Susan M. Cashman

Strain Partitioning between structural domains in the forearc of the Hikurangi Subduction Zone, New Zealand

The Pacific plate obliquely converges with the Australian plate at latitude 39°50′S along the Hikurangi margin off the east coast of the North Island of New Zealand. An extensive and youthful subaerially exposed forearc on the east coast of the North Island in the Hawkes Bay area provides the opportunity to document contemporaneous forearc deformation in this obliquely convergent margin setting. Geologic mapping and analysis of strain at both mesoscale and megascale indicates that strain is partitioning into domains of extension, contraction, and strike-slip. The domains are elongate and trend parallel to the margin. Measurements of net shortening and transcurrent slip in the forearc show that the obliquely convergent motion is transferred across the plate interface. Deformation rates calculated for the past 1–2 m.y. for structures in all six forearc domains account for 50–70% of the margin-parallel motion required by Pacific-Australian plate convergence and about 6% of the plate motion perpendicular to the plate boundary. At the surface in the forearc, this obliquely convergent motion is manifest not by transpressional faults but rather by paired structural domains that consist of a strike-slip fault zone and an accompanying contractional fault-and-fold zone on the trenchward side. There are two such transcurrent faulting-and-contraction couplets, one where the backstop daylights at the arcward edge of the forearc and another couplet trenchward of a relatively undisturbed forearc basin. The small amount of shortening, relative to strike-slip, in the onshore part of the forearc suggests that shortening perpendicular to the plate boundary may be concentrated offshore and that most of the component of plate motion perpendicular to the plate boundary may be accommodated by slip along the subduction zone megathrust.

Cataclasis and deformation-band formation in unconsolidated marine terrace sand, Humboldt County, California

Zones of deformation bands occur in unconsolidated late Pleistocene marine terrace sand in the footwall of the active McKinleyville thrust fault in Humboldt County, California. Individual deformation-band shear zones are as much as 8 cm wide and accommodate ∼50 cm of reverse-dip separation. Like deformation bands described in Mesozoic sandstone of the Colorado Plateau, these structures formed in fine-grained, well-sorted, porous sand. We assessed the relative importance of compaction, grain breakage, and grain rotation during shear-zone development by measuring grain size, grain shape, grain orientation, porosity, and bulk strain both within and outside deformation-band shear zones. Sand grains within shear zones are smaller, more compacted, and have stronger preferred orientations and more elongate shapes than grains outside of deformation-band shear zones. Bulk strain analyses of sand within shear zones give strain ellipses that are compatible with dextral shear strains of ∼0.1–0.5 and volume loss of 5%–15%. On the basis of these observations, we conclude that compaction, grain rotation, and extensive cataclasis all contribute to deformation-band shear-zone formation in these unconsolidated sands, despite very low confining pressures. In addition, the position of these deformation-band shear zones adjacent to an active fault with a history of episodic slip during large earthquakes suggests that they may form in conjunction with slip events.

Structural evolution along the inner forearc of the obliquely convergent Hikurangi margin, New Zealand

The accretionary margin of the Hikurangi forearc on the southeast coast of the North Island of New Zealand is part of the leading edge of the Australian plate, which is overriding the obliquely converging Pacific plate. We investigate the last 10 m.y. of deformation history of the innermost (western) quarter of the total width of the forearc through analysis of the sedimentologic and structural evolution of the Eketahuna area on the east coast of the North Island. The Eketahuna area is ideal for such a study because emergence of the margin in the Quaternary has exposed a complete late Neogene rock record. This record has allowed us to chronicle the strain history. From 10 Ma to about 2.5 Ma this forearc region was the locus of subsidence and marine deposition. In the latest Pliocene this part of the margin began to shorten through folding and reverse faulting, bringing an end to basin filling. The period of shortening was brief, and by the late Pleistocene, reverse faulting had ceased and was immediately succeeded by dextral strike-slip faulting, in some cases along the same faults. Presently, the dominant strain regime in the inner quarter of the forearc is strike-slip faulting. This structural history illustrates that, over time, the pattern of strain partitioning has changed in the Hikurangi forearc. The switch from crustal shortening to dextral shear along the major faults in this area in the last 1 m.y. may be a response to more than 10° of clockwise rotation in the southern Hikurangi forearc in Pliocene and Pleistocene time. This rotation is a consequence of the fact that the accretionary margin is undergoing continuous deformation between the obliquely converging Australian and Pacific plates in this area at the southernmost end of the Kermadec-Hikurangi subduction system. The inboard portion of this young accretionary margin is exceptionally well exposed today, probably in part because of the late Neogene subduction of relatively thick, buoyant crust of the Hikurangi-Chatham plateau.

Microstructures developed by coseismic and aseismic faulting in near-surface sediments, San Andreas fault, California

Evaluation of microstructures in unlithified near-surface sediments provides promising results for differentiating between earthquake rupture-related (coseismic) and creep-related (aseismic) structures formed by the San Andreas fault. Paleoseismic trenches excavated at two sites, Flook Ranch on the creeping section, and Alder Creek on the 1906 rupture trace, show contrasting fault-zone structures and microstructures in near-surface, late Holocene sand. At Alder Creek, the 1–2-m-wide fault zone consists of both faults and 2–10-mm-thick deformation bands. Deformation bands have preferred grain orientations ∼30° counterclockwise from the fault (viewed in the slip-parallel direction), broken and disaggregated grains, smaller than average grain size, and lower porosity than control samples. In contrast, at Flook Ranch, two 4–6-m-wide fault zones consist of multiple faults but lack deformation bands. Silty sand in the fault zone at Flook Ranch has preferred grain orientations ∼10° clockwise from the fault, lacks broken grains, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: cataclasis at Alder Creek, and distributed deformation at Flook Ranch. Deformation bands on the 1906 rupture trace of the San Andreas fault at Alder Creek demonstrate that these structures, with their grain rotation, grain breakage, and localized porosity variations, can form coseismically in unlithified sediment. The grain bridge model accounts for fundamental microstructural characteristics of Alder Creek deformation bands, and it provides a connection between these microstructures and laboratory studies of stick-slip instability. Deformation bands are easily recognizable in field and trench exposures and may be a useful indicator of coseismic slip.

Paleoseismology of an active reverse fault in a forearc setting: The Poukawa fault zone, Hikurangi forearc, New Zealand

The Poukawa fault zone, on the North Island of New Zealand within the forearc of the Hikurangi subduction zone, consists of a series of en echelon reverse faults and companion hanging-wall anticlines. The geomorphically expressed length of the fault zone is 34 km. However, on the basis of coseismic deformation associated with an M s 7.8 earthquake in 1931 and the presence of blind faults north of the geomorphically expressed fault zone, it appears that the seismogenic length of the fault zone may be as much as 130 km. On the basis of chronostratigraphic horizons identified in each of three trenches evenly distributed along the exposed fault zone, from which a paleoseismological record for the past ∼25 k.y. can be determined, there is not a characteristic rupture length for earthquakes. Some slip events are confined to the ∼10–20-km-long southern part of the fault zone, whereas other slip events may have ruptured the entire 34 km length of the geomorphically expressed fault zone. At least two slip events that occurred in the northern part of the fault zone did not occur in the southern part of the zone. The largest earthquake recorded in the trenches had a maximum reverse slip in excess of 10 m. We infer that this prehistoric earthquake, similar to the 1931 earthquake, entailed slip on faults along the geomorphically expressed fault zone and on blind faults to the north. This prehistoric earthquake may have had a rupture length (surface plus subsurface) in excess of 100 km. Average earthquake repeat times on the fault zone range from 3–7.5 k.y. for the southern and middle part of the zone to 7–12 k.y. for the northern part of the fault zone. Average single-event slip ranges from 3 m to as much as 6 m. Slip was initially accommodated at the surface primarily by folding. With successive slip events, however, coseismic displacements propagated to the surface and surface deformation became increasingly dominated by reverse slip on fault planes. The Poukawa fault zone is part of a foreland-propagating fold and thrust belt in the forearc of the Hikurangi subduction zone. Older, actively eroding hanging-wall anticlines are present to the west of the fault zone toward the volcanic arc, whereas younger folds are developing above blind reverse faults east of the main fault trace. In addition to propagating to the east, the fault zone is propagating northward beneath the Heretaunga Plains. This active propagation testifies to ongoing and evolving contractional forearc deformation in response to oblique plate convergence.

Deformation band formation and strength evolution in unlithified sand: The role of grain breakage

[1] We report on laboratory experiments designed to investigate the strength evolution and formation mechanisms of cataclastic deformation bands hosted in unlithified sand, with particular focus on the role of grain breakage. Cataclastic deformation bands are characterized by particle size reduction and increased resistance to weathering compared to parent material. We recovered bands intact from late Quaternary, nearshore marine sand in the footwall of the active McKinleyville thrust fault, Humboldt County, California. Tabular samples 3–5 mm thick and 5 cm × 5 cm in area were sheared at normal stresses representative of in situ conditions, 0.5–1.8 MPa, sliding velocities from 10 mm/s to 10 mm/s, and to shear strain up to 20. Cataclastic deformation bands are stronger than parent material (coefficient of internal friction mi = 0.623 and mi = 0.525, respectively) and exhibit a peak strength followed by weakening. Parent material exhibits significant strain hardening; the frictional yield strength increases up to 9% for a shear strain of 10. Detailed particle size analyses show that strain hardening in parent material is coincident with increased fine particle abundance, resulting from pervasive grain breakage. Our results support the hypothesis that cataclastic deformation bands are stronger than the surrounding parent material due to shear‐driven grain breakage during their formation. We suggest that the combination of strain localization during band formation and strain hardening on individual bands results in dense networks of deformation bands.

Active faults, paleoseismology, and historical fault rupture in northern Wairarapa, North Island, New Zealand

Abstract Active faulting in the upper plate of the Hikurangi subduction zone, North Island, New Zealand, represents a significant seismic hazard that is not yet well understood. In northern Wairarapa, the geometry and kinematics of active faults, and the Quaternary and historical surface‐rupture record, have not previously been studied in detail. We present the results of mapping and paleoseismicity studies on faults in the northern Wairarapa region to document the characteristics of active faults and the timing of earthquakes. We focus on evidence for surface rupture in the 1855 Wairarapa (MW 8.2) and 1934 Pahiatua (MW7.4) earthquakes, two of New Zealands largest historical earthquakes. The Dreyers Rock, Alfredton, Saunders Road, Waitawhiti, and Waipukaka Faults form a northeast‐trending, east‐stepping array of faults. Detailed mapping of offset geomorphic features shows the rupture lengths vary from c. 7 to 20 km and single‐event displacements range from 3 to 7 m, suggesting the faults are capable of generating M >7 earthquakes. Trenching results show that two earthquakes have occurred on the Alfredton Fault since c. 2900 cal. BP. The most recent event probably occurred during the 1855 Wairarapa earthquake as slip propagated northward from the Wairarapa Fault and across a 6 km wide step. Waipukaka Fault trenches show that at least three surface‐rupturing earthquakes have occurred since 8290–7880 cal. BP. Analysis of stratigraphic and historical evidence suggests the most recent rupture occurred during the 1934 Pahiatua earthquake. Estimates of slip rates provided by these data suggest that a larger component of strike slip than previously suspected is occurring within the upper plate and that the faults accommodate a significant proportion of the dextral component of oblique subduction. Assessment of seismic hazard is difficult because the known fault scarp lengths appear too short to have accommodated the estimated single‐event displacements. Faults in the region are highly segmented, disconnected, and probably structurally immature, which implies that apparent geometric discontinuities at the surface may not be significant barriers to rupture propagation at depth and that the surface rupture record significantly under‐represents the seismic slip on faults in the region.

Devonian metamorphic event in the northeastern Klamath Mountains, California

Shared characteristics including parental rock types, lithologic associations, isoclinal folding accompanied by greenschist to lower amphibolite facies metamorphism, parallel structural trends, and Devonian metamorphic age support a correlation between metamorphic rock units previously assigned to the Duzel Formation (“Eastern Klamath belt”) and Grouse Ridge Formation (Central Metamorphic belt) in northern California. Contact relations and mineral assemblages suggest that Devonian metamorphism and deformation occurred during juxtaposition of the Central Metamorphic belt and ultramafic rocks from the Trinity mafic-ultramafic complex, perhaps during eastward subduction. Ages and contact relations of two unmetamorphosed Eastern Klamath belt units (the Gazelle Formation and the Moffett Creek Formation) suggest that these units were faulted into their present positions subsequent to the Devonian metamorphic event.

Finite-strain patterns of Nevadan deformation, western Klamath Mountains, California

Finite-strain analyses of Galice Formation metagraywackes from the footwall block of the Orleans fault, western Klamath Mountains, California, show a dramatic increase in strain ratios structurally upward, approaching the Orleans fault. R xz strain values range from less than 3 at a distance of 2 km from the fault to 6.5 adjacent to the fault. Strain-ellipse orientations are relatively consistent; X-directions are commonly northwest-southeast and subhorizontal and trend subparallel to the trace of the Orleans fault. Finite-strain ellipse orientations and the clockwise rotational strain recorded by microscopic kinematic indicators are both compatible with the interpretation of dextral oblique convergence on the Orleans fault during the Nevadan orogeny.

Deformational history and regional tectonic significance of the Redwood Creek schist, northwestern California

Structural, petrologic, and geochemical data support the correlation of the Redwood Creek schist of northwesternmost California with the South Fork Mountain Schist of northern California and the Colebrooke Schist of southwestern Oregon. Metamorphism to lawsonite-albite-chlorite facies and development of secondary structures recording two penetrative deformations are shared by all three units. A later deformational event recorded by the Redwood Creek schist, but absent in the South Fork Mountain Schist, consists of differential rotation of domains several kilometres in size within the schist belt. This deformation is significant, because it records an event that occurred after the separation of the schist bodies. The rotation most probably occurred in conjunction with northward strike-slip movement of the Redwood Creek schist.