Early Stages Of Collision Research Articles

Time-resolved fragmentation cross-section simulation of C60++C60 collisions.

We have performed density functional theory--linear combination of atomic orbitals molecular-dynamics statistical trajectory ensemble simulations of the 500 eV cross-beam collision of ${\mathrm{C}}_{60}^{+}$+${\mathrm{C}}_{60}$, where the charged collision partner is thermally excited with an average vibrational energy of 30 eV. The time-resolved fragmentation cross section shows the immediate sputtering of carbon dimers, and the formation of small fragments (N\ensuremath{\le}9) during the early stage of collision where there are still large cluster compounds (110\ensuremath{\le}N\ensuremath{\le}120). Medium-size cluster fragments (N\ensuremath{\approxeq}10--40) are formed mainly at the very end of the collision, i.e., as products of a multifragmentation process. The mean coordination number cross section is introduced as a means of characterizing structures of fragmentation products. Though our simulation statistics are calculated from a limited number of trajectories only (n=96--1024), odd-even alterations in the fragmentation cross section are clearly resolved.

Intramolecular energy flow and bond dissociation in iodoacetylene and iododiacetylene

Intermolecular and intramolecular energy flow and subsequent bond dissociation in collinear collisions I–C≡C–H+Ar and I–C≡C–C≡C–H+Ar have been studied by classical trajectory techniques over the collision energy range of 0 to 10 eV. When the molecule is initially in the ground state, the overall energy transfer in I–C≡C–H+Ar is very small, but in I–C≡C–C≡C–H+Ar it is large. The collisionally perturbed C–H bond stores a large amount of energy from translation for a brief period during the early stage of collision and transfers most of it to the inner region of the molecule, specifically to the low frequency C–I vibration. Thus the high-frequency vibration of the perturbed C–H bond during the collision plays a crucial role in determining the extent of intramolecular energy transfer and, in turn, C–I dissociation. But in nondissociative collisions, there is another series of the C–H vibration at the latter stage of collision, transferring energy back to translation. This study also considers collision-induced intramolecular energy flow in the molecule with an initially excited C–H bond. The relaxation of the low-lying C–H excitation is very slow on a nanosecond time scale. However, when the excitation is high, the vibrational frequency of the C–H bond is significantly weakened, thus becoming comparable to that of the triple bond, in which case the isolating effect of the adjacent C≡C bond is no longer important and intramolecular energy flow becomes efficient.

Comparison between BUU and IRS models for nucleus-nucleus collisions in the Fermi domain

This article intends to examine quantitatively the dynamic aspect of the phenomenological moving-source model in nucleus-nucleus collisions in the Fermi domain. The reactions, 36Ar+108Ag (35 MeV u-1)and14N+108Ag (35 MeV u-1), have been studied in terms of the BUU dynamic program. It has been found that the intermediate rapidity source (IRS) components of emitted neutron data extracted from the moving-source fitting approach do come from the very early stage of collisions, and are identified with pre-equilibrium emissions. One may define two time intervals as the pre-equilibrium emission time scale: the first chance collision time tau fcc and the classical transit time tau T=2(RT+RP)/v0, where v0 is the beam velocity, and RT and RP are the radii of the target and projectile nucleus, respectively. The two time scales are indicated to be of nearly the same values for the two reactions concerned. The IRS components quantitatively correspond to the major part of emitted neutrons within this time interval in the BUU frame.

Evolution of retreating subduction boundaries formed during continental collision

Retreating subduction boundaries, formed where the rate of subduction exceeds the rate of overall plate convergence, appear to be commonly developed features within regions of early or incomplete continent‐continent collision. They are characterized by regional extension within the overriding plate and, at their leading edge, by thin‐skinned arcuate thrust belts that are concave towards the overriding plate. As is illustrated by examples from the Mediterranean region, the formation of retreating subduction boundaries is intimately related to the process of continental collision. During the early stages of collision, retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary. Retreating plate boundaries can also form before the main collision, and the associated thrust belts emplaced as precollisional accretionary assemblages. Because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab (slab pull), subduction usually ceases when, and only when, thick buoyant continental crust enters the subduction zone. Thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted. In some cases, retreating subduction boundaries may “escape” into the open ocean, where they form nearly isolated, local tectonic systems. In these systems the rate of subduction is approximately compensated by the rate of upper plate extension, and migration of the system across the oceanic region may be very rapid. For example, the Horseshoe Seamounts, located about 800 km offshore in the eastern North Atlantic, may be the active expression of an east dipping, westwardly migrating retreating subduction boundary that has evolved from the Betic Cordillera‐Rif system active in Miocene time and may now be progressing across the Atlantic at approximately 50 mm/yr. An analogous situation may be represented by the Scotia Arc system, a westward dipping retreating subduction system located between the South American and Antarctic plates, which may have “escaped” into the South Atlantic ocean from a zone of crustal shortening in the Andes and is now progressing across the Atlantic at a rate of about 80 mm/yr.

Holocene uplift in West Timor

First-order 14C dating of molluscs from fossil shorelines in western Timor indicates emergence averaging <0.3 mm yr −1 during the Holocene. The finding tallies with rates derived from U-series dating of coral reefs in the area and contrasts markedly with the uplift history of the Sumatran forearc. It supports the contention that the 6 km of releif on Timor was attained largely by uplift during the early stages of collision.

Sedimentology of a Late Cenozoic collisional sequence: the Misis Complex, Adana, southern Turkey

This study is concerned with Miocene sequences formed within part of the Cukurova Basin of southern Turkey, a major downwarp created during the early stages of collision between the Afro-Arabian plate and the Tauride-Anatolian microplate assemblage. We report here the results of a combined stratigraphical, sedimentological, structure and geochemical study of the Misis Complex, a structurally elevated segment of Cukurova basin-fill. This analysis has demonstrated the structurally imbricated nature of the Misis sequences and the tectonic juxtaposition of originally well-separated coeval facies. The initial discernible phase of basin-filling (pre-Burdigalian) was marked by deep marine conditions and marginal tectonic instability (probably related to a late phase of Tauride nappe advance). The subsequent phase of crustal extension (Burdigalian/Early Tortonian) produced expansion of the Cukurova basin limits and an upwards-shoaling succession, in which sediment transport towards the southwest gradually became dominant, as a result of earlier suturing and uplift in northeasterly sectors of the collision zone. The Miocene arenites of the Misis Complex are petrographically diverse (with common siliciclastic and carbonate detrital admixtures) and mineralogically immature and their position in petrotectonic fields indicates a compound provenance from both recycled orogen and magmatic arc sources. A noteworthy petrographic feature is the compositional similarity of arenites from both shallow and deep marine facies. The general succession of Neogene facies observed in the Cukurova Basin and Misis Complex is consistent with their evolution within a perisutural foreland basin, but this interpretation is complicated by petrographic and geochemical features attributed to remnants of an older magmatic arc. Later stages of basin-filling are also marked by the local occurrence of “anomalous” palaeoenvironmental associations (including olistostromic units) attributed to contemporaneous intra-basinal tectonism.

Metamorphic constraints on the thermal evolution of the central Himalayan Orogen

Recent studies that integrate conventional thermobarometry of pelitic mineral assemblages with thermodynamic modeling of garnet zoning reveal complex Tertiary P -T paths for the Greater Himalayan metamorphic sequence in the central Himalaya. Viewed in light of our current understanding of the structural evolution of the Himalaya, these data provide insights into the relations between tectonic and thermal processes during orogenesis. In this paper, we present an interpretive model for tectonothermal evolution of the Greater Himalaya in the central part of the range. This model involves: (1) middle Eocene—early Oligocene burial to depths of more than 30 km during the early stages of collision between India and Asia; (2) early—late Oligocene uplift and cooling; (3) late Oligocene heating and renewed burial synchronous with the early stages of anatectic melting and leucogranite plutonism; (4) latest Oligocene-middle Miocene rapid uplift and continued leucogranite production associated with ramping on the structurally lower Main Central Thrust and tectonic denudation on structurally higher low-angle detachment systems; and (5) middle Miocene—Recent rapid cooling during the final stages of uplift to the surface.

Escape hypothesis for the Stikine block

Research Article| May 01, 1988 Escape hypothesis for the Stikine block Brian Wernicke; Brian Wernicke 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar David W. Klepacki David W. Klepacki 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Search for other works by this author on: GSW Google Scholar Author and Article Information Brian Wernicke 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 David W. Klepacki 2Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (5): 461–464. https://doi.org/10.1130/0091-7613(1988)016<0461:EHFTSB>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brian Wernicke, David W. Klepacki; Escape hypothesis for the Stikine block. Geology 1988;; 16 (5): 461–464. doi: https://doi.org/10.1130/0091-7613(1988)016<0461:EHFTSB>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Comparison of stratigraphic, faunal, and paleomagnetic characteristics of the Stikine terrane of British Columbia with other terranes in the Cordilleran collage reveals broad similarities with a group of terranes that formed a volcanic belt marginal to North America in Paleozoic and early Mesozoic time. Unlike Stikine, these terranes lie inboard of another belt of terranes that represents an early Mesozoic subduction complex or melange belt. Thus, in British Columbia the marginal volcanic belt is apparently doubled. In the Columbia embayment region, the marginal volcanic and melange belts are missing, and rocks of the outermost major component of the collage, the Wrangellia superterrane, are juxtaposed directly against cratonic rocks. We propose that Stikine is the missing fragment from the Columbia embayment; its northward "tectonic escape" was driven by the early stages of collision of the Wrangefia superterrane with North America in Middle to Late Jurassic time. The escaped fragment was then trapped between melange and more northerly, later arriving parts of the Wrangellia superterrane in Early to mid-Cretaceous time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

River profiles along the Himalayan arc as indicators of active tectonics

Longitudinal profiles along sixteen major transverse Himalayan rivers add important constraints to models of active continental subduction and its evolution. These profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases. The base of the zone of increased gradients correlates with (1) the topographic front between the Lesser and High Himalayas, (2) the narrow belt of intermediate-magnitude thrust earthquakes, (3) the Main Central Thrust zone (MCT). These features define a small circle in the central portion of the Himalayan arc. These correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising High Himalayas and the Lesser Himalayas. No sharp increases in gradient are observed near the Main Boundary Thrust (MBT), except on a few rivers, such as the Jhelum or Kundar, where the MBT lies close to both the MCT and the seismic belt. Thus, it is unlikely that the MBT is a major tectonic boundary. The diversion of river courses along the MBT and around anticlines in the Sub Himalayas has probably been caused by aggradation near the rosion-deposition boundary, upstream of uplifts in the Mahabharat range and Sub Himalayas. A parallel is drawn between the Himalayas and New Guinea based on the hypothesis that continent-arc collision, of the type occurring in northern Australia, preceded continent-continent collision in the Himalayas. The present sedimentary/tectonic phase in New Guinea resembles the Subathu (Paleocene-Eocene) phase in the Himalayas. Incipient counterparts of the major Himalayan structures, including the MCT and the MBT, are recognized in New Guinea. The drainage patterns in the Himalayas and in New Guinea bear a similar relation to major structures. This suggests that (1) the tectonic evolution of the Himalayas has been rather uniform since early stages of collision, and (2) the Himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the High Himalayas.

Structural evolution of an A‐type subduction zone, lofoten‐Rombak Area, northern Scandinavian Caledonides

The climactic phase of the Caledonian orogeny in Scandinavia involved collision of the Baltic and Greenland cratons in late Ordovician (?) to Silurian time. We infer that the collision resulted in large‐scale subduction of the leading edge of the Baltic craton beneath an ‘Andean‐type’ Greenland continental margin. Deep erosional levels in the Lofoten‐Rombak area, Norway‐Sweden (˜69°N) permit direct examination and evaluation of the structural evolution of a continental or A‐type subduction zone. The early stages of collision involved en bloc underthrusting of the continental margin to depths of at least 30 km. During this process, deformational effects in the lower plate were restricted to its upper few hundred meters. Ultimately, en bloc A‐type subduction should be limted by buoyancy effects. As convergence continued, the lower plate became involved in a series of imbricate thrusts with the same vergence as the intial subduction zone. The temporal and spatial relationships between these basement‐involved thrusts are similar to those observed in foreland fold and thrust terrains. Field and geophysical data bearing on lower plate behavior elsewhere in the Caledonldes and in other collisional orogens suggest that the structural features observed in the Lofoten‐Rombak area may be generally characteristic of A‐type subduction in continent‐continent collisional settings.