Evolution of the boundary between the western and central Lachlan Orogen: implications for Tasmanide tectonics
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Abstract:
Differences in oblique overprinting, along‐strike complexity as well as structural, metamorphic and timing constraints suggest that the boundary between the western and central subprovinces of the Lachlan Orogen, currently designated by the Governor Fault, cannot be a single structure. Previously limited data on the nature and kinematics of the fault/shear systems defining the boundary have led to varying scenarios for the tectonic evolution of the Lachlan Orogen. These scenarios either involve large‐scale strike‐slip displacement along the boundary with subsequent overthrusting or convergence of oppositely vergent thrust‐systems with limited strike‐slip translation. Geometrical constraints, fabric chronology and kinematic indicators in both the Mt Wellington (Melbourne Zone) and Governor (Tabberabbera Zone) Fault Zones indicate that maximum displacements relate to thrusting and duplex formation, followed by minor strike‐slip faulting perhaps in response to slightly oblique collision of the Melbourne and Tabberabbera structural zones. Collision of these zones took place between ca 400 and 390 Ma. At Howqua, structural relationships indicate that collision involved northeast‐directed thrusting of the Melbourne Zone (Mt Wellington Fault Zone) over the Tabberabbera Zone (Governor Fault Zone), and was followed by regional, northwest‐trending, open folding. These structures overprint the dominant fabrics and metamorphic assemblages that are interpreted to relate to disruption and underthrusting of Cambrian oceanic/arc crust during closure of a marginal basin. Major deformation in the Tabberabbera Zone took place from ca 445 Ma and was associated with mélange formation, underplating and imbrication or duplexing (Governor Fault Zone, East Howqua segment). At slightly higher crustal levels, and following deposition of Upper Ordovician black shale and chert sequences ( ca 440 Ma), Tabberabbera Zone evolution included offscraping of a serpentinite body (Dolodrook segment) that may have been either a Marianas‐style seamount or transform fault zone within the Cambrian oceanic/arc crust. Major thrusting in the Mt Wellington Fault Zone was underway sometime after ca 420 Ma, and in contrast to the Governor Fault Zone, no mélange or broken formation was produced, metamorphism was at slightly higher temperatures and deformation probably occurred under higher strain states.Keywords:
Transpression
Collision zone
Overprinting
Transform fault
Transpression
Transform fault
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Elastic-rebound theory
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Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D 1 was not associated with major structures, D 2 is related to north–northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D 2 deformation subsequently became the locus of intense D 3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S 2 fabrics being transposed by S 3 resulting in a composite foliation over large areas. Similarly, the L 2 mineral lineation is abruptly transposed by L 3 over relatively small distances indicating high D 3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D 4 ) which was increasingly localized and focused into earlier (D 2 −D 3 ) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F 4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.
Overprinting
Lineation
Dalradian
Transpression
Strain partitioning
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Transform fault
Transtension
Transpression
Ridge push
Mid-Atlantic Ridge
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Abstract Delimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F 1 , F 2 and F 3 , are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F 1 folds by F 2 folds; and (ii) the parallelism between the GBF and F 2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F 1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.
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Overprinting
Strain partitioning
Lineament
Necking
Transtension
Indian Shield
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Dena Fault is one of the fundamental and main structures with more than 130 km. One of the most important structural properties of this fault is changing in its trend so that at least three structural trends are detectable along this fault. Some continental transpression evidences along fault Dena are checked. These evidences are: Fold’s axis has configuration step and mutual desire. Fold’s axis average preferred orientation makes angles less than 45 degrees with preferred orientation of the boundary faults. Strike-slip faults are arranged overlapping and territory. P harmonic sections are more than harmonic sections R. In the central and southern parts the type is transpression Trust and in the northern part the type is transpression shear. In the present strike-slip component right lateral of this fault is dominant to its trust component and Fault function in the present era, is right lateral reverse.
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ATALLAH, M., MUSTAFA, H., EL-AKHAL, H. & AL-TAJ, M. 2005. Dhahal structure: an example of transpression associated with the Dead Sea transform in Wadi Araba, Jordan. Acta Geologica Polonica, 55 (4), 361-370. Warszawa. The Dhahal Mountains located at the eastern margin of the northern Wadi Araba sinistral fault represent an example of a transpression associated with the Dead Sea transform, which is a sinistral wrench fault. This structure was formed as a result of right bending of the Humrat Fidan active sinistral fault, which is a parallel strand of the Dead Sea transform located east of the main Wadi Araba fault. The fault bend caused uplift and squeezing of the Cretaceous and Tertiary rocks of the transpression. Positive flower structures, folds, fault-bounded wedge-shaped pop-ups and reverse faults are the main structural elements that characterized the Dhahal transpression. Folds are found as sets or as single anticlines and synclines. The major trend of the fold axes is N50°; the principal stress axis (σ1) is perpendicular to this trend (N140°). This trend deviates 26° anticlockwise from that of the Dead Sea stress system (DSS) (as obtained from fault slip data north of the study area), which is responsible for the formation of the Dead Sea transform. Evidence of active uplift in the Dhahal structure is provided by the sharp topography relative to the surrounding areas, and the low mountain front sinuosity index of the western margin of the Dhahal mountains.
Transpression
Transtension
Syncline
Transform fault
Clockwise
Dead sea
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Transform fault
Transpression
Transtension
Echelon formation
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The Rodgers Creek–Maacama fault system in the northern California Coast Ranges (United States) takes up substantial right-lateral motion within the wide transform boundary between the Pacific and North American plates, over a slab window that has opened northward beneath the Coast Ranges. The fault system evolved in several right steps and splays preceded and accompanied by extension, volcanism, and strike-slip basin development. Fault and basin geometries have changed with time, in places with younger basins and faults overprinting older structures. Along-strike and successional changes in fault and basin geometry at the southern end of the fault system probably are adjustments to frequent fault zone reorganizations in response to Mendocino Triple Junction migration and northward transit of a major releasing bend in the northern San Andreas fault.
Triple junction
Overprinting
Transform fault
Neotectonics
Pacific Plate
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Oblique continent—continent collision between Iranian microcontinent and Arabian plate is the main cause of transpression and transtension regimes in this area. Zagros orogeny in this area resulted in formation of various structures including thrust fault-related folds, dextral and sinistral, strike slip faults, normal fault related to dextral strike slip fault. Thrust faults within the area under study are duplex with general trend of NW-SE and dip toward the north-east with right slip component. Strike slip fault regime acts as tear fault and it is active yet. Therefore, simultaneous presence of thrusts and strike slip faults illustrates convergent dextral transpressional tectonic regime while this transpressional regime accompanied with transtension as well; since normal faults are also seen in alluvium around depression of Sirjan which can be as a result of extension stresses due to strike slip faults activity of the district. The results achieved from geometry and kinematic analysis of west of Sirjan structures indicate that structures of the area have characteristics of internal part of Zagros orogeny.
Transtension
Transpression
Orogeny
Alpine orogeny
Lineament
Pull apart basin
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