Showing papers in "Tectonics in 2001"

Stratigraphy, structure, and tectonic evolution of the Himalayan fold-thrust belt in western Nepal

Abstract: Regional mapping, stratigraphic study, and 40Ar/39Ar geochronology provide the basis for an incremental restoration of the Himalayan fold-thrust belt in western Nepal. Tectonostratigraphic zonation developed in other regions of the Himalaya is applicable, with minor modifications, in western Nepal. From south to north the major structural features are (1) the Main Frontal thrust system, comprising the Main Frontal thrust and two to three thrust sheets of Neogene foreland basin deposits; (2) the Main Boundary thrust sheet, which consists of Proterozoic to early Miocene, Lesser Himalayan metasedimentary rocks; (3) the Ramgarh thrust sheet, composed of Paleoproterozoic low-grade metasedimentary rocks; (4) the Dadeldhura thrust sheet, which consists of medium-grade metamorphic rocks, Cambrian-Ordovician granite and granitic mylonite, and early Paleozoic Tethyan rocks; (5) the Lesser Himalayan duplex, which is a large composite antiformal stack and hinterland dipping duplex; and (6) the Main Central thrust zone, a broad ductile shear zone. The major structures formed in a general southward progression beginning with the Main Central thrust in late early Miocene time. Eocene-Oligocene thrusting in the Tibetan Himalaya, north of the study area, is inferred from the detrital unroofing record. On the basis of 40Ar/39Ar cooling ages and provenance data from synorogenic sediments, emplacement of the Dadeldhura thrust sheet took place in early Miocene time. The Ramgarh thrust sheet was emplaced between ∼15 and ∼10 Ma. The Lesser Himalayan duplex began to grow by ∼10 Ma, simultaneously folding the north limb of the Dadeldhura synform. The Main Boundary thrust became active in latest Miocene-Pliocene time; transport of its hanging wall rocks over an ∼8-km-high footwall ramp folded the south limb of the Dadeldhura synform. Thrusts in the Subhimalayan zone became active in Pliocene time. The minimum total shortening in this portion of the Himalayan fold-thrust belt since early Miocene time (excluding the Tibetan zone) is ∼418–493 km, the variation depending on the actual amounts of shortening accommodated by the Main Central and Dadeldhura thrusts. The rate of shortening ranges between 19 and 22 mm/yr for this period of time. When previous estimates of shortening in the Tibetan Himalaya are included, the minimum total amount of shortening in the foldthrust belt amounts to 628–667 km. This estimate neglects shortening accommodated by small-scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening.

Extension and basin formation in the southern Andes caused by increased convergence rate: A mid-Cenozoic trigger for the Andes

Abstract: The southern Andes between 33° and 45°S latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin, which itself was characterized by volcanic activity. The basins are filled with thick (up to 3000 m) accumulations of interbedded sedimentary and volcanic strata of late Oligocene-early Miocene age. We interpret that these basins developed during a phase of moderate extension within the plate margin system, triggered by an increased rate of convergence of the Farallon (Nazca) and South American plates between 28 and 26 Ma. This history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling. Although extensional basins of this age are only well-described in the southern Andes, the convergence history and volcanic chronology are similar farther north in the central Andes (18°–33°S), leading to the speculation that extension may have characterized the late Oligocene-early Miocene interval regionally. We hypothesize that this extension was a necessary condition to subsequent building of the modern Andes Mountains.

Southward extrusion of Tibetan crust and its effect on Himalayan tectonics

Abstract: The Tibetan Plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during India-Asia collision, and the contrast in potential energy between the Plateau and its surroundings strongly influences the modern tectonics of south Asia. The distribution of potential energy anomalies across the region, derived from geopotential models, indicates that the Himalayan front is the optimal location for focused dissipation of excess energy stored in the Plateau. The modern pattern of deformation and erosion in the Himalaya provides an efficient mechanism for such dissipation, and a review of the Neogene geological evolution of southern Tibet and the Himalaya shows that this mechanism has been operational for at least the past 20 million years. This persistence of deformational and erosional style suggests to us that orogens, like other complex systems, can evolve toward “steady state” configurations maintained by the continuous flow of energy. The capacity of erogenic systems to self-organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far-field plate interactions.

Crustal reworking at Nanga Parbat, Pakistan: Metamorphic consequences of thermal-mechanical coupling facilitated by erosion

Abstract: Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.

Style and history of Andean deformation, Puna plateau, northwestern Argentina

Abstract: Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE-SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line-balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW-ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE-SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip-slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.

Shallow slab detachment as a transient source of heat at midlithospheric depths

Abstract: Slab detachment (or breakoff) has been proposed as a cause of temperature-related processes associated with subduction, such as postcollisional magmatism, mineralization, and metamorphism In this study, we quantitatively investigate the breakoff process and the subsequent thermal evolution of a plate boundary region involving continental collision after a prolonged period of oceanic lithosphere subduction Our two-dimensional time-dependent thermomechanical modeling shows that the dense, oceanic part of the slab can become detached at depths as shallow as 35 km The detached part of the slab sinks into the mantle, creating a gap in the lithospheric system which is filled with upwelling hot asthenosphere The resulting temperature increase in the overlying material can be more than 500°C It allows for partial melting of the asthenosphere and the overriding metasomatized lithosphere for a timespan of a few millions of years Crustal anatexis and related magmatism and mineralization can proceed over a considerably longer period The quantification of the conditions required for shallow slab detachment will contribute to warranted assessments concerning the role of slab detachment (relative to other proposed heat sources, such as tectonically accreted radioactive material) in the geodynamical evolution of former convergent plate boundaries

Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern Madagascar, with implications for Gondwana studies

Abstract: Southern Madagascar comprises a complex Precambrian terrain of high-grade metamorphic rocks with a history of polyphase deformation and metamorphism. Two prominent N-S trending late Neoproterozoic ductile shear zones, the Ampanihy and Vorokafotra shears, each with projected strike length of > 450 km and between 10 and 20 km in width, crosscut the region. A third set of en echelon shears forms part of the early Paleozoic Ranotsara Shear Zone that cuts the basement in a NW-SE direction over a combined strike length of > 400 km. The host rocks of these shears comprise paragneisses (metasediments) with detrital zircons ranging in age between 720 and 1900 Ma. A felsic layer, interpreted as a metavolcanic rock, gives a date of 722±1 Ma. Remnants of late Archean orthogneisses in the central part of the study area may represent basement to the paragneisses. Four episodes of deformation and metamorphism have been recognized on the combined basis of field observations, petrogenesis, and U/Pb analyzes of zircons, monazites, sphenes, and rutiles. Two episodes of early simple shear deformation (D1 and D2) at midcrustal levels occurred between 627 and 647 Ma, during which northeast verging recumbent sheath folds and ductile thrusts were formed and peak prograde metamorphism reached 7–12 kbar at 750°–900°C. Early prolate mineral fabrics (L1/L2) are preserved in massif-type anorthosite bodies and their marginal country rocks. D1 occurred between 630 and 647 Ma, while D2 occurred at 627-628 Ma. This was followed by a 10–15 Myr period of static, annealing metamorphism until 609–614 Ma when bulk shortening (D3) took place. D2 and D3 are coaxial but are separated in time by leucocratic dykes that intruded between 610 and 620 Ma. D3 was focused zonally, forming the prominent N-S shear zones between 607 and 609 Ma; its oblate strain resulted in a strong composite D2/D3 fabric defined by subvertical S-tectonites and subhorizontal intersection lineations. A variety of post-D3 pegmatites accompanied ∼85 Myr of relatively static annealing and metasomatic/metamorphic mineral growth, during which numerous occurrences of phlogopite, uranium, and rare earth elements formed. A continuum of concordant monazite dates suggests that this thermal event is part of an extended period of low-pressure (3–5 kbar) charnockite-producing processes between 520 and 605 Ma. The continuum, however, appears to be punctuated at ∼580, 550, and 520 Ma. Deformation (D4) recorded within the Ranotsara Shear Zone overlaps with the youngest parts of the regional metamorphic conditions between 520 and 550 Ma. Prevailing low-pressure, high-temperature amphibolite-granulite facies rapidly gave way to greenschist facies conditions between 490 and 530 Ma, as is evident from overlapping ages of zircon, monazite, sphene, and rutile. We conclude that D1 to D3 represents a period of 40 Myr of compressional deformation that we interpret to be related to collisional events during the amalgamation of Gondwana. The first part of the thermal continuum between 550 and 605 Ma reflects ∼55 Myr of slow cooling and annealing at midcrustal levels, while the onset of the last episode, between 520 and 530 Ma, heralds accelerated exhumation accompanied by extensional tectonics between 490 and 520 Ma. We believe that this postcollisional time span represents a prolonged period of evolution of a Tibetan-style plateau into an Aegean-style extensional terrain. This ∼100 Myr event in southern Madagascar is similar to that recorded throughout large sectors of the East African Orogen between ca. 500 and 600 Ma. We believe that this type of postconvergent thermotectonism best represents the original definition of “Pan-African” [Kennedy, 1964], which in today's terminology equates with “postorogenic extensional collapse” [Dewey, 1988], or “destabilization of an orogen” [Lipps, 1998]. Kennedy's Pan-African was widespread throughout the interior a supercontinent, when Gondwana's periferal margins were subjected to far-field tensional forces. This suggests that neither gravitational collapse of the Pan-African-Braziliano Orogens nor delamination were the sole or even the dominant driving forces for the postconvergent extension.

Tectonic erosion and consequent collapse of the Pacific margin of Costa Rica: Combined implications from ODP Leg 170, seismic offshore data, and regional geology of the Nicoya Peninsula

Abstract: The convergent margin off the Pacific coast of the Nicoya Peninsula of Costa Rica exhibits evidence for subduction erosion caused by the underthrusting Cocos plate. Critical evidence for efficacy of this process was recovered at the Ocean Drilling Program (ODP) drilling Site 1042 (Leg 170), positioned ∼7 km landward of the Middle America trench axis off the Nicoya Peninsula. The primary drilling objective at this site was to identify the age and origin of a regionally extensive and prominent seismic discontinuity, the so-called base-of-slope sediment (BOSS) horizon or surface. The BOSS horizon, which can be traced landward from near the trench to the Nicoya coastal area and parallel to it for hundreds of kilometers, separates a low-velocity (∼ 2.0–2.5 km s−1) sequence of slope sediment, from an underlying sequence of much higher-velocity (>4–4.5 km s−1) rock. Site 1042 reached the acoustically defined BOSS horizon at a below sea level depth of ∼ 3900 m and yielded a carbonate-cemented calcarenitic breccia of early-middle Miocene age. Sedimentological, geochemical, paleontological, and cement paragenesis data document that the breccia accumulated in a shallow water depositional environment. On the basis of coastal exposures, the BOSS horizon, as a margin-wide geologic interface, can be temporally and lithostratigraphically correlated to a regional angular unconformity. This unconformity, known as the Mal Pais unconformity, separates Neogene and younger shelf-to-littoral beds from the underlying mafic units of the Mesozoic Nicoya Complex and Cretaceous and early Tertiary sedimentary sequences. At Site 1042 it is inferred that tectonism caused the vertical subsidence of the early Neogene breccia from a shallow to a deep water setting. The Mal Pais unconformity of the BOSS horizon thus connects the rock fabric of the outermost part of margin to that of coastal Nicoya and implies that in the early Neogene the Nicoya shelf extended seaward to near the present trench axis. This circumstance requires that the early Neogene trench axis was at least 50 km seaward of where it is now located. The long-term effects of subduction erosion, similar to those described for the scientifically drilled Japan, Tonga, and Peru margins, best account for offshore and onshore evidence for a post-Paleogene history of crustal thinning and landward trench migration of Costa Rica's Pacific margin. During the past 16–17 Myr the calculated mass removal and landward migration rates are 34–36 km³ Myr−1 km−1 of margin, and 3 km Myr−1, respectively. These values are similar to those found for other Pacific margins dominated by nonaccretionary subduction zone processes.

A reappraisal of the Sibson‐Scholz fault zone model: The nature of the frictional to viscous (“brittle‐ductile”) transition along a long‐lived, crustal‐scale fault, Outer Hebrides, Scotland

Abstract: The widely cited Sibson-Scholz conceptual fault zone model suggests that seismically active, upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10–15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep. The crustal-scale Outer Hebrides Fault Zone (OHFZ) in NW Scotland has been described as the type example of such a continental fault zone. It cuts Precambrian basement gneisses and is deeply exhumed, allowing direct study of the deformation products and processes that occur across a wide range of crustal depths. A number of fault rock assemblages are recognized to have formed during a long-lived displacement history lasting in excess of 1000 Myr. During Caledonian movements that are recognized along much of the 190 km onshore fault trace, brittle, cataclasite-bearing faults in the west of the OHFZ are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones. Field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust-related crustal thickening or shear heating. Microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids. This led to retrogression, with the widespread development of new finegrained phyllosilicate-bearing fault rocks (“phyllonites”), and the onset of fluid-assisted, grain size-sensitive diffusional creep in the most highly deformed and altered parts of the fault zone. Phyllonitic fault rocks also occur in older, more deeply exhumed parts of the fault zone, implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature (depth) range within crustal-scale faults. Our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal-scale faults containing interconnected networks of phyllosilicate-bearing fault rocks will be characterized by long-term relative weakness and shallow (∼5 km) frictional-viscous transition zones. Similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the San Andreas Fault.

Mesozoic-Cenozoic denudation history of the Patagonian Andes (southern Chile) and its correlation to different subduction processes

Abstract: Fission track (FT) analysis is applied to assess the Mesozoic and Cenozoic thermal and denudational history of the Patagonian Andes between 44° and 51°S and the geologic and geomorphic response of late Cenozoic subduction of the active Chile rise mid-oceanic spreading center on the overriding plate. Seventy-two FT ages from 43 samples are presented. Zircon FT ages indicate fast post intrusion cooling of Cretaceous parts of the Patagonian batholith and previously unreported Miocene magmatic rocks south of 48°S. Metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early Carboniferous and Late Permian. Apatite FT data reveal initiation of accelerated cooling and denudation at ca. 30 Ma at the western margin of southern continental South America followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca. 12–8 Ma at the position of the present-day main topographic divide. This migration is proposed to be related to either coeval eastward migration of the retroarc deformation, the effects of subduction erosion in the overriding plate at the Peru-Chile trench or less likely, shallowing of the angle of subduction. East of the divide, <3 km of denudation has occurred since the Late Cretaceous. Enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca. 28–26 Ma that triggered orographically enhanced precipitation on the west side of the Patagonian Andes allowing increased erosion by fluvial incision and mass transport processes. The actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation.

Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Abstract: Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure-temperature (P-T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat-lying ductile to brittle extensional shear zone. The upper complex consists of a nappe-like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite-bearing sequence showing typical high-P/low-T parageneses (Mg-carpholite and Na-amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene-Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep-seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post-orogenic Neogene basin sedimentation (middle-upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high-P/low-T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene-Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.

GPS‐determination of along‐strike variation in Cascadia margin kinematics: Implications for relative plate motion, subduction zone coupling, and permanent deformation

Abstract: High-precision GPS geodesy in the Pacific Northwest provides the first synoptic view of the along-strike variation in Cascadia margin kinematics. These results con- strain interfering deformation fields in a region where typical earthquake recurrence intervals are one or more orders of mag- nitude longer than the decades-long history of seismic moni- toring and where geologic studies are sparse. Interseismic strain accumulation contributes greatly to GPS station veloci- ties along the coast. After correction for a simple elastic dis- location model, important residual motions remain, especially south of the international border. The magnitude of northward forearc motion increases southward from western Washington (3-7 mm/yr)to northern and central Oregon (-9 mm/yr), con- sistent with oblique convergence and geologic constraints on permanent deformation. The margin-parallel strain gradient, concentrated in western Washington across the populated Puget Lowlands, compares in magnitude to shortening across the Los Angeles Basin. Thus crustal faulting also contributes to seismic hazard. Farther south in southern Oregon, north- westward velocities reflect the influence of Pacific-North America motion and impingement of the Sierra Nevada block on the Pacific Northwest. In contrast to previous notions, some deformation related to the Eastern California shear zone crosses northernmost California in the vicinity of the Klamath Mountains and feeds out to the Gorda plate margin.

Pretectonic and posttectonic emplacements of the granitoids in the south central Okchon belt, South Korea: Implications for the timing of strike-slip shearing and thrusting

Abstract: Structural analyses of three granitic plutons in the south central Okchon belt of the Korean peninsula reveal that the Baegnok and Cheongsan plutons are pretectonic with respect to right lateral strike-slip ductile shearing along the Cheongsan shear zone and later top to the ESE thrusting, whereas the Boeun pluton is posttectonic with respect to these two deformation events. U-Pb sphene age data from the three plutons indicate that the intrusion ages of the Baegnok, Cheongsan, and Boeun plutons are 222.7 ± 2.1 Ma, 216.9 ± 2.2 Ma, and 171.7 ± 1.4 Ma, respectively. Microstructural evidence preserved in deformed rocks from the NE striking Cheongsan shear zone and adjacent, NNE striking thrusts suggests that the right lateral ductile shearing event along the shear zone occurred before the ESE directed thrusting. Since these thrusts crosscut the late Triassic - Early Jurassic Taedong Group, we estimate that the thrusts developed at ∼180 Ma during the Daebo tectonic event, just prior to the intrusion of the Boeun pluton. We also constrain the age of the Cheongsan shear zone at Middle - Late Triassic time, corresponding to the Songrim orogeny, so that the shear zone is not related to deformation along the Middle Jurassic Honam shear zone (∼176 Ma). The Honam shear zone is not a simple tectonic boundary between the Okchon belt and the Yongnam massif but an anastomosing ductile shear zone not only affecting the Okchon belt and the Yongnam massif but also crosscutting the boundary between them at a low angle. We suggest that the right lateral Cheongsan shear zone is a major boundary between the Okchon zone and the Yongnam massif with Paleozoic cover rocks. The Kyonggi massif and the Okchon zone (South China block) were juxtaposed against the Yongnam massif and Taebaeksan basin (North China block) along the Cheongsan shear zone during a late stage of the Middle Triassic Songrim orogeny, probably related to the late Paleozoic - early Mesozoic collision between the North and South China blocks.

Oligo‐Miocene midcrustal subhorizontal shear zone in Indochina

Abstract: New structural observations of Oligo-Miocene deformation in north Vietnam along the Red River Shear Zone suggest that left-lateral strike-slip shear was restricted to the upper and middle crust above a horizontal shear zone. Left-lateral shear deformation is associated with low-pressure-low-temperature parageneses. High-temperature deformation is restricted to zones where the foliation has a low dip in the core of the Dai Nui Con Voi antiformal dome. These observations complete those made earlier in the Bu Khang extensional dome farther south. Above the horizontal shear zone left-lateral transpression was active during the first stage and changed to transtension some 33 Myr ago. Purely extensional metamorphic domes (Bu Khang) or transtensional domes (Dai Nui Con Voi) were exhumed during this younger stage. Our observations plead for caution when interpreting strike-slip structures at lithospheric scale.

Dynamic modeling of the transition from passive to active rifting, application to the Pannonian Basin

Abstract: We examine a number of first-order features of Pannonian basin evolution in terms of the feedback relation between passive far-field-induced extension and active Raleigh Taylor instable upwelling of the astheno- sphere. We show that active mantle upwelling following a phase of passive extension are viable mechanisms ex- plaining the Pannonian basin formation. The dynamic interplay between far-field-driven passive extension and active thinning of the mantle lithosphere by convective upwelling beneath the rift zone is modeled using ther- momechanical finite element methods. Our modeling results predict a first phase of passive lithospheric thin- ning which is followed by a second phase of late synrift to postrift active mantle lithosphere thinning due to buoyancy-induced flow beneath the rift zone. We argue that the pattern of coeval extension in the thinning re- gion and compression in the flanking regions may be explained by the buoyancy forces due to lithosphere thinning. It is demonstrated that timescales of and stresses generated by both processes are comparable. The model appears also to explain the occurrence of late shallow mantle-related decompression melts in the Pannonian region and late regional doming.

Synthrusting metamorphism, cooling, and erosion of the Himalayan Kathmandu Complex, Nepal

Abstract: In this paper we tackle some of the outstanding problems of the Himalaya, in particular the external zone in the Kathmandu Complex, using an integrated approach involving field mapping, microstructure, thermobarometry, and geochronology. The result is a new model showing the evolution of one major Main Central Thrust: therefore we refute suggestions that the Kathmandu Complex is a klippe or separate thrust sheet. Compared to the Main Central Thrust sheet in the High Himalaya, the Kathmandu Complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some 100 km south of the High Himalaya, fairly near the leading edge. Unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the Main Central Thrust wedge was ∼40 km thick on the northern side of the Kathmandu Complex and <20 km thick on the southern margin. Initiation of the Main Central Thrust occurred at ∼22 Ma, possibly during the closing stages of peak amphibolite facies metamorphism; slip at elevated temperature (500°–600°C) continued until ∼14 Ma. This is a little longer than has previously been proposed. In marked contrast to the famous inverted metamorphism on the Main Central Thrust in the High Himalaya, the metamorphic zonal scheme in the Kathmandu Complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base. These mylonites postdate the high-grade regional amphibolite metamorphism and give an illusion of inverted metamorphism. A likely reason for the contrast is that the Main Central Thrust cut up section toward the foreland and therefore at Kathmandu, carries high levels in the metamorphic structure. Our model involves reactivation of the Main Central Thrust at 7–8 Ma as inferred from published monazite and mica ages, but because the Kathmandu rocks show no evidence for high-temperature reactivation at this time, we presume that the late reactivation involved only the internal High Himalaya zone while the Main Central Thrust was inactive in the external Kathmandu zone. We attempt to quantify rates of cooling, exhumation and thrusting during time period 22 Ma to the present.

Late Oligocene‐Neogene evolution of Europe‐Adria collision: New structural and geochronological evidence from the Giudicarie fault system (Italian Eastern Alps)

Abstract: The Giudicarie fault system (Giudicarie sensu stricto, Meran-Mauls, Passeier, Thurnstein, and Jaufen faults) represents a sharp break in the generally E-W strike direction of the orogen-scale Periadriatic fault system and is a key element for understanding the late Oligocene-Neogene evolution of the Alpine chain. The kinematics, timing and magnitude of movements on the Giudicarie fault system are presented here, on the basis of a detailed structural study of the component fault segments combined with zircon and apatite fission track analysis from closely spaced samples. Two main tectonic phases are established: (1) back thrusting of the Austroalpine units over the Southern Alps around 32 Ma, recorded by basement and limestone mylonites along the Giudicarie and Meran-Mauls faults with transport directions toward 100°–110°, and (2) later sinistral transpressive displacement, characterized by structures at the ductile-brittle transition, which overprinted the top to E/ESE thrust-related mylonites but also partitioned into a major system of transcurrent faults in the Southalpine domain. It was during this later event that the Periadriatic fault attained its present-day geometry. However, the amount of sinistral displacement along the Giudicarie system was only ∼15–20 km. The magnitude is established here from the ∼15 km sinistral offset of the Jaufen mylonites across the Passeier brittle fault and the ∼20 km long gap in the otherwise continuous distribution of Oligocene tonalitic lamellae along the Giudicarie fault. A direct structural connection is also established between the Brenner and the Jaufen faults. This constrains the timing of the second phase, since it must postdate the main exhumation phase of the Tauern Window at 20–18 Ma. The results of this study argue against an originally straight Periadriatic fault. The Giudicarie fault formed a restraining bend in this part of the Periadriatic fault system since at least the late Miocene and probably since the late Oligocene.

Geometry and structural evolution of the central Andean backthrust belt, Bolivia

Abstract: The central Andean backthrust belt is a large-scale west vergent thrust system along the western side of the Eastern Cordillera in the generally east vergent Andean fold-thrust belt of Bolivia. Although west vergent structures in the central Andes have been recognized previously, we describe the backthrust belt at a regional scale, emphasizing its implications for the kinematic development of the Andes and the subsequent influence of these kinematics on amounts of tectonic shortening. We use techniques such as line length balancing, restorability, and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and Altiplano. The cross sections are taken to a regional depth of detachment (basement) to examine the relationship between mapped surface structures and inferred subsurface structures. The relationship of the backthrust belt to the Altiplano suggests that the Altiplano basin is a crustal-scale piggyback basin created as a basement megathrust propagated up and over a half-crustal scale ramp located just west of the physiographic boundary of the Eastern Cordillera. This basement megathrust was the means by which a narrow Paleocene fold-thrust belt located to the west of the Altiplano propagated eastward and emerged in the present Eastern Cordillera. The relationship between the basement thrusts and the physiographic boundaries of the Central Andean plateau (as defined by Isacks [1988]) suggests that extensive megathrust sheets (involving crystalline basement or quartzite) may play an important role in the formation of orogenic plateaus. The kinematic development of the Andean fold-thrust belt indicates that the backthrust belt developed as a taper-building mechanism after the basement megathrust overextended the system eastward. The mechanism proposed in this study for the development of the central Andean backthrust belt requires a minimum of 200 km of shortening within the Altiplano/Eastern Cordillera alone. This increases minimum shortening estimates across the fold-thrust belt in Bolivia to as much as 300–340 km.

Metamorphic soles from the Albanian ophiolites: Petrology, 40Ar/39Ar geochronology, and geodynamic evolution

Abstract: In the eastern Mediterranean region the Albanian ophiolites are remnants of one branch of the Tethyan oceanic basin obducted on carbonate margins during the Mesozoic. Pressure, temperature, time, and structural data in the metamorphic soles of these ophiolites place new constraints on the closure of this basin and the emplacement history of oceanic rocks. The petrological and structural work documents an “apparently inverted metamorphic gradient” with granulite facies at the top, decreasing downward through amphibolite, then greenschist facies until unmetamorphosed rocks. Thermobarometry indicates that the granulite facies formed at peak temperatures of 800°–860°C for pressures close to 1 Gpa. These conditions point to the subduction of the ophiolites beneath young oceanic lithosphere to a depth of 30–40 km. A large set of 40Ar/39Ar data has been obtained on the metamorphic soles and some ophiolite intrusives using single grains and bulk mineral populations. Ages range from 160 to 174 Ma, corresponding to the middle Jurassic (Bajocian-Bathonian). Within the same sole the convergence of results using single grains and mineral populations of amphibole and muscovite and the lack of internal age gradient within muscovite are interpreted to result from fast cooling of metamorphic soles. The main result documented by this study is a systematic younging of metamorphic sole ages from south to north, with a difference of 14 Ma along the 140 km length of the belt. The age of igneous rocks, such as plagiogranites and mafic dikes in the ophiolites, is equivalent to that of metamorphic soles, which indicates that the ophiolite was still hot and young at the time of metamorphic sole formation. The emplacement of the Albanian ophiolites is the consequence of a complex tectonic evolution: continental rifting, slow mid-ocean spreading followed by west dipping intraoceanic subduction initiated at the ridge axis, symmetric intraoceanic thrusting with opposite vergence, and at last, emplacement onto the continental margin. The geochronological and stratigraphic data suggest that this evolution lasted only a few million years. Moreover, they indicate that this evolution stopped earlier to the south with the closure of a narrow basin than to the north, where the basin was wider due to faster oceanic spreading.

U-Pb zircon ages from the Indian plate in northwest Pakistan and their significance to Himalayan and pre-Himalayan geologic history

Abstract: U-Pb zircon isotopic ages of seven rock samples are combined with field data to provide constraints on deposition, intrusion, and metamorphism in the northwestern part of the Indian plate (Pakistani) hinterland south of the Indus suture zone in Pakistan. The results suggest deformation, intrusion, and regional metamorphism at ca. 2174 Ma, an event that may correlate with granulite facies metamorphism in the Nanga Parbat area. This was followed by erosion and deposition prior to a second Proterozoic deformation at ca. 1850 Ma, which was associated with widespread intrusion and possibly with low-grade regional metamorphism. Metasedimentary and intrusive rocks of this age are present within the Lesser Himalaya of Nepal but are apparently absent in the High Himalayan crystalline slab north of the MCT. This intrusive/deformational event was followed by erosion and Late Proterozoic (?) deposition with development, in the Cambrian, of an epicontinental, shallow marine shelf. Intrusion in Late Cambrian-Middle Ordovician resulted only in minor uplift/erosion and development of a disconformity. Marine shelf conditions were reestablished in the Middle Paleozoic prior to a widespread intrusive event in the Carboniferous-Permian and normal faulting, erosion, and syndeformational deposition and volcanism in the Late Permian. Marine shelf conditions were reestablished in the Triassic prior to the Himalayan orogeny. Zircons with a concordant age of 89 Ma coupled with field and published isotopic age data suggest that Himalayan deformation and metamorphism in the Pakistani hinterland began between 90 and 75 Ma due to subduction of the Indian plate beneath Indus ophiolitic melange and reached peak amphibolite facies conditions between 70 and 48 Ma. This metamorphism precedes Eocene (54–50 Ma) collision of India with the Kohistan arc complex. Field data suggest that the presently exposed Pakistani hinterland from Afghanistan to Babusar was never significantly overthrust or buried by the Kohistan arc. Rather than initiating the metamorphism, the collision of Kohistan with India resulted in uplift, exhumation, and cooling of the metamorphic pile.

Ductile and brittle shortening, extension‐parallel folds and maintenance of crustal thickness in the central Aegean (Cyclades, Greece)

Abstract: In contrast to previous studies that concentrated on the two-dimensional crustal strain in the central Aegean region (Cyclades), it is shown that NNE-SSW extension via ductile and brittle stretching and low-angle detachments was accompanied and/or alternated with horizontal shortening perpendicular to the stretching direction since at least the early Miocene. Roughly E-W directed ductile shortening produced large-scale overturned and upright folds having axes parallel or slightly oblique to the stretching lineation. Upright folding and arching of low-angle normal faults occurred above the brittle-ductile transition and brittle E-W compression culminated with vertical axis block rotations, strike-slip faults and minor thrusts. Since at least the early Miocene, the structure of the Cycladic massif evolved through alternating and/or coeval increments of horizontal shortening and vertical thinning associated with an approximately constant NNE-SSW stretching. Exact magnitudes of the vertical and horizontal deformational components are difficult to assess. Nevertheless, we note that extensional tectonics that affected the Cyclades during the last 15–20 m.y. have produced no net crustal thinning. We suggest that crustal thickness was maintained by extension-parallel folds, which represent true contractional structures, and that crust was fed into the extended region from its margins.

Role of topography-induced gravitational stresses in basin inversion: the case study of the Pannonian basin

Abstract: Numerical stress models suggest that gravitational body forces associated with elevated topography around sedimentary basins can significantly influence the stress and strain pattern in basin interiors. In the absence of tectonic forces, basins surrounded by high-altitude mountain ranges experience net horizontal compression. Owing to gravitational forces pointing from areas of high gravitational potential energy to subsided basin areas, further lithospheric extension can eventually terminate, leading to a gradual late stage inversion of the entire basin system. Modeling results suggest that the state of recent stress in the Pannonian basin, particularly in its western part, is controlled by the interplay of plate boundary forces, i.e., the counterclockwise rotation and northward indentation of the Adriatic microplate against the Alpine-Dinaric belt, and buoyancy forces associated with the elevated topography and related crustal thickness variation of the Alpine-Dinaric belt. Model calculations show that uplifted regions surrounding the basin system can exert compression on the thinned Pannonian lithosphere of ~ 40-60 MPa that is of the order of the assumed far-field tectonic stress magnitudes. The combined analysis of stress sources of tectonic and gravitational origin helps estimating the magnitude of maximum horizontal compression. High levels of compressional stresses (up to >100 MPa) are concentrated in the elastic core of the lithosphere, consistent with the ongoing structural inversion of the Pannonian basin system.

Alpine plate kinematics revisited: The Adria Problem

Abstract: Tectonic evolution of the Alpine Tethys is controlled by the plate movements of Africa, Europe, and the Adriatic microplate. It is, however, unclear to which extent and at what times the motion of Adria was related to Africa. Kinematic models which assume a rigid connection between Africa and Adria have difficulties in explaining the Alpine rock record. Reconstructions based on the Alpine record are, on the other hand, in conflict with the involved kinematics. To resolve these conflicts, they require complicated motions or the introduction of additional microplates. Here we present a solution which is based on a rigid connection between Africa/Adria during Jurassic/Cretaceous times. Our model requires only four plates, involving Africa, Europe, Iberia, and Adria. It describes a self-consistent kinematic evolution of the western Tethyan microplate motions from Jurassic to Miocene times. The initial (Early Jurassic) plate configuration was found by iterative forward modeling. The resulting Jurassic plate configuration is unusual and provides new insights into Alpine geology. The obtained model is, however, in good agreement with the available geological data and suggests that the assumption of independent movements of Adria during Jurassic/Cretaceous times is not a necessity.

Cenozoic tectonics of the Cape Roberts Rift Basin and Transantarctic Mountains Front, Southwestern Ross Sea, Antarctica

Abstract: We conducted a multichannel seismic reflection survey offshore Cape Roberts, Antarctica, and combined our findings with the results of the Cape Roberts International Drilling Project (CRP). This allows us to interpret Cenozoic tectonics in the southwest sector of the Ross Sea including the history of uplift of the Transantarctic Mountains (TAM) and subsidence of the Victoria Land Basin (VLB). Seismic stratigraphic sequences mapped offshore Cape Roberts arc tilted eastward and thicken into the VLB where they comprise more than half the fill seen on seismic records. Normal faults a few kilometers offshore cut these sequences and define a north trending rift graben. Drilling results from the CRP show that these strata are latest Eocene (?), Oligocene, and younger in age; much younger than previously inferred. We interpret this pattern to be due to an episode of E-W extension and related subsidence that occurred across the major basins in the western Ross Sea during the early Cenozoic. The rift graben offshore and adjacent to Cape Roberts is bounded on the west by a major north trending fault zone. At Cape Roberts this fault system may have from 6 to 9 km of vertical separation. This fault system is part of a larger zone along the coastline in southern Victoria Land that accommodated uplift of the TAM in Oligocene time. We name it here the McMurdo Sound Fault Zone. A late Oligocene angular unconformity that is seen in seismic data and sampled by CRP drilling marks the end of east tilting of the stratigraphic sequences. We interpret this as the end of the main uplift of the TAM coinciding with a change from E-W extension to NW-SE oblique rifting at that time. Uplift of the TAM and subsidence in the VLB may be linked with seafloor spreading on the Adare Trough to the northwest of the Ross Sea between 43 and 26 Ma. This would imply a plate boundary between East and West Antarctica crossing through the western Ross Sea in Eocene and Oligocene time.

Neogene to Quaternary sedimentary basins in the south Adriatic (Central Mediterranean): Foredeeps and lithospheric buckling

Abstract: Up to several kilometers deep sedimentary basins of Neogene to Quaternary age are found in the south Adriatic between the Apennines and the Dinarides/Albanides. They are the Apennines foredeep, the Central Adriatic Basin, and the South Adriatic Basin. Two regional seismic lines, 260 and 170 km long, across the entire system provide hitherto unavailable constraints on the kinematics and dynamics of basin development. The Apennine foredeep imaged along the lines, formed in middle to late Pliocene times through a rapid westward tilting of the basin floor. A paleobathymetry of few kilometers was created in which allochthonous bodies were emplaced and sediments deposited unaffected by deformation and farther rotations. The Central and South Adriatic Basins formed in Neogene to Quaternary times and are both characterized by strong subsidence in their central parts, gradually diminishing toward the edges in the SW and NE. Depocenters did not shift laterally through time. Subsidence rates increase through time, and subsidence continued also following the end of main shortening episodes in the Dinarides/Albanides. The Central and South Adriatic Basins form two crustal-scale synclines with subsidence concentrated in their central parts. Basins' geometries and subsidence patterns indicate that the Adriatic crust has undergone (episodes of) buckling since Miocene times. Wavelengths and amplitudes of such folds change along strike corresponding to mechanical changes of the folded medium.

Syncontractional extension and exhumation of deep crustal rocks in the east Greenland Caledonides

Abstract: Tectonic models for the North Atlantic Caledonides typically show Baltica being subducted below Laurentia prior to late orogenic collapse and exhumation of deep crustal rocks, conveniently explaining the high- and ultrahigh-pressure provinces in the Scandinavian Caledonides. However, these models offer no tectonic explanation for the deformation and overthickening of the overriding Laurentian plate. Here we document major anatectic midcrustal ultraductile top to the (N)W shear zones in the east Greenland Caledonides that may account for this orogenic thickening. U-Pb dating of monazite and 40Ar/39Ar dating suggests that high-grade fabrics (∼ 435–424 Ma) and granites (∼427 and 422 Ma) involved in these zones show that these were contemporaneous with syntectonic granites involved in the Fjord Region Detachment Zone (FRDZ), a major top to the east extensional fault. More than 100 km of top to the NW overthrusting has been documented in the foreland of the orogen, yet we show that rocks in the hanging wall of the thrust were thinned by orogen-parallel extension rather than thickened during emplacement, thereby exhuming the underlying active midcrustal thrust zones. This is documented by Late Silurian (∼419 Ma) to Early Devonian (∼401 Ma) 40Ar/39Ar-muscovite cooling dates from rocks uplifted along major normal faults and intrusion of mafic dikes (∼ 419 Ma) into extensional faults. Folding and thrusting in the foreland probably continued well beyond this time, and underthrust high-grade nappes probably were exhumed in the triangular zone between the thrust faults (west) and the structurally higher FRDZ (east) before final NW vergent thrust emplacement over low-grade sedimentary rocks. Final cooling of the metamorphic core below closure temperatures for Ar diffusion in K-teldspar occurred in the early Carboniferous. Major folds previously regarded as having formed during the main Caledonian event at ∼425 Ma may in part be up to 100 m.y. younger. Some of these folds also deform sedimentary rocks associated with collapse basins of Middle Devonian to Carboniferous age.

Paleostress magnitudes associated with development of mountain belts: Insights from tectonic analyses of calcite twins in the Taiwan Foothills

Abstract: Determinations of stress magnitudes based on inversion of calcite twin data from fault-related folds in the Taiwan Foothills show a decrease of differential stresses during synfolding erosion and exhumation, suggesting that paleodepth of burial largely controlled the levels of differential stresses sustained by rocks. Regional frictional conditions linked to shallow and/or deep decollement tectonics probably also influenced stress magnitudes. Combination of the twin inversion technique with fracture analysis and rock mechanics data provides first-order estimates of principal stress values related to the major Pleistocene shortening event in Taiwan. These stress estimates are compared to previous stress estimates in fold-thrust belts and to available data on differential stress magnitudes in foreland and hinterland domains of orogens. At the scale of an entire orogenic system the most striking point is a decrease of differential stresses from the hinterland toward the foreland. This decrease not only reflects tectonic stress attenuation away from the plate boundary but also suggests a major control by the depth of deformation. Differential stresses estimated from natural deformation consequently provide an independent support to the depth-dependent strength and the frictional behavior of the upper continental crust deduced from laboratory experiments and stress measurements in deep boreholes.

Southeast Baffin volcanic margin and the North American-Greenland plate separation

Abstract: Plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at Moho level and extruded as thick piles of seaward dipping lava formations. The Disko-Svartenhuk area in west Greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward. We present a quantitative tectonic study of this margin, which we tentatively restored in its geodynamic position during the different stages of plate separation between Greenland and North America. Our structural data are constrained with recently published 39Ar/40Ar and new and coherent 40K/40Ar geochronology in dikes of different orientations. The first-order structure is that of a tectonically-driven seaward crustal flexure linked to definitive plate breakup between Greenland and Baffin Island during the Eocene, coeval with the formation of the upper part of the exposed seaward dipping volcanic prism. This flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults. This across-strike structure is correlated to a fundamental along-strike segmentation with the three “segments” adopting a “zigzag” strike. There is a clear increase of extensional strain at the extremities of the segments. Eocene extension trended N060 on average in the northern and southern segments but was NW trending in the central NE trending Nugssuaq segment. We discuss the interpretation of such an extension perpendicular to the different margin segments. From a regional point of view the N060 extension is distinct in orientation from the approximately N-S trend of plate separation between North America and Greenland in the Baffin Bay during the Eocene. However, the extension recorded in the margin is nearly perpendicular to the obliquely spreading Eocene accretion axis in the Baffin Bay. This latter point suggests a mantle or ridge control of the development of the margin over regional far-field lithospheric stress models.

Development of volcanic passive margins: Two‐dimensional laboratory models

Abstract: [1] Continental breakup above an anomalously hot mantle may lead to the development of volcanic margins. Volcanic margins are characterized by (1) thick seaward dipping lava flow sequences, (2) central intrusive complexes associated with dyke swarms parallel to the coast, and (3) high seismic velocity bodies in the lower crust attributable to magma underplating. A conceptual model for volcanic margins development has recently been proposed based on onshore studies of the Greenland margins and the British Tertiary Igneous Province. It is proposed that the long-lived central intrusions are genetically linked to underlying persistent zones of mantle fusion. These localized melting domains (or soft spots), equivalent to small mantle diapirs, may locally soften the extending continental lithosphere. The low-viscosity diapirs would (1) localize tectonic strain and (2) feed the volcanic margin with magma. Thus such soft spots can control the along-strike magmatic and tectonic segmentation of volcanic margins. Recent geophysical investigations appear to show that the along-strike structure of volcanic passive margins is compatible with such a segmentation process. Here we present a set of scaled experiments designed to study how such localized rheological heterogeneities in the sub-Moho mantle may have a mechanical effect on continental breakup. Four-layer models were constructed using sand and silicone putties to represent the brittle and ductile layers of both crust and mantle. The soft spots are simulated by low-viscosity silicone putty emplaced within the brittle material. At the scale of the entire breakup zone, the soft spots display an oceanic-type strength profile defining low-strength zones where continental breakup is initiated. The rift orientation and segmentation are strongly controlled by the distribution of the low-viscosity heterogeneities, rather than by the direction of regional extension. The experiments are compared with the geometry and segmentation of the onshore part of the Greenland volcanic margins.

Numerical modeling of Cenozoic stress patterns in the mid‐Norwegian margin and the northern North Sea

Abstract: Numerical modeling of Cenozoic stress patterns in the northern North Sea and the mid-Norwegian margin is presented, and the sense of potential slip along major fault planes belonging to the two areas is restored. We assume that the main regional source of stresses is the Atlantic ridge push as demonstrated by previous studies. Furthermore, we also assume a nearly consistent NW-SE strike for the far-field stress from continental breakup between Greenland and Norway (earliest Eocene) to present day. First, we applied the commercial two-dimensional distinct element method (UDEC) to simulate Cenozoic stress and displacement patterns in the study area. Variations in rheology and major fault zones were introduced into the model. The More-Trondelag Fault Complex and its inferred continuation into the Shetland Platform forms the major mechanical discontinuity in the model. Second, we used the SORTAN method, developed at the University of Paris VI, to predict the sense of potential slip along major fault planes. The input for the SORTAN model was constrained by the geometry of the selected fault planes and local principal stress directions extracted from the UDEC modeling. Our results show that the More-Trondelag Fault Complex and its inferred continuation into the Shetland Platform act as a weak fault zone. This fault zone divides the study area into two different stress provinces: the continental margin and the northern North Sea. This result agrees well with the observed differences in Cenozoic structural evolution of the two areas. Compressive structures are observed along the continental margin, whereas relative tectonic quiescence characterizes the northern North Sea during the Tertiary. The restored stress patterns in the northern North Sea and the mid-Norwegian margin also agree well with the observed present-day stress configuration. Our analysis demonstrates a method to reconstruct the sense of slip on major fault planes by combining two complementary numerical tools (UDEC and SORTAN). As a result, it is demonstrated that oblique-slip motions are mainly expected, in particular, strike-slip and reverse dip-slip faulting are simulated.