deformation and sedimentation along a developing terrane suture eastern sunda forearc indonesia the collision of the eastern sunda arc with northwest australia has resulted in the development of a suture between the sumba ridge and sawu timor terranes along a zone of intraforearc convergence the developing suture varies from the low angle sawu thrust with attendant mud diapirs in the sumba basin to high angle reverse faults near a basement high of the underthrust sumba ridge terrane bottom currents associated with the flow of pacific ocean deep water into the indian ocean have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture this study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages

geometry and mechanism of faulting of the 1980 el asnam algeria earthquake from inversion of teleseismic body waves and comparison with field observations the el asnam earthquake of october 10 1980 ms 7 3 provided a wealth of geological and seismological data and is an ideal event for comparing geologically and seismologically derived models the event produced extensive surface faulting in addition to the main tectonic deformation which is clearly a thrust widespread secondary normal faulting was observed at the surface in the southwest region the surface break of a thrust fault could be traced for 24 km in the northeast part of the fault zone normal faults with throws of several meters were observed although no clear thrust type surface breaks were observed in this region geodetic measurements and aftershocks indicate that thrusting was nonetheless the main tectonic mode of deformation in this paper the rupture process of the el asnam earthquake is investigated by inversion of teleseismic p and sh waves the effort is concentrated on the late part of the waveforms which carries information about the faulting in the least understood north eastern segment of the fault zone during the earthquake rupture initiated at the southern terminus of the southwestern fault segment and propagated northward as indicated by the epicentral location and the observable azimuthal directivity of the body wave shapes and amplitudes the seismogenic faulting in the southwestern segment has a thrust mechanism with the following average parameters seismic moment 3 9 1019 n m depth 6 km strike 220 dip 46 and rake 72 the duration of rupture on this fault segment was approximately 10 s which together with the observed fault length of 24 km indicates a rupture velocity of 2 4 km s unconstrained inversions suggest normal faulting in the northeast part of the source region that began immediately following the arrival of the rupture to the northeast region the normal faulting mechanism is in agreement with that observed for the large surface fault scarps in this region guided by the evidence from aftershocks and geodetic measurements a solution with a thrust type mechanism was also sought using a priori constraints based on the field observations the observed strike and slip angle we obtain the following thrust source mechanism for the northeast segment seismic moment 3 6 1019 n m depth 6 km strike 230 dip 20 and rake 91 this solution requires that the thrusting in the northeast be initiated 10 s after the arrival of the rupture from the southwest suggesting that some time is required to overcome the geometric barrier marked by the abrupt change in the dip and azimuth inversions are nonunique with respect to the seismic moment associated with normal faulting the ambiguity being caused by the limited bandwidth of the observed body waves the moment estimated from the surface waves at periods about 300 s however favors thrust faulting to be dominant in the northeast region

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

seismological evidence for shallow thrusting north of the timor trough summary the eastern sunda arc in indonesia is the site of subduction of the continental crust of australia and its shallow seismicity is manifested by infrequent large ms 7 earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc in the same region volcanic activity has ceased and small shallow earthquakes are rare we examine two of the larger earthquakes the events of 1962 may 15 ms 7 2 and 1977 august 27 ms 7 0 the 1977 event occurred north of central timor and the inversion of long period p and sh waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea level and a seismic moment of 4 1 1026 dyne cm the p wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone the wetar thrust zone at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone a broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred the 1962 earthquake occurred approximately 300 km north east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the banda arc the similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism modelling of pdif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km for which a seismic moment of 5 1026 dyne cm is estimated these fault plane solutions indicate that most of the seismic energy released by the collision between australia and the sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent we suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin the banda sea southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc s polarity

seismic response to slab rupture and variation in lithospheric structure beneath the savu sea indonesia article i nfo variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc continent collision in the banda arc are constrained by focal mechanism solutions from the cmt earthquake catalogue in particular the slab under the western savu sea is unusual in that intermediate depth 70 300 km events indicate that at this depth range the slab is largely in down dip compression this contrasts with the intermediate depth down dip tension that typifies the sunda slab to the west and the far eastern banda slab to the east down dip compression beneath the savu sea reflects subduction of transitional crust of the scott plateau more buoyant than the indian ocean crust subducting further west in this region enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc and suggests that down dip compression reflects not only more buoyant transitional crust but also a reduction in slab wedge coupling induced by enhanced magma flux east of the savu sea the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where australian continental crust has collided with the arc differences in seismic moment release around this slab window indicate asymmetric rupture propagating to the east at a much faster rate than to the west

thrusting of the hindu kush over the southeastern tadjik basin afghanistan evidence from two large earthquakes we infer from the mechanisms and depths of two large earthquakes that the hindu kush is actively thrusting northwest over the tadjik basin and that the basin is closing rather than being displaced to the west teleseismic body waves were used to determine focal mechanisms and depths for the two largest shallow earthquakes on the southern edge of the basin the two earthquakes on june 24 1972 mb 6 0 and december 16 1982 mb 6 2 have seismic moments of 2 1018 n m and 6 1018 n m respectively focal mechanisms of both events indicate almost pure thrust faulting with nodal planes striking northeast southwest the inferred fault planes dip southeast at 20 for the first event and 50 for the second the p axes for both events are oblique to the direction of relative motion between india and asia suggesting that the pamir is overthrusting the basin to the west depths for both earthquakes are between 20 and 25 km and place them well below the tadjik basin sediments the depths and steep fault planes suggest that these earthquakes represent a downdip extension within the basement of shallow folding and thrusting seen in the sediments northwest of the events thus convergence in afghanistan between india and eurasia is taken up along southeast dipping thrust faults north of the hindu kush as well as by northward subduction under the southern part of the range

structure along the northwest edge of the snake river plain interpreted from seismic refraction the results of a seismic refraction survey at the idaho national engineering laboratory along the northwest boundary of the snake river plain show that velocities of the volcanic rocks beneath the plain increase from 1 5 km s at the surface to 5 2 5 5 km s at depths between approximately 2200 and 2500 m an exploration well in the area inel 1 encountered cenozoic volcanic rock with some interbedded sediments to a depth of 2460 m underlain by 700 m of a rhyodacite porphyry the refraction data demonstrate a nearly vertical faultlike discontinuity of about 1000 m displacement beneath the plain about 1 8 km from the southeast flank of the arco hills lost river range whether this discontinuity represents the northwest flank of the snake river plain graben the east flank of the lost river range fault or a caldera wall is not known the results further indicate that paleozoic rocks may extend beneath the plain as far as 5 6 km southeastward from the arco hills

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

the wetar back arc thrust belt eastern indonesia the effect of accretion against an irregularly shaped arc the 350 km long east west trending wetar back arc thrust belt in eastern indonesia is a result of the collision of australia with the indonesian island arc along the northwest margin of wetar island a short 50 km section of the thrust belt trends northeast coincident with the offset of the indonesian arc by the newly discovered left lateral wetar atauro fault which runs along the shelf region and trends northeast the wetar atauro fault may be viewed as a large scale lateral ramp or wrench fault separating the eastern wetar thrust belt from the western wetar thrust belt the interaction of strike slip faulting and back arc thrust faulting creates several arc blocks whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the wetar back arc thrust belt the varying orientations of the arc backstop make the wetar back arc thrust belt a perfect laboratory for the study of oblique convergence along most of its length the wetar thrust belt parallels the arc slope however along the northeast trending offset in the thrust belt seamarc ii side scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope rather it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction the side scan images allow us to map the geometry of four main thrust faults the wetar liran atauro and alor faults the wetar liran and atauro faults trend northeast parallel to the thrust front and do not have an en echelon relationship the westernmost fault in the survey area the alor fault trends almost east west the orientation of the wetar atauro fault is consistent with a maximum principal stress within the arc oriented between 8 and 23 west of north depending on the internal coefficient of friction this agrees well with the p axes of nearby earthquake focal mechanisms which trend consistently west of north suggesting that the regional principal tectonic compression is oriented west of north because the thrust belt is oblique to the arc slope we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress the tectonic compression due to arc continent collision in this region may be modified by the arc geometry in several ways 1 the stress necessary to support the arc topography may be significant close to the arc 2 the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress and 3 the difference in material properties between the relatively rigid arc bulldozer and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc the structure of the wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate convergence

accretion of the southern banda arc to the australian plate margin determined by global positioning system measurements global positioning system geodetic measurements at thirteen locations in indonesia and four in australia reveal that the australian continent has accreted the banda island arc to its margin small relative velocities of five sites on west java south kalimantan bali and south sulawesi define a rigid sunda shelf that moves relative to northern australia in a manner consistent with pole locations from nuvel 1 australia eurasia but at a rate that is about 7 slower block like northward motion of the southern banda arc toward the sunda shelf at nearly the same rate as australia suggests that the timor trough is now inactive as a thrust little of the convergence of australia with eurasia is accommodated by strain within the banda arc structure most of the convergence appears to occur as northward translation of the rigid arc with shortening on the flores and wetar thrusts and possibly on faults within the back arc basin

chemistry and stromatolites of the sea linked satonda crater lake indonesia a recent model for the precambrian sea abstract the abundance of in situ calcified stromatolites in the precambrian paleontological record and their absence from modern seas is still an enigma we report on the discovery of the first recent in situ calcified stromatolites growing in a sea linked environment the stromatolites are produced by coccoid cyanobacteria and started to grow 4000 yr ago in the crater lake of satonda island indonesia compared to seawater the lake has gained alkalinity and lost ca its ph 8 45 as well as its saturation with respect to calcite is significantly higher than in seawater the additional alkalinity is provided by biogenic co2 and subsequent weathering of volcanic silicates satonda crater lake may provide an analogue to the stromatolite sustaining precambrian marine environments which therefore may have been more alkaline than present day seawater this conclusion is congruent with the recently advanced hypothesis of an early soda ocean

a dual doubly vergent orogen in the banda arc continent arc collision zone as observed on deep seismic reflection profiles new deep seismic reflection profiles across the banda arc of indonesia reveal reflectors in the uppermost 50 km of the lithosphere combined with existing earthquake hypocenter locations and focal mechanisms the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m y and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge a clearly defined wadati benioff zone and recent deformation of shelf sediments observed on older shallow seismic profiles indicated to previous workers that substantial convergence occurred at the timor trough a few focal mechanisms seafloor escarpments and recent geodetic surveying indicate that convergence at 7 cm yr 1 currently last 200 kyr occurs at the northern margin of the now inactive volcanic arc the wetar thrust zone reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the timor trough and wetar thrust specifically basement reflectors beneath the toe of the accretionary complex have reverse sense offsets that imply blind thrusts the whole crust is horizontally shortened not only the sedimentary cover rocks that previously deformed into duplexes above a decollement reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting thus each arc represents a doubly vergent fold and thrust belt but only the northern one is currently active crustal thicknesses inferred from seismic velocities reflectors and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3 4 km of topographic relief in the region of eastern timor

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

the geometry of back arc thrusting along the eastern sunda arc indonesia constraints from earthquake and gravity data the flores earthquake of december 23 1978 represents the first seismological evidence for active back arc thrusting behind the eastern sunda arc this mb 5 8 earthquake occurred north of flores island where seismic reflection profiling has revealed back arc thrusting interpreted to be a reaction to compression of the arc following collision with the australian continent we investigate the source mechanism and depth of the flores earthquake by inversion of long period p waveforms and relocate the earthquake s epicenter incorporating arrival time data from local stations we find that this event occurred beneath the accretionary prism south of the back arc thrust zone at a depth of 11 5 km 7 km below the seafloor the best fitting fault plane solution is consistent with active southward thrusting of the floor of the flores basin beneath the volcanic arc along a 30 dipping fault plane the morphology of the thrust zone and the free air gravity profile over the flores basin in the epicentral region resemble those of oceanic trenches and the observed gravity field is best interpreted if the crust of the flores basin dips smoothly below the accreted wedge at an average angle of about 6 the position of the earthquake hypocenter south of the flores thrust and the fault plane solution suggest that the 1978 earthquake is analogous to thrust events at subduction zones and represents slip between the subducting and overriding plates the flores thrust is thus the surface expression of a deep seated thrust zone and may represent the initial stage of polarity reversal of the eastern sunda arc

microearthquake seismicity and fault plane solutions related to arc continent collision in the eastern sunda arc indonesia the collision of australia with the eastern sunda and banda arcs of indonesia represents a modern example of the early stages of arc continent collision in order to obtain a more detailed view of the tectonics of the collision zone a microearthquake survey was conducted around the savu sea the region that encompasses the transition from subduction of indian ocean lithosphere on the west to collision or subduction of australian continental litho sphere on the east intermediate depth earth quakes were concentrated near pantar island the easternmost active volcano of the sunda arc and outline a northwest dipping zone that strikes n65oe from 70 to 150 km depth the seismic zone probably marks the western edge of the collision zone and its more northerly strike and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km because the convergence rate and the timing of collision from geologic observations on timor suggest that the oart of the slab now at 150 km depth was at the trench when australia first collided with timor we infer that continental crust or at least rifted continental margin crust caused the bend in the convergent margin and has been subducted to 150 km depth fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone along which a northwest side down sense of dis placement is indicated these events are concentrated at the southern top side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50 100 km depth range beneath the eastern savu sea fault plane solutions for earthquakes within the upper part of the detaching slab 70 100 km depth have p and t axes whose trends lie parallel to the strike of the seismic zone and horizontal southeast trending b axes these earthquakes are interpreted as due to stresses caused by the bend in the subducting plate

active tectonics of the eastern sunda and banda arcs the tectonics of the eastern sunda and banda arcs of eastern indonesia are inferred from the centroid depths seismic moments and fault plane solutions of 63 large earthquakes that occurred between 1962 and 1984 source parameters are estimated by inverting long period teleseismic p and sh waves earthquakes show that the collision of the australian continent with the banda arc shortens the upper plate beneath the forearc and back arc basin over the broad area between the timor and seram troughs along the eastern end of the sunda arc and along the southern banda arc earthquakes reveal that the arc and forearc respond to collision by shortening in the direction of convergence by elongating in the direction perpendicular to convergence and by thrusting over the back arc basin shallow thrust and strike slip earthquakes beneath the banda basin the back arc basin demonstrate that deformation within the back arc accommodates some of the northward motion of the australian continent the rate of north south shortening of the entire upper plate at the longitude of timor calculated from the seismic moments of the earthquakes over the 22 year period is roughly 20 of the predicted convergence rate between australia and southeast asia strike slip faulting within the banda basin results also in eastward motion of the banda arc with respect to western indonesia and australia eastward extrusion of the banda arc results in thrusting at the aru trough earthquakes due to thrusting of the back arc basin beneath the volcanic arc are too shallow to corroborate that the arc s polarity has reversed although the wetar back arc thrust zone at present has a high seismic slip rate earthquakes and their mechanisms provide evidence that the lithosphere of the weber basin a 7500 m deep forearc basin rests on the subducting lithosphere without intervening asthenosphere so that the weber basin subsides in response to sinking of the subducting lithosphere closure of the banda basin is geometrically incompatible with the australian plate subducting simultaneously beneath both the timor and seram troughs instead the bird s head which subducts beneath seram is decoupled from the australian plate in western new guinea and probably moves west or southwest with respect to australia in the eastern indonesian example strike slip faulting plays a major role in the evolution of the collision zone and clearly demonstrates the complexity in three dimensions of continental accretion and mountain building by arc continent collision

seismicity fault plane solutions depth of faulting and active tectonics of the andes of peru ecuador and southern colombia the intracontinental seismicity of the andes of peru ecuador and southern colombia is concentrated along the easternmost flank of the cordillera beneath the western margin of the sub andes the focal depths and fault plane solutions of the largest events were constrained by comparing the observed long period p waves with synthetic waveforms in general the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest southeast and reflect crustal shortening perpendicular to the range probably in response to the subduction of the nazca plate to the west earthquakes in the sub andes occur at depths of between 8 and 38 km indicating that much of the crust deforms in a brittle manner the seismicity seems to reflect antithetic underthrusting of the brazilian shield beneath the eastern margin of the andes however the earthquakes are too deep and their nodal planes are too steep to be associated with the thin skinned decollement of the sedimentary cover instead they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub andes hence the earthquakes on the east side of the andes may reflect the style of deformation in the hinterland of fold and thrust belts such as in the canadian rockies while much of the decollement farther east occurs aseismically in contrast normal faulting occurs in the high andes on planes parallel to the strike of the mountain belt thus there is a delicate balance between the compressive stress applied to the andes in the direction of subduction which causes thrust faulting in the sub andes and the gravitational body forces acting on the topographically higher parts and the crustal root of the andes which may cause normal faulting at high elevation

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

paleomagnetic evidence for the paleoposition of sumba island indonesia abstract jurassic paleomagnetic directions have been obtained for the sedimentary rocks in sumba the mean direction is 59 2 in declination and 44 2 in inclination the reliability of the direction is ascertained through thermal demagnetization and the presence of normal and reversed polarities comparison with the permian paleomagnetic direction of timor indicates that sumba was subjected to a clockwise rotation through 79 4 relative to timor since the jurassic when sumba is restored by a counter clockwise rotation the sumba pole is gradually approaching the jurassic pole of australia as well as the timor permian pole this implies that at least until the jurassic sumba and timor were situated at the australian continental margin and that sumba rotated clockwise during or after the jurassic