slab. Mantle flows generated from our geodynamic models suggest strong upwellings under Indochina during the late Eocene to Oligocene. Our models generated strong downwellings under the South China Sea in the late Cenozoic that did not support a deep-origin &#8216;Hainan plume&#8217;.&#160;The following plate models variants were assimilated in the geodynamic models: (1) southward vs. double-sided PSCS subduction; (2) early Borneo counterclockwise rotations during the Oligocene to Early Miocene vs. later rotations (mid- to Late Eocene and Early Miocene); (3) a smaller Philippine Sea plate restored with a shorter ~1000 km northern slab vs. a longer >3000 km slab. This study assimilates four different plate models into the numerical model TERRA (Bunge et al., 1998). We digitally re-built in GPlates (Boyden et al., 2011) the implemented the plate models as a set of continuously closing plates in order to generate a global self-consistent velocity field to be assimilated into the convection models. The temperature fields were converted to seismic velocities assuming a Pyrolite composition and equilibrium mineralogy. We quantify the correlation between our geodynamic models and seismic tomography within SE Asia. For the tomography models S40RTS and LLNL-G3Dv-JPS we explicitly accounted for their finite resolution (Ritsema et al., 2011; Simmons et al. 2019).

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