Nigel Woodcock

Strike-slip duplexes

Strike-slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip-slip, thrust and normal, fault systems. They are evident in map view rather than in vertical sections. Examples of duplexes are cited from both active and ancient systems and from theoretical and physical models. Duplexes may form at bends on strike-slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip-slip faults. However some may form, and many may initiate, as non-sequential ‘Riedel’ fractures at fault offsets or on straight fault segments. This process is more marked than in dip-slip systems where primary anisotropy such as bedding exerts more control on fault geometry. Strike-slip duplexes may be shunted along the fault system parallel to the regional slip vector. However, duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system. These factors mean that no section through a strike-slip system should be expected to area balance. The faults of strike-slip duplexes and imbricate fans may root in kinematically necessary low-dip faults or may converge downwards and appear in vertical sections as flower structures.

Specification of fabric shapes using an eigenvalue method

Eigenvalues, derived from the “orientation tensor” method for analyzing directional data, are useful indicators of fabric shape. Three possible methods of graphing these eigenvalues are discussed. These provide a convenient visualization of fabric shapes and strengths. Examples are given of the use of such graphs for representing field data and for tracing progressive deformation of fabrics.

Silurian collision and sediment dispersal patterns in southern Britain

The evidence is reviewed for the timing of collision between the microcontinent of Eastern Avalonia (southern Britain and adjacent areas) and the Laurentian continent. Recent palaeomagnetic results placing Eastern Avalonia in a high (50°) southern latitude in mid Ordovician time are now consistent with faunal evidence for the first time. The resulting apparent polar wander path is evaluated and suggests that Eastern Avalonia detached itself from a southern peri-Gondwanan latitude in the early Ordovician, moved northwards, and approached Laurentia by the late Ordovician. Its western corner probably impinged on Laurentia in the early Silurian and it docked against the Laurentian margin during Silurian and early Devonian time with a component of anticlockwise rotation. This kinematic history is supported by a compilation of sediment dispersal patterns on Eastern Avalonia. A low-volume Ordovician and earliest Silurian supply from within the microcontinent was overwhelmed in late Llandovery time by a large volume of southwest-derived turbidites, probably from the uplifting impact zone to the west. This source was later augmented by a high-volume clastic supply to the north margin of the microcontinent. Eastward migration of this source through Wenlock and Ludlow time reflects the progressive anticlockwise docking of Eastern Avalonia against the Laurentian margin. The earliest sign of a large-volume supply from Baltica is in the late Wenlock, arguing against any earlier hard collision.

Structural style in slump sheets: Ludlow Series, Powys, Wales

The slump deformation in sediments of the Montgomery Trough is described and compared with tectonic deformation. The structures within each slump sheet are systematically oriented with respect to the inferred palaeoslope. The slump folds have lognormal size distributions and their range of styles closely matches that of tectonic folds. Similar folds are rare, and deformed lineations show that buckling was important during slump fold formation. Axial microfold lineations and axial plane cleavages are associated with the slump folds. Also present is a spaced cleavage, thought to have formed in soft sediment. Structural style cannot discriminate between slump and tectonic structures. The distinction may in many cases rely on subjective and uncommon features of soft-sediment deformation, such as the absence from folds of tension cracks and veins.

Classification of fault breccias and related fault rocks

Despite extensive research on fault rocks, and on their commercial importance, there is no non-genetic classification of fault breccias that can easily be applied in the field. The present criterion for recognizing fault breccia as having no ‘primary cohesion’ is often difficult to assess. Instead we propose that fault breccia should be defined, as with sedimentary breccia, primarily by grain size: with at least 30 % of its volume comprising clasts at least 2 mm in diameter. To subdivide fault breccias, we advocate the use of textural terms borrowed from the cave-collapse literature –crackle, mosaic and chaotic breccia – with bounds at 75 % and 60 % clast content. A secondary breccia discriminant, more difficult to apply in the field, is the ratio of cement to matrix between the clasts. Clast-size issues concerning fault gouge, cataclasite and mylonite are also discussed.

Faulting mechanisms in high-porosity sandstones; New Red Sandstone, Arran, Scotland

Summary Faults in the ‘New Red’ aeolian sandstones of Arran are unusual, firstly for occurring as closely-spaced (less than 1 m) often conjugate sets affecting large volumes of rock, and secondly for forming upstanding fault zones with numerous anastomosing strands of granulated rock, each preserving a small increment of slip. Anisotropy, such as bedding and cross-bed sets, has no discernible effect on fault behaviour. In contrast to the underlying Carboniferous rocks, large displacements are rarely concentrated on a single fault plane within the high-porosity sandstones. The proposed cause is slip-hardening of each fault after a very small displacement (less than 10 mm) causing the next slip increment to be taken up through undeformed rock rather than on the original plane. The common factor in recent records of similar faults elsewhere is their occurrence in high-porosity sandstones. Because of the low grain-contact strength, these rocks are partly analogous to unconsolidated sediment. The high porosity promotes high grain-contact stresses which induce rapid cataclasis during initial slip. Grain fracture and spalling of iron oxide coatings and quartz overgrowths produce a seam with reduced grain size, poorer sorting, higher angularity and lower porosity than the unfaulted rock. These factors collectively strengthen the seam because the coefficient of friction is increased, even though cohesion is reduced. This results in a Mohr failure envelope that lies outside the envelope of the undeformed rock for most stress states. A transient pore pressure increase in the fault seam may be important during slip. Rocks deformed by this slip-hardened faulting preserve a record of each increment of strain. If the displacement on each individual fault seam is the same, the geometry of the total fault systems is directly related to the bulk strain. Quadrimodal systems observed by us in Arran, and by others elsewhere, are probably a response to triaxial strain and show that bimodal ‘Andersonian’ fault systems are only special plane strain cases. If the bulk strain is irrotational, both the orientation and relative magnitude of the principal strains might be estimated.

RIFTING DURING SEPARATION OF EASTERN AVALONIA FROM GONDWANA : EVIDENCE FROM SUBSIDENCE ANALYSIS

Subsidence curves for Cambrian-Ordovician sequences from the Anglo-Welsh segment of the paleocontinent of Avalonia reveal two periods of regionally enhanced basement subsidence: Early Cambrian (545–518 Ma) and Late Cambrian to early Tremadocian (505–490 Ma). The earlier event may record transtension following the Avalonian-Cadomian orogeny. The second event may be a transtensional precursor to the late Tremadocian volcanic arc on Eastern Avalonia. However, paleomagnetic, faunal, volcanic, and sedimentary evidence suggests that the main separation of Eastern Avalonia from Gondwana occurred after middle Arenigian time. Rifting during separation is probably recorded by localized middle Arenigian to Llanvirnian (480–462 Ma) subsidence along the Welsh basin margin, but rifting must have occurred mainly on the now-obscured southern margin of the Avalonian continent. Pronounced Caradocian (462–449 Ma) subsidence is associated with back-arc rifting after separation from Gondwana.

A Rheic cause for the Acadian deformation in Europe

The Acadian (mid-Devonian) deformation in NW Europe has typically been interpreted as the culminating event of the Silurian closure of the Iapetus Ocean. This view has been challenged by the recognition of an intervening early Devonian transtensional event across part of the assembled Laurussian continent. Instead, the Acadian shortening must be driven by a renewed ‘push from the south’, involving subduction of the Rheic Ocean, and either flat-slab subduction or impingement of another Gondwana-derived continental fragment. A problem with either hypothesis is the lack of Acadian deformation or even correlative unconformity in the segment of the Rhenohercynian Zone between the Acadian belt and the Rheic suture. The possibility is explored that this Rhenohercynian segment was juxtaposed with the Acadian belt and the Midland Microcraton only during latest Acadian and/or Variscan tectonics. If so, a major lithospheric suture lies buried just south of the Variscan Front, along the Bristol Channel Fault Zone, and the missing Acadian terranes must now lie elsewhere along the orogen. A case is made that they are related to the allochthonous terranes of NW Iberia. In any case, the Acadian event in Europe should properly be regarded as proto-Variscan rather than late Caledonian.

Transpressive duplex and flower structure: Dent Fault System, NW England

Abstract Revised mapping along the Dent Fault (northwest England) has improved the resolution of folds and faults formed during Variscan (late Carboniferous) sinistral transpression. A NNE-trending east-down monocline, comprising the Fell End Syncline and Taythes Anticline, was forced in Carboniferous cover above a reactivated precursor to the Dent Fault within the Lower Palaeozoic basement. The Taythes Anticline is periclinal due to interference with earlier Acadian folds. The steep limb of the monocline was eventually cut by the west-dipping Dent Fault. The hangingwall of the Dent Fault was dissected by sub-vertical or east dipping faults, together forming a positive flower structure in cross-section and a contractional duplex in plan view. The footwall to the Dent Fault preserves evidence of mostly dip-slip displacements, whereas strike-slip was preferentially partitioned into the hangingwall faults. This pattern of displacement partitioning may be typical of transpressive structures in general. The faults of the Taythes duplex formed in a restraining overlap zone between the Dent Fault and the Rawthey Fault to the west. The orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly. Kinematic constraints provided by the Dent and neighbouring Variscan faults yield a NNW–SSE regional shortening direction in this part of the Variscan foreland.

Sequence stratigraphy of the Palaeozoic Welsh Basin

The timing and extent of unconformities in the Welsh Basin are investigated using ‘rock preservation curves’ derived from outcrop stratigraphic logs. Four basin-wide unconformities occur, focussed in late Precambrian, late Tremadoc, Pusgillian (early Ashgill) and mid-Devonian times. These bound three megasequences, equating with newly defined lithostratigraphic units, the Dyfed, Gwynedd and Powys Supergroups. Less extensive unconformities bound 18 component sequences. The majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea-level change. The megasequence boundaries are attributed to late Precambrian to early Cambrian onset of rifting to form a passive margin, Tremadoc onset of subduction with intra-arc then back-arc extension, late Caradoc end to subduction, and late Early Devonian collisional deformation. The megasequences and controlling events can be tentatively matched with other basins on the Avalonian margin. More generally, this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data, and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins.