Late Cenozoic Deformation Research Articles

Structure and neotectonics of the Blackstone Hill Antiform, Central Otago, New Zealand

Blackstone Hill is a northeast‐trending ridge of schist basement rising between the Ida Burn and Manuherikia River valleys. Deformation of young sediments and uplift of strath river terraces cut into both sides of the ridge indicate that its rise is an ongoing, late Cenozoic phenomenon. Deformation of poorly exposed cover sediments at the ridge margins appears to be relatively localised and complex. In the schist basement, a new generation of fractures has developed to accommodate late Cenozoic deformation. The basement also displays a prominent lineation and foliation of Mesozoic age. Lineation defined by quartz rodding trends c. 020°. The orientation of foliation varies and defines the Blackstone Hill Nappe, a large synmetamorphic fold of Mesozoic age with a hinge trending c. 167°. The Otago Peneplain, the regional unconformity between schist basement and overlying sedimentary cover, was a subhorizontal erosion surface before the onset of ridge formation. It is now warped around the late Cenozoic Blackstone Hill Antiform hinge, which trends 035°, rather than offset discontinuously along a range‐scale fault. The antiform is box‐shaped, asymmetric, and open, although the peneplain may be steeply tilted at depth. Data from a strath terrace system indicate that, on the flanks of the antiform, the peneplain has been progressively tilted during folding. Drainage patterns incised into the peneplain suggest that the southeastern flank of the antiform began rising before the northwestern flank. These observations are consistent with a fault‐propagation fold model for the Blackstone Hill Antiform, whose geometry is best explained by reverse motion along two steep faults at depth. Variations in basement fabric related to the Mesozoic Blackstone Hill Nappe have mechanically influenced the development and topographic expression of the antiform.

Growth of late Quaternary folds in southwest Santa Clara Valley, San Francisco Bay area, California: Implications of triggered slip for seismic hazard and earthquake recurrence

We combine analysis of geologic and geomorphic data with observations of deformation during the 1989 Loma Prieta earthquake in northern California to evaluate the contribution of aseismic triggered slip and creep processes to cumulative late Cenozoic deformation along the northeastern Santa Cruz Mountains range front. Deformed late Pleistocene alluvial fans and terraces provide evidence for localized late Quaternary uplift above range-bounding reverse faults within the southwestern Santa Clara Valley adjacent to the range front. On the basis of offset of late Quaternary surfaces, the long-term average slip rate on the primary range-bounding structure (Monte Vista fault) is estimated to be ∼0.2 mm/yr. Northeast of the range front are several discontinuous northwest-trending folds, indicated by alignment of late Pleistocene alluvial-fan apices, anomalous stream-channel convexities, and topographic and vegetation lineaments within a 3–5-km-wide, northwest-trending corridor. Subsurface geologic and geophysical data support the interpretation that the surface folds are a result of blind reverse faulting along the Cascade fault beneath the Santa Clara Valley. From stream incision rates we estimate an average uplift rate of 0.2 ± 0.05 mm/yr for the Cascade fault. Measurements of triggered and postseismic slip following the 1989 Loma Prieta earthquake, combined with estimates of the average return period for Loma Prieta–type events, suggest a long-term average deformation rate along the range front of 0.25–0.4 mm/yr associated with these aseismic processes. This range of values is comparable to rates of late Quaternary deformation on the range-front faults derived from geologic and geomorphic data, and it suggests that growth of the overlying folds is at least partially the result of triggered slip and postseismic creep associated with nearby Loma Prieta–type earthquakes. If this inference is correct, then the return period for independent events on the range-front faults probably is greater than that suggested by the long-term average geologic slip rates.

Lithospheric controls on late Cenozoic construction of the Mongolian Altai

The Altai is one of the great Cenozoic intracontinental and intraplate mountain ranges of central Asia, but it is one of the less studied mountain belts on Earth from a modern structural geology standpoint, and few western scientists are familiar with its tectonic evolution. The range is located dominantly in Mongolia with important sectors in Russia, China, and Kazakhstan and structurally links with the Chinese Tien Shan and Russian Sayan ranges. The Altai is tectonically active and is best understood as two kinematically distinct mountain belts that intersect at 46°N, 96°E: the right‐lateral transpressive western Altai and the left‐lateral transpressive Gobi Altai. Transpressional deformation dominates the late Cenozoic deformation of the Altai and is manifested by throughgoing strike‐slip faults, restraining bends, thrust fault‐bounded ranges linked by strike‐slip faults, and possibly inverted Mesozoic graben. Regionally, there is strong correlation between Cenozoic fault trends and older basement strike trends. Cenozoic deformation regimes appear to be dictated by the angular relationship between preexisting basement structural trends and the prevailing NE directed maximum compressive stress. The Altai is constructed on Paleozoic terranes dominantly consisting of arc and subduction complex assemblages and passive margin sedimentary rocks juxtaposed to the west, southwest and south of the Precambrian block that underlies the Hangay Dome area of central Mongolia. Because the Hangay Dome is characterized by diffuse extension and not transpression, regionally upwarped topography, late Cenozoic volcanism, and elevated heat flow, it is kinematically separate from the Altai region and has previously been interpreted to overlie a mantle plume or asthenospheric diapir. It is proposed that anomalously hot mantle beneath the Hangay Dome coupled with the regionally domed surface topography creates a radial horizontal stress field that acts against the regional NE directed maximum horizontal stress within the Altai. These conditions may deflect postulated NE flowing lithospheric mantle/lower crust that is believed to be driving the shallow upper crustal deformation in the Altai.

Three‐dimensional model of the late Cenozoic history of the Death Valley region, southeastern California

The accumulation of a large database on the timing and kinematics of late Cenozoic deformation in the Death Valley region of southeastern California indicates a complex three‐dimensional history. On the basis of paleogeographic reconstructions we suggest the system was initiated as a localized pull‐apart between two conjugate strike‐slip faults, the Garlock and Furnace Creek faults, and evolved into a system characterized by distributed transtension related to the eastern California shear zone. Our reconstructions differ from previous models in the incorporation of significant vertical axis rotations of a number of crustal blocks to explain paleomagnetic data from the region. The model may resolve (1) a long‐standing problem of the eastern termination of the Garlock fault which is explained here as a complex system of splays that initially terminated in the pull‐apart between the Furnace Creek and Garlock systems; and (2) the complex architecture of the Black Mountains which is explained here in terms of initial extreme attenuation between the Garlock and Furnace Creek systems with overprinting by a fold and normal fault system that operated simultaneously as a result of distributed transtension. This model suggests much of the displacement field is taken up in rotations and translations, and the actual crustal thinning in our model is relatively small (50–66% of original thickness).

Late Cenozoic crustal deformation of the north-central Basin and Range Province, western US

Late Cenozoic (mid-Miocene and younger) deformation of the Basin and Range Province in north-central Nevada is studied by an analysis of mesoscopic faults. Correlations between fault strikes, fault types and senses of displacement were found for this area. Normal faults strike N–S, and right-lateral and left-lateral faults strike NNW and NNE respectively. The existence of strike-slip faulting, mainly right-lateral, is consistent with vertical-axis counterclockwise rotation interpreted from paleomagnetic declination data. These data suggest that mid-Miocene and younger crustal deformation in north-central Nevada was accompanied by both E–W extension, and N–S compression and shortening. Moreover, the contribution of strike-slip faulting to the net extension of this area, although hard to estimate, is significant.

Kinematic interpretation of the 1992 Landers earthquake

Abstract The M w 7.3 Landers earthquake produced approximately 80 km of surface faulting along a preexisting, integrated system of faults separated by complex intersection points. The earthquake was accompanied by distributed seismicity and triggered slip across a large area of the central Mojave desert known as the Mojave block. We synthesized surface mapping, geodetic, and seismicity data to develop a kinematic model for regional co-seismic deformation. We interpret that the earthquake simultaneously accommodated north-directed motion of blocks within the San Bernardino mountains, dextral shear on northwest-striking faults in the central Mojave block, and minor northwest-southeast extension of the eastern Mojave block. This pattern of distributed heterogeneous co-seismic deformation is similar to a kinematic model proposed for cumulative late Cenozoic deformation of the Mojave block in which dextral shear, north-south shortening, and northwest-southeast extension are partitioned into discrete domains bounded principally by strike-slip faults. Based on the similarity between the co-seismic deformation and patterns of cumulative late Cenozoic deformation, we propose that infrequent multiple fault-segment ruptures similar to those associated with the 1992 Landers earthquake are part of the process by which the Mojave block has changed size and shape during the late Cenozoic. This model suggests that combinations of multiple-fault or multiple-segment rupture may be assessed as relatively more or less likely, depending on whether a particular rupture scenario contributes proportionally to the cumulative regional deformation.

Late cenozoic deformation in the Central Andes: fault kinematics from the northern Puna, northwestern Argentina and southwestern Bolivia

Fault kinematics from the northern Puna region of the central Andean Plateau show two phases of deformation. The older deformation involved thrusting on NNE-SSW striking faults prior to about 9 Ma. This produced an average shortening direction (120°±20°) which does not overlap the Miocene-to-Recent convergence direction between the Nazca and South American Plates (82°±4°). Reactivation of normal faults associated with a Cretaceous rift and strike-slip faulting in the weakened magmatic arc can explain the ∼40° discrepancy. Deformation younger than 9 Ma is on normal and strike-slip faults that indicate diverse Plio-Quaternary strain orientations, possibly due to the influence of both gravitational and subduction forces. While the normal faults are minor, five orogen parallel (NS) strike-slip faults are marked by ten- to sixty-kilometer-long scarps. Striae indicate right-lateral strike-slip with minor normal displacement. At least two of these faults reactive older thrusts. By analogy with trench-linked forearc strike-slip faults, the kinematics of these back-arc strike-slip faults can be explained by the decoupling of oblique foreland convergence into two mutually orthogonal components.

Late Cenozoic tectonic evolution of the Puna Plateau and adjacent foreland, northwestern Argentine Andes

Kinematic analysis of ∼ 1500 fault-slip measurements from the Puna plateau and adjacent foreland of northwestern Argentina suggests that two regional kinematic regimes characterize late Cenozoic deformation: a thrust phase with ∼WNW-ESE shortening and subvertical extension followed by a strike-slip phase with ∼ENE-WSW shortening and ∼NNW-SSE extension. Radiometric dating combined with field relationships demonstrate that thrust faulting started by 13 Ma and lasted, at least locally, until 1 Ma, and that strike-slip faulting started by 2 Ma and is still active. The shortening direction of the thrust phase, which accounts for most of the Andean shortening, differs from the coeval plate tectonic convergence direction and probably cannot be explained by later oroclinal bending. Paleostructural control of deformation kinematics and/or strike-slip faulting along the thermally weakened volcanic arc might explain the Mio-Pliocene shortening direction. The subhorizontal extension direction of the strike-slip phase is evident at all elevations studied, suggesting that local body forces do not drive it. A decrease in South America-Nazca plate convergence rate and/or complex three-dimensional effects, possibly including kinematic variation with depth in the crust, might provide a satisfactory explanation.

Kinematics associated with late Cenozoic deformation in central Costa Rica: Western boundary of the Panama microplate

We present kinematic data for late Cenozoic deformation in central Costa Rica that marks the western margin of the Panama microplate (i.e., the Caribbean-Panama boundary). This boundary extends from the North Panama deformed belt, west through the Valle Central in Costa Rica, and then southwest along the East Nicoya Fracture Zone to intersect the Middle America Trench. Terrace correlation and basin asymmetry indicate a major change in tectonic evolution across the boundary, where three regional northeast-striking faults intersect the Pacific coast. Mesoscopic fault populations are consistent with transtension across these north-east-striking faults and with transpression within the east-trending Valle Central. This late Tertiary and Quaternary transcurrent deformation links the North Panama deformed belt to the east with the Middle America Trench to the west. Earthquake focal mechanisms are consistent with mesoscopic fault data, suggesting that fault populations characterize the present-day stress field. This deformation marks the western extent of the Panama microplate, a fragment of volcanic arc that separated from the Caribbean plate in the late Tertiary or early Quaternary and is currently advancing northward due to collisions with South America to the east and the indenting Cocos Ridge on the Cocos plate to the south.

A New Tectonic Model for the Development of the Eastern Cordillera, Altiplano, and Subandean Zones, Bolivian Central Andes, 20 degrees S Latitude

Construction of a regional transect across the central Andes at 20[degrees]S sheds new light on the relationship between the Altiplano, Eastern Cordillera (EC), and Subandean zones and allows us to refine the two-stage model of Isacks (1988) for the growth of the Central Andes. This new model is based on examination of the regional geology and geophysics, coupled with field investigations, satellite image analysis, and new Ar-Ar geochronology. In this model, widespread Oligocene to mid-Miocene compressional deformation in the Altiplano and EC was followed in the late-Miocene and Pliocene by thrusting localized east of the EC within the Subandean fold-thrust belt. During the first stage of deformation, the Altiplano basin underwent important subsidence and internal deformation. The EC was both deformed internally and thrust westwards over the Altiplano basin, while the present Subandean zone was the site of an early, broad foreland basin which received material eroded from the EC. During the second stage, beginning at [approximately]10 ma, deformation terminated within the EC and became concentrated within the fold-thrust belt in response to large scale overthrusting of the EC above the Brazilian shield; this resulted in major thrusting along the Cabalgamiento Frontal Principal (CFP), which soles into the master Subandeanmore » decollement, and [approximately]100 km of telescoping within the early, broad foreland basin. In the EC, this second stage is marked by the elaboration of a regionally extensive erosion surface, ponding of gravels in shallow basins, and the emplacement of giant ignimbrite sheets. The Eastern Cordillera can thus be thought of as a crustal-scale wedge which has been extruded upward and outward on alternate sides during successive stages of late Cenozoic deformation. This motion has served to drive subsidence in both the Altiplano and Subandean foreland basins, as well as shortening in the fold-thrust belt.« less

Tectonic structures developed in Oligocene limestones: Implications for New Zealand plate boundary deformation in North Canterbury

Abstract Systematic analysis of brittle and semibrittle tectonic structures developed in two Oligocene limestones provides a good record of the post middle Cenozoic low‐strain deformational history in North Canterbury. These structures imply two periods of deformation. (1) Mild mid‐late Oligocene shortening along a NNE horizontal axis is inferred from fractures at a low angle to bedding. These are widespread throughout the northeastern South Island and indicate a regional tectonic event associated with the distal effects of major Oligocene extension in western New Zealand. (2) Late Pliocene ‐ Recent shortening was associated with the development of orthogonal fracture sets (many containing stylolites), macrofractures, small‐scale faults, and macroscopic folding. The late Cenozoic deformation is characterised by northwest‐southeast shortening, which is locally overprinted by east‐west and north‐south shortening associated with folding and the activation of macroscopic faults in basement. These late Pliocen...

Late Cenozoic kinematics of the Calabrian Arc, southern Italy

Based on structural analysis, seismicity, and paleomagnetic data a model is presented for the late Cenozoic kinematics of the southern Italian mountain belt. The model predicts that late Cenozoic deformation of the internal part of the belt involved extension and lateral bending of the mantle and lower crust and that in the upper crust faulting was accompanied by semirigid block rotation. In the external part of the belt thrusting dominated. In the north where the arc joins up with the southern Apennines regional shear was dominantly left lateral and distributed between left‐slip along WNW trending cross faults, a component of right‐slip on north to NNE trending extensional faults, and counterclockwise rotation of upper crustal blocks, including the basin fill and north to NNE trending basin margin faults. In the southern part of the arc, overall right lateral shear was accommodated by right‐slip on WNW trending cross faults, a component of left‐slip on NE trending extensional faults, and clockwise rotation of upper crustal blocks, basins, and NE trending faults. These styles of deformation were probably confined to the upper plate of the Tyrrhenian subduction system. They are considered to have been active from the beginning of extension in the Tyrrhenian Basin (circa 11 Ma) and are still active today (based on recent seismicity).

Amount and style of Late Cenozoic Deformation in the Liupan Shan Area, Ningxia Autonomous Region, China

The structures of the Liupan Shan area are characterized by numerous active thrust and strike‐slip faults that suggest thin‐skinned deformation. The structural history of this area can be divided into three phases that probably overlap one another in time and are parts of a single protracted deformation. The oldest Cenozoic deformational phase occurred probably between late Pliocene and early Quaternary time and produced some of the folds and thrust faults in the Liupan Shan and Yueliang Shan. During this phase, deformation was the result of approximately N50°E shortening, and the amount of shortening seems to have been about 1–2 km. The second phase of deformation was dominated by left‐lateral strike‐slip faulting (left slip) on the N60°W striking Haiyuan fault zone and shortening on north‐south trending structures; shortening was associated with a transfer of the left‐slip displacement on the Haiyuan fault zone to shortening in areas farther east. Shortening occurred by thrust faulting in the Liupan Shan and Xiaoquan Shan and by folding in the Madong Shan. During this phase the orientation of shortening changed to N60°W. The average amount of shortening on the north‐south trending folds in the Madong Shan is about 6.3–7.8 km. Most of the shortening on the Liupan Shan and Xiaoguan Shan thrust faults also occurred during this phase and amounted to a minimum of 4.8–6.3 km and 6.6–7.6 km, respectively, also with an orientation of N60°W. During the third phase of deformation, about 1–1.5 km of late Pleistocene to Recent left slip occurred on the Xiaokou fault, which was transferred into oblique left‐slip thrusting in the Liupan Shan. At this time, deformation in the Madong Shan and Xiaoguan Shan ceased or was reduced to a very slow rate. The present, active left‐slip on the Haiyuan fault zone is accommodated by shortening in the Liupan Shan area. The total displacement along the Haiyuan fault is essentially the same as the total amount of shortening in the Liupan Shan area. The sequence and interaction of strike‐slip and thrust faults in the Liupan Shan area seem to apply to the folds and thrust faults farther north in the southern Ningxia region. Thus the northeastern margin of the Tibetan Plateau appears to grow by shortening oriented northeast and by left slip faulting that transfers material from farther to the west. The total left‐slip in the entire southern Ningxia region probably is less than 20–25 km and may be absorbed by shortening within this region. Thus the eastward translation of crustal fragments in the northern part of the Tibetan Plateau may not extend east of southern Ningxia, and if large‐scale eastward displacement has occurred, it must lie farther south.

Middle to late cenozoic magmatism of the southeastern Colorado plateau and central Rio Grande rift (New Mexico and Arizona, U.S.A.) : a model for continental rifting

The region of the present Rio Grande rift and southeastern Colorado Plateau underwent lithospheric extension during middle to late Cenozoic deformation affecting the entire southwestern U.S. Lithospheric mantle was disrupted, and in many regions displaced or replaced by asthenospheric mantle at depths from which basaltic magmas were derived and erupted to the surface. Study of the igneous rocks erupted or intruded during this deformation yields insights into processes of magmatism associated with extension of continental lithosphere. Magmatic rocks associated with an early (late Oligocene-early Miocene) ductile phase of extension are dominantly basaltic andesites and related, calc-alkaline intermediate to silicic derivative rocks. Mafic magmas were probably derived from isotopically “enriched” lithospheric mantle. Igneous rocks associated with a later (middle Miocene-Holocene), more brittle phase of extension include widespread basaltic rocks and localized central volcanoes of intermediate to silicic composition. Isotopic compositions of mafic rocks, which include both tholeiitic and alkalic basalts, correlate strongly with tectonic setting and lithospheric structure. Basalts erupted in areas of greatest crustal extension, such as the central and southern rift and Basin and Range province, were derived from isotopically “depleted” (correlated with “asthenospheric”) mantle. Also, isotopic compositions of Pliocene to Holocene basalts are slightly more depleted than those of Miocene basalts, suggesting that subcrustal lithospheric mantle was thinned during late Miocene extension. Intermediate rocks of the central volcanoes formed by a complex combination of processes, probably dominated by fractional crystallization and by assimilation of upper and lower crust in isolated, small magma chambers. The petrologic, geochemical, and isotopic data are compatible with a model, derived first from geophysical data, whereby lithosphere is thinned beneath the central rift and southeastern Colorado Plateau, with greatest thinning centered beneath the axis of the rift. A lithospheric model involving uniform-sense simple shear does not appear compatible with the data as presently understood.

Structural Deformation in the Offshore Santa Maria Basin, California

The authors divide the offshore Santa Maria basin into the southern, central, and northern provinces based on the pattern, style, and timing of late Cenozoic deformation. From their analyses of over 2,000 km of CDP seismic data and offshore well data they prepared time structure contour maps for three basin-wide unconformities: top of basement, top of Miocene, and the unconformity between early and late Pliocene chronostratigraphic units. Isochron maps were constructed between these horizons and between the early/late Pliocene unconformity and the sea floor to evaluate timing of the deformation. All maps were converted to depth and isopach values based on a three-dimensional velocity model. The regional structure contour and isopach maps show that the offshore Santa Maria basin is characterized by localized crustal shortening orthogonal to the Pacific/North America plate margin and indications of post-Miocene to recent lateral slip along the eastern basin boundary, the San Simeon/Hosgri fault system.

Comment on “The style of Late Cenozoic deformation at the eastern front of the California Coast Ranges” by C.M. Wentworth and M.D. Zoback

Comment on “The style of Late Cenozoic deformation at the eastern front of the California Coast Ranges” by C.M. Wentworth and M.D. Zoback

Reply [to “Comment on ‘The style of Late Cenozoic Deformation at the eastern front of the California Coast Ranges’”

Reply [to “Comment on ‘The style of Late Cenozoic Deformation at the eastern front of the California Coast Ranges’”

Late Cenozoic deformation of the Hellenide foreland, western Greece

Research Article| May 01, 1989 Late Cenozoic deformation of the Hellenide foreland, western Greece JOHN R. UNDERHILL JOHN R. UNDERHILL 1Department of Geology and Geophysics, Grant Institute, University of Edinburgh, West Mains Road, Edinburgh, EH9 3JY, Scotland, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (1989) 101 (5): 613–634. https://doi.org/10.1130/0016-7606(1989)101<0613:LCDOTH>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN R. UNDERHILL; Late Cenozoic deformation of the Hellenide foreland, western Greece. GSA Bulletin 1989;; 101 (5): 613–634. doi: https://doi.org/10.1130/0016-7606(1989)101<0613:LCDOTH>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The late Cenozoic structural evolution of western Greece involved the shortening of Mesozoic passive continental-margin and Cenozoic foreland basin sequences in a foreland-propagating, fold-and-thrust system and superimposed neotectonic deformation related to the development of an active island arc. Although the Pre-Apulian zone has been regarded as the undeformed, western foreland to the thrust belt, three-dimensional exposures in the Ionian islands of Kephalonia and Zakinthos afford evidence of significant late Neogene and Quaternary shortening, including thrusting on reactivated normal faults. Time relations and fault kinematics show that these structures are compatible with the westward migration of the locus of Hellenide thrusting beyond its previously recognized limit and outer-arc compression related to active subduction. The presence of the Pre-Apulian zone in the west contrasts with other Aegean foreland areas, where no such intermediate block lay between the thrust pile and old (?Mesozoic) oceanic crust. Consequently, the Pre-Apulian block took up continued Hellenide shortening in the late Miocene while oceanic crust was being overridden (subducted) in southern parts of the arc to the east. The Pre-Apulian zone subsequently overrode the ocean-floored Ionian Sea during the late Neogene. The confusion in determining driving mechanisms for late Cenozoic deformation in western Greece and the apparent conflict between evidence for the age of initial subduction in different parts of the arc are explained by this model, which is consistent with regional considerations, including structuration in adjacent areas and important paleomagnetic rotations in the arc. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The style of Late Cenozoic deformation at the eastern front of the California Coast Ranges

The 1983 Coalinga earthquake occurred at the eastern boundary of the California Coast Ranges in response to northeast directed thrusting. Such movements over the past 2 Ma have produced Coalinga anticline by folding above the blind eastern tip of the Coalinga thrust zone. The 600‐km length of the Coast Ranges boundary shares a common structural setting that involves westward upturn of Cenozoic and Cretaceous strata at the eastern front of the Coast Ranges and a major, southwest facing step in the basement surface beneath the western Great Valley. Like Coalinga anticline, Pliocene and Quaternary folding and faulting along the rest of the boundary also result from northeast–southwest compression acting nearly perpendicular to the strike of the San Andreas fault. We suggest that much of this deformation is related to active thrusts beneath the eastern Coast Ranges. The step in the basement surface beneath the Great Valley seems to have controlled the distribution of this deformation and the shape of the Coast Ranges boundary.

Seismic expression of shallow structures in active tectonic settings in New Zealand

Late Cenozoic deformation along the Australian/Pacific plate boundary is seen in onshore New Zealand as zones characterised by extension- or transcurrent- or contraction-related structures. High-resolution multichannel seismic reflection data were acquired in several of these tectonic zones and successfully reveal the shallow structures within them. Thirty kilometres of dynamite reflection data in the Rangitaiki Plains, eastern Bay of Plenty, define a series of NE-trending normal faults within this extensional back-arc volcanic region. The data cross surface ruptures activated during the 1987 Edgecumbe earthquake. In the southern North Island, a 20 km Mini-Sosie? seismic profile details the Quaternary sedimentation history and reveals the structure of the active strike-slip and thrust fault systems that form the western and eastern edges of the Wairarapa basin, respectively. This basin is considered to sit astride the boundary between a zone of distributed strike-slip faults and an active accretionary prism. In the Nelson area, northwestern South Island, previously unrecognised low-angle thrust faults of Neogene or Quaternary age are seen from Mini-Sosie data to occur at very shallow depths. Crustal shortening here was previously thought to arise from movement on high-angle reverse faults, and the identification of these low-angle faults has prompted a reassessment of that model. A grid of 18 km of Mini-Sosie seismic data from the central eastern South Island delineates Neogene or Quaternary thrust faults in Cenozoic sediments. The thrusts are interpreted as reactivated Early Eocene normal faults, and the thrust fault geometry is dominated by these older structures.