Crustal Slices Research Articles

Archean granulite gneisses from eastern Hebei Province, China: rare earth geochemistry and tectonic implications

The granulite gneisses and their retrograded products of the Qianxi Group from eastern Hebei Province, China, have been investigated for their isotope and trace element geochemistry. A consistent age of about 2.5 AE has been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods, in agreement with the zircon U-Pb data (Pidgeon 1980; D.Y. Liu, unpubl.). Geochemical arguments from initial isotopic ratios (ISr and INd) and elemental distribution patterns have led us to conclude that this age of about 2.5 AE represents the time of granulite facies metamorphism, which must have followed closely the primary emplacement of their protoliths. Previous claims for early Archean ages (>3.5 AE) of these granulites are not substantiated. The mineral isotope systematics register an important thermal event at about 1.7 AE, roughly corresponding to the time of the widespread Luliang Orogeny (Ma and Wu 1981) or Chungtiao Movement (Huang 1978). The granulites of the Qianxi Group have diverse compositions ranging from ultrabasic through basic-intermediate to acid. Discriminant function calculations suggest that most analyzed samples have igneous parentage. Only a few show characteristics of metasedimentary rocks. The igneous protoliths apparently belong to two series — tholeiitic and calc-alkaline, with the latter dominating in abundance. The majority of the acid granulites have compositions corresponding to tonalite-granodiorite. Except for ultrabasic and metasedimentary rocks, all REE patterns are significantly fractionated with LREE enrichment. The degree of fractionation, as measured by the (La/Yb)N ratios, is most important in the acid granulites. These rocks often show positive Eu anomalies and HREE depletions that are typical of Archean TTG rocks (tonalitetrondhjemite-granodiorite). The existence of komatiites has been previously reported in this region. Although a few rocks have a major element chemistry similar to that for peridotitic komatiites, the lack of associated members in a komatiitic series and the scarcity of REE data have not confirmed the true komatiite occurrence in this region. Many Qianxi granulites are highly depleted in Rb relative to K and Sr. This preferential Rb depletion during granulite facies metamorphism has led to very high K/Rb and very low Rb/Sr ratios. The most comparable case is found in Lewisian granulites. Although the fractionated REE patterns of the basic granulites somewhat resemble those of continental flood basalts, the highly different abundances in other incompatible elements (Ti, Zr, and Ba) readily distinguish them from each other. Nevertheless, the LREE enriched patterns of the basic granulites may suggest an origin of their protoliths by partial melting of LREE-enriched mantle sources. On the other hand, the REE patterns of acid granulites suggest that their protoliths could be derived by partial melting of quartz eclogite, amphibolite or basic granulite. The close time relationship for a series of geologic events, namely, from initial melting of mantle peridotites, through fractional crystallisation of basaltic magmas, to granulite facies metamorphism, seems to occur in many granulite terrains. This relationship, together with the juxtaposition of lithologies of different origins and the exceptionally high pressure conditions (>10 Kb) can be best explained by crustal underplating combined with intracrustal thin-skinned thrusting and stacking of crustal slices. The “andesitic or island arc” model for the formation of the lower continental crust is not in good agreement with the present geochemical data.

Marfa Basin of West Texas: Foreland Basin Subsidence and Depocenter Migration: ABSTRACT

The Marfa basin, encompassing approximately 6,000 mi2 (15,539 km2) of Presidio and Brewster Counties in west Texas, is a foreland basin that formed in the late Paleozoic in response to the encroaching Ouachita-Marathon thrust belt. The basin is one of several, including the Arkoma, Fort Worth, and Val Verde basins, that developed along the southern margin of the North American craton during convergence of North America and Africa-South America in Pennsylvanian to Permian time. We present a model of the formation of the Marfa basin in which basin subsidence is effected by compression from plate convergence and by loading owing to the emplacement of the Marathon fold-thrust complex. A model of foreland basin evolution by thrust loading as applied to the Idaho-Wyoming thrust belt can be applied with some modification to the Marfa foreland basin. Preexisting northwest-trending faults in the Marfa region were reactivated by the Marathon thrust belt as the latter advanced onto the continental margin toward the craton. Subsidence owing to compression and thrust loading first formed the Tesnus basin, a Pennsylvanian basin now buried beneath the Marathon overthrust. In the later stage of thrust-sheet emplacement, the depocenter split into two prongs, and the Marfa and Val Verde basins collected thick sections of Wolfcamp sediments. Preexisting northwest trends, which result from a Precambrian rifting event and the late Precambrian to Cambrian development of the Delaware aulacogen, controlled the location of subsidence in front of the thrust sheet. The fragmented craton was composed of northwest-trending high and low areas including the Diablo platform and the Delaware basin. These fragments behaved much like piano keys, subsiding first in a central region to form the Tesnus basin and later in adjacent regions forming the Marfa and Val Verde basins. The model is supported with data from 63 well logs that indicate the position of the depocenters through time and that suggest the differential elevation of crustal slices controlling the formation and location of the three Pennsylvanian-Permian foreland basins. End_of_Article - Last_Page 117------------

Initiation of subduction zones: implications for arc evolution and ophiolite development

Summary The initial location of trenches with respect to continental margins is a critical factor in the nature of the forearc region and its subsequent evolution, as well as for the interpretation of ophiolites. Although recent studies of ancient margins and highly evolved arcs suggest that wide strips of oceanic crust are commonly trapped between the trench and volcanic arc, review of young active arcs does not support this conclusion. Neogene trenches have formed predominantly very close to the crustal interface between oceanic crust and continental or older island arc crust. A few arcs have developed along transform zones, but none can be identified as having formed by breakage within an oceanic plate. Apparently the high strength of normal oceanic lithosphere causes the initial fracture to utilize pre-existing zones of weakness or pre-stressing, such as the downbowed and fractured crustal interfaces along passive margins. There seems to be no physical basis for the initial rupture to form smooth, large radius arcs, thereby trapping oceanic crust in re-entrants. Moreover, oceanic crust or ophiolites originating adjacent to a continental or arc margin ought to betray that heritage by a high terrigenous or volcaniclastic component in their overlying sediment. Large ophiolite sheets that can be interpreted as forearc slabs may develop in areas with relatively unusual plate geometries, as where oceanic spreading zones cut diagonally across the forearc. Disrupted ophiolites could, in many cases, represent slices of oceanic crust and upper mantle accreted to and responsible for the growth of oceanic arcs. Evidence for accreted slices in the Mariana arc and the range of forearc geometries among oceanic arcs allow construction of an evolutionary sequence in which an initial steep and narrow trench shape is differentiated into upper and lower sections by intermittent accretion of oceanic crustal slices. As a result, the slope apron on the upper section rotates arcward and can rise to form a basin as is displayed in the southern Middle America arc.

The Himalayan Main Central Thrust pile and its quartz-rich tectonites in central Nepal

As a consequence of the India—Asia collision, a deeply rooted crustal slice has been moved southwards over the northern boundary of the Indian plate. At the present level of observation the Main Central Thrust pile, a rock-pile of several kilometers thickness, is affected by shear deformation which imprinted a flat foliation and a constant N—S trending lineation. These tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field. The M.C.T. shear zone is studied from the view-point of the quartz-rich rocks: over 100 samples helped to define a microstructural zonation, roughly parallel to the thermal one; 35 samples, particularly rich in quartz, have been used for the preferred orientation study of their c-axes, and a few of them for their 〈a〉 axes. Strong lattice orientations, when undisturbed by (mainly) secondary recrystallization, attest to large plastic strains in quartz. The c-axes are mostly distributed along unequally populated crossed-girdles which may evolve into an inclined single girdle. The 〈a〉 axes concentrate perpendicular to the more populated girdle. Concerning quartz the following is suggested: 1. (1) the deformation has a dominant shear component responsible for the dominant stretching lineation. It could be simple shear in case of inclined single girdles 2. (2) one set of 〈a〉 axes line up parallel to the shear direction. Slip on various planes in the 〈a〉 direction may be an important deformation mechanism 3. (3) asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the Himalayan thrusting in more than 80% of cases.

Kaledonisk tektonik i Jämtland

Abstract Recent investigations in the Swedish Caledonides have proved the existence of a series of NW. striking transform faults. They are relatively broad and are in the nature of deep zones of movement in the parautochthonous basement. Stratigraphical differences between the blocks delimited by faults demonstrate vertical movements in the faults at latest Precambrian and early Silurian. To the west the transform faults disappear or are re-orientated at the line of large culminations. A hypothesis is presented that the parautochthonous zone with transform faults constitutes a large Caledonian slice in the crust, the zone with “horsts” forms a more westerly second crustal slice, etc. In accordance with this, some geophysical relationships are discussed.