Brittle Deformation Adjacent to the Awatere Strike-Slip Fault in New Zealand: Faulting Patterns, Scaling Relationships, and Displacement Partitioning

Home , Awatere Fault

Brittle deformation adjacent to the Awatere strike-slip fault in New Zealand: Faulting patterns, scaling relationships, and displacement partitioning

Timothy A. Little Department of Geology, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand

ABSTRACT is a poor index of slip sense on the mature (Chester et al., 1993; Cowie and Scholz, fault. Close to the Awatere fault, dextral- 1992). Brittle fabrics in fault zones also may On the coast of South Island, New Zea- reverse faults are synthetic to that mature contain a record of early displacement inland, the active dextral-reverse Awatere structure. Their disposition indicates incre- crements along faults with complex slip his- fault has <2 km finite dextral slip and in- mental compression acting Ϸ45؇–50؇ clock- tories. Recent papers have cited aspects of itiated <4 Ma. Fault spacing along Ϸ2km wise to the strike of the Awatere fault. These the fractal nature of fault populations to of coastal sea cliffs decreases from >22 m to youngest faults mirror the slip kinematics constrain models on fault growth and fault- <0.5 m as one approaches the fault from the of the adjacent mature fault. Their attitude ing-related strain, but field data on scaling north. The pattern of fault attitudes and is controlled apparently by local kinematics, relationships for natural faults are sparse slip directions is independent of scale. A not by far-field stress orientations. (Cowie and Scholz, 1993; Gillespie et al., power-law relationship between slip and cu- 1993; Scholz et al., 1993). mulative frequency of faults has an expo- INTRODUCTION The active Marlborough fault system in nent of Ϸ0.8 for this linear outcrop sample. the northeastern part of the South Island of Near the Awatere fault, faulting-related Displacement along major faults is not New Zealand splays northeastward from the strains are large, reflecting partitioning of confined to a single plane, but is distributed Alpine fault to comprise an Ϸ100-km-wide slip into late-stage faults near the core of into the adjacent rock volume by arrays of zone of distributed deformation between the active fault; farther away, they are very subsidiary faults (e.g., Tchalenko, 1970; the Pacific and Australian plates (Fig. 1). small, where most of the small-scale fault- Wojtal, 1986). Structures bordering faults Pliocene–Holocene motion of the Pacific ing is relict. Strain softening led to local- preserve a record of deformation mecha- plate relative to the Australian plate trends ization of slip into (and near) the fault’s nisms and stress distributions and may pro- nearly west (DeMets et al., 1990), resulting core and was assisted by suprahydrostatic vide insight into the fault growth process in transpression across the Marlborough pore-fluid pressure gradients, but evidence for fault-orthogonal compression and extreme fault zone weakness is lacking. Two styles of gouge on faults both have linear scaling ratios relating gouge thickness to finite slip. Different ratios for each may indicate coseismic and aseismic gouge gener- ation processes acting at crustal depths of 1–2 km. Early tensile cracking of pebbles is interpreted to have occurred in the process zone of the Awatere fault as it propagated upward as a mode III crack. If so, the fault has a process zone width/finite slip ratio of >1.5, consistent with an elastic-plastic model of fault growth. Younger domino- style oblique-normal faults resulted in oblique extension of the Awatere fault zone, with mean ␴1 oriented Ϸ30؇ clockwise from its strike. Transtension may have been caused by divergence between rotating, crustal-scale fault blocks, the result of a Figure 1. Simplified tectonic map of the New Zealand region showing relative motion of mode II fault tip to the east, which caused a Pacific plate relative to Australian plate, the actively deforming plate boundary zone (stip- local rotation of stress trajectories. The pled), the Alpine fault, and the Marlborough faults. Adapted from Walcott (1989) and early faults are relict, and their kinematics DeMets et al. (1990).

GSA Bulletin; November 1995; v. 107; no. 11; p. 1255–1271; 16 ﬁgures.

1255

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

biozones with a detailed magnetostratig- raphy established for the Awatere basin (Roberts et al., 1994) indicates that the rocks at White Bluffs were deposited ca. 7–4 Ma. On both sides of the fault, strata dip on average Ϸ10Њ to the northeast (Figs. 3a and 3b) and are at least 600 m thick. Upright folds with gently northeast plunging hinges warp the Neogene sequence. Beds dip on average Ϸ10Њ to the southeast or south, but are variably tilted in detail (Fig. 3c). Detailed mapping near the coast indicates that the Awatere fault strikes 063, dips 70Њ–90Њ to the northwest, and defines a gentle releasing bend with respect to the mean 055 strike of more inland parts of the fault (Fig. 2). A series of stream chan- nels near the coast are offset dextrally Figure 2. Tectonic map of the Marlborough faults, South Island, New Zealand. For 150–200 m by the fault. South of the active location, see Figure 1. Active fault traces and estimates of fault slip rates (mm/yr, in part of the Awatere fault, an inactive splay parentheses) compiled from Van Dissen and Yeats (1991) and Kneupfer (1992). Direction of that fault strikes slightly clockwise from of maximum incremental horizontal shortening (PHS) from geodetic surveys shown with the active trace (Lensen, 1963). Its trace is ball-and-bar symbol and taken from Bibby (1976, 1981), Lamb and Bibby (1989), and defined topographically by a range of hills Walcott (1984); PHS inferred from earthquake focal mechanism solutions (double arrows), on its northern side, but is concealed be- from Arabasz and Robinson (1976), and Anderson et al. (1993). Velocity of Pacific plate neath a flight of Holocene alluvial terraces with respect to Australian plate near Figure 3 calculated from Nuvel-1 pole of DeMets et (Eden, 1983). .Blarich Stream ؍ Nina Brook and bl ؍ al. (1992). Abbreviations nb Over 260 m high, the White Bluffs provide superb exposure of faults and other brittle faults, which have undergone dextral-reverse from the active Awatere fault. This data has structures on the north side of the active slip during late Neogene time. Pebble rind implications for (1) displacement-gouge trace of the Awatere fault. The distribution dating of offset terraces at two sites along thickness and displacement-frequency scal- and orientation of faults and fault rocks the Awatere fault (Fig. 2) indicates Holo- ing relationships in natural fault popula- were recorded in a compass-pace/detailed cene dextral-slip rates of Ϸ4–7 mm/yr and tions, (2) the nature and width of zones of mapping traverse along the north- to north- horizontal/vertical displacement ratios Ͼ10 small-scale faulting bordering major upper west-trending coastal bluffs. These comprise (Kneupfer, 1992). In the upper Awatere crustal strike-slip faults and variation in time an essentially linear sample of the fault pop- Valley, Silberling et al. (1988) inferred a fi- of the directions of principal stress/incre- ulation at high angle to the strike of the nite dextral displacement of Ϸ16 km for the mental strain within such zones, (3) the Awatere fault. Data were projected (by Awatere fault from offset of a near-vertical kinematics of faulting in zones of oblique computer) onto a transect normal to the melange unit within basement rocks of the extension, and (4) models for brittle defor- northeast strike of the Awatere fault. A cu- Mesozoic Torlesse terrane. Farther north- mation and fault-zone evolution associated mulative width of 250 m was covered by east in the lower Awatere Valley, Little with the rupture and finite growth of large slumps or colluvium, and 750 m occurred (1994) inferred Ͼ32 km of dextral slip from faults. within a reach of inaccessible cliffs. In the the minimum offset of a marker line defined remaining 2 km of composite exposure, 307 by the intersection of bedding and a facies WHITE BLUFFS EXPOSURE OF THE faults were observed and measured in detail. boundary in a late Miocene (ca. 6–8 Ma) AWATERE FAULT ZONE Figure 4 summarizes the types of observa- marine basin transected by the fault (Fig. 2, tions made. A computer program calculated stipple pattern). Near the coast, a sequence of late Mio- displacements for each fault, based on offset The study region for this paper is located cene–early Pliocene marine conglomerate of bedding on outcrops and slip-lineation along the active trace of the Awatere fault and mudstone occurs on both sides of the pitch (Little, in press). on the northeast coast of the South Island Awatere fault (Fig. 3). Planktonic and (Fig. 3). North of the fault, Neogene rocks benthic foraminiferal assemblages in 20 INCEPTION AGE OF AWATERE are superbly exposed in sea cliffs, allowing samples from both sides of the fault are FAULT SEGMENTS detailed cross-sectional study of Ͼ300 small- late Tongaporutuan to possibly earliest er-scale faults. In this paper, I describe the Kapitean (New Zealand stages, late Mio- Inception of the Awatere fault is inferred pattern, distribution, and sequential devel- cene) and, at the highest preserved levels, to postdate the 6.5–8 Ma age (Roberts et al., opment of tensile fractures and faults vary- early Pliocene. The assemblages are char- 1994) of the late Tongaporutuan coastal sec- ing in scale over several orders of magnitude acteristic of outer shelf to upper bathyal tion, as these marine rocks were deposited and quantify faulting-related displacement paleodepths (P. Vella, 1995, personal across the present fault and contain the off- gradients as a function of distance away commun.). Correlation of foraminiferal set facies boundary, which strikes at a high

1256 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

side-up sense of throw, and the unconformity dips gently northeast, this offset is a maximum estimate of ﬁnite dextral slip on the active coastal segment. For a mean slip rate of Ϸ5 mm/yr, such a displacement would take Ͻ500 k.y. to accumulate. Occur- rence of similar marine facies of foraminifera-bearing early Pliocene rocks (P. Vella, 1995, personal commun.) on both sides of the Awatere fault at the coast suggest that the presently active segment did not exist, at least as a major bathymetric feature, until less than ca. 4 Ma. These relationships are important because they imply that White Bluffs underwent brittle deformation at about the same time as the adjacent coastal segment of the Awatere fault was initiated.

BRITTLE DEFORMATION STYLES AND FAULT ROCKS

The density of faulting and style of fault rocks change with proximity to the Awatere fault. Inland from the coast (at location ‘‘bl,’’ Fig. 2), a bedrock exposure of the Awa- tere fault uplifts Torlesse terrane metabasalt on the northwest against fractured Pliocene mudstone. The fault is marked by a 5-cm- wide band of black gouge bordered to the northwest by 30 cm of white ultracataclasite interleaved with gouge seams. This core zone is bordered to the north by Ϸ20mofme- tabasaltic protocataclasite-cataclasite (us- Figure 3. Geologic map of northeastern Awatere Valley showing regional structure and age of Sibson, 1977). The cataclasites have a active and inactive traces of Awatere fault. Conglomeratic rocks are shown with stipple fault-parallel foliation defined by preferred pattern; mudstone-sandstone units with line pattern, and Torlesse terrane basement in alignment of porphyroclasts. Ultracatacla- gray. Star pattern is trace of late Miocene–early Pliocene unconformity defined by foram- site seams, 2–40 mm wide, surround blocks iniferal assemblages and mapping. White Bluffs coastal transect indicated by double ar- of microbrecciated host rock and are cut by rows and thick gray line along shore. Bedding of Pliocene rocks exposed along Awatere zeolite veins. Farther east (location ‘‘nb,’’ River bank from Russel (1959). Equal-area stereograms of poles to bedding of late Mio- Fig. 2), the fault cuts Holocene gravels and cene–early Pliocene sequence. (a) Bedding poles for inland region north of the Awatere is a simple contact marked by Ϸ5cmof fault. Here, beds immediately above the basal unconformity (open circles) dip north locally gouge. -at up to 57؇. Up-section, dips are more gentle, suggestive of a stratal fanning relationship At the coast, the trace of the active Awa caused by syntectonic deposition. (b) Bedding poles from south of the Awatere fault. tere fault is concealed. North of a 9-m-wide (c) Bedding poles from White Bluffs (north of the Awatere fault). landslide covering the fault, the next 53 m of shoreline exposes densely faulted, friable mudstone, labeled as ‘‘damage zone’’ in Fig- angle to that structure. In the central Awa- ied, inactive strand across which the late ure 5. There, faults are spaced at 10–30 cm tere Valley, post–late Miocene alluvial fan Miocene facies boundary has been offset. and bound slickensided chips and slices of deposits on the south side of the fault (Max- Hunt (1969) noted the absence of a gravity mudstone. Dismembered sandstone beds well, 1990) were by derived in part by ero- field gradient across the active strand, con- strike parallel to the fault and are steepened sion of a narrow metabasaltic unit that today cluding that it had only a minor finite sep- to a dip of 50ЊSE. Northwest of the damage borders the northern side of the Awatere aration, in contrast to the inactive one to the zone, faults in the mudstone-sandstone se- fault scarp, suggesting that uplift along the south, which coincides with a well-defined quences of zones 1 and 2 (Figs. 3 and 5) have fault had begun by that time (see also Lamb gradient. The active strand cuts and offsets a an average spacing of Ϸ0.6 m and are locally and Bibby, 1989). slightangularunconformitybetweenlateMio- up to 3 m wide. Thin sandstone markers are The active strand of the fault near White cene and early Pliocene rocks with a dextral repeatedly cut and offset in profile view. Cal- Bluffs has a much smaller dextral separation strike separation of Ϸ2 km (Fig. 3, star pat- culated displacements are Ͻ1–40 m (Little, and is inferred to be younger than the bur- tern). Because the fault has a northwest- in press). Farther north, beyond a reach of

Geological Society of America Bulletin, November 1995 1257

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

MPa and temperatures of Ϸ25–50 ЊCat their time of maximum burial (using a modern temperature-depth profile for Marlbor- ough from Pandey, 1981). Fault-rock deformation mechanisms include intergranular slip, fracture, and cata- clasis. Most faults in both mudstone and conglomerate are smooth surfaces barren of mineralized coatings, and effects of veining or pressure solution are not obvious at outcrop scale. The faults were probably active above the Ϸ3 km upper cutoff in seismicity attributed to a downward change from velocity strengthening to weakening behavior in gouge (Sibson, 1986; Marone and Scholz, 1988), and thus may have been largely aseismic. Offset of mudstone concretions indicates that faulting postdated diagenesis. Figure 4. Diagram of faulted outcrop. Types of structural measurements made on White Slickenlines are defined by fine (Ͻ1 mm) Bluffs transect shown in italic lettering. These include fault plane attitude, outcrop attitude, corrugations in mudstone and asperity slip-lineation pitch, and bedding attitude and offset, from which a computer program grooves in conglomerate. Although they lo- algebraically and trigonometrically calculated the net-slip vector for each fault. cally truncate clasts, most faults avoid pebbles (which are unpitted) to follow the inaccessible cliffs, zone 3 consists of clast- White Bluffs (Hunt, 1969). In the Wairau coarse sandy matrix between them. Inter- supported units of pebble-cobble conglom- basin, offshore and to the northeast of White granular slip (and dilation) between detrital erate interbedded with lenses of finer- Bluffs (Fig. 2), industry seismic reflection grains was thus an important deformation grained rocks. There, faults typically have a data (Carter et al., 1988; Uruski, 1992) re- mechanism. Within 20 cm of meter-scale spacing of Ϸ22 m and displacements of veals Ͼ2.5 km (1.6 s two-way travel time) of fault planes, pebbles are commonly offset by Ϸ0.5–3 m. late Miocene(?)–Holocene rocks, which small, clast-scale faults. Rocks at White Bluffs are inferred to have contrast with the Ͻ600 m section preserved Fault surfaces are coated with two styles been exhumed by Ϸ1–2 km of post–early at White Bluffs. Evidence for 1–2 km of dif- of gouge. Tacky, white or light-gray gouge is Pliocene uplift and erosion. South of the ferential burial suggest that, prior to being most common in all wall-rock types and is Awatere fault, gravity data suggests the uplifted and eroded in the hanging wall of massive in texture. Friable black gouge oc- presence of Ϸ1.8 km of late Miocene–Hol- the Awatere fault, the White Bluffs section curs in seams up to 1 cm thick along larger ocene strata not present across the fault at was subjected to confining pressures of Ͻ50 faults, and in some fault zones occurs in-

Figure 5. Cumulative number of faults observed in linear transect plotted against distance normal to Awatere fault, showing brittle deformation zones outlined in text. Total length of unexposed rock for each zone shown with ruled pattern at right of each zone. Con- glomeratic rocks indicated with stippled pattern. Zones 1, 2, and 3 are differentiated on the basis of density and style of faulting (see text).

1258 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

Figure 6. (A) Photograph of sawn slab of pebbly clay-silt occurring as dike in fault. Scale bar is 1 cm. Note angular, fractured pebble fragments scattered in silt-clay matrix. Inset (at same scale) shows in situ tensile fracturing of pebble fragment previ- ously entrained into matrix. Note termination of central crack at clast boundary. (B) Photograph of conglomerate pebble split by central swarm of tensile fractures. Pencil is 15 cm long. (C) Tensile fractures in conglomerate. Note transgranular nature of some fractures. Pencil is 15 cm long. C

terlaced with thicker zones of light-gray TENSILE FRACTURING cracks are spaced across clast centers, not gouge. Black gouge is typically finely lam- localized at point contacts; (3) some cracks inated (foliated), with a well-developed Tensile fractures have two occurrences: are transgranular (Fig. 6C); and (4) rare composite (s/c) planar fabric (e.g., Chester cracks in conglomerate pebbles and joints in boulders up to 1.5 m in diameter are split by and Logan, 1987), which Sibson (1989) at- mudstone. Cracks in pebbles occur as planar cracks orders of magnitude longer than the tributes to slow aseismic shearing. Some mode I (dilatant) fractures that are barren grain size of the surrounding matrix. Pebbles faults cutting conglomerate are filled by of mineral coatings (thin quartz films are and matrix consist uniformly of fine- to me- pebbly mud rock in tabular sheets, 3–10 rare). The cracks split well-rounded, fine- to dium-grained quartzofeldspathic sandstone. cm thick. Angular pebble fragments in medium-grained graywacke sandstone clasts, This homogeneity counters the pebble fracture model of Eidelman and Reches (1992), these sheets are scattered in a distinctly 3–50 cm in diameter. Commonly, two to four cracks define a swarm in clast centers which calls upon contrasts in shear modulus finer-grained matrix of light-gray clay-silt that accomplish a longitudinal extension of (in their example, 100:1) and Poisson’s ratio (Fig. 6A). I interpret the sheets as forming 2%–4% along the clast (Fig. 6B). The num- (up to 1:2) between pebbles and matrix to by implosion of wall-rock pebble frag- ber of cracks is independent of clast size, but generate locally high elastic tensile stresses ments into fluidized slurries of mud and/or their width increases with clast diameter inside pebbles to cause them to fracture. gouge that were injected as dikes along from 1–2 mm in pebbles to 15 mm in rare Tensile-fractured pebbles are widely but fault planes. Later tensile fracturing of meter-scale boulders. heterogeneously distributed in conglomer- consolidated dikes resulted in further in Tensile cracks in pebbles probably ate within 3 km of Awatere fault. Fractured situ fragmentation of ‘‘floating’’ pebbles formed approximately orthogonal to the pebbles are most common in more tightly (Fig. 6A, inset). The fractures end at grain mean direction of least principal stress (␴3). packed, indurated conglomerate. Tensile boundaries, indicating that stress concen- This is based on the following observations: pebble cracks predate adjacent meter-scale trations at crack tips were blunted by duc- (1) the cracks have a mean attitude that is faults, as they are commonly reactivated by tile deformation in the surrounding clay- regionally consistent on both sides of the shear that is synthetic to the adjacent faults. rich matrix. Awatere fault (Figs. 7a and 7b); (2) most Where pebble cracks are not adjacent to

Geological Society of America Bulletin, November 1995 1259

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

Figure 7. Lower-hemisphere equal-area stereographic projections of poles to tensile fractures in late Miocene–early Pliocene rocks adjacent to the Awatere fault. (a) Tensile pebble fractures in conglomerate on north side of Awatere fault at White Bluffs; (b) tensile pebble fractures in conglomerate on south side of Awatere fault; (c) meter-scale orthogonal joints in bedded mudstone at White Bluffs (north side of fault).

faults, which is most common, their sense of fault surface striae or from the net slip of bers of faults cutting mudstone and con- shear reactivation is consistently sinistral. adjoining displaced pebbles (Fig. 4). Where glomerate. These different scaling ratios

Thus, either ␴1 rotated counterclockwise no slip-lineation was measured, one was suggest two different mechanical processes with respect to ‘‘fixed’’ rock, or the fractures later assigned to the fault based on relation- at work (see discussion). rotated clockwise with respect to a ‘‘fixed’’ ships between slip-vector pitch, fault strike, The above rates of gouge thickening were direction of ␴1, or both. The cracks probably and slip sense observed in the subset of used to infer displacements on faults across did not form as ‘‘wing cracks’’ growing in the faults that did have slip lineations (see be- which a bedding offset could not be ob- local stress field near the propagating tips of low). Length and sense of bedding offset was served (chiefly faults with Ͼ3 m of dip sep- adjacent meter-scale faults, because crack observed on 90% of the faults, and a gouge aration, including the 14 largest faults, or orientation is insensitive to the attitude and thickness (defined by edge of undeformed faults cutting massive conglomerate units). slip sense of adjacent faults, even where rock under a hand lens) on most. Gouge Figure 9 is a log-log plot of fault displace- such structures occur in proximity to the thicknesses on faults for which net slips have ment versus cumulative frequency of faults pebble fractures (e.g., Hancock, 1985; Pol- been calculated (Little, in press) from stratal of equal or greater displacement. The lard and Segall, 1987). offsets and slip lineations (Fig. 4) are pre- Mudstone beds are cut by bedding-or- curved ends to the distribution can be at- thogonal joints. These are decorated with sented in Figure 8. The marked difference in tributed, at least in part, to sampling trun- plumrose-hackle structures and terminate gouge thickness/displacement ratio between cation of the largest and smallest faults, against sandstone or conglomerate beds black gouge (1:Ϸ1030) and gray gouge (1: whereas the central linear segment suggests (see Fig. 10A below). Joint spacing averages Ϸ54) is not related to wall-rock lithology, as a power-law scaling relationship over two 10–20 cm in meter-thick beds, increasing data was collected from nearly equal num- orders of magnitude. with bed thickness. At White Bluffs, most joints are east-striking and subvertical (Fig. 7c), oblique to the southeast-striking pebble fractures. This jointing at least in part predated initiation of the earliest faults at White Bluffs (see below). Figure 8. Graphs of fault displace- DISTRIBUTED FAULTING ment calculated from outcrop observations of stratal offset, bedding dip, Scaling Relationships fault dip, and pitch of slip-lineation, plotted on a linear scale against Well exposed in profile, faults at White gouge thickness. Squares, data from Bluffs range in displacement magnitude faults having only massive gray over three orders of magnitude. Most tra- gouge; circles, data from faults hav- verse the visible width of the local exposure ing only laminated black gouge. (3–100 m) or intersect other faults. Of the 307 faults observed on the transect, 56 (18%) yielded slip lineations, either from

1260 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

drag folding and accommodation structures at fault jogs and tilt domain boundaries. Drag folds, Ͻ30 cm wide, have a normal Figure 9. Log-log plot of fault sense of flexure (Fig. 10A, locations 5–7). displacement (in cm) against Fault blocks are gently tilted about the strike cumulative frequency of faults of bedding because of domino-style faulting of equal or greater displace- (see below). In some cases, differential ment, as observed on the linear rotation between adjacent tilt domains is transect at White Bluffs. See accommodated by deformation of wedge- text for further discussion. shaped zones of breccia (Fig. 10A, location 9), in others by gentle warping (Fig. 10B, location 2). Other tilt domain boundaries are simply steep strike-slip faults that juxtapose variably dipping blocks Fault Arrays and Intrablock Deformation Many faults are curviplanar, flattening up- (Fig. 10B, locations 3, 4). ward (Fig. 10A, location 4), and some Most faults are steeply dipping and have steepen sharply as they refract from mud- branching traces in profile (Fig. 10A). Faults stone into sandstone (Fig. 10B, location 1). Fault Kinematic Pattern bifurcate upward, but splays that diverge ‘‘Horsetail’’ splays occur near fault tips. down-dip and fault-bounded lenses are also Faults are densely and uniformly spaced The basic kinematic pattern of faulting is common (Fig. 10A, locations 1–3). Splays in the mudstone-sandstone sequence of remarkably consistent geographically and commonly branch at 20Њ–40Њ in serial fash- zones 1 and 2. In zone 3, clusters of several across the sampled range of displacements. ion from a less steeply dipping master seg- faults tend to alternate with blocks of intact Most faults strike approximately west and ment. Most merge with (or are cut by) other conglomerate, 40–100 m wide. dip steeply to the north or are subvertical. faults at both ends to form connected arrays. Internal deformation of blocks includes Some strike northwest or southwest (Fig. 11a).

Figure 10. Line drawings of fault arrays and selected joints from outcrop photomosaic. These proﬁles are from zone 2, but the geometries and cross-cutting relationships are representative of White Bluffs as a whole. Numbers refer to locations cited in text. ‘‘S’’ and ‘‘D’’ refer to oblique-slip faults, with sinistral and dextral strike-slip components, respectively. Late-stage dextral-slip faults are represented by faults 8 and 9 in (A), and faults 3, 4, and 5 in (B).

Geological Society of America Bulletin, November 1995 1261

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

Figure 11. Summary of kinematic data for faults with trace lengths >1 m cutting Neogene sequence near White Bluffs. Class interval is 2؇ for all histograms, except (c), where the interval is 5؇. (a) Histogram showing strike distribution of 370 faults, where strike direction .(is deﬁned by right-hand rule. Mean strike is 268؇ ؎ 9؇ (␣95). Strike of Awatere fault and selected modes in the data are labeled (see text 45؇ ؎ 6؇). (d) Strike direction of faults for ؍ b) Dip angle of 370 faults (mean is 65؇ ؎ 2؇). (c) Slip-lineation pitch of 54 faults (mean) which a sense of strike slip was observed. Includes data from 54 faults and 55 faults with trace lengths <1 m (mostly shear-fractured pebbles). Dextral faults shown in black, sinistral faults in stipple pattern.

The average fault dip is 65Њ (Fig. 11b), but cumulative displacement. Dextral faults Pebble-scale faults generally terminate in this value has been reduced by back-tilting strike east-northeast to northeast, in part the sandy matrix surrounding a clast and of fault blocks (e.g., Fig. 10B). Most slip lin- subparallel to the Awatere fault, whereas have displacements of Ͻ15 mm. eations have moderate plunge and pitch sinistral faults strike west-northwest to (Fig. 11c), a relationship that is implied northwest (Fig. 11d). In proﬁle, arrays of dex- Early, Dominant Fault Set also by the consistency of dip-separation tral- and sinistral-oblique faults are inter- sense across arrays of subparallel faults spersed in apparently random fashion About 80% of the observed faults are (Fig. 10). In proﬁle view, oblique slip is (Fig. 10, ‘‘S’’ and ‘‘D’’ symbols). oblique-normal faults referred to here as the revealed by variable dip separation of This basic kinematic pattern is apparently dominant fault set, most of which are mod- markers on the outcrop face (Fig. 10A, lo- independent of scale, as attitude data (not erately to steeply north-dipping (Fig. 10). cation 4). Of the 307 faults observed, 80% shown here) for 62 shear-fractured pebbles South of the White Bluffs anticline, bedding have a normal component of dip slip, an with lengths Ͻ20 cm mirror that for the dips are variable to the south (Fig. 3c). Bed- unexpected result given the transpressive longer faults shown in Figure 11. The net- ding-fault angles average Ϸ75Њ and do not nature of the late Quaternary Awatere slip pitch for pebble-scale faults is generally correlate with bedding dip (Fig. 12a). Rota- fault. Dextral faults outnumber sinistral oblique, resembling the pitch of striations tion of bedding about its strike to horizontal faults 2:1, contributing Ϸ75% of the total on throughgoing faults of similar strike. improves and steepens the clustering of

1262 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

Figure 12. Bedding-fault geometric data for early and late faults near White Bluffs. Data on left side (a, b, and c) refer to dominant, early fault set, and on right side (d and e) to late fault set. (a) Graph of bedding dip plotted against bedding-fault angle for dominant fault set. (b) Lower hemisphere equal-area projection of poles to faults in dominant fault set. Large arrow is inferred direction of maximum incremental shortening (PHS). (c) Poles to dominant faults restored to their inferred original attitude by rotating local bedding back to horizontal. Large open circle is pole to Awatere fault. (d) Graph of bedding dip plotted against bedding-fault angle for late fault set. (e) Lower hemisphere equal area projection of poles to faults in late fault set. Stipple pattern, orientation sector for sinistral faults (labeled R؅); diagonal pattern, orientation sector of dextral faults (range from R to P). Large arrow is inferred direction of maximum incremental shortening (PHS). Small open circles represent attitude of thrust fault swarm observed in zone 1. See text for further explanation.

fault poles (cf. Figs. 12b and 12c). These re- fault strikes deviate from either of these two ogy. Mean slip and spacing on faults are lationships imply differential tilting of fault azimuths, slip-lineation pitches progres- greater in conglomerate than in mudstone. blocks about axes subparallel to the mean sively increase, reaching Ϸ90Њ for strikes of fault strike. Early faults commonly cut joint 090–105. This empirical relationship was Late Fault Set planes or reactivate them (Fig. 10A, location used to assign slip-vectors to faults without 10). Near-90Њ bedding-fault angles are thus a measured slip-lineation (Little, in press). Faults could be assigned to this set where interpreted as fault-reactivated joints. A few Normal-slip faults (pitch ϾϮ80Њ) comprise they cut and offset older faults. Most (81%) faults have been tilted to dips as low as 20Њ 14% of the early fault set. Oblique reverse- are north- to northwest-dipping to subver- (Fig. 10B). slip faults make up Ϸ12% of the faults as- tical dextral-reverse faults (e.g., Figs. 10A, The early fault set is a gradational mixture signed to the dominant set. Some of these location 9; 10B, location 5). Fault attitudes of dextral and sinistral oblique-normal faults, are probably late-stage faults for which a in part overlap with the early set, suggesting in which dextral faults outnumber sinistral cross-cutting relationship was not observed reactivation of older structures (Fig. 12e). faults, 5:4. Faults striking at 240Њ–250Њ, sub- in outcrop (see below). Whereas the mean strike of early faults is parallel to the Awatere fault, are dextral-slip Faults of this early set are distributed east-west, 30Њ counterclockwise from the faults. These deﬁne an apparent mode punc- across all three zones at White Bluffs but are Awatere fault, the mean strike of the late set tuating a bell-curve–like frequency distribu- most densely spaced in zone 1 adjacent to is northeast, subparallel to it. The late set is tion of fault strikes (Figs. 11a, 11d). Faults the Awatere fault. Their mean displacement a mixture of (1) faults subparallel to the striking at 300Њ–310Њ are sinistral-slip. As is 0.5–1 m, but this is dependent on lithol- Awatere fault, (2) apparent Riedel (R)

Geological Society of America Bulletin, November 1995 1263

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

Late-stage faults typically have large displacements (Ͼ1 m). Damage zones, 20–75 cm wide, consist of densely fractured mudstone laced with gouge and may contain lenses of intact rock (Fig. 10B, location 5). Confined to zones 1 and 2 near the Awatere fault, late faults constitute only Ϸ20% of the faults at White Bluffs but account for Ϸ63% of the total cumulative displacement. Late faults in zone 1 are spaced 0.5–3 m apart; in zone 2, 2–10 m apart. The 11 largest late- stage faults have a mean displacement of 17 m and a maximum of 40 m (inferred from gouge thickness); the rest have an average displacement of Ϸ2 m. Thus late faults postdated tilting, had relatively large dextral-reverse displacements, and were localized in a narrow zone adjacent to the Awatere fault. Significantly, the best-fit great circle through poles to the late faults (excluding the thrusts) intersects the Awatere fault at its mean late Quaternary slip vector, and the bisector between sinistral and dextral faults implies an incremental compression acting at Ϸ45Њ to the Awatere fault (Fig. 12e).

PRINCIPAL STRESS ORIENTATIONS DURING EARLY FAULTING EVENT

The consistent disposition of fault attitudes and slip directions for each of the two sets of faults across the entire exposure suggests that the orientations of the principal deviatoric stresses acting on these faults were approximately uniform across the Figure 13. Results of stress inversion using program of Gephart (1990b) on 52 fault- width of the transect. This conclusion was plane/slip-direction/slip-sense observations along the White Bluffs transect. The program tested using the stress inversion technique of applies a grid search over the four model parameters (three stress directions and the stress Gephart (1990a, 1990b). All techniques for inverting for the deviatoric stress tensor -␴2–␴1)/(␴3–␴1), adjusting each systematically through a wide range of pos) ؍ ratio, R sibilities and assessing the misfit of each fault-slip datum relative to each model by de- from fault-slip data are limited by assump- termining the smallest rotation necessary to bring the observed fault plane and slip di- tions that the stress tensor acting on each rection into coincidence with the model. These misfits of the data are summed to arrive at fault was uniform and that slip was parallel a measure of that model’s fitness, from which the best of the tested models may be selected. to the resolved shear traction on each fault. (A) Raw fault-slip data showing slip-vector of hanging wall with respect to ‘‘fixed’’ footwall. These, in turn, imply coaxial deformation, (B) Distribution of acceptable stress models using the approximate minimum pole rotation small fault-slip magnitudes relative to fault spacing, and lack of fault rotations resulting method: stereonet plot of ␴1 and ␴3 directions (left) and histogram of R values (right). Average misfit values and confidence limits are shown in box at upper right, with the 0% from variable tilting or internal deformation confidence limit being the best-fit model. (C) Dimensionless Mohr circle diagram of data of blocks. Thus solutions at best may repre- for the best-fit model showing relationship between observed fault geometries (circles) and sent a spatial and temporal average of de- predicted geometries (crosses). viatoric stress related to an incremental strain (Pollard et al., 1993; Wojtal and Per- shing, 1991). shears striking clockwise at up to 30Њ to that the northeast constitute 6% of the late set. If Data and results for the subset of White structure, and (3) a few north-northeast– these are conjugates to the R-shears, the Bluffs faults Ͼ1 m long that have slicken- striking faults, possibly ‘‘P-shears’’ (Figs. 11a normal sense of dip slip observed on many lines are shown in Figure 13. Of these, 45 and 12e). Striations and gouge s/c fabrics on of these (e.g., Fig. 10B, location 3) may sim- were assigned in the field to the early set of late faults reveal dextral-reverse slip on vec- ply be related to the northwest dip of the faults, and only 7 to the late set. Using the tors pitching 5Њ–40Њ. Zone 1 contains a clus- Awatere fault. Correlation of bedding-fault combined sample of 52 faults (Fig. 13A), the

ter of six gently northwest-dipping thrusts angle with bedding dip afﬁrms a post-tilting best-ﬁt solution yields a ␴1 that plunges gen- (Fig. 12e, circles). Sinistral faults dipping to age for late faults (Fig. 12d). tly toward 090, Ϸ30ЊϮ15Њ clockwise from

1264 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

dant late dextral-reverse faults have large mean displacements (Ϸ6 m), and accomplish Ϸ86% of the displacement across zone 1. The steep, positive slope of the curve for ⌺⌬u in zone 1 reﬂects the dominant effect of these (Fig. 14). Because the largest fault displacements were inferred from gouge thickness data (not from fault-slip data), displacements inferred within this zone are probably only order-of-magnitude estimates (Evans, 1990). The ratio, ⌺⌬v/⌺⌬u, indicates a transpressive net displacement vector trending Ϸ5Њ to the Awatere fault. Zone 2, although almost as densely faulted as zone 1, has smaller displacement gradients, because it contains relatively Figure 14. Cumulative fault displacement (in meters) versus orthogonal distance away fewer late-stage faults. More detailed plots from the Awatere fault (y direction), as summed across 307 faults observed in the White than Figure 14 reveal linear displacement Bluffs transect. The dimension, y, is the composite width of exposures perpendicular to the gradients intermediate in magnitude be- 063-striking Awatere fault trace, excluding slumped or covered intervals. The origin of the tween those of zones 1 and 2 (Little, in y axis is the northwest edge of the damage zone (see Fig. 5). Displacements for 90% of the press). Of the 120 faults exposed in zone 2, faults were computed from fault attitudes, bedding offsets, and outcrop orientations only 10% are late-stage faults. Based on (Fig. 4). Gouge thickness–displacement relationships derived from Figure 8 were used to gouge thickness data, these have mean dis- estimate displacements for the other 10%. Slip-vector pitches on 18% of the faults were placements of Ϸ8 m and still contribute measured directly from striae or displacement of adjacent pebbles. For the remaining 82% Ϸ55% of the cumulative displacement of the faults, pitches were assigned on the basis of empirically observed relationships across zone 2. The ⌺⌬v/⌺⌬u ratio indicates between fault strike and slip-vector pitch described in Little (in press). See text for further net dextral divergence across the zone at explanation. Ϸ49Њ from the Awatere fault’s strike. This transtension reﬂects slip on the abundant early set of oblique-normal faults. This cal- the Awatere fault’s strike (Fig. 13B). Values tance away from the Awatere fault. Fig- culated displacement angle increases by a of R, the stress ellipsoid shape parameter, ure 14 shows cumulative displacement for few degrees if extension caused by fault are near 0, denoting a nearly uniaxial (pro- the entire transect plotted as a function of block is considered.

late) state of stress (i.e., ␴2 and ␴3 are nearly distance from the Awatere fault. The curve Zone 3 consists chiefly of conglomerate equal in magnitude). Although constrained labeled ‘‘total magnitude’’ is a running sum and is cut by widely spaced early faults with- by a range in fault attitudes (Fig. 13C), ac- of fault displacements (all taken to be pos- out significant overprinting by younger ceptable models are relatively insensitive to itive). Note the sharp decline in displace- faults. As in zone 2, down-to-the-north R, and its best-fit value of 0.1 may not be ment gradient (slope) at the boundary be- oblique-normal faults, both sinistral and significant (Fig. 13B, histogram). The direc- tween zones 2 and 3. The other three curves dextral, result in a net dextral slip parallel to

tion of least compression, ␴3, plunges gently are running sums of displacement in three the Awatere fault, a net downthrow to the to the south. Faults in the best-fit model coordinate directions: ⌺⌬u, parallel to the north, and a net widening or extension of the have a mean fault misfit of Ϸ14Њ, a good strike of the Awatere fault (defined as pos- deformation zone (Fig. 14). Strain magni- result. Running the program on a subset of itive for dextral slip); ⌺⌬v, the horizontal tude is very small (Ͻ5%). The net displace- the data with late-stage faults removed re- direction normal to the strike of the Awa- ment vector across the zone is again diver- duces the mean misfit angle but does not tere fault (positive for fault-normal exten- gent, making an Ϸ40Њ angle to the strike of significantly change the best-fit solution, as sion); and ⌺⌬w, vertical (positive for north- the Awatere fault. This angle increases it is steered by the abundant early faults. west–side-up throw). The near-linear shape slightly if the effect of fault-block tilting is

The analysis thus predicts that ␴1 during for- of these gradients indicates that the bulk considered. mation of the early fault set trended Ϸ30Њ to faulting-related strain within each zone at the strike of the Awatere fault. Note that the scale of Figure 14 is nearly homogene- DISCUSSION AND INTERPRETATION this direction is neither parallel nor perpen- ous. At the Ͻ5 m scale of observation, gra- OF DATA dicular to the Awatere fault, and is subpar- dients are jagged, and strain is heterogeneous allel to the strike of joints in mudstone Little (in press). Evolution of Brittle Structures Adjacent (Fig. 7c). Displacement in the complex damage to the Awatere Fault zone (where y Ͻ 0) is unknown. Zone 1, BULK DISPLACEMENT GRADIENTS adjacent to it, is 260 m wide and contains Pebble tensile fractures are inferred to be RELATED TO FAULTING 121 m of total outcrop length in which 114 the oldest brittle structures present near the faults were observed. Here, faults are Awatere fault and to have presaged upward Like fault spacing (Fig. 4), cumulative densely spaced and accomplish most of the propagation of that structure (Fig. 15A). fault displacement falls off rapidly with dis- total displacement on the transect. Abun- The cracks indicate the direction of maxi-

Geological Society of America Bulletin, November 1995 1265

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

Figure 15. Inferred evolution of brittle structures in Neogene rocks adjacent to the Awatere fault. (Inset) Angle, ␣, is measured between strike of Awatere fault and trend of bulk displacement vector for faulting. PHS refers to direction of maximum incremental horizontal shortening. Its inferred angle with respect to the strike of Awatere fault is shown inside white sector. (A) Distributed tensile cracking in conglomerate and wrench folding of late Neogene sequence prior to development of throughgoing Awatere fault. (B) Onset of dextral-oblique divergence with associated counterclockwise rotation of direction of maximum incremental extension, and initiation of east-west–striking joints and normal faults (early fault set). (C) Continued dextral-oblique divergence accommodated by normal and oblique-slip faulting. Deformation is focused in narrow zone along to the present Awatere fault, suggesting previous inception of that structure. (D) Late-stage dextral-reverse faults restricted to narrow zone parallel to the Awatere fault.

mum incremental horizontal shortening commodation of the DeMets et al. (1990) that locally reactivated and tilted those (PHS) that trended at Ϸ109, Ϸ46Њ clockwise Paciﬁc-Australia relative plate motion vec- joints. The mean, Ϸ70Њ bedding-fault angle from the present Awatere fault plane. tor is predicted to result in an incremental of the early faults and their normal dip-slip Oblique folds near the coast imply a short- PHS at 115Њ (e.g., Walcott, 1978). Thus the suggest that these structures initiated as nor-

ening at Ϸ120Њ. These wrench folds are pos- incremental structures are consistent with mal faults, and that incremental direction e3 sibly developed coevally, with the pebble the relative plate motion being accommo- (least principal extension) was vertical. Fur- cracks, but being finite structures are ar- dated by oblique slip, rather than its parti- ther oblique extension was accomplished by ranged slightly clockwise of those incremen- tioning into separate domains of margin- finite oblique slip and domino-style fault- tal structures because of the rotational na- parallel and margin-orthogonal faulting. block tilting of the early fault arrays. Coun- ture of the bulk deformation. Geodetic and Fault density and faulting related finite terclockwise rotation of the incremental earthquake focal mechanism data indicate strain increase markedly toward the Awa- PHS resulted in widespread reactivation of that the modern PHS in the Marlborough tere fault, suggesting that the main fault had earlier pebble cracks by sinistral shear region has an azimuth of Ϸ108Њ–113Њ reached the surface by the time of distrib- (Fig. 15B). For an ideal shear zone parallel (Fig. 2). Stress inversions from fault-slip uted faulting at White Bluffs. Small-scale to the Awatere fault, this direction of incre- data elsewhere in Marlborough also yield a faulting was associated with a Ϸ20Њ coun- mental PHS implies bulk oblique extension southeast-trending PHS (Nicol and Wise, terclockwise rotation of the incremental at ␣ϭϷ35Њ to the Awatere fault. The ratio 1992; Pettinga and Wise, 1994). In a con- PHS in the Awatere fault zone (Fig. 15B). of the dextral and fault-perpendicular cu- stant-volume, transpressional shear zone This direction is inferred from the strike of mulative displacements in zone 3 (Fig. 14) parallel to the Awatere fault, uniform ac- (1) joints in mudstone and (2) early faults indicates a bulk dextral-oblique extension at

1266 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

a mean displacement angle, ␣,at40Њ–50Њ to value of 1.0 implies an equal contribution of the immature Awatere fault (see below), the Awatere fault, with net downthrow to moment or strain by each magnitude range then the width, P, of the fractured process the northwest. of faults (Marrett and Almendinger, 1990, zone (at least 3 km) relative to ﬁnite fault Analysis of paleostress (this paper) and 1991), one infers from this data that small displacement, d, on the active segment faulting-related strain (Little, in press) indi- faults at White Bluffs make a subordinate (Ͻ1.5 km) suggests a scaling ratio, P/d,of

cate that the incremental PHS (e3) associ- (but still signiﬁcant) contribution in compar- Ͼ2. This ratio is consistent with the Dug- ated with this brittle deformation was ap- ison to the largest ones. Other linear sam- dale-Baranblatt elastic-plastic fracture me- proximately east-west–trending and nearly ples of fault displacement populations have chanics model of fault growth, which predicts equal to the intermediate principal strain yielded slopes between –0.4 and –1.2 (Mar- a P/d of 1–10 (Scholz et al., 1993).

(e2), which was subvertical. This disposition rett and Almendinger, 1991; Peacock and of incremental strain axes implies obliquely Sanderson, 1994; Walsh and Watterson, Fault Zone Kinematics and Faulting- divergent deformation along the Awatere 1992; Wojtal, 1994). These data thus affirm Related Displacement Gradients fault, and they are analogous to those doc- that ‘‘small’’ faults can contribute signifi- umented near the Dead Sea transform (Eyal cantly to bulk strain in complex fault zones. The basic slip-kinematic pattern for faults and Reches, 1983; Eidelman and Reches, One explanation for the abundance of at White Bluffs is apparently independent of 1992). This transtensive structural pattern is small faults is that they developed during an fault length, displacement, magnitude, spac- not consistent with uniform accommodation early period of strain hardening along the ing, or proximity to the Awatere fault. Al- of the relative plate motions. Awatere fault. Hardening may have been though the attitude and slip pattern of the Late dextral-reverse faults are inferred to caused by interference or ‘‘tangling’’ of dominant faults do not change with proxim- postdate inception of the Awatere fault, be- cross-cutting faults, requiring increased dif- ity to the Awatere fault, their mean spacing cause they are focused in a 200-m-wide zone ferential stress to fracture progressively decreases in that direction (Fig. 4). A similar bordering that structure (Fig. 15D). Their smaller fault blocks, and increasing the fault inward intensification of brittle fabric has mean throw is northwest-up, synthetic to the surface/volume ratio. Thus the width of the been described adjacent to many other ma- Awatere fault, and the associated direction densely faulted zone may have expanded jor faults (e.g., Wallace and Morris, 1986; of incremental PHS is Ϸ110Њ, subparallel to outward Ͼ2 km into relatively unfractured Chester and Logan, 1986; Wojtal, 1986). the strike of early pebble cracks. A quasi- rock until softening processes led to a local- Displacement gradients at White Bluffs conjugate fault pattern (Fig. 12e) implies ization of displacement onto a small subset are approximately linear at length scales

that e2 was subvertical. of the existing faults (Wojtal and Mitra, Ͼ5 m. These gradients imply bulk homoge- 1986; Wojtal, 1994). neous deformation resulting from superpo- Fault Scaling Laws Displacements of faults with measurable sition of two sets of faults. Gradients are slip lineations and offsets at White Bluffs in- very small in zone 3, at 0.4–3 km from the Small displacement faults are more abun- dicate that gouge thickness scales linearly Awatere fault, whereas strains closer to the dant than large displacement faults. A linear with displacement, as is consistent with a fault, in zones 1 and 2, are finite. Wojtal slope of the central segment of the log-log steady-state frictional wear mechanism of (1986) noted a similar intensification of fi- plot of cumulative number of faults against fault gouge or cataclasite development nite displacement gradients toward the displacement (Fig. 9) indicates a power-law (Scholz, 1987). Other studies indicate that Cumberland Plateau thrust fault. Inferred relationship over at least two orders of mag- gouge thickness/displacement ratios vary bulk strain ratios at White Bluffs adjacent to nitude. The mean slope of –0.8 for the cen- widely between 10–1 and 10–3 (e.g., Evans, the Awatere fault are locally as large as 20:1 tral part of the curve indicates that ‘‘small’’ 1990; Hull, 1988; Marrett and Almendinger, in zone 1 (Little, in press), whereas the max- (Ͻ1 m) faults provide a large contribution to 1990). At White Bluffs, the thickness of mas- imum values measured adjacent to the the total brittle deformation in the Awatere sive gouge indicates a ratio of Ϸ55, includ- Cumberland Plateau fault are Ϸ7:1 (Wojtal, fault zone. For a power-law relationship being data from both mudstone or conglom- 1986). Cumulative displacement data indi- tween cumulative numbers of faults and erate (Fig. 8). Black, laminated gouge is cates that total slip accommodated by dis- their moments, slopes much less than –1.0 associated with a ratio of Ϸ1030 and typi- tributed faulting is relatively small com- indicate that bulk strain is dominated by the cally coats larger faults. As the two types of pared to the fault’s finite slip and is contribution of larger faults (Marrettt and gouge are locally interlaced with one an- concentrated along the main fault. This re- Almendinger, 1991; Scholz and Cowie, other, this change does not correspond to a lationship reflects marked partitioning of 1990). Interpretation of the slope in terms of simple change in wall rock lithology, but in- slip adjacent to the core of a mature fault. A strain depends on the dimensionality of the stead suggests two competing processes of similar conclusion was obtained qualita- sample, whether the faults penetrate the gouge formation (e.g., Scholz et al., 1993). tively by Chester and Logan (1987, 1993) for seismogenic layer, and the power-law expo- Their textural difference suggests the mas- strike-slip faults in the San Andreas system. nent, n, for scaling of fault length to finite sive gouge formed by frictional wear during Experimental deformation studies predict displacement (Marrett and Almendinger, coseismic slip events, whereas the compos- that distributed deformation zones near 1991; Walsh and Watterson, 1992). In this ite-foliated black gouge formed by grain- strike-slip faults will contain conjugate sets case, the sample is one-dimensional (linear). boundary diffusion processes during aseis- of Riedel faults (R and RЈ shears) (e.g., For shallow crustal faults, and n of 1–2, a mic-creep. Further work is warranted on the Naylor et al., 1986; Schreurs, 1994). The slope of –0.8 implies a power-law exponent, differing composition and microstructures acute bisector of these secondary faults is b,ofϷ0.9 for scaling between cumulative associated with the two types of gouge. predicted to be at Ϸ45Њ to the master fault, frequency and geometric moment of faults If the pebble fractures are relicts of a pro- and their line of intersection with the main (Marrett and Almendinger, 1991). As a b cess zone wake related to finite growth of fault plane to be at right angles to its slip

Geological Society of America Bulletin, November 1995 1267

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

Wairau fault (Fig. 2), with an extra 10Њ clockwise rotation affecting the coastal region between the Awatere and Clarence faults. He modeled the extra rotation as a result of distributed strike-slip deformation occurring north of the coastal termination of the Clarence fault (Fig. 16). The inactive strand of the Awatere fault, striking slightly clockwise from the active strand, and oblique-normal character of faulting observed at White Bluffs support this simple model. The active trace thus appears to ‘‘straighten out’’ the bend caused by the slight clockwise rotation of the older strand. This kinematic mechanism implies that a locally imposed component of fault-orthogonal extension was distributed across a several-km-wide zone of oblique-normal faulting adjacent to the Awatere fault. Unlike the early faults, the late set has a slip-kinematic pattern that faithfully reflects the transpressive kinematics of the adjacent Awatere fault. Dextral-reverse faults domi- nate the array, occurring as a gradational mixture of fault ‘‘classes.’’ These include faults parallel to the main structure, apparent dextral ‘‘R shears’’ striking Ͻ30Њ clock- Figure 16. Schematic fault block kinematic model of lower Awatere Valley region con- wise from that plane, and apparent ‘‘P sistent with paleomagnetic data for late Miocene–early Pliocene rocks. Adapted from Rob- shears’’ striking counterclockwise from it erts (1995). Distributed strike-slip shear results in rigid rotation of lower Awatere Valley (Figs. 11a and 12e). The horizontal bisector fault block, which is pinned to seaward termination of Clarence fault. Offshore trace of separating dextral faults from sinistral faults Awatere fault and location of Wairau basin from Carter et al., 1988. Fault terminations is disposed at Ϸ45Њ to the strike of the Awa- indicated by ‘‘T’’. tere fault. A best-fit great circle to the poles to all these faults is coplanar with the late Quaternary slip direction on the Awatere direction (e.g., Hancock, 1985). Experi- Awatere fault (Fig. 11d), and the best-fit so- fault (Fig. 12e). Chester and Logan (1987, ments with analogue materials commonly lution for the deviatoric principal stresses 1993) obtained a very similar pattern for

produce more complicated fault arrays than places ␴1 at Ϸ30Њ clockwise from the Awa- small-scale faulting adjacent to strands of this—for example, in which Riedel faults are tere fault (Fig. 13). The acuity of this angle the San Andreas system. These results imply helicoidal, or younger Riedel faults, splays, (Ͻ45Њ) and apparent constrictional shape of that the pattern of late-stage distributed or ‘‘P-shears’’ define acute bisectors at an- the incremental strain/stress ellipsoid are faulting is controlled by the kinematics of gles other than 45Њ to the master fault be- expressions of oblique extension occurring the adjacent master fault. At White Bluffs cause of stress field modifications near the within the Awatere fault zone (e.g., Sander- the incremental compression direction, in- tips of early faults (Naylor et al., 1986). For son and Marchini, 1984; Withjack and Jami- ferred as the bisector of opposite slip-sense natural fault zones in anisotropic rocks a son, 1986; Tron and Brun, 1991). Given the faults, makes an Ϸ45Њ angle to the Awatere range of fault attitudes, rather than true transpressive nature of the late Quaternary fault. Chester et al. (1993) imply that fault- conjugate sets, are therefore anticipated. Awatere fault, this is a remarkable result. slip data indicating an Ϸ65ЊϮ25Њ obliquity This is certainly true of the fault arrays at Although some fault-normal extension at of the compression direction in damage- White Bluffs. White Bluffs may be in part related to the zone rocks indicate fault-normal compres- Near the Awatere fault, most of the small- slight (Ͻ10Њ) right-stepping bend in the sion along a weak San Gabriel fault. These scale faults, especially the early set, do not Awatere fault near the coast, the cumulative data are also consistent with an Ϸ45Њ angle. follow a conjugate pattern. Normal oblique displacement gradient data imply a bulk di- Motion of rigid fault blocks imposes simple slip is dominant and occurs together with vergence angle Ͼ40Њ (ratio of ⌺⌬vto⌺⌬u shear deformation in weaker fault zone domino-style fault block rotation. Oblique on Fig. 14). A more compelling mechanism rocks. Such a kinematic constraint will result extension is demonstrated by the finite, for the fault-normal extension observed in principal incremental strain/stress axes positive slope of the ⌺⌬v component on near the coast involves local divergence be- arrayed at Ϸ45Њ to the fault (e.g., Lockner the cumulative displacement gradient tween crustal-scale fault blocks. Roberts and Byerlee, 1993). Fault-slip data from the plot (Fig. 14). Arrays of dextral- and (1992, 1995) measured a mean of 20Њ of re- damage zones of mature fault zones, such as sinistral-oblique faults have an acute bisec- gional post–early Pliocene clockwise rota- the Awatere and San Andreas, thus reflect tor that trends only Ϸ30Њ clockwise from the tion in the coastal region south of the slip-sense on the large faults and are not

1268 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

sensitive indicators of far-field stress normal faulting event at White Bluffs, as of Ϸ40Њ (steepening to Ϸ70Њ at low confining trajectories. compared with the present-day regional pressures), and cohesions of 5–20 MPa. A PHS azimuth of Ϸ110Њ (Fig. 2). According mean uniaxial tensile strength of Ϸ5–10 Implications for Fault Growth to this crack-tip model, brittle fabrics at the MPa can be estimated by graphically extrap- coast record a transitory elastic crack-tip olating the failure envelope of those data In isotropic rock, faults are thought to stress field related to one or more coseismic into the tensile field. Tensile fracturing of grow by a process involving (1) distributed rupture surfaces that were arrested near the Torlesse graywacke pebbles, similar in di- tensile cracking in a process-zone region of Wairau Basin dilational jog in Cook Strait mension to the test cores, at 1–2 km depth intensified stress near the crack tip, (2) link- (see, for example, Sibson, 1989). Early fault would require pore fluid/lithostatic pressure

age across a mesh of obliquely inclined ten- arrays may be relict from one or more tran- ratios of 0.4–0.7 MPa if ␴1 is assumed to be sile cracks to form throughgoing, frictional sient deformation increments acting locally vertical and equal to the overburden pres- shear failure surfaces, and (3) lengthening along a discontinuous or pinned rupture sur- sure (e.g., Suppe, 1985, p. 193). For a more

of faults by propagation of fault tips through face, a conclusion consistent with the occur- likely case in which ␴2 is vertical, higher the process zone (e.g., Cox and Scholz, rence of transpressive incremental deforma- (i.e., suprahydrostatic) ratios would be re- 1988). Given the post–early Pliocene incep- tions of both older and younger age. quired. Thus elevated pore fluid pressures tion age for the Awatere fault, brittle struc- Whatever their origin, a conclusion of this may have played a role in weakening the tures in Pliocene rocks at White Bluffs are study is that early distributed faults are a fault. probably related, at least in part, to devel- poor index of the finite slip-sense on an ad- Heat-flow and in situ stress data for the

opment of that fault. The Awatere fault ap- jacent, major fault (see also Wojtal, 1986; San Andreas fault system indicate that ␴1 parently propagated upward as a mode III Wojtal and Pershing, 1991; Peacock and near the fault is approximately normal to its (screw) dislocation, rather than laterally Sanderson, 1994). strike (e.g., Zoback et al., 1987; Mount and as an edge (mode II) dislocation, because Suppe, 1987, 1992) and that the San An- adjacent to a mode II crack tip aerially Implications for Fault Weakening dreas fault is weak (see Hickman, 1991, for asymmetric principal stress trajectories will a review). Fault-normal compression along create bimodally striking ‘‘wing crack’’ ori- Fault density, displacement gradients, the San Andreas fault system also has been entations (e.g., Scholz et al., 1993). For an and strain increase toward the Awatere inferred from fault-slip data from exhumed upwardly propagating mode III strike-slip fault, and cross-cutting relationships indi- fault zone rocks (Chester et al., 1993) and fault, however, fracture mechanics theory cate that brittle deformation associated with from aftershock focal mechanism data predicts that cracks will develop in a process that fault has narrowed with time. These ob- (Zoback and Beroza, 1993). Data presented

zone at Ϸ45Њ on both sides of the fault. Data servations indicate that strain softening pro- here indicate that the angle between ␴1 and for pebble cracks near White Bluffs strongly cesses ultimately led to localization of dis- the strike of the Awatere fault has fluctuated support this interpretation (Figs. 7a, 7b). placement into a narrow region surrounding between Ϸ30Њ and 50Њ. Arguments for ex- After breaching the surface as a mode III the fault’s core. Chester et al. (1993) docu- treme fault zone weakness thus do seem to crack near the coast during the early pebble mented a similar inward intensification of hold for the Awatere fault, where, despite its cracking event, the coastal segment of the brittle deformation in shallowly exhumed complicated structural history, evidence for Awatere fault may have continued to grow parts of San Andreas fault system in Cali- fault-orthogonal compression is lacking. as a mode II crack tip propagating eastward fornia. There, localized strain in ultracata- Further, marked partitioning of plate mo- into Cook Strait. On the basis of poor-qual- clasite core rocks occurred by particulate tion components does not occur across the ity seismic reflection data, Carter et al. flow of clays at low mean confining pres- oblique-slip Marlborough faults in New (1988) infer that the Awatere fault bends sures, and possibly by minor pressure solu- Zealand (Anderson et al., 1993). eastward offshore of White Bluffs before ter- tion (Chester et al., 1993). Extreme fine minating in Cook Strait along the southern grain size, alteration of grains to hydrous CONCLUSIONS edge of the Wairau basin (Fig. 16). The clay minerals, flux of water through low-per- Wairau basin is a probable late Miocene– meability fault rocks, and sealing of pores by The active Awatere fault is Ͻ4 m.y. old Pliocene feature cut by down-to-the-north vein mineralization during post-seismic pe- and has accumulated Ͻ2 km of finite dextral oblique normal faults (Carter et al., 1988; riods may have contributed to locally high slip. Mean fault density along coastal sea Uruski, 1992). The early fault set at White pore fluid pressures and localized weaken- cliffs increases from Ϸ1 fault/22 m at a dis- Bluffs is approximately synthetic to faults in ing of the core zone rocks (Byerlee, 1990; tance of Ͼ2 km north of the active fault to the Wairau basin. Elastic fracture mechan- Rice, 1992; Chester et al., 1993). Ϸ2 faults/m within 400 m of the fault, to 10 ics models (e.g., Segall and Pollard, 1980; Pore fluid pressure may have helped faults/m in a complex, 50-m-wide damage Pollard and Segall, 1987; Scholz et al., 1993) weaken the mature Awatere fault. Dikes of zone bordering the active fault. The pattern predict that maximum stress trajectories pebbly claystone or gouge intruded into the of fault attitudes and slip directions is ap- near the ‘‘compressional’’ end of a discon- early faults at White Bluffs indicate local su- parently independent of scale across several tinuous strike-slip fault will be deflected prahydrostatic pore fluid pressure gradients, magnitude orders of fault sizes and does not counterclockwise in the tip region by as possibly a result of fault-valve behavior (Sib- change with proximity to the fault. A power- much as 30Њ from their far-field orientation. son, 1990). Testing by Rowe (1980) and Bry- law relationship between displacement and This relationship could explain the region- ant (1977) on 25- to 50-mm-diameter cores cumulative frequency of faults exists over ally anomalous, 090 trend of the greatest of fine- to medium-grained Torlesse sand- two orders of magnitude and has an expo- compressive stress on the northwest side of stone indicate uniaxial compressive strengths nent of Ϸ–0.8 for this linear outcrop sam- the Awatere fault during the early, oblique- of 100–200 MPa, an internal friction angle ple. Two styles of gouge coat fault surfaces.

Geological Society of America Bulletin, November 1995 1269

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 T. A. LITTLE

For both, gouge zone thickness scales lin- Within 200 m of the Awatere fault, late- Eyal, Y., and Reches, Z., 1983, Tectonic analysis of the Dead Seas region since the Late Cretaceous based on mesostructures: early with displacement, but at a different stage dextral-reverse faults occur in a syn- Tectonics, v. 2, p. 167–185. Gephart, J. W., 1990a, Stress and the direction of slip on fault rates, suggesting distinct gouge-forming pro- thetic relationship with the main fault. Their planes: Tectonics, v. 9, p. 845–858. cesses acting at crustal depths of 1–2 km. kinematic pattern indicates transpression, Gephart, J. W., 1990b, FMSI: A FORTRAN program for inverting fault/slickenside and earthquake focal mechanism data to Massive gouge has a scaling ratio of Ϸ1/50 with incremental compression acting at obtain the regional stress tensor: Computers and Geo- sciences, v. 16, p. 953–989. and may have been associated with seismic- Ϸ45Њ–50Њ to the strike of the Awatere fault. Gillespie, P. A., Walsh, J. J., and Watterson, J., 1993, Limitations of dimension and displacement data from single faults and slip events, whereas composite-foliated Unlike the early fault set, these faults faith- the consequences for data analysis and interpretation: Jour- gouge has a scaling ratio of Ϸ1/1000 and fully mirror the present-day geodetic strain nal of Structural Geology, v. 14, p. 1157–1172. Hancock, P. L., 1985, Brittle microtectonics-principles and prac- may have formed more slowly during aseis- pattern and the late Quaternary slip kin- tice: Journal of Structural Geology, v. 7, p. 437–457. Hickman, S. H., 1991, Stress in the lithosphere and the strength of mic creep. Displacement gradients are ap- ematics of the mature Awatere fault. Their active faults, in Brett, R., and Dalrymple, G. B., eds., U.S. proximately linear at scales of Ͼ5 m, imply- orientation was controlled by local kinemat- national report to International Union of Geodesy and Geo- physics, 1987–1990: Washington, D.C., American Geophysi- ing statistically homogeneous deformation. ics, not far-field stress orientations. cal Union Contributions in Tectonophysics, p. 759–775. Hull, J., 1988, Thickness-displacement relationships for deforma- These gradients increase in magnitude with tion zones: Journal of Structural Geology, v. 10, p. 431–435. proximity to the Awatere fault. Near it, they ACKNOWLEDGMENTS Hunt, T. M., 1969, Gravity survey of the lower Awatere district, Marlborough, New Zealand: New Zealand Journal of Ge- are finite and large, reflecting partitioning of ology and Geophysics, v. 12, p. 633–642. Kneupfer, P. L. K., 1992, Temporal variations in latest Quaternary slip into late-stage faults near the weakened, I thank J. W. Gephart for sharing his com- slip across the Australian-Pacific plate boundary, northeast- gouge-filled core of the active fault. Farther puter program and P. Vella for analysis of ern South Island, New Zealand: Tectonics, v. 11, p. 449–464. Lamb, S. H., and Bibby, H. M., 1989, The last 25 Ma of rotational away, displacement gradients are very small, foraminifera in mudstone samples. Criti- deformation in part of the New Zealand plate boundary zone: Journal of Structural Geology, v. 11, p. 473–492. where faults are relict from an early period cism by J. R. Pettinga and R. I. Walcott Lensen, G. J., 1963, Sheet 16, Kaikoura: Wellington, New Zealand of possible fault zone hardening. Strain soft- improved the paper. The New Zealand Department of Scientific and Industrial Research, Geolog- ical Map of New Zealand, scale 1:250 000. ening processes ultimately led to localiza- Foundation for Research Science and Tech- Little, T. A., 1994, Late Cenozoic tectonics of the Awatere fault, New Zealand [abs.]: New Zealand Geological Society Mis- tion of displacement into a narrow region nology and Victoria University of Welling- cellaneous Publication 80A, p. 117. Little, T. A., in press, Faulting-related displacement gradients and surrounding the fault’s core. Supra-hydro- ton Internal Grants Committee provided strains adjacent to the Awatere strike-slip fault in New Zea- static pore-fluid pressure gradients allowed funding for this research. land: Journal of Structural Geology. Lockner, D. A., and Byerlee, J. D., 1993, How geometrical con- tensile fractures to form in pebbles, muddy straints contribute to the weakness of mature faults: Nature, v. 363, p. 250–252. dikes to be intruded along faults, and may REFERENCES CITED Marrett, R., and Almendinger, R. W., 1990, Kinematic analysis of have helped weaken the mature Awatere fault-slip data: Journal of Structural Geology, v. 12, Anderson, H., Webb, T., and Jackson, J., 1993, Focal mechanisms p. 973–986. fault. Fault-slip data indicate stress and of large earthquakes in the South Island of New Zealand: Marrett, R., and Almendinger, R. W., 1991, Estimates of strain Implications for accommodation of Pacific-Australia plate due brittle faulting: Sampling of fault populations: Journal maximum incremental shortening axes that motion: Geophysical Journal International, v. 115, of Structural Geology, v. 13, p. 735–738. are disposed at Ͻ50Њ to the main fault plane, p. 1032–1054. Marone, C., and Scholz, C. H., 1988, The depth of seismic faulting Arabasz, W. J., and Robinson, R., 1976, Microseismicity and ge- and the upper transition from stable and unstable slip weak- not nearly perpendicular to it as has been ologic structure of the northern South Island, New Zealand: ening: Geophysical Research Letters, v. 15, p. 621–624. New Zealand Journal of Geology and Geophysics, v. 19, Maxwell, F. A., 1990, Late Miocene to Recent evolution of the suggested for parts of the San Andreas fault p. 569–601. Awatere Basin, Medway and Middle Awatere Valleys, Marl- system. Arguments for extreme fault weak- Bibby, H. M., 1976, Crustal strain across the Marlborough faults, borough [M.Sc. thesis]: Wellington, New Zealand, Victoria New Zealand: New Zealand Journal of Geology and Geo- University of Wellington, 133 p. ness, therefore, do not apply to the physics, v. 19, p. 407–425. Mount, V. S., and Suppe, J., 1987, State of stress near the San Bibby, H. M., 1981, Geodetically determined strain across the Andreas fault: Implications for wrench tectonics: Geology, oblique-slip Awatere fault, where evidence southern end of the Tonga-Kermadec-Hikurangi subduc- v. 15, p. 1143–1146. for slip partitioning and fault-orthogonal tion zone: Geophysical Journal of the Royal Astronomical Mount, V. S., and Suppe, J., 1992, Present-day stress orientations Society, v. 66, p. 513–533. adjacent to active strike-slip faults: California and Sumatra: compression is lacking. Bryant, J. M., 1977, Some properties of Kaimanawa greywacke: Journal of Geophysical Research, v. 97, p. 11995–12013. New Zealand Ministry of Works and Development, Central Naylor, M. A., Mandl, G., and Sijpeseijn, C. H. K., 1986, Fault Tensile cracks in conglomerate pebbles Laboratories Report no. 2-77/1, 21 p. geometries in basement induced wrench faulting under dif- Byerlee, J., 1990, Friction, overpressure, and fault normal com- ferent initial stress states: Journal of Structural Geology, strike at Ϸ45Њ to the Awatere fault and may pression: Geophysical Research Letters, v. 17, p. 2109–2112. v. 8, p. 737–752. have developed in the process zone of that Carter, L., Lewis, K. B., and Davey, F., 1988, Faults in Cook Strait Nicol, A., and Wise, D. U., 1992, Paleostress adjacent to the Al- and their bearing on the structure of central New Zealand: pine fault of New Zealand: Fault vein and stylolite data from structure as it propagated upward as a mode New Zealand Journal of Geology and Geophysics, v. 31, Doctor’s Dome area: Journal of Geophysical Research, p. 431–446. v. 97, p. 17685–17692. III crack. If so, the fault has a process zone Chester, F. M., and Logan, J. M., 1986, Implications for mechan- Pandey, O. P., 1981, Terrestrial heat flow in New Zealand [Ph.D. width/finite slip ratio of Ͼ1.5, consistent ical properties of brittle faults from observations of the thesis]: Wellington, New Zealand, Victoria University of Punchbowl fault zone, California: Pure and Applied Geo- Wellington, 194 p. with the Dugdale-Baranblatt elastic-plastic physics, v. 124, p. 79–106. Peacock, D. C. P., and Sanderson, D. J., 1994, Strain and scaling Chester, F. M., and Logan, J. M., 1987, Composite planar fabric of of faults in the chalk at Flamborough Head, U.K.: Journal fracture mechanics model for fault growth gouge from the Punchbowl fault, California: Journal of of Structural Geology, v. 16, p. 97–107. (Scholz et al., 1993). Younger fault arrays Structural Geology, v. 9, p. 621–634. Pettinga, J. R., and Wise, D. U., 1994, Paleostress adjacent to the Chester, F. M., Evans, J. P., and Biegal, R. L., 1993, Internal Alpine fault: Broader implications from fault analysis near are dominated by an early, nonconjugate set structure and weakening mechanisms of the San Andreas Nelson, South Island, New Zealand: Journal of Geophysical fault: Journal of Geophysical Research, v. 98, p. 771–786. Research, v. 99, p. 2727–2736. of branching, curviplanar, domino-style, Cowie, P. A., and Scholz, C. H., 1992, Physical explanation for Pollard, D. D., and Segall, P., 1987, Theoretical displacements and oblique-normal faults, both dextral and sin- displacement-length relationship of faults using a post-yield stresses near fractures in rock: With applications to faults, fracture mechanics model: Journal of Structural Geology, joints, veins, dikes, and solution surfaces, in Atkinson, B., istral, and accomplish an oblique extension v. 14, p. 1133–1148. ed., Fracture mechanics of rock: London, Academic Press, Cowie, P. A., and Scholz, C. H., 1993, Displacement-length scaling p. 121–156. of the Awatere fault zone. One explanation relationship for faults: Data synthesis and discussion: Jour- Pollard, D. D., Saltzer, S. D., and Rubin, A. M., 1993, Stress in- nal of Structural Geology, v. 14, p. 1149–1156. version methods: Are they based on faulty assumptions?: for the component of fault-orthogonal ex- Cox, S. J. D., and Scholz, C. H., 1988, On the formation and growth Journal of Structural Geology, v. 15, p. 1045–1054. tension is differential rotation between of faults: An experimental study: Journal of Structural Ge- Rice, J. R., 1992, Fault stress states, pore pressure distributions, ology, v. 10, p. 413–430. and the weakness of the San Andreas fault, in Evans, B., and crustal-scale fault blocks. Another possibil- DeMets, C., Gordon, R. G., Argus, D. F., and Stein, S., 1990, Wong, T. F., eds., Fault mechanics and transport properties Current plate motions: Geophysical Journal International, of rock: London, Academic Press, p. 475–503. ity is that the early faults formed near a v. 101, p. 425–478. Roberts, A., 1995, Tectonic rotation about the termination of a mode II fault tip in Cook Strait, which Eden, D. N., 1983, A late Quaternary history of the Awatere and major strike-slip fault, Marlborough fault system, New Zea- Wairau Valleys, Marlborough, New Zealand [Ph.D. thesis]: land: Geophysical Research Letters, v. 22, p. 187–190. caused stress trajectories to be locally ro- Wellington, New Zealand, Victoria University of Welling- Roberts, A. P., 1992, Paleomagnetic constraints on the tectonic ton, 128 p. rotation of the southern Hikurangi margin, New Zealand: tated counterclockwise Ϸ20Њ. Kinematics of Eidelman, A., and Reches, Z., 1992, Fractured pebbles—A new New Zealand Journal of Geology and Geophysics, v. 35, the widely preserved early fault set is a poor stress indicator: Geology, v. 20, p. 307–310. p. 311–323. Evans, J. P., 1990, Thickness-displacement relationships for fault Roberts, A. P., Turner, G. M., and Vella, P. P., 1994, Magnetostrat- index of slip sense of the mature fault. zones: Journal of Structural Geology, v. 12, p. 1061–1065. igraphicchronologyoflateMiocenetoearlyPliocenebiostrat-

1270 Geological Society of America Bulletin, November 1995

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021 AWATERE STRIKE-SLIP FAULT, NEW ZEALAND

igraphic and oceanographic events in New Zealand: Geo- logical Society of America Bulletin, v. 106, p. 665–683. Rowe, G. H., 1980, Applied geology of Wellington rocks for ag- gregate and concrete [Ph.D. thesis]: Wellington, New Zea- land, Victoria University of Wellington, 397 p. Russel, W. A. C., 1959, A geological reconnaissance of northeast Marlborough: New Zealand Geological Survey Petroleum Report, v. 279, 49 p. Sanderson, D. J., and Marchini, R. D., 1984, Transpression: Jour- nal of Structural Geology, v. 6, p. 449–458. Statement of Ownership, Management, and Circulation Schreurs, G., 1994, Experiments on strike-slip faulting and block (Required by 39 U.S.C. 3685) rotation: Geology, v. 22, p. 567–570. Scholz, C. H., 1987, Wear and gouge formation in brittle faulting: The Geological Society of America Bulletin (Publication No. 0016-7606) is Geology, v. 15, p. 493–495. published monthly by The Geological Society of America, Inc., (GSA) with Scholz, C. H., and Cowie, P. A., 1990, Determination of total geologic strain from faulting: Nature, v. 346, p. 837–839. headquarters and offices at 3300 Penrose Place, Boulder, Colorado 80301 Scholz, C. H., Dawers, N. H., Yu, J.-Z., and Anders, M. H., 1993, U.S.A.; and mailing address of Post Office Box 9140, Boulder, Colorado Fault growth and scaling laws: Preliminary results: Journal of Geophysical Research, v. 98, p. 21951–21961. 80301-9140 U.S.A. The Publisher is Donald M. Davidson, Jr.; the Editor is Segall, P., and Pollard, D. D., 1980, Mechanics of discontinuous Larry Bowlds; the Managing Editor is Faith E. Rogers; their office and mail- faults: Journal of Geophysical Research, v. 85, p. 4337–4350. ing addresses are the same as above. The annual subscription prices are: for Sibson, R. H., 1977, Fault rocks and fault mechanisms: Geological Members and Associate-Student Members of GSA, $60; for non-members Society of London Journal, v. 133, p. 191–214. Sibson, R. H., 1986, Earthquakes and rock deformation in crustal $205. The publication is wholly owned by The Geological Society of fault zones: Annual Review of Earth and Planetary Sci- America, Inc., a not-for-profit, charitable corporation. No known stockholder ences, v. 14, p. 149–175. Sibson, R. H., 1989, Earthquake faulting as a structural process: holds 1 percent or more of the total stock. CEDE & Company, 55 Water Journal of Structural Geology, v. 11, p. 1–14. Street, New York, NY 10041, holds all outstanding bonds; there are no known Sibson, R. H., 1990, Conditions for fault valve behavior, in Knipe, R. J., and Rutter, E. H., eds., Deformation mechanisms, mortgagees or holders of other securities. The purpose, function, and non- rheology, and tectonics: Geological Society of London Spe- profit status of The Geological Society of America, Inc., has not changed dur- cial Paper 54, p. 15–28. ing the preceding twelve months. The average number of copies of each issue Silberling, N. J., Nichols, K. M., Bradshaw, J. D., and Blome, C. D., 1988, Limestone and chert in tectonic blocks from the Esk during the preceding twelve months and the actual number of copies pub- Head subterrane, South Island, New Zealand: Geological lished nearest to the filing date (September 1995 issue) were: Society of America Bulletin, v. 100, p. 1213–1223. Stirling, C., 1991, Late Neogene deformation associated with the Actual No. Awatere fault, Marlborough, New Zealand [B.Sc. thesis]: Wellington, New Zealand, Victoria University of Welling- Item Avg. Copies of ton, 74 p. No. No. Copies Single Issue Suppe, J., 1985, Principles of structural geology: Englewood Cliffs, New Jersey, Prentice-Hall, 537 p. from PS Each Issue Published Tchalenko, J. S., 1970, Similarities between shear zones of differ- Form in past Nearest to ent magnitudes: Geological Society of America Bulletin, v. 81, p. 1625–1640. 3526 Extent and Nature of Circulation 12 Months Filing Date Tron, V., and Brun, J.-P., 1991, Experiments on oblique rifting in a.Total No. Copies (Net press run) 7,803 7,000 brittle-ductile systems: Tectonophysics, v. 188, p. 71–84. Uruski, C., 1992, Sedimentary basins and structure of Cook Strait: b.Paid and/or Requested Circulation New Zealand Institute of Geological and Nuclear Sciences (1) Sales through dealers and carriers, Science Report, v. 93-3, 17 p. Van Dissen, R., and Yeats, R. S., 1991, Hope fault, Jordan thrust, street vendors,and counter sales and uplift of the seaward Kaikoura Range, New Zealand: (not mailed) 0 0 Geology, v. 19, p. 393–396. Walcott, R. I., 1978, Present tectonics and late Cenozoic evolution (2) Paid or Requested Mail Subscriptions, of New Zealand: Royal Astronomical Society Geophysical (Including advertisers proof copies and Journal, v. 52, p. 137–164. Walcott, R. I., 1984, The kinematics of the plate boundary zone exchange copies) 6,877 6,158 through New Zealand: A comparison of long and short term c.Total Paid and/or Requested Circulation deformation: Royal Astronomical Society Geophysical Journal, v. 79, p. 613–633. (Sum of b (1) and b (2)) 6,877 6,158 Wallace, R. E., and Morris, H. T., 1986, Characteristics of faults d.Free Distribution by Mail (Samples, and shear zones in deep mines: Pure and Applied Geophys- ics, v. 124, p. 107–125. complimentary, and other free) 80 80 Walsh, J. J., and Watterson, J., 1992, Populations of faults and e.Free Distribution Outside the Mail fault displacements and their effects on estimates of fault- (Carriers or other means) 0 0 related extension: Journal of Structural Geology, v. 14, p. 701–712. f. Total Free Distribution (Sum of d and e) 80 80 Withjack, M. O., and Jamison, W. R., 1986, Deformation pro- g.Total Distribution (Sum of c and f) 6,957 6,238 duced by oblique rifting: Tectonophysics, v. 126, p. 99–124. Wojtal, S., 1986, Deformation within foreland thrust sheets by h.Copies Not Distributed populations of minor faults: Journal of Structural Geology, (1) Office use, leftovers, spoiled 846 762 v. 8, p. 341–360. Wojtal, S., and Mitra, G., 1986, Strain hardening and strain soft- (2) Returned from news agents 0 0 ening in fault zones from foreland thrusts: Geological So- i. Total (Sum of g, h (1), and h (2)) 7,803 7,000 ciety of America Bulletin, v. 97, p. 674–687. Wojtal, S., and Pershing, J., 1991, Paleostress associated with faults Percent Paid and/or Requested Circulation of large offset: Journal of Structural Geology, v. 13, (c/g x 100) 99% 99% p. 49–62. Wojtal, S. F., 1994, Fault scaling laws and the temporal evolution This information taken from PS Form 3526, signed September 20, 1995 of fault systems: Journal of Structural Geology, v. 16, p. 603–612. by the Publisher, Donald M. Davidson, Jr., and filed with the United States Zoback, M. D., and Beroza, G. C., 1993, Evidence for near-fric- Postal Service in Boulder, Colorado. tionless faulting in the 1989 (M 6.9) Loma Prieta, California earthquake and its aftershocks: Geology, v. 21, p. 181–185. Zoback, M. D., and eight others, 1987, New evidence on the state of stress of the San Andreas fault system: Science, v. 238, p. 1105–1111.

MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 18, 1994 REVISED MANUSCRIPT RECEIVED APRIL 20, 1995 MANUSCRIPT ACCEPTED MAY 9, 1995

Printed in U.S.A.

Geological Society of America Bulletin, November 1995 1271

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/107/11/1255/3382102/i0016-7606-107-11-1255.pdf by guest on 29 September 2021