Geophysical and structural analyses of the Middlesboro impact structure,Kentucky, USA: Reactivation of a thrust detachment of the Appalachian foreland fold-and-thrust belt     

Lars Wihanto  Thomas Kenkmann 《Meteoritics & planetary science》2023,58(12):1848-1871

This paper reports on the Middlesboro impact structure (36°37′03″ N; 83°43′39″ W), a complex impact structure located in the Appalachian foreland fold-and-thrust belt of southeast Kentucky, USA. The structure forms a basin approximately 5.5 km in diameter exposing intensely deformed Pennsylvanian sediments. Based on field data, microstructural observation, and geophysical analyses we qualitatively assess degree and distribution of fragmentation of target rocks within the impact structure. Shock deformation features, especially feather features, are reported for the impact structure. Shallow seismic refraction data were acquired along a radial profile from outside the impact structure to the center of the structure. Seismic (P-wave) velocity trends indicate that fracture intensity increases toward the center, reducing the overall seismic velocity. However, intense rock sealing in some parts of the central uplift increases the strength of rock samples and locally increases seismic velocities. We present a modified geologic map of the Middlesboro impact structure based on high-resolution lineament analysis, field work, and reevaluation of existing structural data from geologic maps. The modified map suggests that crater circularity at Middlesboro is skewed by the reactivation of pre-existing zones of structural anisotropy in the target. We propose that due to an oblique impact from northerly direction the sub-horizontal Pine Mountain thrust plane beneath the impact structure became reactivated with a top-to-the south shear component. The impact-induced formation/reactivation of the Doublings Fault Zone as a thrust ramp appears to have deflected the otherwise straight strike of the Cumberland Mountains southward. The Rocky Face fault approaches the crater rim fault, and we propose that this sinistral strike-slip fault was formed, or at least reactivated, by the impact and accommodated the oblique impact's horizontal momentum.  相似文献   

Empirical constraints for the magnitude and composition of galactic winds     

H. Jabran Zahid  Paul Torrey  Mark Vogelsberger  Lars Hernquist  Lisa Kewley  Romeel Davé 《Astrophysics and Space Science》2014,349(2):873-879

Galactic winds are a key physical mechanism for understanding galaxy formation and evolution, yet empirical and theoretical constraints for the character of winds are limited and discrepant. Recent empirical models find that local star-forming galaxies have a deficit of oxygen that scales with galaxy stellar mass. The oxygen deficit provides unique empirical constraints on the magnitude of mass loss, composition of outflowing material and metal reaccretion onto galaxies. We formulate the oxygen deficit constraints so they may be easily implemented into theoretical models of galaxy evolution. We parameterize an effective metal loading factor which combines the uncertainties of metal outflows and metal reaccretion into a single function of galaxy virial velocity. We determine the effective metal loading factor by forward-fitting the oxygen deficit. The effective metal loading factor we derive has important implications for the implementation of mass loss in models of galaxy evolution.  相似文献   

The origin of young mare basalts inferred from lunar meteorites Northwest Africa 4734, 032, and LaPaz Icefield 02205     

Stephen M. Elardo  Charles K. Shearer Jr.  Amy L. Fagan  Lars E. Borg  Amy M. Gaffney  Paul V. Burger  Clive R. Neal  Vera A. Fernandes  Francis M. McCubbin 《Meteoritics & planetary science》2014,49(2):261-291

Northwest Africa (NWA) 4734 is an unbrecciated basaltic lunar meteorite that is nearly identical in chemical composition to basaltic lunar meteorites NWA 032 and LaPaz Icefield (LAP) 02205. We have conducted a geochemical, petrologic, mineralogic, and Sm‐Nd, Rb‐Sr, and Ar‐Ar isotopic study of these meteorites to constrain their petrologic relationships and the origin of young mare basalts. NWA 4734 is a low‐Ti mare basalt with a low Mg* (36.5) and elevated abundances of incompatible trace elements (e.g., 2.00 ppm Th). The Sm‐Nd isotope system dates NWA 4734 with an isochron age of 3024 ± 27 Ma, an initial ε_Nd of +0.88 ± 0.20, and a source region ¹⁴⁷Sm/¹⁴⁴Nd of 0.201 ± 0.001. The crystallization age of NWA 4734 is concordant with those of LAP 02205 and NWA 032. NWA 4734 and LAP 02205 have very similar bulk compositions, mineral compositions, textures, and ages. Their source region ¹⁴⁷Sm/¹⁴⁴Nd values indicate that they are derived from similar, but distinct, source materials. They probably do not sample the same lava flow, but rather are similarly sourced, but isotopically distinct, lavas that probably originate from the same volcanic complex. They may have experienced slightly different assimilation histories in route to eruption, but can be source‐crater paired. NWA 032 remains enigmatic, as its source region ¹⁴⁷Sm/¹⁴⁴Nd definitively precludes a simple relationship with NWA 4734 and LAP 02205, despite a similar bulk composition. Their high Ti/Sm, low (La/Yb)_N, and Cl‐poor apatite compositions rule out the direct involvement of KREEP. Rather, they are consistent with low‐degree partial melting of late‐formed LMO cumulates, and indicate that the geochemical characteristics attributed to urKREEP are not unique to that reservoir. These and other basaltic meteorites indicate that the youngest mare basalts originate from multiple sources, and suggest that KREEP is not a prerequisite for the most recent known melting in the Moon.  相似文献   

Internal and forced variability along a section between Greenland and Portugal in the CLIPPER Atlantic model     

Anne Marie Treguier  Claire Gourcuff  Pascale Lherminier  Herle Mercier  Bernard Barnier  Gurvan Madec  Jean-Marc Molines  Thierry Penduff  Lars Czeschel  Claus Böning 《Ocean Dynamics》2006,56(5-6):568-580

Numerical models are used to estimate the meridional overturning and transports along the paths of two hydrographic cruises, carried out in 1997 and 2002 from Greenland to Portugal. We have examined the influence of the different paths of the two cruises and found that it could explain 0.4 to 2 Sv of difference in overturning (the precise value is model-dependent). Models show a decrease in the overturning circulation between 1997 and 2002, with different amplitudes. The CLIPPER ATL6 model reproduces well the observed weakening of the overturning in density coordinates between the cruises; in the model, the change is due to the combination of interannual and high-frequency forcing and internal variability associated with eddies and meanders. Examination of the -coordinate overturning reveals model–data discrepancies: the vertical structure in the models does not change as much as the observed one. The East Greenland current variability is mainly wind-forced in the ATL6 model, while fluctuations due to eddies and instabilities explain a large part of the North Atlantic Current variability. The time-residual transport of dense water and heat due to eddy correlations between currents and properties is small across this section, which is normal to the direction of the main current.  相似文献   

Controls on upper Campanian–Maastrichtian chalk deposition in the eastern Danish Basin          下载免费PDF全文

Myriam Boussaha  Nicolas Thibault  Kresten Anderskouv  Julien Moreau  Lars Stemmerik 《Sedimentology》2017,64(7):1998-2030

Detailed facies analysis of a 350 m long core of upper Campanian–Maastrichtian chalk at Stevns Peninsula, eastern Denmark, shows that four mudstone and wackestone chalk facies account for close to 95% of the succession, and that bioturbated mudstone chalk alone accounts for nearly 55% of the sediment. Sedimentation took place in deep water, below the photic zone and storm‐wave base, and is characterized by decimetre to metre‐scale variations in facies and trace fossil assemblages indicating repeated shifts in depositional environment. Integration of facies with published data on sea‐surface temperature and accumulation rates suggests that sea‐surface temperature is the most important parameter in controlling stratification of the water column and thereby, indirectly, the observed variations in depositional facies. However, bioturbated mudstone chalk occurs in all stratigraphic levels independent of accumulation rates and sea temperatures and is interpreted to represent a very broad set of deep water environmental conditions with an ample supply of calcareous nannofossil debris and intense bioturbation. Longer term shifts in deposition are best expressed by distribution of clay, flint and bioturbated micro‐wackestone, bioturbated wackestone and laminated mudstone chalk facies, whereas the trace fossil assemblages appear less useful. The data set indicates overall shallowing over time with two distinctive events of clay influx to the basin during the late Campanian–earliest Maastrichtian and late Maastrichtian.  相似文献   

The First Reported Pleistocene Beaver,Castor fiber (Castoridae,Rodentia), from the Gobi Altai Region of Mongolia     

Klementiev  A. M.  Khatsenovich  A. M.  Bazargur  D.  Tserendagva  Y.  Marchenko  D. V.  Rybin  E. P.  Gunchinsuren  B.  Olsen  J. W.  Krivoshapkin  A. I. 《Doklady Earth Sciences》2023,507(3):S420-S423

Doklady Earth Sciences - Here, we report a fragmentary Castor fiber (Linnaeus, 1758) proximal left ulna from Tsagaan Agui Cave (Gobi Altai Mountains, southern Mongolia). This constitutes the first...  相似文献