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Deformation Front (deformation + front)

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Earth and Environmental Science	100%

Selected Abstracts

Contemporary kinematics of the southern Aegean and the Mediterranean Ridge

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2004
Corné Kreemer
SUMMARY This study focuses on the kinematics of the southern Aegean and the Mediterranean Ridge (MR). A quantification of the deformation of the MR is essential for both evaluating physical models of accretionary wedges in general and for obtaining a self-consistent model of the surface deformation over the entire Nubia,Eurasia (NU,EU) plate boundary zone in the eastern Mediterranean. Previous kinematic studies have not properly considered the deformation field south of the Hellenic arc. Although this study focuses on the deformation field of the MR, we also discuss the kinematics of the southern Aegean, because the geometry and movement of the Hellenic arc determine to a large extent the kinematic boundary conditions for kinematic studies of the MR. We calculate a continuous velocity and strain rate field by interpolating model velocities that are fitted in a least-squares sense to published Global Positioning System (GPS) velocities. In the interpolation, we use information from a detailed data set of onshore and offshore active faulting to place constraints on the expected style and direction of the model strain rate field. In addition, we use the orientations of tracks left by seamounts travelling into the wedge to further constrain the offshore deformation pattern. Our model results highlight the presence of active shear partitioning within the Mediterranean ridge. High compressional strain rates between the ridge crest and the deformation front accommodate approximately 60,70 per cent of the total motion over the wedge, and the outward growth rate of the frontal thrust is , 4 mm yr,1. Strain partitioning within the wedge leads to 19,23 mm yr,1 of dextral motion at the wedge,backstop contact of the western MR, whereas the Pliny and Strabo trenches in the eastern MR accommodate 21,23 mm yr,1 of sinistral motion. The backstop of the western MR is kinematically part of the southern Aegean, which moves as a single block [the Aegean block (AE)] at 33,34 mm yr,1 in the direction of S24°W ± 1° towards stable Nubia (NU). Our model confirms that there is a clear divergence between the western and eastern Hellenic arc and we argue for a causal relation between the outward motion of the arc and the gradient in the regional geoid anomaly. Our results suggest that a significant driving source of the surface velocity field lies south of the Hellenic arc and only for the southeastern Aegean could there be some effect as a result of gravitational collapse associated with density differences within the overriding plate. [source]

Evolution of a crustal-scale transpressive shear zone in the Albany,Fraser Orogen, SW Australia: 2.

JOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2004
Mawson cratons, Tectonic history of the Coramup Gneiss, a kinematic framework for Mesoproterozoic collision of the West Australian
Abstract Within the Albany,Fraser Orogen of southwestern Australia, the Coramup Gneiss is a NE,SW trending zone of high-strain rocks that preserves a detailed record of orogenesis related to Mesoproterozoic convergence of the West Australian and Mawson cratons. New structural, metamorphic and U,Pb SHRIMP zircon age data establish that the Coramup Gneiss underwent high-grade tectonism during both Stage I (c. 1290 Ma) and Stage II (c. 1170 Ma) of the Albany,Fraser Orogeny. Stage I commenced with c. 1300 Ma high- T, low- P M1a metamorphism during extension, and the formation of small-scale ptygmatic folds within a subhorizontal S1a gneissosity. High- P M1b metamorphism at c. 1290 Ma was accompanied by the transposition and shearing of S1a into a composite, shallow SE-dipping S1b foliation, and the development of tight recumbent F1b folds with S1-parallel axial surfaces and asymmetries indicating NW-directed thrusting. The preservation of a similar P,T,time record in the Fraser Complex (NE of the Coramup Gneiss) is consistent with large-scale, NW-directed Stage I thrusting of the Mawson Craton margin over the south-eastern edge of the West Australian Craton. Stage II tectonism in the western Coramup Gneiss involved high- T, low- P M2a metamorphism and the formation of subvertical SE-dipping D2 shear zones, shallow SW-plunging L2 mineral stretching lineations, and NW-verging F2 folds with S2-parallel axial surfaces. A synkinematic pegmatite dyke emplaced into a D2 shear zone yielded a U,Pb SHRIMP zircon age of 1168 ± 12 Ma. Kinematic indicators suggest a combination of pure shear flattening perpendicular to S2, and dextral simple shear. However, contemporaneous structures elsewhere in the Albany,Fraser Orogen are consistent with continued NW,SE convergence at craton-scale during Stage II, and oblique compression in the Coramup Gneiss is attributed to the arcuate geometry of the orogen-scale deformation front. [source]

Evolution of the late Cenozoic Chaco foreland basin, Southern Bolivia

BASIN RESEARCH, Issue 2 2006
Cornelius Eji Uba
ABSTRACT Eastward Andean orogenic growth since the late Oligocene led to variable crustal loading, flexural subsidence and foreland basin sedimentation in the Chaco basin. To understand the interaction between Andean tectonics and contemporaneous foreland development, we analyse stratigraphic, sedimentologic and seismic data from the Subandean Belt and the Chaco Basin. The structural features provide a mechanism for transferring zones of deposition, subsidence and uplift. These can be reconstructed based on regional distribution of clastic sequences. Isopach maps, combined with sedimentary architecture analysis, establish systematic thickness variations, facies changes and depositional styles. The foreland basin consists of five stratigraphic successions controlled by Andean orogenic episodes and climate: (1) the foreland basin sequence commences between ,27 and 14 Ma with the regionally unconformable, thin, easterly sourced fluvial Petaca strata. It represents a significant time interval of low sediment accumulation in a forebulge-backbulge depocentre. (2) The overlying ,14,7 Ma-old Yecua Formation, deposited in marine, fluvial and lacustrine settings, represents increased subsidence rates from thrust-belt loading outpacing sedimentation rates. It marks the onset of active deformation and the underfilled stage of the foreland basin in a distal foredeep. (3) The overlying ,7,6 Ma-old, westerly sourced Tariquia Formation indicates a relatively high accommodation and sediment supply concomitant with the onset of deposition of Andean-derived sediment in the medial-foredeep depocentre on a distal fluvial megafan. Progradation of syntectonic, wedge-shaped, westerly sourced, thickening- and coarsening-upward clastics of the (4) ,6,2.1 Ma-old Guandacay and (5) ,2.1 Ma-to-Recent Emborozú Formations represent the propagation of the deformation front in the present Subandean Zone, thereby indicating selective trapping of coarse sediments in the proximal foredeep and wedge-top depocentres, respectively. Overall, the late Cenozoic stratigraphic intervals record the easterly propagation of the deformation front and foreland depocentre in response to loading and flexure by the growing Intra- and Subandean fold-and-thrust belt. [source]

Frontal accretion and thrust wedge evolution under very oblique plate convergence: Fiordland Basin, New Zealand

BASIN RESEARCH, Issue 4 2002
P. M. Barnes
ABSTRACT A thrust wedge with unusual geometry has developed under very oblique (50,60°) convergence between the Pacific and Australian Plates, along the 240-km length of the Fiordland margin, New Zealand. The narrow (25 km-wide) wedge comprises three overlapping components, lying west of the offshore section of the Alpine Fault, and straddles a change of > 30° in the regional strike of the plate boundary. Swath bathymetry, marine seismic reflection profiles, and dated samples together reveal the stratigraphy, structure, and evolution of the wedge and the underthrusting, continental, Caswell High (Australian Plate). Lateral variations in the composition and structure of the accretionary wedge, and the depth of the décollement thrust, result partly from variations in crustal structure and basement relief of the underthrust plate, and from associated variations in the thickness of turbidites available for frontal accretion. In the southern Fiordland Basin the underthrust plate is undergoing flexural uplift and extension, and a thick turbidite section is available for accretion. Along-strike, a structurally elevated portion of the underthrust plate is very obliquely colliding with the central part of the accretionary wedge, the turbidite section available for accretion is condensed, and structural inversion occurs in the underthrust plate. ,Growth of the thrust wedge is inferred to have commenced in the Pliocene prior to 3 ± 1 Ma, but much of the wedge developed in the Quaternary. The spatial distribution of thrusting has varied through time, with most late Quaternary shortening occurring on structures within 10 km of the right-stepping deformation front. Estimates of the magnitude and rates of deformation indicate that the wedge accommodates a significant component of the oblique convergence between the Pacific and Australian Plates. Shortening of up to 7.3 ± 1.4 km and 9.1 ± 1.8 km within the southern and central parts of the wedge, respectively, represent about 5,15% of the total 70,140 km of shortening predicted across the plate boundary since 6.4 Ma, and about 10,30% since 3 Ma. Late Quaternary shortening rates of the order of 1,5 mm yr,1, estimated across both the northern and southern parts of the wedge, represent about 10,50 and 5,21% of the total NUVEL-1 A shortening across the plate boundary at these respective latitudes, implying that most shortening is occurring onshore. Furthermore, possible oblique-slip thrusting within the wedge may be accommodating boundary-parallel displacement of 0,6 mm yr,1, representing 0,17% of the total predicted within the plate boundary. [source]