The transition from complex crater to peak-ring basin on the Moon: New observations from the Lunar Orbiter Laser Altimeter (LOLA) instrument     

David M.H. Baker  James W. Head  Seth J. Kadish  Maria T. Zuber  Gregory A. Neumann 《Icarus》2011,214(2):377-393

Impact craters on planetary bodies transition with increasing size from simple, to complex, to peak-ring basins and finally to multi-ring basins. Important to understanding the relationship between complex craters with central peaks and multi-ring basins is the analysis of protobasins (exhibiting a rim crest and interior ring plus a central peak) and peak-ring basins (exhibiting a rim crest and an interior ring). New data have permitted improved portrayal and classification of these transitional features on the Moon. We used new 128 pixel/degree gridded topographic data from the Lunar Orbiter Laser Altimeter (LOLA) instrument onboard the Lunar Reconnaissance Orbiter, combined with image mosaics, to conduct a survey of craters >50 km in diameter on the Moon and to update the existing catalogs of lunar peak-ring basins and protobasins. Our updated catalog includes 17 peak-ring basins (rim-crest diameters range from 207 km to 582 km, geometric mean = 343 km) and 3 protobasins (137-170 km, geometric mean = 157 km). Several basins inferred to be multi-ring basins in prior studies (Apollo, Moscoviense, Grimaldi, Freundlich-Sharonov, Coulomb-Sarton, and Korolev) are now classified as peak-ring basins due to their similarities with lunar peak-ring basin morphologies and absence of definitive topographic ring structures greater than two in number. We also include in our catalog 23 craters exhibiting small ring-like clusters of peaks (50-205 km, geometric mean = 81 km); one (Humboldt) exhibits a rim-crest diameter and an interior morphology that may be uniquely transitional to the process of forming peak rings. A power-law fit to ring diameters (D_ring) and rim-crest diameters (D_r) of peak-ring basins on the Moon [D_ring = 0.14 ± 0.10(D_r)^1.21±0.13] reveals a trend that is very similar to a power-law fit to peak-ring basin diameters on Mercury [D_ring = 0.25 ± 0.14(D_rim)^1.13±0.10] [Baker, D.M.H. et al. [2011]. Planet. Space Sci., in press]. Plots of ring/rim-crest ratios versus rim-crest diameters for peak-ring basins and protobasins on the Moon also reveal a continuous, nonlinear trend that is similar to trends observed for Mercury and Venus and suggest that protobasins and peak-ring basins are parts of a continuum of basin morphologies. The surface density of peak-ring basins on the Moon (4.5 × 10⁻⁷ per km²) is a factor of two less than Mercury (9.9 × 10⁻⁷ per km²), which may be a function of their widely different mean impact velocities (19.4 km/s and 42.5 km/s, respectively) and differences in peak-ring basin onset diameters. New calculations of the onset diameter for peak-ring basins on the Moon and the terrestrial planets re-affirm previous analyses that the Moon has the largest onset diameter for peak-ring basins in the inner Solar System. Comparisons of the predictions of models for the formation of peak-ring basins with the characteristics of the new basin catalog for the Moon suggest that formation and modification of an interior melt cavity and nonlinear scaling of impact melt volume with crater diameter provide important controls on the development of peak rings. In particular, a power-law model of growth of an interior melt cavity with increasing crater diameter is consistent with power-law fits to the peak-ring basin data for the Moon and Mercury. We suggest that the relationship between the depth of melting and depth of the transient cavity offers a plausible control on the onset diameter and subsequent development of peak-ring basins and also multi-ring basins, which is consistent with both planetary gravitational acceleration and mean impact velocity being important in determining the onset of basin morphological forms on the terrestrial planets.  相似文献   

Estimation of the age of impact craters and other adjacent geological subdivisions within the Thetis Regio on Venus     

E. N. Guseva  A. T. Basilevsky  J. W. Head 《Solar System Research》2013,47(3):159-169

Based on the results of photogeological analysis of the Thetis Regio on Venus, 13 material and 3 structural subunits were mapped and the relative sequence of their formation was determined. Two subunits are represented by impact craters, which were subdivided into relatively older (C1, 7 craters) and younger (C2, 8 craters) ones, according to the presence and clarity of the associated radio-dark halo. Taking into account the data from previous studies, it was established that the age of the first group was older than 1/2T or 1/3T, where T is the average age of the surface of Venus, and the age of the second group was younger than these values. Analyzing the age relationships between these craters and other geological subunits, it was determined that the formation of one of the largest rift zones in this area of Venus occurred later than at 1/3T up to the present time.  相似文献   

Wild articulations: environmentalism and indigeneity in Northern Australia     

Lesley Head 《The Australian geographer》2019,50(3):407-408

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Comment: Community resilience,natural hazards and climate change     

Lesley Head 《Norsk geografisk tidsskrift. Norwegian journal of geography》2020,74(3):134-137

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Subdividing the Holocene Series/Epoch: formalization of stages/ages and subseries/subepochs,and designation of GSSPs and auxiliary stratotypes     

MIKE WALKER  Martin J. Head  John Lowe  Max Berkelhammer  Svante Bj rck  Hai Cheng  Les C. Cwynar  David Fisher  Vasileios Gkinis  Antony Long  Rewi Newnham  Sune Olander Rasmussen  Harvey Weiss 《第四纪科学杂志》2019,34(3):173-186

The Holocene, which currently spans ~11 700 years, is the shortest series/epoch within the geological time scale (GTS), yet it contains a rich archive of evidence in stratigraphical contexts that are frequently continuous and often preserved at high levels of resolution. On 14 June 2018, the Executive Committee of the International Union of Geological Sciences formally ratified a proposal to subdivide the Holocene into three stages/ages, along with their equivalent subseries/subepochs, each anchored by a Global boundary Stratotype Section and Point (GSSP). The new stages are the Greenlandian (Lower/Early Holocene Subseries/Subepoch) with its GSSP in the Greenland NGRIP2 ice core and dated at 11 700 a b2k (before 2000 CE); the Northgrippian (Middle Holocene Subseries/Subepoch) with its GSSP in the Greenland NGRIP1 ice core and dated at 8236 a b2k; and the Meghalayan (Upper/Late Holocene Subseries/Subepoch) with its GSSP in a speleothem from Mawmluh Cave, north‐eastern India, with a date of 4250 a b2k. We explain the nomenclature of the new divisions, describe the procedures involved in the ratification process, designate auxiliary stratotypes to support the GSSPs and consider the implications of the subdivision for defining the Anthropocene as a new unit within the GTS.  相似文献   

The Earth Galleries: an inside view     

Head of Exhibition Research  Design  ROBERT M. BLOOMFIELD 《Geology Today》1997,13(3):96-96

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Processes of crustal formation and evolution on Venus: An analysis of topography,hypsometry, and crustal thickness variations     

Head  James W. 《Earth, Moon, and Planets》1990,(1):25-55

On Venus, present evidence indicates a crust of predominantly basaltic composition and a relatively young average age for the surface (several hundreds of millions of years). Estimates of crustal thickness from several approaches suggest an average crustal thickness of 10–20 km for much of the lowlands and rolling plains and a total volume of crust of about 1 × 10¹⁰ km³, approximately comparable to the present crustal volume of the Earth (1.02 × 10¹⁰ km³). The Earth's oceanic crust is thought to have been recycled at least 10–20 times over Earth history. The near-coincidence in present crustal volumes for the Earth and Venus suggests that either: (1) the presently observed crust of Venus represents the total volume that has accumulated over the history of the planet and that crustal production rates are thus very low, or (2) that crustal production rates are higher and that there is a large volume of missing crust unaccounted for on Venus which may have been lost by processes of crustal recycling.Known processes of crustal formation and thickening (impact-related magma ocean, vertical differentiation, and crustal spreading) are reviewed and are used as a guide to assess regional geologic evidence for the importance of these processes on Venus. Geologic evidence for variations in crustal thickness on Venus (range and frequency distribution of topography, regional slopes, etc.) are outlined. The hypothesis that the topography of Venus could result solely from crustal thickness variations is assessed and tested as an end-member hypothesis. A map of crustal thickness distribution is compiled on the basis of a simple model of Airy isostasy and global Venus topography. An assessment is then made of the significance of crustal thickness variations in explaining the topography of Venus. It is found that the distinctive unimodal hypsometric curve could be explained by: (1) a crust of relatively uniform thickness (most likely 10–20 km thick) comprising over 75% of the surface, (2) local plateaus (tessera) of thickened crust (about 20–30 km) forming less than 15% of the surface, (3) regions of apparent crustal thicknesses of 30–50 km (Beta, Ovda, Thetis, Atla Regiones and Western Ishtar Terra) forming less than 10% of the surface and showing some geologic evidence of crustal thickening processes (these areas can be explained on the basis of geologic observations and gravity data as combinations of thermal effects and crustal thickening), and (4) areas in which Airy isostasy predicts crustal thicknesses in excess of 50 km (the linear orogenic belts of Western Ishtar Terra, less than 1% of the surface).It is concluded that Venus hypsometry can be reasonably explained by a global crust of generally similar thickness with variations in topography being related to (1) crustal thickening processes (orogenic belts and plateau formation) and (2) local variations in the thermal structure (spatially varying thermal expansion in response to spatially varying heat flow). The most likely candidates for the formation and evolution of the crust are vertical differentiation and/or lateral crustal spreading processes. The small average crustal thickness (10–20 km) and the relatively small present crustal volume suggest that if vertical crustal growth processes are the dominant mechanism of crustal growth, than vertical growth has not commonly proceeded to the point where recycling by basal melting or density inversion will occur, and that therefore, rates of crustal production must have been much lower in the past than in recent history. Crustal spreading processes provide a mechanism for crustal formation and evolution that is consistent with observed crustal thicknesses. Crustal spreading processes would be characterized by higher (perhaps more Earth-like) crustal production rates than would characterize vertical differentiation processes, and crust created earlier in the history of Venus and not now observed (missing crust) would be accounted for by loss of crust through recycling processes. Lateral crustal spreading processes for the formation and evolution of the crust of Venus are interpreted to be consistent with many of the observations derived from presently available data. Resurfacing through vertical differentiation processes also clearly occurs, and if it is the major contributor to the total volume of the crust, then very low resurfacing rates are required.Although thermal effects on topography are clearly present and important on both Venus and the Earth, the major difference between the hypsometric curves on Earth (bimodal) and Venus (unimodal) is attributed primarily to the contrast in relative average thickness of the crust between the two terrains on Earth (continental/oceanic; 40/5 km = 35 km, 8:1) and Venus (upland plateaus/lowlands; about 30/15 km = 15 km, 2:1) (35 – 20 km = a difference of 20 km). The Venus unimodal distribution is thus attributed primarily to the large percentage of terrain with relatively uniform crustal thickness, with the skewness toward higher elevations due to the relatively small percentage of crust that is thickened by only about a factor of two. The Earth, in contrast, has a larger percentage of highlands (continents), whose crust is thicker by a factor of eight, on the average, leading to the distinctive bimodal hypsometric curve.Data necessary to firmly establish the dominant type of crustal formation and thickening processes operating and to determine the exact proportion of the topography of Venus that is due to thermal effects versus crustal thickness variations include: (1) global imaging data (to determine the age of the surface, the distribution and age of regions of high heat flux, and evidence for the nature and global distribution of processes of crustal formation and crustal loss), and (2) high resolution global gravity and topography data (to model crustal thickness variations and thermal contributions and to test various hypotheses of crustal growth).'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).  相似文献   

The Early-Middle Pleistocene Transition： characterization and proposed guide for the defining boundary   总被引：1，自引：0，他引：1  

Martin J. Head Brad Pillans Sarah A. Farquhar 《《幕》》2008,31(2):255-259

The mid-Pleistocene transition （MPT, c. 1.2 to 0.5 Ma） records fundamental changes in Earth＇s climate state, where low-amplitude 41-kyr obliquity-dominated cycles gave way progressively to the high-amplitude, quasiperiodic （c. 100-kyr） fluctuations that characterize the later Pleistocene and Holocene. We use wavelet analysis on the LR04 δ^l8O benthic foraminiferal stack to confirm low-frequency power as early as 1.25-1.20 Ma, determine the persistence of obliquity-dominated cyclicity through and beyond the MPT, and reveal new levels of complexity in climate evolution.  相似文献   

Culture, nature and landscape in the Australian region     

Noel Castree  Lesley Head   《Geoforum》2008,39(3):1255-1257

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Trans-Atlantic responses of Calanus finmarchicus populations to basin-scale forcing associated with the North Atlantic Oscillation     

C. H. Greene  A. J. Pershing  A. Conversi  B. Planque  C. Hannah  D. Sameoto  E. Head  P. C. Smith  P. C. Reid  J. Jossi  D. Mountain  M. C. Benfield  P. H. Wiebe  E. Durbin 《Progress in Oceanography》2003,58(2-4):301

Populations of the copepod species Calanus finmarchicus often dominate the springtime biomass and secondary production of shelf ecosystems throughout the North Atlantic Ocean. Recently, it has been hypothesised that interannual to interdecadal fluctuations observed in such populations are driven primarily by climate-associated changes in ocean circulation. Here, we compare evidence from the North Sea and Gulf of Maine/Western Scotian Shelf (GoM/WSS) linking fluctuations in C. finmarchicus abundance to changes in ocean circulation associated with the North Atlantic Oscillation (NAO). A particularly striking contrast emerges from this Trans-Atlantic comparison: whereas the North Sea C. finmarchicus population exhibits a negative correlation with the NAO index, the GoM/WSS population exhibits a more complex, positive association with the index. The physical processes underlying these contrasting population responses are discussed in the context of regional- to basin-scale circulation changes associated with the NAO.  相似文献