Dip-slip Component Research Articles

Active fault survey on the Tanlu fault zone in Laizhou Bay

Shallow-depth acoustic reflection profiling survey has been conducted on the Tanlu fault zone in Laizhou Bay. It is found that the Tanlu fault zone is obviously active during the late Quaternary and it is still the dominating structure in this region. The Tanlu fault zone consists of two branches. The KL3 fault of the western branch is composed of several high angle normal faults which had been active during the period from the the latest Pleistocene to early Holocene, dissected by a series of northeast or approximate east-west trending fault which leaped sediment of the late Pleistocene. The Longkou fault of the eastern branch consists of two right-laterally stepped segments. Late Quaternary offsets and growth strata developed along the Tanlu fault zone verify that the fault zone retained active in the latest Pleistocene to the early Holocene. The Anqiu-Juxian fault that passes through the middle of Shandong and corresponds to the Longkou fault is composed of a series of right-laterally stepped segments. The active faults along the eastern branch of the Tanlu fault zone from the Laizhou bay to the north of Anqiu make up a dextral simple shear deformation zone which is characterized by right-lateral strike-slip movement with dip-slip component during the late Quaternary.

Long-period seismic source inversions using global tomographic models

SUMMARY We investigate the effect of laterally varying earth structure on centroid moment tensor inversions using fundamental mode mantle waves. Theoretical seismograms are calculated using a full formulation of surface wave ray theory. Calculations are made using a variety of global tomographic earth models. Results are compared with those obtained using the so-called great-circle approximation, which assumes that phase corrections are given in terms of mean phase slowness along the great circle, and which neglects amplitude effects of heterogeneity. Synthetic tests suggest that even source parameters which fit the data very well may have large errors due to incomplete knowledge of lateral heterogeneity. The method is applied to 31 shallow, large earthquakes. For a given earthquake, the focal mechanisms calculated using different earth models and different forward modelling techniques can significantly vary. We provide a range of selected solutions based on the fit to the data, rather than one single solution. Difficulties in constraining the dip-slip components of the seismic moment tensor often produce overestimates of seismic moment, leading to near vertical dip-slip mechanisms. This happens more commonly for earth models not fitting the data well, confirming that more accurate modelling of lateral heterogeneity can help to constrain the dip-slip components of the seismic moment tensor.

Coseismic deformation of the 2002 Denali Fault earthquake: Insights from GPS measurements

We estimate coseismic displacements from the 2002 Mw7.9 Denali Fault earthquake at 232 GPS sites in Alaska and Canada. Displacements along a N‐S profile crossing the fault indicate right‐lateral slip on a near‐vertical fault with a significant component of vertical motion, north‐side up. We invert both GPS displacements and geologic surface offsets for slip on a three‐dimensional (3‐D) fault model in an elastic half‐space. We restrict the motion to right‐lateral slip and north‐side‐up dip slip. Allowing for oblique slip along the Denali and Totschunda faults improves the model fit to the GPS data by about 30%. We see mostly right‐lateral strike‐slip motion on the Denali and Totschunda faults, but in a few areas we see a significant component of dip slip. The slip model shows increasing slip from west to east along the Denali Fault, with four localized higher‐slip patches, three near the Trans‐Alaska pipeline crossing and a large slip patch corresponding to a Mw7.5 subevent about 40 km west of the Denali‐Totschunda junction. Slip of 1–3 m was estimated along the Totschunda Fault with the majority of slip being at shallower than 9 km depth. We have limited resolution on the Susitna Glacier Fault, but the estimated slip along the fault is consistent with a Mw7.2 thrust subevent. Total estimated moment in the Denali Fault earthquake is equivalent to Mw7.89. The estimated slip distribution along the surface is in very good agreement with geological surface offsets, but we find that surface offsets measured on glaciers are biased toward lower values.

Effects of fault geometry and slip style on near-fault static displacements caused by the 1999 Chi-Chi, Taiwan earthquake

We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements. D 2005 Elsevier B.V. All rights reserved.

Synchronous vertical and horizontal tectonism in the Neoarchean: Kinematic evidence from a synclinal keel in the northwestern Superior craton, Canada

The long and narrow Carrot River greenstone belt in the northwestern Superior craton is a synclinal keel between two broad granitoid domes. Its unique structural geometry and kinematics place significant constraints on the timing relationship between vertical and horizontal tectonism. A high-strain zone spatially coincident with the belt records evidence for synchronous greenstone-down/granitoid-up dip-slip and dextral strike-slip components. Kinematic and regional considerations lead to the conclusion that the dip-slip component was a result of vertical tectonism and the strike-slip component was a result of horizontal tectonism, and therefore that the vertical and horizontal tectonism were synchronous. It is emphasized that vertical and horizontal tectonism do not have to act exclusively of each other. It is not necessary to downplay the significance of vertical tectonism to validate horizontal tectonism, and vice versa. Evidence for synchronous vertical and horizontal tectonism reported here makes it necessary that both processes (as opposed to either one or the other) and their potential interactions are considered in interpretations of Archean terrains. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and might represent a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic. It might be an important process for gold mineralization at the late stages of Archean cratonization.

Average slip rate, earthquake rupturing segmentation and recurrence behavior on the Litang fault zone, western Sichuan Province, China

The Litang fault zone (LFZ) is an active fault zone within the northwestern Sichuan sub-block. Field investigation reveals new evidence for its late Quaternary offset, neoteric earthquake ruptures and surface-rupturing segmentation, from which long-term slip-rates, characteristic earthquake magnitudes and recurrence intervals on the fault zone are estimated. This study shows that the LFZ consists of three subordinate faults, and they are the northern Maoyaba fault, Litang fault and Kangga-Dewu fault, respectively. All of them are dominated by left-lateral strike-slip with reverse dip-slip component on different segments. Based on offset landforms and ages of relevant deposits collected from seven sites, it is estimated that the average left-lateral slip-rate on the LFZ reaches 4.0±1.0 mm/a in the past 14 ka BP, and vertical (reverse) dip-slip rate in the range of 0.1―1.8 mm/a. Three subordinate faults are independent earthquake rup-turing segments, on which the maximum moment magnitude of the characteristic earthquakes is estimated to be 7.0―7.3, and their average recurrence interval to be 500―1000 a. The latest earthquake ruptures occurred 119±2 years ago on the northern Maoyaba fault, about 1890 AD on the Litang fault, and in 1948 AD on the Kangga-Dewu fault, and this may indicate a unidirectional migration for surface rupturing earthquakes along the Litang fault zone, related to stress trigger-ing between the segments.

Late Holocene activity of the Dead Sea Transform revealed in 3D palaeoseismic trenches on the Jordan Gorge segment

Three-dimensional excavations of buried stream channels that have been displaced by the Jordan Fault, the primary strand of the Dead Sea fault zone in northern Israel, demonstrate that late Holocene slip has been primarily strike–slip at a minimum rate of 3 mm/yr. The palaeoseismic study was carried out in the Bet-Zayda Valley, the delta of the Jordan River at the north shore of the Sea of Galilee. The site was chosen where a north-striking scarp with up to 1-m vertical expression crosses the flat valley. One group of trench excavations was located where a small stream crosses the scarp. The active stream, which is incised into the scarp, is not offset by the fault. However we found two palaeo channels about 2 m below the surface offset sinistrally 2.7±0.2 m by the fault and two younger nested channels offset 0.5±0.05 m. Based on radiocarbon dates we attribute the last 0.5 m rupture to the earthquake of October 30, 1759. The older offset of 2.2 m most probably occurred in the earthquakes of May 20, 1202. These two events correlate with the findings at Ateret, about 12 km north of Bet-Zayda, where the 1202 earthquake produced 1.6 m of lateral displacement in E–W-striking defence walls of a Crusader castle, and an Ottoman mosque was offset 0.5 m in the earthquake of 1759. In the second group of trenches some 60 m farther south we found another offset channel. Its northern margin is displaced 15 m sinistrally whereas the southern margin shows only 9 m of sinistral offset. The dip slip component is 1.2 m, west side down. The different amounts of margin offset can be explained by erosion of the southern margin during the first 6 m of displacement. Additional slip of 9 m accrued after the stream had been abandoned and buried by a 2-m-thick lacustrine clay layers. Radiocarbon dates on organic residue provide the age control which indicates that the 15 m of slip has accrued over the past 5 kyr, yielding a short-term slip rate of 3 mm/yr for the late Holocene. It is possible that our study covers only part of the fault zone, hence we regard this mean slip rate to be a minimum for the DST. Based on other palaeoseismic studies the best estimate for Quaternary slip rate is 4±1 mm/yr.

Fault zone development and strain partitioning in an extensional strike-slip duplex: A case study from the Mesozoic Atacama fault system, Northern Chile

Upper crustal strike-slip duplexes provide an excellent opportunity to address the fundamental question of fault zone development and strain partitioning in an evolving system. Detailed field mapping of the Mesozoic Atacama fault system in the Coastal Cordillera of Northern Chile documents the progressive development of second- and third-order faults forming a duplex at a dilational jog between two overstepping master faults: the sinistral strike-slip, NNW-striking, Jorgillo and Bolfin faults. These are constituted by a meter-wide core of foliated S-C ultracataclasite and cataclasite, flanked by a damage zone of protocataclasite, splay faults and veins. Lateral separation of markers along master faults is on the order of a few kilometers. Second-order, NW-striking, oblique-slip subsidiary fault zones do not show foliated ultracataclasite; lateral sinistral separations are in the range of ∼ 10 to 200 m with a relatively minor normal dip-slip component. In turn, third-order, east–west striking normal faults exhibit centimetric displacement. Oblique-slip (sinistral–normal) fault zones located at the southern termination of the Bolfin fault form a well-developed imbricate fan structure. They exhibit a relatively simple architecture of extensional and extensional-shear fractures bound by low displacement shear fractures. Kinematic analysis of fault slip data from mesoscopic faults within the duplex area, document that the NW-striking and the EW-striking faults accommodate transtension and extension, respectively. Examination of master and subsidiary faults of the duplex indicates a strong correlation between total displacement and internal fault structure. Faults started from arrays of en echelon extensional/extensional-shear fractures that then coalesced into throughgoing strike-slip faults. Further displacement leads to the formation of discrete bands of cataclasite and ultracataclasite that take up a significant part of the total displacement. We interpret that the duplex formed by progressive linkage of horsetail-like structures at the southern tip of the Bolfin fault that joined splay faults coming from the Jorgillo and Coloso faults. The geometry and kinematics of faults is compared with that observed in analog models to gain an insight into the kinematic processes leading to complex strike-slip fault zones in the upper crust.

From transpression to transtension: changes in morphology and structure around a bend on the North Anatolian Fault in the Marmara region

The east–west-trending North Anatolian Fault makes a 17° bend in the western Marmara region from a mildly transpressional segment to a strongly transtensional one. We have studied the changes in the morphology and structure around this fault bend using digital elevation models, field structural geology, and multi-channel seismic reflection profiles. The transpression is reflected in the morphology as the Ganos Mountain, a major zone of uplift, 10 km wide and 35 km long, elongated parallel to the transpressional Ganos Fault segment west of this bend. Flat-lying Eocene turbidites of the Thrace Basin are folded upwards against this Ganos Fault, forming a monocline with the Ganos Mountain at its steep southern limb and the flat-lying hinterland farther north at the flat limb. The sharp northern margin of the Ganos Mountain coincides closely with the monoclinal axis. The strike of the bedding, and the minor and regional fold axes in the Eocene turbidites in Ganos Mountain are parallel to the trace of the Ganos Fault indicating that these structures, as well as the morphology, have formed by shortening perpendicular to the North Anatolian Fault. The monoclinal structure of Ganos Mountain implies that the North Anatolian Fault dips under this mountain at 50°, and this ramp terminates at a decollement at a calculated depth of ∼8 km. East of this fault bend, the northward dip of the North Anatolian Fault is maintained but it has a normal dip-slip component. This has led to the formation of an asymmetric half-graben, the Tekirdağ Basin in the western Sea of Marmara, containing a thickness of up to 2.5 km of Pliocene to Recent syn-transform sediments. As the Ganos uplift is translated eastwards from the transpressional to the transtensional zone, it undergoes subsidence by southward tilting. However, a morphological relic of the Ganos uplift is maintained as the steep northern submarine slope of the Tekirdağ Basin. The minimum of 3.5 km of fault-normal shortening in the Ganos Mountain, and the minimum of 40 km eastward translation of the Ganos uplift indicate that the present fault geometry has existed for at least the last 2 million years.

Focal Mechanisms of Intermediate-depth Earthquakes Beneath Southeastern Hokkaido, Japan Implications of the Double Seismic Zone

In this study, the focal mechanism solutions and spatial distributions of 100 intermediate-depth earthquakes are examined and discussed. These earthquakes occurred from 1985 to 1993 mostly in the upper and lower planes of the double seismic zone beneath the southeastern part of Hokkaido. The focal-mechanism solutions along with the spatial distribution of their epicenters as a function of depth and the minimum- and maximum-compressive stress directions confirm the following essential features; A double-planed deep seismic zone between the depths from 60 to 120 km has been clearly revealed in this region and the number of earthquakes located in the upper plane are less than those for the lower one. Low-angle thrust fault mechanism is dominant to a depth of about 55–60 km in the zone. At depths deeper than about 60 km focal mechanism solutions are mostly characterized by down-dip extension type or down-dip compression type in both lower and upper planes of the zone. Most of the earthquakes deeper than 60 km have dip-slip components and some of the solutions have significant strike-slip components, as well. Although the down-dip extensional-type events are dominant, seven events that are located on the lower plane are constrained as down-dip compression mechanism. The T axes of earthquake focal mechanisms on the lower plane show extension at about N40°E and N35°W, and the dip-angle between 25° and 70°, almost coincide with the dip of the subducting Pacific Plate in this region. We also investigate the statistical features of the stress directions of the earthquakes occurring both on the lower and upper plane after converting all the events onto the plane having the same strike and dip angle as the subducting lithosphere in the region. Seventy-two percent of the minimum compressive stress directions are oriented mainly in the NS or NNE- SSW directions (within 45° of the dip direction) at the plate surface of the lower plane and are almost parallel to the dip of the local slab in this region. The number of earthquakes located in the upper plane is less than that for the lower one. Those which occurred on the upper plane show various types of mechanism solutions. Weak systematic orientation is found in the direction distribution of stress in the upper plane.

Phanerozoic strike-slip faulting in the continental interior platform of the United States: examples from the Laramide Orogen, Midcontinent, and Ancestral Rocky Mountains

Abstract The continental interior platform of the United States is that part of the North American craton where a thin veneer of Phanerozoic strata covers Precambrian crystalline basement. N- to NE-trending and W- to NW-trending fault zones, formed initially by Proterozoic/Cambrian rifting, break the crust of the platform into rectilinear blocks. These zones were reactivated during the Phanerozoic, most notably in the late Palaeozoic Ancestral Rockies event and the Mesozoic-Cenozoic Laramide orogeny — some remain active today. Dip-slip reactivation can be readily recognized in cross section by offset stratigraphic horizons and monoclinal fault-propagation folds. Strike-slip displacement is hard to document because of poor exposure. Though offset palaeochannels, horizontal slip lineations, and strain at fault bends locally demonstrate strike-slip offset, most reports of strike-slip movements for interior-platform faults are based on occurrence of map-view belts of en echelon faults and anticlines. Each belt overlies a basement-penetrating master fault, which typically splays upwards into a flower structure. In general, both strike-slip and dip-slip components of displacement occur in the same fault zone, so some belts of en echelon structures occur on the flanks of monoclinal folds. Thus, strike-slip displacement represents the lateral component of oblique fault reactivation; dip-slip and strike-slip components are the same order of magnitude (tens of metres to tens of kilometres). Effectively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinear crustal blocks. In this context, the sense of slip on an individual strike-slip fault depends on block geometry, not necessarily on the trajectory of regional σ 1 . Strike-slip faulting in the North American interior differs markedly from that of southern and central Eurasia, possibly because of a contrast in lithosphere strength. Weak Eurasia strained significantly during the Alpine-Himalayan collision, forcing crustal blocks to undergo significant lateral escape. The strong North American craton strained relatively little during collisional-convergent orogeny, so crustal blocks underwent relatively small displacements.

DIPSLIP: a QuickBasic stress inversion program for analysing sets of faults without slip lineations

A simple computer program is described for estimating palaeostress tensors from orientation data from a set of fault planes. The computation is based on a novel technique that allows the tensor to be estimated in situations where directions of slip on the faults cannot be determined, but where the senses of the dip-slip component of slip on the faults are known. The new technique greatly broadens the scope of palaeostress analysis, permitting the analysis of faults lacking slickenlines but exhibiting offsets of horizontal marker beds.

The 1998 March 14 Fandoqa earthquake (Mw6.6) in Kerman province, southeast Iran: re-rupture of the 1981 Sirch earthquake fault, triggering of slip on adjacent thrusts and the active tectonics of the Gowk fault zone

SUMMARY The 1998 March 14 Fandoqa earthquake (Ms 6.6) was the penultimate in a series of five substantial earthquakes on the Gowk fault system of southeast Iran since 1981, all of which were associated with co-seismic surface ruptures. We use observations of surface faulting, analysis of P and SH body waves, SAR interferometry and geomorphology to investigate the ruptures in these earthquakes and how they are related both to each other and to the regional active tectonics. The 1998 Fandoqa earthquake produced 23 km of surface faulting with up to 3 m right-lateral strike-slip and 1 m vertical offsets. SAR interferometry and seismic waveforms show that the main rupture plane dipped west at ∼50° and had a normal component, although the surface ruptures were more complicated, being downthrown to both the east and the west on steep faults in near-surface sediments. In addition, SAR interferometry shows that a nearby thrust with a similar strike but dipping at ∼6°W moved about 8 cm in a time interval and in a position that makes it likely that its slip was triggered by the Fandoqa earthquake. The 1998 surface ruptures in the Gowk valley followed part of a much longer (∼80 km) set of co-seismic ruptures with smaller offsets that were observed after larger earthquakes in 1981 (Mw 6.6 and 7.1). The main ruptures in these 1981 earthquakes probably occurred on different, deeper parts of the same fault system, producing only minor reactivation of the shallower faults at the surface. Although the 1981–1998 earthquake sequence apparently ruptured parts of the same fault system repeatedly, these earthquakes had very different rupture characteristics: an important lesson for the interpretation of both palaeoseismological trenching investigations and historical accounts of earthquakes. The regional kinematics, which involve oblique right-lateral and convergent motion, are evidently achieved by a complex configuration of faults with normal, reverse and strike-slip components. Some of the complexity at the surface may be related to a ramp-and-flat fault geometry at depth, but could also be related to the large topographic contrast of ∼2000 m across the fault system, which separates the high Kerman plateau from the low Dasht-e-Lut desert. Details of the fault geometry at depth remain speculative, but it must be unstable and evolve with time. It may be this requirement that causes the principal features of geological ‘flower structures’ to develop, such as series of subparallel faults which accommodate dip-slip components of motion.

Origin of a vertical lineation in conjugate transcurrent shear-zones at Broken Hill, Australia

Shear-zones at Little Broken Hill are interpreted as conjugate transcurrent structures. They developed before dolerite dyke emplacement in response to N–S horizontal shortening. They were reactivated as conjugate transcurrent structures after dyke emplacement in response to horizontal E–W shortening. Shear-zone deformation was accompanied by retrograde metamorphic conditions for at least the later part of the history. Displacement on the shear-zones was accommodated by coeval deformation within the shear-zone-bound blocks. This deformation involved shear on the pre shear-zone S 1 foliation and resulted in incomplete retrogression of earlier peak metamorphic mineral assemblages. It also resulted in a vertical extension and a component of dip slip on the shear-zones. A near-vertical stretching-lineation occurs parallel to the intersection of the shear-zones and is interpreted as a localised response to an overall vertical extension. Localisation of the lineation is explained in terms of grain-scale partitioning of deformation mechanisms. In the country rock, vertical extension was achieved by shear on S 1 with very little intragranular deformation. Consequently this deformation failed to produce a lineation. In the shear-zones, where all fabric elements were tending to vertical, the only deformation mechanism available for vertical extension was intragranular deformation, which resulted in a shape fabric, viz the stretching-lineation. Shear within the zones had both horizontal and vertical components parallel to the shear-zone foliation. Because the shear was achievable by slip on the foliation and did not involve intragranular mechanisms, it did not modify the lineation. Late shear-zone movement resulted in local crenulation of the shear-zone foliation about the early-formed stretching-lineation. The result was vertical plunging folds and a more pronounced “stretching” lineation. It is concluded that stretching-lineations in shear-zones should not be considered indicators of movement direction or of instantaneous or finite strain axes.

Surface rupture and rupture mechanism of the October 1, 1995 (Mw=6.2) Dinar earthquake, SW Turkey

A moderate earthquake (Mw=6.2) caused substantial damage in the town of Dinar at 17:57 UT, on October 1, 1995. Surface rupture has been observed to be discontinuous for 10km along the NW-trending Dinar fault segments. The surface rupture has consisted of nearly vertical cracks up to several tens of meters long and a meter wide that display linear, sigmoidal and anastomosing geometries indicating the mechanism of displacement on the fault plane. Most of these cracks display a dip-slip component of displacement (0.05–0.6m); those with anastomosing geometries indicate a large dip-slip (>0.25m). Sigmoidal crack patterns show both right and left strike-slip (rotational) component of displacement and a small dip-slip (<0.25m). The geometry of the cracks and related modes of slip suggest a 325° oriented horizontal principal stress direction. This direction makes an acute angle with the NW-trending plane of the Dinar fault. This angular difference is the cause of the strike-slip (rotational) movement manifested in the surface rupture and indicated by the focal mechanism solution of the earthquake. Under the influence of this horizontal principal stress direction of compression, firstly en echelon cracks were formed. Then these cracks were modified by a predominantly dip-slip displacement on the Dinar fault; the initial cracks were connected to each other by anastomosing cracks of a second rotational phase. The rotation led the hanging-wall block of the Dinar fault to depart from its dip-slip orientation to an oblique southwesterly direction towards the Mediterranean Sea as suggested by strike and dip-slip indicating cracks.

Did the Makarska earthquake of 1962 generate a tsunami in the central Adriatic archipelago?

The Makarska earthquakes of 1962 represent one of the most important seismic sequences in Croatia in the last 50 years. The mainshock ( M L =6.1) occurred in the area between the mainland and the islands of Hvar and Brač. Fault plane solutions for the three largest events indicate reverse faulting on a steeply dipping, WNW–ESE striking fault, which also agrees with the position of the relocated hypocentres. The dip-slip component of the motion on the fault was large enough to cause maximum vertical sea-bottom displacement of about 15 cm, which in turn could have generated a small tsunami. The horizontal dimensions of the source area are estimated by empirical formulas to be of the order of 10 km. As the bottom depth in the area ranges between 10 and 100 m, a linear shallow-water model may be used to simulate the tsunami. The model equations have been solved using a finite-difference numerical scheme (resolution 500 m/10 s). The modelled tsunami reveals some interesting features: short waves are detained in the source area, probably due to scattering caused by the irregularities of the coastline; long waves propagate outwards, through the Brač, Hvar and Neretva Channels; and the tsunami is somewhat amplified near the coasts. The simulated tsunami is compared to the corresponding tide-gauge record from Split. The observed arrival time and amplitudes agree well with those of the synthetic tsunami, if it is assumed that initial disturbance was due to the subsidence of the foot wall of the seismogenetic fault — in agreement with tectonic motions in the area.

Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland (Published in Scottish Journal of Geology, 35(1), 1999, pp. 31–50)

R. G. Park and J. Tarney write: Droop et al . (1999) make a welcome addition to the Lewisian literature in providing the first rigorous published estimates of FT conditions at the Laxfordian metamorphic peak in the Loch Maree Group (530–630°C, 6.5 kbar, using various methods including calcite-dolomite and garnet-biotite geothermometers and garnet-plagioclase geobarometer). In their discussion of the implications of the relatively high pressure-temperature estimates, they note an apparent contradiction with the tectonic model for the early Laxfordian deformation presented by Coward and Park (1987) and Park et al . (1987). The 1987 tectonic model for the Gairloch area resulted from the recognition that the strong early Laxfordian deformation was associated with a WNW-ESE movement direction implying oblique slip on gently inclined planes, with both strike-slip and dip-slip components. The reason for choosing the dextral/normal-slip option for these movements then was because of the observation that certain of the more prominent shear zones exhibited dextral, NE-down movement-sense criteria. However subsequent more detailed work on the Gairloch shear zones (Lei and Park 1993) showed that the movement history on the shear zones was more complex than first thought, that both sinistral/normal and dextral/reverse movement criteria existed for the early shear zones, but that the main early movements involved overthrusting to WNW, i.e. compressional, in agreement with Droop et al .’s conclusions. Droop et al .’s estimates of the metamorphic conditions provide timely additional evidence in support of new work by ourselves, together with J.N. Connelly, recently accepted for publication (Park et al . in press), . . .

Recent tectonics of the Blanco Ridge, eastern blanco transform fault zone

Bathymetric, hydro-acoustic, seismic, submersible, and gravity data are used to investigate the active tectonics of the eastern Blanco Transform Fault Zone (BTFZ). The eastern BTFZ is dominated by the ∼150 km long transform-parallel Blanco Ridge (BR) which is a right-lateral strike-slip fault bordered to the east and west by the Gorda and Cascadia Depressions. Acoustic locations, fault-parameter information, and slip vector estimates of 43 earthquakes (Mw≥3.8) that occurred along the eastern BTFZ over the last 5 years reveal that the Blanco Ridge is a high-angle right-lateral strike-slip fault, with a small component of dip-slip motion, where the Juan de Fuca plate is the hanging wall relative to the Pacific plate. Furthermore, the Cascadia and Gorda basins are undergoing normal faulting with extension predominantly oblique to the transform trend. Seafloor submersible observations agree with previous hypotheses that the active transform fault trace is the elongate basin that runs the length of the BR summit. Brecciated and undeformed basalt, diabase, and gabbro samples were collected at the four submersible survey sites along the Blanco Ridge. These petrologic samples indicate the Blanco Ridge is composed of an ocean crustal sequence that has been uplifted and highly fractured. The petrologic samples also appear to show an increase in elevation of the crustal section from east to west along the Blanco Ridge, with gabbros exposed at a shallower depth farther west along the southern (Pacific plate side) BR ridge flank. Further supporting evidence for BR uplift exists in the seismic reflection profiles across the BR showing uplift of turbidite sequences along the north and south ridge base, and gravity and magnetics profiles that indicate possible basement uplift and a low-density zone centered on the ridge's Pacific plate side. The BR formation mechanism preferred here is first, uplift achieved partially through strike-slip motion (with a small dip-slip component). Second, seawater penetration along the fault into the lower crust upper mantle, which then enhanced formation and intrusion of a mantle-derived serpentinized-peridotite diapir into the shallow ocean crust, causing further uplift along the fault.

Quantifying the timing and sense of fault dip slip: New application of biostratigraphy and geohistory analysis

The timing and sense of the dip-slip component of a fault9s motion can be quantified by using geohistory analysis. Geohistory analysis traditionally is used to quantify tectonic subsidence or uplift of marine basins, but is newly applied to evaluate phases of activity of individual faults, including blind faults. Intermittent fault activity and changing sense of dip slip emerge, particularly for faults reactivated under changing kinematic regimes. Through the use of outcrop or core data from two fault blocks on either side of a major fault, the vertical component of displacement can be isolated. Steps of geohistory analysis include generation of (1) a rock-accumulation curve consisting of stratal thicknesses as a function of age, (2) a decompacted sediment-accumulation curve as a function of age, and (3) a total-subsidence curve with paleobathymetry inferred from fossil assemblages and sedimentary structures. Because errors on the inferred water depths are considered to be 20% of the water depth, the method is limited to large-scale faults. The analysis is repeated on both sides of the fault to determine which block subsided or was uplifted faster, thus isolating both the sense of dip slip and its relative magnitude. If both blocks display parallel subsidence curves within the error margins, a period of fault inactivity or pure strike slip is inferred. If the sense of dip slip changed, the subsidence curves cross. If the dip direction of the fault plane is interpreted from field or subsurface data, a change—for example, from an extensional component to a contractional component—also emerges, and the timing of basin inversion is detectable. As a case study, fault activity was evaluated for a mosaic of faults cutting the Messaras basin of Crete, Greece, a Neogene forearc basin that underwent extension followed by transpression. This approach is applicable to neotectonics or ancient tectonics and pertains to subsurface or exhumed faults as well as active faults offshore that generate contrasting water depths across a fault zone.

Source processes and seismotectonic implications of the 1995 and 1996 Cyprus, Eastern Mediterranean region, earthquakes

The teleseismic body waves of two earthquakes were inverted for their sources to obtain a rupture process for each. The February 23, 1995 ( M s=5.8) and October 9, 1996 ( M s=6.8) events took place to the northwest and southwest of Cyprus. Both the earthquakes generated complex body waveforms which were inverted using the method of Kikuchi and Kanamori (1991). The inversion results suggest that the 1995 and 1996 events were multiple events associated with subevents taking place on different faults located nearby the source regions. Three subevents were necessary to explain the waveforms of the 1995 earthquake. The first subevent showed a predominantly normal faulting mechanism while the second subevent, located 20 km to the north of the first subevent, had a strike-slip mechanism with dip-slip component. Despite the difference in mechanisms both the subevents had slip vectors oriented nearly E–W. The last subevent was located in the proximity of the first subevent and had a normal faulting mechanism with a slip vector in N–S direction. The different depths and slip-vector directions of the first and third subevents may suggest strain partitioning with depth in the source region of the 1995 event. The complex waveforms of the 1996 earthquake were modeled with three subevents, too. Combining the hypocentral distribution of the aftershocks with the inversion results revealed that the first subevent took place on a fault plane dipping about 20° to the east-southeast. The mechanism of the first subevent, together with the azimuthal pulse-width distribution suggest that the sense of motion on the fault plane was right-lateral strike-slip and unilateral rupture propagation. We estimate from the major subevent a slip rate of 10 mm/year for the African plate. The second and third subevents were located 35 km to the southeast of the main shock and had a predominantly normal faulting mechanism with slip vectors in north-northwest direction. The rupture process analysis of the 1995 and 1996 events suggests that the region to the west of Cyprus experiences extensional tectonics.