Positive Flower Structure Research Articles

The Klagenfurt Basin in the Eastern Alps: an intra-orogenic decoupled flexural basin?

The Klagenfurt Basin is an E-W-trending narrow Sarmation to Quaternary flexural basin formed by flexure of the Austro-Alpine lithosphere by loading through the southerly adjacent Karawanken Mountains in the Eastern Alps (Austria). The tectonic evolution of the basin is kinematically connected to Miocene brittle transpressive dextral strike-slip deformation along the Periadriatic Fault which separates the South Alpine unit from the Austro-Alpine unit in the Eastern Alps. The final NW-directed overthrust of the Karawanken Mountains onto the foreland is characterised by a positive flower structure, which is kinematically linked to dextral transpressive shearing along the Periadriatic Fault. Only a small part of the lithosphere appears to support the regional isostatic response to the load by the Karawanken Mountains. The area is characterised by a crustal thickness between 40 and 45 km, elevated heat flow and a strength distribution which suggests that only the upper crust elastically supports the topographic load. Ductile flow of the lower crust is also supported by the absence of a crustal root beneath the Karawanken chain. Rheologic models for this lithospheric configuration indicate a mechanical decoupling of upper crust, lower crust and mantle and generally low strengths for the lower crust and mantle.

Neotectonic and seismic activity in the Armorican and Cornubian Massifs: Regional stress field with glacio-isostatic influence?

In Brittany and SW England, evidence for low magnitude Quaternary seismicity can be found in sand pit exposures and beach sections. Deformation is especially well seen in alluvial and estuarine complexes resting on Late Pliocene sands or thick saprolite. The deformations are shallow, dominantly hydroplastic (high water-table) and led to overconsolidated sands, silts or clays. They reveal normal loading at micro (millimetric) and macro (decametric) scales as controlled by the local rheological properties of the sediments, by strikeslip faults associated with positive flower structures, folding, and intraformational water expulsion or hill slope sliding with reverse microfaulting. All the sites where these features occur, are in the vicinity of presumed active faults or steep slopes in highly fractured Proterozoic basement rocks indicating a possible shear zone. In most cases, these features are not associated with synsedimentary deformation, as strong superficial red soils are generally reworked by them. All these features are reworked by microfaulting after overconsolidation. Additional periglacial phenomena are superimposed on them and are often confused with them. Deformation occurred after the development of Holstenian peats (isotopic stage 11,400 ka BP) at Crozon (Brittany), after 317 ka BP (beginning of isotopic stage 9) in the Vilaine estuary, and in most other sites before the last rubified pedogenesis in the Middle Pleistocene (presumed isotopic stage 9). These dates correspond to the same episode that gave rise to the last main reactivation of the fossil cliff around 300 ka BP and to local uplift. One or several seismic clusters have taken place, probably due to delayed crustal rebound after a major glacial event (stage 10) and to rapid loading resulting from younger ice sheet growth (stage 8). Similar events occurred in late stage 7 and late stage 5. These events might have locally amplified the crustal deformation of the old Brittany and Cornubian Hercynian massifs resulting from a regional stress field especially from 400 ka to 200 ka BP. Stratigraphical, geodynamical and paleoclimatological implications are discussed.

Variable deep structure of a midcontinent fault and fold zone from seismic reflection: La Salle deformation belt, Illinois basin

Deformation within the United States midcontinent is frequently expressed as quasilinear zones of faulting and folding, such as the La Salle deformation belt, a northwest-trending series of folds cutting through the center of the Illinois basin. Seismic reflection profiles over the southern La Salle deformation belt reveal the three-dimensional structural style of deformation in the lower Paleozoic section and uppermost Precambrian(?) basement. Individual profiles and structural contour maps show for the first time that the folds of the La Salle deformation belt are underlain at depth by reverse faults that disrupt and offset intrabasement structure, offset the top of interpreted Precambrian basement, and accommodate folding of overlying Paleozoic strata. The folds do not represent development of initial dips by strata deposited over a preexisting basement high. Rather, the structures resemble subdued “Laramide-style” forced folds, in that Paleozoic stratal reflectors appear to be flexed over a fault-bounded basement uplift with the basement-cover contact folded concordantly with overlying strata. For about 40 km along strike, the dominant faults reverse their dip direction, alternating between east and west. Less well expressed antithetic or back thrusts appear to be associated with the dominant faults and could together describe a positive flower structure. The overall trend of this part of the La Salle deformation belt is disrupted by along-strike discontinuities that separate distinct fold culminations. Observations of dual vergence and along-strike discontinuities suggest an original deformation regime possibly involving limited transpression associated with distant late Paleozoic Appalachian-Ouachita mountain building. Moderate-magnitude earthquakes located west of the western flank of the La Salle deformation belt have reverse and strike-slip mechanisms at upper crustal depths, which might be reactivating deep basement faults such as observed in this study. The La Salle deformation belt is not necessarily typical of other well-known major midcontinent fault and fold zones, such as the Nemaha ridge, over which Paleozoic and younger sediments appear to simply be draped.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vänern (Värmlandsnäs, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes, during which the complex rift segment formed upon the multiphase Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile‐brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.

Strike-slip deformation in the Confidence Hills, southern Death Valley fault zone, eastern California, USA

The Confidence Hills form a well exposed, composite, positive flower structure developed in Pliocene to Recent lacustrine and alluvial fan sediments. The structure has developed along the current trace of the southern Death Valley dextral strike-slip fault zone. California, USA. NW–SE-striking fault zones bound the Confidence Hills. In 3D, these fault segments are inferred to link at depth to a common basal fault system. The flower structure is formed by doubly plunging anticlines that roughly parallel the bounding fault segments. Fold development was aided by: (a) the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile and (b) buttressing and uplift along and against the bounding oblique-slip reverse faults. Structural and palaeomagnetic evidence indicates that the anticlines developed with axial surfaces parallel and sub-parallel to the trace of both fault zones. Folding appears to have initially developed adjacent to and above the southeastern fault segment and then propagated outwards and to the northwest. The latest displacements along the current southern Death Valley fault zone are probably only in the orders of hundreds of metres and as young as 0.9 Ma to Recent. This young fault zone is highly segmented along the length of the floor of southern Death Valley: the high proportion of oblique-slip faulting consistent with the strike-slip zone being immature. The Confidence Hills structure displays close geometric similarities with other natural examples of positive flower structures and to the features found in scaled. analogue sandbox experiments of strike-slip faulting.

The southern Whidbey Island fault: An active structure in the Puget Lowland, Washington

Information from seismic-reflection profiles, outcrops, boreholes, and potential field surveys is used to interpret the structure and history of the southern Whidbey Island fault in the Puget Lowland of western Washington. This northwest-trending fault comprises a broad (as wide as 6–11 km), steep, northeast-dipping zone that includes several splays with inferred strike-slip, reverse, and thrust displacement. Transpressional deformation along the southern Whidbey Island fault is indicated by along-strike variations in structural style and geometry, positive flower structure, local unconformities, out-of-plane displacements, and juxtaposition of correlative sedimentary units with different histories. The southern Whidbey Island fault represents a segment of a boundary between two major crustal blocks. The Cascade block to the northeast is floored by diverse assemblages of pre-Tertiary rocks; the Coast Range block to the southwest is floored by lower Eocene marine basaltic rocks of the Crescent Formation. The fault probably originated during the early Eocene as a dextral strike-slip fault along the eastern side of a continental-margin rift. Bending of the fault and transpressional deformation began during the late middle Eocene and continues to the present. Oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation. Evidence for Quaternary movement on the southern Whidbey Island fault includes (1) offset and disrupted upper Quaternary strata imaged on seismic-reflection profiles; (2) borehole data that suggests as much as 420 m of structural relief on the Tertiary-Quaternary boundary in the fault zone; (3) several meters of displacement along exposed faults in upper Quaternary sediments; (4) late Quaternary folds with limb dips of as much as ≈9°; (5) large-scale liquefaction features in upper Quaternary sediments within the fault zone; and (6) minor historical seismicity. The southern Whidbey Island fault should be considered capable of generating large earthquakes (M s ≥7) and represents a potential seismic hazard to residents of the Puget Lowland.

The geometry and evolution of a transpressional strike-slip system: the Carboneras fault, SE Spain

The Carboneras fault system is a 40 km long, 1 km wide Neogene NE-SW-trending left-lateral transpressional strike-slip fault system which is part of the Trans-Alboran shear zone in SE Spain. The Carboneras fault system is an anastomosing array of sub-vertical, individual fault planes or fault zones which surround pods or lenses of less intensely strained rocks. Displacements along the individual fault planes exhibit reverse components of slips and form positive flower structures. Faults have either sharp boundaries or wider bands of gouge, typically a few metres thick and are hundreds of metres in length. Second-order fault splays are well developed and usually exhibit P-shear rather than Riedel-shear orientations. These are interpreted to be related to the transpressional displacement and may also characterize other oblique convergent zones elsewhere. The second-order faults are interpreted to have formed as shear strain increased along the first-order fault and was transferred laterally to branch segments. This process produced pods, or shear lenses, bounded by the fault segments. The length to width aspect ratios of the shear lenses were found to be scale-independent across five orders of magnitude with the most common values between 3:l and 6:l. A model is proposed for the development of the fault zone by incremental finite displacements along the segmented fault surfaces. This model is based on field evidence that displacement switched with time from one fault strand to another.

Plio-pleistocene fault movement as evidence for mega-block kinematics within the Hyblean—Malta Plateau, Central Mediterranean

This paper examines the structural kinematics from the late Miocene to recent across the Hyblean—Malta Platform. Seismic data show that the E—W trending North Gozo Graben formed by right-lateral transtension during the Upper Pliocene. Structural mapping on the top of the Miocene in the western half of the Malta Platform shows both compressional and extensional features. The compressional features are generally right-stepping, with en echelon ridges and positive flower structures, all of which support a zone of dextral strike-slip fault movement north of the island of Gozo. This recent episode of transtension has reactivated the older ENE-WSW and NW-SE trending normal faults to create a series of small pull-apart grabens near the islands of Gozo and Malta. By mapping the orientation of these regional faults and timing their relative movement, we have developed a schematic fault block model that attempts to provide a tectonic framework for the Hyblean—Malta Plateau.

Late Cretaceous and Cenozoic tectono-stratigraphic development of the East Java Sea Basin, Indonesia

The East Java Sea Basin is underlain by a metamorphic basement complex. Subsidence of this basement during the Late Cretaceous resulted in accumulation of up to 3 km of marine Upper Cretaceous sediments (megasequence 1). Contraction and near peneplanation of the Upper Cretaceous sediments and underlying basement occurred before the middle Early Eocene, producing a regional unconformity which defines the base of megasequence 2. The Cenozoic East Java Sea Basin started to form during the Early Eocene by crustal extension on both planar normal faults and extensionally reactivated Cretaceous thrusts. Normal faulting was pulsed from the Early Eocene to Early Oligocene and affected a progressively larger area with time. Complex structural geometries evolved in response to local extensional reactivation of obliquely orientated pre-existing structures. The resultant Palaeogene fault-controlled sub-basins were filled with fluvial, coastal plain and shelf clastic and carbonate sediments recording an overall transgressive evolution. Regional subsidence became dominant over fault-controlled subsidence during the Early Oligocene. Basin-fill during this time was dominated by deep marine, fine-grained clastic sediments. A regional unconformity of intra-Oligocene age is recognized and is overlain by Oligocene to lowermost Miocene deep water calcareous mudrocks and limestones which locally onlap erosionally truncated Eocene rocks. The Lower Eocene to lowermost Miocene sediment package comprises megasequence 2. Regional inversion of Palaeogene sub-basins commenced in the Early Miocene and continues to the present day. The syn-inversion Lower Miocene to Recent sediments comprise megasequence 3. Inversion has resulted from regional contraction and resultant reverse reactivation of Palaeogene normal faults and Cretaceous thrusts. Regional subsidence has been continuous during the inversion history, resulting in a gradual reversal of depocentre location; Palaeogene depocentres became Neogene highs, whereas Palaeogene platforms became Neogene depocentres. Miocene deposition during inversion was dominated by the alternation of fine-grained carbonate-dominated and clastic-dominated cycles. Subsequent Pliocene sandstone deposition followed fluvial incision during an Early Pliocene lowstand. The depositional history, during the Pleistocene to Recent, records rapid relative sea-level changes; progradation of clastic and carbonate sediments, erosional truncation, channelling and slumping. The Tertiary structural geometrical evolution and preserved sediment distribution can be explained by dominantly dip-slip fault movement during the extensional and contractional phases of basin development and deformation. Basin-scale cross-sectional geometries similar to classical positive flower structures have evolved by the reverse reactivation of a geometrically complex extensional fault system.

Late Neogene kinematics of intra‐arc oblique shear zones: The Petilia‐Rizzuto Fault Zone (Calabrian Arc, Central Mediterranean)

The kinematics of intra‐arc shear zones play a key role in the secondary shaping of orogenic arcs such as the Calabrian Arc (central Mediterranean). Comparison of the Neogene structural development of the Petilia‐Rizzuto Fault Zone and the basement structure of the bordering Sila massif reveals that the fault zone is the surface expression of a deep NW–SE trending sinistral crustal oblique shear zone. This shear zone continues over a length of more than 130 km across the northern segment of the Calabrian Arc and shows a post‐Eocene sinistral displacement of about 50 km. The late Neogene forearc basin development and syndepositional tectonics along the fault zone are reconstructed in great detail by analyzing the middle Miocene‐Recent tectonic sequence stratigraphy. A strike‐slip cycle can be recognized whereby the subsequent activity of Riedel shears, tensional faults, and P shears, positive flower structures and principle displacement wrench faults, can accurately be traced in time. Observed phenomena are discussed in terms of the activity of a conjugate system of oblique thrust zones within the growing accretionary complex. The evolution of special types of thrust belt basins is illustrated. These include oblique thin‐skinned pull‐apart basins, oblique rhomboidal “harmonica” basins, and “detached slab” basins (new terms introduced here), evolving one into the other. A new feature illustrated is the recurrent basin inversion which generated passive roof duplexes through back‐shear motion and out‐of‐sequence thrusting along the wedge. The fault patterns and the style of inversion tectonics imply an E–W directed axis of effective compressive stress in this part of the arc. This resulted from an interaction of (1) local E–W directed compression related to a differential displacement of two parallel segments of the arc (generated by the migration to the southeast of the Calabrian Arc and opening of the Tyrrhenian backarc basin); (2) alternating NW–SE directed compression and extension (related to pulsating thrust wedge dynamics with phases of accretion and underthrusting respectively) and (3) regional, compressive interplate stress (middle Messinian‐middle Pliocene). All structures are overprinted by post middle Pleistocene extensional faulting (related to rapid uplift of intra‐arc massifs) and reversal along thrust planes and transcurrent faults. This extensional collapse reflects isostatic adjustments in response to plate rupture which was provoked by regional compressive stress.

Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation

Reprocessing of line PR3 proprietary seismic reflection data has delineated Grenvillian, Paleozoic, and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians in Virginia and West Virginia. The eastern portion of PR3 can be correlated along strike with the western portion of line I-64, reprocessed earlier at Virginia Tech. The combined seismic reflection data image the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont, and Atlantic Coastal Plain provinces. Within the Piedmont, large (as much as 10 km wide) reflective structures imaged on both lines PR3 and I-64 are interpreted to be thrust sheets that might be composed of deformed Catoctin, Evington Group, and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a thrust sheet imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported north-west along the Blue Ridge thrust, which ramps upward ∼12 km east of the surface exposure of the Mountain Run Fault. Westward along line PR3, the Blue Ridge thrust maintains an undulating geometry; the maximum thickness of the Blue Ridge allochthon is interpreted to be ∼4.5 km. The Blue Ridge allochthon is generally acoustically transparent and overlies lower Paleozoic shelf strata. The maximum thickness of these strata is ∼8 km. Shelf strata are interpreted to extend in the subsurface 5 km east of the surface exposure of the Mountain Run Fault, the northeastward extension of the Brevard Fault Zone, where they are truncated by the Blue Ridge thrust at a depth of 10.5 km (3.5 s). Various folds and blind thrusts are imaged beneath the Appalachian foreland; however, the foreland has not experienced the same degree of deformation as observed in the eastern provinces. A basement uplift ∼45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Late Cambrian time. Farther west, reflections imaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep (>9 km) crustal reflections are imaged along line PR3. The majority of reflections that do exist at these depths are observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas might be the result of the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.

Ouachita-Appalachian Juncture: a Paleozoic Transpressional Zone in the Southeastern U.S.A.

The northern margin of Gondwana collided with the southern margin of the North American craton in the late Paleozoic (330-265 Ma). Near-normal compressional stresses, generated where the trend of the North American margin was nearly perpendicular to Gondwana's plate-motion vector (closure direction), caused extensive crustal shortening and decollement thrusting within the sedimentary section. The Appalachian and Ouachita fold belts are the erosional remnants of this collision on the North American margin. Transecting the Ouachita fold belt are several zones of late Paleozoic transcurrent faulting: the Val Verde, Ardmore-Anadarko, and Reelfoot zones. They occur where the precollision margin of the North American craton was oriented oblique or parallel to the closure direct on with Gondwana. Stresses generated within these zones during the collision were transpressional rather than simply compressional, and gave rise to high-angle faulting, high-amplitude vertical displacements, and the emplacement of positive flower structures. In the Val Verde and Ardmore-Anadarko basins, these features have proven to be major structural traps for oil and gas. On the basis of seismic and well data, this study identifies a zone of complex structures, including positive flower structures, in the subsurface Paleozoic section of Kemper County in east-central Mississippi. The region is considered to be another transpressional zone analogous to the Val Verde, Ardmore-Anadarko, and Reelfoot zones. The Kemper County zone separates and truncates the Appalachian Valley and Ridge fo d belt in the subsurface from the Mississippi Ouachita deformed belt to the west and is, therefore, also identified as the juncture between the Ouachita and Appalachian fold-belt systems in the southeastern U.S.A.

Compression- and transpression-related deformation in the Kohat Plateau, NW Pakistan

Abstract Detailed surface mapping and recently acquired subsurface data from the Kohat Plateau of northwestern Pakistan show complex structural styles created by the overprinting by transpression-related structures of pre-existing compression-related structures. The Kohat Plateau is roughly divided into two structurally distinct regions; a northwest region (NW) and a southeast region (SE). Surface structures in the Plateau include doublyoverturned folds (both limbs overturned) which incorporate a lower Eocene to Pleistocene stratigraphic succession. Intensely folded with the stratigraphy in the NW region is a relict thrust belt, representing an earlier compressional episode of deformation. In the SE region many folds (related to wrench faulting at depth) have had either their southern or northern limbs faulted out by high-angle reverse faults which flatten near the surface along a Lower Eocene shale and evaporite horizon. In the NW region, lower Eocene evaporites and limestones are replaced by thick shales which served as a fault-flat forming horizon. This is believed to have hindered the surfaceward propagation of faults, leaving detachment folds exposed at the surface. Steeply dipping strata at depths of over 4 km, steeply dipping faults that become shallow near the surface forming positive flower structures, large vertical displacements across faults, and left-stepping folds suggest transpressional rather than purely compressional deformation. Regions adjacent to the Kohat Plateau also contain transpression-related structures, rather than thin-skinned imbricate thrusts. In view of the position of the Kohat Plateau near a major syntaxial bend in the Himalayan orogen, both north-south and east-west convergence played a dominant role in creating structural styles, and the combination of these two convergence directions influenced the development during the Plio-Pleistocene of transpression-related structures.

Small-scale positive flower structures

Field relations indicate that small-scale positive flower structures along sub-regional strike-slip faults localize ore in particular 4 m thick, bedded ore zones in Mississippi Valley-type Pb-Zn deposits of the Viburnum Trend, southeast Missouri, U.S.A. Outwardly divergent, shallow-dipping, duplex-deformed fault splays control ore and merge inwardly with sub-vertical fault strands. The characteristics of both duplex-deformed splays and ore suggest that the flower structures acted as drains for fluids being moved vertically along the strike-slip faults. This ore control differs from pipe-like conduits of transtensional dilation jogs in that duplex-deformed splays form under transpression and develop horizontal veins adjacent to strike-slip faults.

U–Pb and 40Ar/39Ar mineral ages from Cape North, northern Cape Breton Island: implications for accretion of the Avalon Composite Terrane

Isotopic studies have been used to determine protolith and tectonothermal ages in rocks of the Cape North and Money Point groups and two plutons, that form part of the Avalon Composite Terrane. Colourless, euhedral zircon crystals from low-grade metarhyolite in the Money Point Group record a concordant U–Pb age of 427.5 ± 4 Ma, which is interpreted to date the time of extrusion. This provides a maximum age of middle Silurian for polyphase deformation and concomitant greenschist–amphibolite-facies metamorphism that affected the Cape North area in the eastern half of a major, sinistral, positive flower structure. Monazite from the Cape North granite, intruded during peak metamorphic conditions, yielded nearly concordant data with a 207Pb/235U age of 414 ± 3 Ma. This dates the time of crystallization of the Cape North granite. Metamorphic hornblende from the Cape North and Money Point groups records variably discordant 40Ar/39Ar age spectra that define plateau and (or) isotope correlation ages of ca. 380–390 Ma. Muscovite from these units displays generally concordant spectra that define plateau ages ranging between ca. 365 and 375 Ma. Muscovites from the syntectonic Cape North granite and the posttectonic Relay Station granite record plateau ages of ca. 380–382 Ma. The 40Ar/39Ar ages suggest that relatively rapid postmetamorphic cooling occurred during the Devonian. These isotopic results and petrological data indicate that burial of the Cape North units to ca. 18 km, metamorphism, uplift, and erosion occurred in ca. 60 Ma. Such rates are comparable with those recorded in modern transpressive tectonic regimes. The calc-alkaline nature of the Money Point volcanic rocks suggests derivation above a subduction zone. A correlative Silurian–Devonian tectonothermal event recorded in southern Newfoundland probably records the accretion of the Avalon Composite Terrane with the Dunnage Zone.

The northeastern portion of the Clarence Fault: Tectonic implications for the late Neogene evolution of Marlborough, New Zealand

The Clarence Fault does not extend northeastward to the Marlborough coast as traditionally mapped. Instead, it joins with a northerly trending shear zone within the Torlesse Supergroup near Peggioh Station, 10 km west of Ward. In this region the Clarence Fault is a blind, west‐dipping moderate‐angle reverse or thrust fault, east of which, on the downthrown side, the Late Cretaceous and Tertiary sedimentary and volcanic succession has been overturned.The Clarence Fault and a number of associated splay faults represent a positive flower structure that formed in association with drape folding. These structures probably began forming in the early Miocene.If the present‐day rate of dextral strike‐slip on the Clarence Fault of 4–8 mm/yr were extrapolated into the past, the total inferred lateral offset of 15 km along the fault could be accomplished in 3 million years. Consequently, it is suggested that the Clarence Fault has acted as a dextral strike‐slip structure only comparatively recently, in the order of the last few million years, and that before this it was a southward‐directed thrust fault.

Seismic imaging of major tectonic features in the crust of Phanerozoic eastern Australia

Crustal architecture can be recognised from Seismic fabric characteristics and velocity information. Across Phanerozoic eastern Australia, three major crustal sub-divisions are identified from data in southern Queensland. They occur under (west to east) the Nebine Ridge, Taroom Trough and New England Fold Belt, with each crustal sub-division having its own internal architecture but separated by major bounding structures (geosutures). These features are assessed against evolutionary models for the region. The significant conclusions are as follows: 1.(1)The Nebine Ridge forms the southeastern part of the Thomson Fold Belt but has a crustal architecture distinctly different from that under the central part of the fold belt, tentatively supporting an evolutionary model involving fragmentation and eastward stretching and thinning of a Precambrian Australian craton. The ridge crust has possibly been considerably altered by Carboniferous plutonism and compressional events.2.(2)Between the Nebine Ridge and the crust under the Taroom Trough of the Bowen Basin there is a major low-angle (5–10°) geosuture (the Foyleview geosuture), which extends from upper to lower crustal levels along a series of prominent horizons. It was probably active during the major Carboniferous tectonism which severely structured Devonian basins west of the Nebine Ridge. The geosuture forms the southeastern boundary of the Thomson Fold Belt.3.(3)The crustal architecture under the Taroom Trough has little obvious structure above an outstanding Moho transition zone. However, the prominent Meandarra Gravity Ridge striking along the trough axis highlights the possibility of high-density mafic volcanics within the crust and the probability of a rift model being appropriate early in the trough's history. The sedimentary basin architecture supports the concept of an early extensional history for the crust within a poorly defined, N-S trending, strike-slip regime.4.(4)Between the Taroom Trough and the New England Fold Belt we interpret a high-angle geosuture extending through the crust (the Burunga-Mooki geosuture). Reactivation during late Palaeozoic-early Mesozoic times produced a series of reverse faults along the uplifted eastern margin of the trough. Some of these structures are tentatively considered to be positive flower structures above deeper faults with strike-slip movement. The geosuture separates regions with distinctly different Seismic fabrics and played an important role in the formation of the Bowen-Gunne-dah-Sydney Basin system.5.(5)Within the New England Fold Belt, crustal domains can be recognized associated with late Palaeozoic oroclinal bending and early Mesozoic trans-tensional basins. Major oroclinal bending of the upper crust in the west of the fold belt is interpreted above a mid-crustal detachment (the Texas detachment). In the central part of the fold belt, the Seismic data support a trans-tensional mechanism for the formation of Triassic basins with steeply dipping bounding faults. Under the east of the fold belt, an imbricate thrust stack/accretionary wedge is clearly imaged above a mid-crustal horizon (the Brisbane mid-crustal detachment). Upper crustal deformation appears to have occurred above this detachment.6.(6)From the Nebine Ridge to the coast, the Moho is clearly defined below a 3-km thick Moho transition zone. Hast of the Thomson Fold Belt, the Moho level is identified as a gently undulating feature at 36–38 km depth and probably re-established after the major late Palaeozoic tectonic events which formed the crust under the Taroom Trough and New England Fold Belt. This contrasts with the middle Palaeozoic lower crustal/Moho features which appear to be preserved under the Thomson Fold Belt.7.(7)The thickest crust under the Nebine Ridge (about 44 km) appears to be associated with lower crustal wedging from the east. There is an apparent thinning of the non-sedimentary crust under the deepest basin (Taroom Trough) of about 30% compared with the crust to the west and east. No crustal thickening is evident in the region of oroclinal bending within the New England Fold Belt.

The post-Devonian tectonic evolution of southern Spitsbergen illustrated by structural cross-sections through Bellsund and Hornsund

AbstractConstruction of cross-sections in Bellsund and Hornsund, southern Spitsbergen, using offshore and onshore structural data illustrate the main tectonic units of the region. From west to east these are: a wedge of late Cenozoic clastic sediments; a series of late Palaeozoic to early Mesozoic grabens controlled by basement faults along the west Spitsbergen margin; the Basement Horst, comprising late Precambrian to early Palaeozoic rocks deformed and metamorphosed during a mid-Palaeozoic orogeny; the Fold Belt, which forms a narrow NNW–SSE striking zone of eastward verging folds and thrusts attributed to Eocene inversion of a pre-Cenozoic basin; the Palaeogene Central Basin, deformed into a broad synclinorium and bound to the east by the Billefjorden Fault Zone. This basement lineament shows evidence of Palaeogene reactivation and may be linked to the Fold Belt by a detachment zone beneath the Central Basin. East of Spitsbergen, there is an offshore basin of possible Carboniferous age controlled by the Storfjorden Fault Zone which shows evidence of later inversion to form a positive flower structure.

Seismic reflection across a continental transform: An example from a convergent segment of the Dead Sea rift

Three seismic reflection lines in the Dead Sea rift in northern Israel cross the transform plate boundary between Arabia and the Sinai block of the African plate. The Arabian plate edge consists of the Meshoshim tilted block, which we suggest to be partly a result of prerift activity. In the Sinai block the elevated Rosh Pinna Saddle is faulted over the Meshoshim block with a vertical separation of 2.5–3‐km. Much of this separation is due to young reverse faulting which is related to the oblique plate boundary in this area. The main fault zone transfers the motion from the western margin of the Kinneret‐Zemah graben to the eastern margin of the Hula graben. Immediately north of Lake Kinneret, the main fault is a 1‐ to 2‐km‐wide zone of extensive deformation and a discrete main fault trace is hard to define. Further to the north the deformation is less extensive and is concentrated in a narrow fault zone. Deformation across the plate boundary is asymmetrical and, while the Meshoshim block is quite coherent, the Rosh Pinna Saddle is a highly faulted and deformed positive flower structure.