Brittle Reactivation Research Articles

Brittle reactivation of ductile basement structures; a tectonic model for the lake Vättern basin, southern Sweden

Brittle reactivation of ductile basement structures; a tectonic model for the lake Vättern basin, southern Sweden

Geometry and kinematics of a major crustal shear zone segment in the Appalachians of southern New Brunswick

In southern New Brunswick the Kennebecasis Fault follows the northern boundary of a crystalline portion of the late Precambrian – Cambrian Avalon terrane. This low-grade crystalline complex forms the basement to a series of Carboniferous through Triassic basins. This complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault, and that with it defines the Pocologan–Kennebecasis fault zone. The orientation of the main composite foliation (S1) and the included mineral (stretching) lineation (L1) indicate that this geometry is a primary feature of the shear zone, representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit. Kinematic indicators show that this allochthon moved parallel to the strike of the north Appalachian orogen, with top towards the west or west-southwest. The bounding shear zone is not uniform, but consists of a mylonite–phyllonite adjacent to the detachment. The upturned segment of the shear zone has been the site of later, brittle reactivation, one episode of which is represented by the Kennebecasis Fault. The main shear zone relic relates to more fundamental events, such as the accretion of the Avalon terrane.

Contrasting metamorphic terranes in northwest Wales

Abstract The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and PT conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Llŷn peninsula (Llŷn Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to c. 550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.

Basement–cover relationships around Aberdaron, Wales, U.K.: the fault-reactivated northwestern margin of the Welsh Basin

AbstractOn mainland North Wales basement rocks emerge from beneath a Lower Ordovician cover along the west side of the Llyn peninsula. The basement contains steeply dipping mylonites (Llŷn shear zone) that separate plutonic and gneissic rocks (Sarn Complex) from a melange (Gwna mélange). The western edge of the Ordovician outcrop follows the basement shear zone, and new trenching data confirm that only a faulted relationship exists between cover and basement along this northwestern extremity of the Welsh Basin. Deformation along this margin has propagated into the Arenig cover to produce southeasterly verging thrusts, asymmetric folds and northwesterly dipping cleavage. A prominent steep fault (Daron Fault) cutting the Ordovician succession follows an eastern splay of the Llŷn shear zone and again therefore records brittle reactivation of an underlying mylonite belt. The likelihood of syn-Arenig fault movements is provided by the presence of a prominent late Arenig coarse clastic unit, containing boulders of the basement mylonites, that is found only to the west of the Daron Fault. Steep basement structures such as the Llŷn shear zone, initially generated as major transcurrent faults, are interpreted as having exerted a strong control over the deposition and subsequent deformation of the Ordovician cover sequence.

Shuswap terrane of British Columbia: A Mesozoic “core complex”

The Shuswap terrane of the northwest Cordillera owes its distinctive characteristics to deformation and high-grade regional metamorphism that occurred primarily during the Mesozoic. Within the terrane, the Monashee Complex, the overlying Selkirk allochthon, and the intervening Monashee decollement record strain and metamorphism that occurred in Middle to Late Jurassic time, during and subsequent to the accretion of a western allochthonous terrane. The Selkirk allochthon moved eastward across the Monashee Complex after emplacement of the western allochthonous terrane but before late Mesozoic to early Cenozoic telescoping of the Rocky Mountain Belt. In this latter event, the Monashee Complex and overlying allochthonous slices were transported eastward relative to the North American craton on a sole fault that developed during listric thrusting of the Rocky Mountain foreland. Synmetamorphic to late-metamorphic elongation lineations and associated fabrics of the mylonitic rocks of the Monashee decollement zone were generated by shear strain during the Middle Jurassic emplacement of the Selkirk allochthon and are not related to any upper crustal extension that occurred in the Tertiary. Uplift, normal faulting, brittle reactivation of the mylonitic decollement zone, widespread resetting of K-Ar and Rb-Sr mineral dates, and arching of the terrane are events that culminated in the Eocene.

Kinematic interpretation of mylonitic rocks in part of the Columbia River fault zone, Shuswap terrane, British Columbia

Mylonitic rocks of the Columbia River fault zone have been examined in a 600 m thick section, 20 km north of Revelstoke, British Columbia. At this locality coarse-grained granite with well-preserved igneous texture lies in the hanging wall of the fault zone. Adjacent to the boundary of the mylonite zone the granite is foliated and mineral segregation gives rise to well-defined compositional layering. The foliation (Sf) is defined by oriented mineral grains and flattened porphyroclasts. In areas of intense strain compositional layering (Sc) is parallel to Sf. At the boundary of the mylonite zone and in weakly deformed areas within the mylonite zone Sc and Sf are inclined at angles up to 30°. Sf is taken to be a plane of flattening, and Sc appears to parallel a plane of shear. After correction for effects of late-stage flexural slip folding the angular relationship of Sc to Sf may be used to determine sense of shear in the mylonitc zone; the hanging-wall rocks have been displaced toward the east relative to the footwall. Formation of mylonitic fabrics and associated displacement of the Selkirk allochthon eastward relative to the underlying Monashee Complex occurred after the Middle Jurassic peak of regional metamorphism and before brittle reactivation and uplift in the Tertiary.

Palaeomagnetic study of the (Late Precambrian) West Greenland kimberlite-lamprophyre suite: definition of the Hadrynian Track

Kimberlite and potassic lamprophyre dykes were intensively intruded into the early Proterozoic Nagssugtoqidian mobile belt of West Greenland during an important phase of brittle reactivation in Late Precambrian-Early Cambrian times (ca. 580-570 Ma) and during at least one other minor phase. Thermal and alternating field demagnetisation studies of 52 of these dykes identify primary components residing in the critical blocking temperature range distributed between shallow westerly and steep positive directions. Near the axis of the Proterozoic shear belt the dykes (predominantly lamprophyres) have closely grouped shallow directions with a reversal; near the margins of the shear belt dykes (predominantly kimberlites) have steeper and distributed directions. The cleaned components of magnetisation appear to be single, and the distribution of directions is interpreted to record a migration of the palaeofield axis which intersecting relationships show to have been from shallow to steep. The dyke directions are grouped to define representative mean palaeopoles of 215°E 3°N (LK1, A 95 = 3.9°), 213°E 18°N (LK2, A 95 = 6.1°), 203°E 46°N (LK3, A 95 = 10.4°) and 259°E 54°N (LK4, A 95 = 11.0°); a subsidiary direction recorded in five dykes near the southern margin of the shear belt (LK5, palaeopole at 297°E 16°S (A 95 = 12.5°)) is derived entirely from lamprophyres and is possibly Silurian in age. An RbSr isochron on three lamprophyres of 1227 Ma and agreement of the remanence direction with ca. 1220 Ma rocks from elsewhere in Greenland suggests that the LK1 component is wholly or partly of that age. The remaining sequence of palaeopoles falls along the Hadrynian Polar Track and the age data relating to this track are re-evaluated. Evidence for a pre-800 Ma age is no longer valid and the new data from West Greenland confirm that the track is latest Precambrian to Early Cambrian in age. It is shown to connect poles of Late Precambrian and Lower Cambrian age and to embrace other data from the Laurentian shield. The rapid passage of the shield across the South Pole is consistent with the sedimentation sequences, and suggests a high-latitude origin for the tillite horizon of this age. The Hadrynian Track is also compared with the contemporaneous record from Gondwanaland and it is shown that the two shields were in juxtaposition in the identical reconstruction to the Proterozoic Supercontinent until earliest Cambrian times. This discovery links the Lower Cambrian marine transgression and the widespread ca. 580-560 Ma alkaline province in the Gondwanaland, Laurentian and Fennoscandian shields to major continental break up, and it conforms with evidence that the Iapetus Ocean did not open until Cambrian times.