Flower structures and Riedel shears at a step over zone along the Alpine Fault (New Zealand) inferred from 2‐D and 3‐D GPR images

Abstract

[1] High-resolution GPS and ground-penetrating radar (GPR) data are used to detect and identify hidden faults along a stretch of the transpressional Alpine Fault (South Island, New Zealand) immediately north of its junction with the Hope Fault. At this location, the Alpine Fault emerges from the basement into a sequence of variably thick late Holocene gravel deposits. Geomorphology and trenching already mapped three principal fault strands and two distinct step over zones at the study site. Our GPR images reveal numerous additional secondary fault strands throughout the region, only some of which are obvious at the surface or in the trench walls. According to the GPR data, the main fault-generated disturbance zone has a width ranging from $40 to $200 m. The secondary fault strands outside of the step over zones likely represent the branches of positive flower structures, whereas the faulting pattern around the step over zones is best explained in terms of linked Riedel shears. Systematic northeastward increases in the width of the main fault-generated disturbance zone and corresponding increases in principal fault-scarp height are the likely consequences of older terraces in the northeast being disrupted and offset by more earthquakes than younger terraces in the southwest. The pattern of complex faulting in this region is distinct from the system of alternating strike-slip and reverse faults characteristic of the Alpine Fault to the south and from the rather simple sequence of faults mapped to the north. GPR surveying has added new information on the distribution and nature of faulting at our study site.

Seismic imaging of the Alpine Fault near Inchbonnie, New Zealand

“…Our interpretations of faults on the seismic sections (Figures and ) are tightly constrained by the near‐surface locations and dips of the three principal Alpine Fault strands AF1–AF3 and several secondary fault strands as defined by surface mapping, paleoseismological studies [ Berryman et al ., ; Langridge et al ., ], and 2‐D and 3‐D GPR investigations that extend to depths of 5–18 m (e.g., Figure ) [ Carpentier et al ., ]. The surface positions of all faults so defined are marked by red dots along the tops of the P wave velocity tomograms and seismic reflection sections in Figures and .…”

“…Earthquakes on the Alpine Fault generated offsets in the Taramakau River terraces that are now recognized in the surface topography by scarps (Figure b) with elevation differences of up to 9 m. Left step‐overs in the fault scarps and offsets and disruptions of GPR reflections are evidence for a locally complicated distribution of fault strands and associated strain [ Langridge et al ., ; Carpentier et al ., ]. In a number of trenches, reverse‐faulted Taramakau gravels are juxtaposed against recent Taramakau overbank sands and silts.…”

“…The seismic sections are aligned along the straight line defined by the opposing red arrows in Figure b. Red dots identify fault locations near or at the surface based on surface mapping and paleoseismological and GPR studies [ Langridge et al ., ; Carpentier et al ., ]. Dashed lines delineate poorly constrained geological boundaries and faults.…”

“…Based on the results of these investigations and guided by structural information derived from laboratory analogue model studies [ McClay and Bonora , ] and geological mapping and seismic reflection surveying of inactive and active strike‐slip faults worldwide [ Sylvester , ; Harding , ; Jones et al ., ; Imren et al ., ; Beyhan et al ., ], Carpentier et al . [] interpret shallow parts of the Alpine Fault at Inchbonnie in terms of primary and secondary fault strands that form branches of positive flower structures and multiple oblique Riedel shears within two step‐over zones. In the present study, we use high‐resolution seismic reflection techniques to image the primary Alpine Fault strands and several secondary fault strands to depths well beyond the capabilities of surface mapping, paleoseismological studies, and GPR surveying.…”

Initiation and growth of strike-slip faults within intact metagranitoid (Neves area, eastern Alps, Italy)

“…The sandbox model had a free upper surface, allowing uplift of material at the contractional stepover to compensate for incoming material during slip. The development of pop-up structures, folding, and reverse faulting has been described in many natural contractional stepovers between strike-slip faults (e.g., Zampieri, 2000), and positive fl ower structures and systems of inverse faults are typical of contractional stepovers in near-surface strike-slip faults (e.g., Woodcock and Fischer, 1986;Carpentier et al, 2012). An "out-of-plane" vertical component of slip is thus well documented in contractional stepovers in strike-slip systems (at least when there is a nearby upper free surface), whereas the "in-plane" horizontal component is less well constrained.…”