Francesca Ghisetti

Peri-Adriatic melanges and their evolution in the Tethyan realm

In the peri-Adriatic region, mélanges represent a significant component of the Apennine and Dinaride–Albanide–Hellenide orogenic belts as well as ancient and present-day accretionary wedges. Different mélange types in this broad region provide an excellent case study to investigate the mode and nature of main processes (tectonic, sedimentary, and diapiric) involved in mélange formation in contrasting geodynamic settings. We present a preliminary subdivision and classification of the peri-Adriatic mélanges based on several years of field studies on chaotic rock bodies, including detailed structural and stratigraphic analyses. Six main categories of mélanges are distinguished on the basis of the processes and geodynamic settings of their formation. These mélange types are spatially and temporally associated with extensional tectonics, passive margin evolution, strike-slip tectonics, oceanic crust subduction, continental collision, and deformation. There appears to have been a strong interplay and some overlap between tectonic, sedimentary, and diapiric processes during mélange formation; however, in highly deformed regions, it is still possible to distinguish those mélanges that formed in different geodynamic environments and their main processes of formation. This study shows that a strong relationship exists between mélange-forming processes and the palaeogeographic settings and conditions of mélange formation. Given the differences in age, geographic location, and evolutionary patterns, we document the relative importance of mélanges and broken formations in the tectonic evolution of the peri-Adriatic mountain belts.

Andersonian wrench faulting in a regional stress field during the 2010–2011 Canterbury, New Zealand, earthquake sequence

Abstract The initial Mw7.1 Darfield earthquake sequence was centred west of Christchurch City in the South Island of New Zealand but aftershocks, including a highly destructive Mw6.3 event, eventually extended eastwards across the city to the coast. The mainshock gave rise to right-lateral strike-slip of up to 5 m along the segmented rupture trace of a subvertical fault trending 085±5° across the Canterbury Plains for c. 30 km, in agreement with teleseismic focal mechanisms. Near-field data however suggest that the mainshock was composite, initiating with reverse-slip north of the surface rupture. Stress determinations for the central South Island show maximum compressive stress σ1 to be horizontal and oriented 115±5°. The principal dextral rupture therefore lies at c. 30° to regional σ1, the classic ‘Andersonian’ orientation for a low-displacement wrench fault. An aftershock lineament trending c. 145° possibly represents a conjugate left-lateral strike-slip structure. This stress field is also consistent with predominantly reverse-slip reactivation of NNE–NE faults along the Southern Alps range front. The main strike-slip fault appears to have a low cumulative displacement and may represent either a fairly newly formed fault in the regional stress field, or an existing subvertical fault that happens to be optimally oriented for frictional reactivation.

Deformation of the Top Basement Unconformity west of the Alpine Fault (South Island, New Zealand): seismotectonic implications

Geometry of the Top Basement Unconformity (TBU) west of the Alpine Fault has been reconstructed through a set of cross-sections linking surface and subsurface geology. Onshore, the TBU shows tectonic relief of several kilometres between antiformal pop-ups and synformal depressions in contrast with a smoother topography offshore. This geometry arises from reverse slip on sets of north–south to NNE–SSW faults, mostly dipping 50–66° both west and east, that control folding of the TBU and overlying cover sequence. Some of these faults are inherited Upper Cretaceous–Palaeogene normal faults that displaced the TBU during the extensional phases and were later reactivated as reverse faults under compression, whereas others appear to be newly propagated Neogene reverse faults. The faults that deform the TBU have vertical displacements of 3–5 km and lengths of >150 km, and have the potential of being reactivated in the present stress field. Currently active faults comprise a set of blind reverse faults that propagate upsection from pre-existing extensional fault fabric in the basement, imposing a short-wavelength undulation on the TBU.

Slip partitioning and deformation cycles close to major faults in southern California: Evidence from small‐scale faults

Geometry and kinematics of small-scale faults adjacent to large seismically active faults have been analyzed for a large region of southern California. The sampled population of small-scale faults is scattered in orientation and slip but possesses distinct trends. Two important characteristics are, first, the paucity of small-scale faults that mimic orientations and mechanisms of the adjacent master faults, and, second, the large variation in slip mechanisms of fault planes oriented E-W and N-S. In order to test the similarity of the sampled fault population to present-day deformations, data on geometry and slip of the small-scale faults have been used for stress inversions. These results are compared with those derived from focal mechanisms of low-magnitude earthquakes in the same region. Stress orientations derived from small-scale faults are spatially heterogeneous, but horizontal axes of stress maintain N-S and E-W trends over the entire region. The similarities between stress inversions from small faults and seismicity support the conclusion that most of the sampled faults are recent and formed during cyclical events of fracturing in a relatively strong crust adjacent to weak master faults.

The last 2 Myr of accretionary wedge construction in the central Hikurangi margin (North Island, New Zealand): Insights from structural modeling

Three depth-converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the central Hikurangi margin. Plio-Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E-verging thrust faults that propagated up-section from the plate boundary decollement. Growth stratigraphy of piggy-back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over c. 60 km in the last 2 Myr. Moderate internal shortening of fault-bounded blocks typically 4-8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out-of-sequence reactivation to maintain critical wedge taper. Multi-stage structural restorations show that forward progression of shortening involves: (1) initial development of a c. 10-25 km wide “proto-thrust” zone, comprising conjugate sets of moderately to steeply dipping low-displacement (c. 10-100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto-thrust faults and propagate up-section with progressive break-through of folds localized above the fault tips. The youngest, still unbreached folds deform the present-day seafloor. Progressive retro-deformations show that macroscopic thrust faults and folds account for less than 50% of the margin-perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to meso- and microscopic scale layer-parallel shortening within the wedge, in the proto-thrust zones, and in the outer decollement zone. This article is protected by copyright. All rights reserved.

New insights into the tectonic inversion of North Canterbury and the regional structural context of the 2010–2011 Canterbury earthquake sequence, New Zealand

The 2010–2011 Canterbury earthquake sequence highlighted the existence of previously unknown active faults beneath the North Canterbury plains and Pegasus Bay, South Island, New Zealand. We provide new insights into the geometry and kinematics of ongoing deformation by analyzing marine seismic data to produce new maps of regional faults and cross-sectional reconstructions of deformation history. Active faulting and folding extends up to 30 km offshore, and involves reactivation of sets of Late Cretaceous-Paleogene normal faults under NW-SE tectonic compression. The active faults consist predominantly of NE-SW striking, SE-dipping reverse faults, and less commonly E-W to NW-SE faults suitably oriented for strike-slip reactivation. Additionally, newly developing reverse faults obliquely segment and overprint the inherited basement fabric and impose geometric and kinematic complexities revealed by mapping and reverse displacement profiles of markers. The Quaternary reverse slip rates decrease from 0.1–0.3 mm/yr beneath northern Pegasus Bay to <0.05 mm/yr approaching Banks Peninsula. Fault growth modeling involving trishear fault-propagation folding mechanisms successfully restores an evolutionary sequence of progressive fault inversion, revealing a history of reactivated individual faults. Tectonic inversion and overprinting processes beneath Pegasus Bay are immature and <1.2 ± 0.4 Ma old, with no evidence of systematic spatial migration of deformation. Our marine data analyses give insights into the structural context of the 2010–2011 Canterbury earthquake sequence, while the combined onshore to offshore data provide an excellent illustration of fault growth associated with immature inversion tectonics, in which selective fault reactivation results from compressive stress imposed across a complex network of inherited faults.

The role of protothrusts in frontal accretion and accommodation of plate convergence, Hikurangi subduction margin, New Zealand

Protothrusts mark the onset of deformation at the toe of large subduction accretionary wedges. They are recognized in seismic reflection sections as small-displacement (tens of meters) faults seaward of the primary frontal thrust fault. Although assumed to reflect incipient accretionary deformation and to mark the location of future thrusts, few studies discuss their displacement properties, evolution, and kinematic role during frontal accretion and propagation of the subduction décollement. We analyze high-quality geophysical and bathymetric images of the spectacular 25-km-wide Hikurangi margin protothrust zone (PTZ), the structure of which varies along strike north and south of the colliding Bennett Knoll seamount. We provide a quantitative data set on protothrust scaling relationships and fractal fault population characteristics. Our analyses lead us to speculate on the importance of stratigraphic heterogeneity in structural development, and highlight the role of protothrust arrays in the formation of the frontal thrust. We document a migrating wave of protothrust activity in association with forward advancement of the décollement and deformation front. Shortening east of the present frontal thrust, calculated from displacements on seismically imaged faults and from subseismic faulting derived from power law relationships, reveal the significant role of the PTZ in accommodating shortening. There is possibly as much as ~7.4 km and ~4.0 km of shortening accommodated by the PTZ south and north, respectively, of Bennett Knoll seamount. As much as ~90% of the total shortening may be accommodated at subseismic scale. These data indicate that the active PTZ, together with older accreted PTZs, may accommodate ~10%–50% of the total margin-normal convergence rate at the Hikurangi margin.

Structural and morpho-tectonic evidence of Quaternary faulting within the Moutere Depression, South Island, New Zealand

ABSTRACT The subsurface structure of the Moutere Depression is obscured by a thick Plio-Quaternary gravel sequence. Surface and subsurface data have been used to define a blind basement pop-up bounded to the west by a NNE-striking reverse fault dipping c. 50°E (Ruby Bay-Moutere Fault, RB-MF). The RB-MF is geometrically and kinematically similar to the adjacent Waimea-Flaxmore Fault System (W-FFS) and linked to the Surville Fault offshore and the Tutaki Fault to the south. Reverse-slip activity on the RB-MF started post 21–18 Ma and continued after deposition of the lower Pleistocene gravel sequence, but the fault tip remains buried. Persistent Quaternary activity with progressive development of a hanging wall anticline is inferred from surface topography and deformation of late Quaternary river terraces. The collected evidence suggests that the RB-MF is capable of being reactivated in the present-day stress field, though slip rates are lower than those of the Holocene-active W-FFS.