Lithospheric Compression Research Articles

Lithospheric Folding Controls the Intracontinental Orogen of South China: Insight from Numerical Simulation

Abstract Orogenic processes worldwide have been attributed to various deformation mechanisms. However, the significance of lithospheric folding in these processes has often been overlooked and underestimated. Within the South China Block (SCB), a region marked by notable temporal and spatial variability in intracontinental deformation, the emergence of fold-and-thrust belts during the Paleozoic and Mesozoic periods has captured a scientific interest. The mechanisms governing the genesis of these belts remain a subject of debate, with no discernible subduction interface accounting for the extensive-scale fold-thrust deformation. Moreover, the SCB presents a substantial variation in lithospheric thickness, exceeding 100 km, offering a plausible mechanism for lithospheric folding. To interrogate this mechanism, we conducted lithospheric compression simulations via two-dimensional finite element methods, incorporating variable viscosity both laterally and vertically within the SCB. Our models elucidate that disparities in lithospheric strength beget distinctive deformational manifestation within the SCB. We observe that a weaker lithosphere tends to uplift, whereas a stronger lithosphere tends to subside during compression. Lithospheric strength also influences the Xuefengshan uplift and the spatial distribution of deformational features. In addition, lithospheric folding can account for crustal shortening and the presence of deep anomaly structures. A compelling correlation emerges between lithospheric folding and fluctuations in Moho depth and lithospheric thickness, suggesting its potential influence over the prolonged topographical evolution and shifts in depositional environments within the SCB. This study sheds new light on the role of lithospheric folding in the complex geodynamic history of the SCB and highlights its importance in understanding the broader context of orogenic processes worldwide.

Multiple tectonic switches in the Eastern Tianshan, Central Asian Orogenic Belt: Implications for crustal growth and metallogenesis

Subduction zones are crucial sites for inputs of juvenile crust and the generation of porphyry copper deposits by arc magmatic processes that generally involve episodic trench advance and retreat. It is well documented that the generation of juvenile crust is associated with lithospheric extension during the retreat of subduction zones, whereas crustal reworking is associated with lithospheric compression in continental arcs during the advance of subduction zones. However, it remains unclear whether such models can be applied to island arc settings and how they relate to metallogenesis. To address this issue, we examine intermediate–felsic igneous rocks (∼460–300 Ma) of the Dananhu–Harlik island arc in the southern Central Asian Orogenic Belt. Our new and compiled data indicate three magmatic episodes in the Dananhu–Harlik island arc during the northward subduction of the Kangguer oceanic plate, with each involving a tectonic switch from compressional trench advance to extensional retreat. Crustal growth occurred mainly during the compressional stages of trench advance, which is in contrast with typical continental arcs. Tectonic transitions also controlled regional mineralization patterns, and the typical porphyry and related skarn-type deposits formed during subduction zone advance, whereas volcanogenic massive sulfide (VMS) deposits and overprinting mineralization were generated during subduction zone retreat. A combination of all data on tectonic settings and physical–chemical conditions (e.g., temperature, oxygen fugacity, and water content) of the pre-mineralization and ore-related intrusions led us to the conclusion that normal intra-crustal arc magmatic processes, involving sulfide pre-enrichment and reactivation, generated the typical Tuwu–Yandong porphyry Cu deposits in the Dananhu arc without the prerequisite of slab melting. Our study provides an excellent example of crustal growth in an island arc setting as well as metallogenesis during the repeated advance and retreat of a subduction zone.

Warm versus cold crust in the Tien Shan orogenic belt revealed by seismic Lg attenuation tomography

SUMMERY Due to the far-field effect of the India–Eurasia collision, the Tien Shan orogenic belt has been undergoing reactivation and modification. Two end-member models of the geodynamic mechanisms are (1) surface uplift due to crustal shortening caused by lithospheric compression and (2) mountain formation resulting from thermal upwelling of asthenospheric mantle materials generated by lithospheric subduction. However, the topography along the Tien Shan orogenic belt changes significantly, and the deep structure and dynamic process are quite different beneath the Tien Shan orogenic belt from both geological and geophysical observations. Therefore, the reactivation and modification of the Tien Shan orogenic belt are likely influenced by both geodynamic mechanisms, which also generate various thermal anomalies in the crust. Seismic Lg-wave attenuation is very sensitive to crustal material composition and status and can point to the presence of partial melting within the crust resulting from mantle upwelling. In this study, we develop a high-resolution Lg-wave attenuation model between 0.05 and 10.0 Hz in Northwest China and use lateral attenuation variations to infer thermal structures in the crust. The central Tien Shan is characterized by prominent low-QLg anomalies, whereas relatively high-QLg distributions are imaged beneath the eastern and western Tien Shan. The surface uplift and crustal deformation are mostly related to the upwelling of hot mantle materials in the central Tien Shan and are likely induced by lithospheric compression in the eastern and western Tien Shan. However, low-Q anomalies are observed in the junction between the Pamir Plateau and western Tien Shan, indicating that the uplift in the south of the western Tien Shan is related to thermal subduction-induced upwelling and intrusion into the crust due to the collision between the Indian and Eurasian plates. The Kazakh Shield, characterized by pronounced high-QLg values, is likely a cold and hard terrane, and hence blocks the far-field effect of the India–Eurasia collision.

Fractal dimension and area of seismicity in the Baikal Rift System: Implications for modern geodynamics

The fractal geometry and extent of seismicity in the Baikal Rift System (BRS) are estimated from data on 52,700 instrumental events of MLH ≥ 2.5 magnitudes for fifty years (1964–2013). The seismic pattern is characterized by the box-counting Hausdorff dimension D0, multifractal spectra f(α), and surface area S of seismicity at three scales: the rift system as a whole, its three zones, and six subzones. The multifractal spectra record a self-similar hierarchical structure of the BRS seismicity pattern. The space and time variations in the fractal dimension (D0) and area of seismicity (S), which are mapped and plotted as a function of time, show good correlation. The two parameters depend on three related factors: progressive increase in the amount of instrumental data (dataset size), structure of seismogenic fault network, and geodynamic activity. They increase as ever more data appear with time and acquire high local values at increasing extent and density of quakes. Moreover, the obtained D0 estimates reflect statistical self-similarity of earthquake patterns being in the range ≈ 1.45–1.55 over most of BRS, except one zone and one subzone in the rift flanks. They are the highest in the southwest and the lowest in the northeast of the rift system (D0 ≈ 1.60 ± 0.02 and D0 ≈ 1.37 ± 0.02 respectively). This dissimilarity indicates that seismogenic faulting occurs by different mechanisms: distributed failure as a result of superposed global-scale collisional compression and regional rifting in the SW flank and quasi-linear rift propagation in the NE flank. In general, D0 decreases toward the northeastern part of the BRS, where the pattern of earthquakes becomes localized along lineaments instead of being distributed over an area. The space and time variations of D0 and S revealed in the earthquake data are consistent with the location and activity pulses of rifting attractors and provide a realistic explanation of BRS geodynamics and tectonophysics. The global lithospheric compression and the regional pulse-like activity of rifting attractors control the network of seismogenic faults which, in turn, govern the fractal geometry and 2D structure of seismicity in the region. The obtained results confirm the oscillatory dynamics of the regional seismicity at a decadal period correlated with activity pulses of rifting attractors. The oscillations stand out against the background of decreasing global low-frequency secular cycle of the BRS seismicity. The BRS lithospheric geodynamics fits the model of a nonlinear oscillator with dissipation. The suggested analysis of the fractal geometry and extent of seismicity as proxies of the faulting evolution provides insights into modern geodynamics of the Baikal Rift System and its constituents.

Geochronology and geochemistry of the granites from the Zhuxi W-Cu ore deposit in South China: Implication for petrogenesis, geodynamical setting and mineralization

The giant Zhuxi tungsten deposit is located in the Taqian-Fuchun Ore Belt in northeastern Jiangxi province, and genetically associated with the Zhuxi granitic stocks and dykes. Three mineralization-related granites including granite porphyry dykes (GP), biotite granitic stocks (BG), and white granitic dykes (WG), were identified in the Zhuxi deposit. SHRIMP zircon U–Pb analysis for the three granitic rocks present ages ranging from 153.5±1.0Ma to 150.4±1.0Ma. The BG mainly contains quartz, microcline, albite, biotite and muscovite with minor accessory minerals including zircon, apatite, monazite, Ti/Fe oxides, and dolerite. However, the WG is mainly composed of quartz, microcline and albite with minor muscovite and accessory minerals. The GP is a medium-grained porphyritic granite and its phenocrysts include quartz, alkali feldspar, muscovite and plagioclase. All the Zhuxi granites have high SiO2 content (71.97wt%–81.19wt%) and total alkali (3.25wt%–9.42wt%), and their valid aluminum saturation index (ASI) values show a wide range of 1.03 to 2.49. High Rb/Sr ratios, low Sr content (<50ppm) and markedly negative Eu anomalies of GP, WG and BG demonstrated that the Zhuxi granites are highly fractioned and intensive crystal differentiated. Because they display the features of both I- and S-types granites, they were confirmed to be I-S transform-type granites. Whole rock εNd(t) and zircon εHf(t) values fall into the ranges of −6.98 to −11.97, and −3.1 to −11.5, and the Nd (TDM2) and Hf two-stage model ages (TDMc) are 1.51–1.92Ga and 1.42–2.01Ga, respectively. Geochemical and isotopic data suggest that these highly fractionated I–S transform-type granites were originated from magmas which showed affinity with the Proterozoic continent and the Shuangqiaoshan Group and little mantle contribution was involved during the generation of Zhuxi granitic rocks. Extreme fractional crystallization resulted in further enrichment of tungsten in the evolved granitic magma. New data, presented together with previously published data, suggest that the Zhuxi granitic complex was likely to be formed during lithospheric compression setting during the late Jurassic to early Cretaceous. The biotite granite stock predominately contributed to the production of skarn alteration and mineralization, followed by the white granite dyke; the granite porphyry dykes have little effect.

Neogene-Quaternary slow coastal uplift of Western Europe through the perspective of sequences of strandlines from the Cotentin Peninsula (Normandy, France)

The Cotentin Peninsula (Normandy, France) displays sequences of marine terraces and rasas, the latter being wide Late Cenozoic coastal erosion surfaces, that are typical of Western European coasts in Portugal, Spain, France and southern England. Remote sensing imagery and field mapping enabled reappraisal of the Cotentin coastal sequences. From bottom to top, the N Cotentin sequence includes four previously recognized Pleistocene marine terraces (T1 to T4) at elevations <40m as well as four higher and older rasas (R1 to R4) reaching 200±5m in elevation. Low-standing marine terraces are not observed in the central part of the Peninsula and a limited number of terraces are described to the south. The high-standing rasas are widespread all over the peninsula. Such strandline distributions reveal major changes during the Late Cenozoic. Progressive uplift of an irregular sea-floor led to subaerial exposure of bathymetric highs that were carved into rocky platforms, rasas and marine terraces. Eventually, five main islands coalesced and connected to the mainland to the south to form the Cotentin Peninsula. On the basis of previous dating of the last interglacial maximum terrace (i.e. Marine Isotopic Stage, MIS 5e), sequential morphostratigraphy and modelling, we have reappraised uplift rates and derived: (i) mean Upper Pleistocene (i.e. since MIS 5e ~122+/−6ka, i.e. kilo annum) apparent uplift rates of 0.04±0.01mm/yr, (ii) mean Middle Pleistocene eustasy-corrected uplift rates of 0.09±0.03mm/yr, and (iii) low mean Pleistocene uplift rates of 0.01mm/yr. Extrapolations of these slow rates combined with geological evidence implies that the formation of the sequences from the Cotentin Peninsula occurred between 3Ma (Pliocene) and 15Ma (Miocene), which cannot be narrowed down further without additional research. Along the coasts of Western Europe, sequences of marine terraces and rasas are widespread (169 preserve the MIS 5e benchmark). In Spain, Portugal, S England and other parts of western France, the sequences morphostratigraphy is very similar to that of Cotentin. The onset of such Western European sequences occurred during the Miocene (e.g. Spain) or Pliocene (e.g. Portugal). We interpret this Neogene-Quaternary coastal uplift as a symptom of the increasing lithospheric compression that accompanies Cenozoic orogenies.

Potassic magma genesis and the Ailao Shan-Red River fault

Two types of K-rich magma of Eocene to Early Oligocene (ca. 40–30) and Plio-Pleistocene (ca. 5–0.1Ma) age were emplaced prior to and following left-lateral slip on the Ailao Shan-Red River (ASRR) fault, a regional shear zone extending between southwest China and the Tonkin Gulf (South China Sea) that accommodated ‘escape’ of the Indochina block. The first type is exposed in the Dali-Lijiang and adjacent regions of western Yunnan and Sichuan and comprises ultramafic potassic to ultrapotassic ‘absarokites’ and their shoshonite, banakite, and SiO2-rich derivatives which were emplaced immediately prior to activation of the ASRR fault. They are characterized by high Mg.-nos, and low contents of fusible oxides (FeO*, CaO, Al2O3), for equivalent MgO content, and pronounced primitive mantle-normalized high-field strength element (HFSE) depletions. In contrast, ‘post-escape’ K-rich magmas were erupted in the Puer, Maguan-Pingbian regions of south and southeast Yunnan. Apart from their relative enrichments in potassium they show typical HFSE-rich intra-plate compositional affinity. Geological and geomorphic evidence, and thermochronologic age dating of metamorphisc events, suggest that left-lateral shearing occurred between ca. 30 and 17Ma; thereby accommodating the southeastward ‘escape’ of Indochina and (possibly) two episodes of spreading in the South China Sea. The southwestern part of Dali-Lijiang magmatic products was detached and offset by ca. 600km and are now located in Phan Xi Pang in northern Viet Nam. The same is true for the Permo-Triassic Emeishan flood basalts, whose western exposures were likewise displaced by the same amount and are now represented by the Song Da complex, also in northern Viet Nam. Here, we report geochemical, isotopic, and 40Ar/39Ar age data for samples from both the ‘pre-escape’ Dali-Lijiang magmas and the ‘post-escape’ K-rich Puer, Maguan-Pingbian basalts and basanites, with a view to comparing and contrasting their interpolated source compositions, estimated conditions of upper mantle melt segregation and, by inference, their mantle dynamic and contamination histories insofar as these were conditioned by the India-Asia collision. Our interpretations yielded two complementary conclusions. The first contends that the pre-escape magmas result from adiabatic melting of crust-contaminated asthenosphere comprising a ‘mélange’ of continental lithospheric mantle (CLM) (hydrated by sab-derived hysdrous fluids released at 0.2–0.5GPa) and lower crust, delaminated from the overriding plate during mantle wedge corner flow and further enriched by metasomatic melts of subducted continental crust. We postulate that incipient H2O-saturated melting of the ‘mélange’ occurs at depths of between ca. 100 and 200km after being ‘dragged’ down by relict oceanic slab fragments, in response to the dehydration of supra-subduction amphibole- and phlogopite. The ensuing viscosity ‘crisis’ and buoyancy relative to ambient ‘fertile’ convecting mantle of such asthenospheric ‘pockets’, and the collision-related change from lithospheric compression to extension, almost certainly predisposes such a refractory yet crust-contaminated ‘pockets’ to rapid adiabatic melting. The second conclusion concerns the post-escape K-rich basalts and basanites and is based on the contention that decompression melting of thermally anomalous K-rich asthenospheric occurred in response to regional post-escape transtension, concomitant with the cessation Indochina escape and contiguous seafloor spreading. However, although these magmas share the HFSE-rich fertile source character of other, widely dispersed, post-escape Cenozoic basalts they more specifically resemble relatively rare examples of intra-plate, K-rich activity observed in northeast China, central Spain, and elsewhere in Asia and Europe, arguably (indirectly) reflecting mantle perturbations caused by major continental collisions.

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins, Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.

Modeling of salt tectonics

In this work a general framework for the simulation of sedimentary basins in presence of salt structures is addressed. Sediments and evaporites are modeled as non-Newtonian fluids and the thermal effects induced by the presence of salt are taken into account. The computational strategy is based on a Lagrangian methodology with intensive grid adaptivity, together with a kinematic modeling of faults and different kinds of boundary conditions representing sedimentation, erosion, basement evolution, lithospheric compression and extension. The proposed methodology is applied to simple test cases as well as to a geological reconstruction of industrial interest.

Mountains on Io: High‐resolution Galileo observations, initial interpretations, and formation models

During three close flybys in late 1999 and early 2000 the Galileo spacecraft acquired new observations of the mountains that tower above Io's surface. These images have revealed surprising variety in the mountains' morphologies. They range from jagged peaks several kilometers high to lower, rounded structures. Some are very smooth, others are covered by numerous parallel ridges. Many mountains have margins that are collapsing outward in large landslides or series of slump blocks, but a few have steep, scalloped scarps. From these observations we can gain insight into the structure and material properties of Io's crust as well as into the erosional processes acting on Io. We have also investigated formation mechanisms proposed for these structures using finite‐element analysis. Mountain formation might be initiated by global compression due to the high rate of global subsidence associated with Io's high resurfacing rate; however, our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains. The large fraction (∼40%) of mountains that are associated with paterae suggests that in some cases these features are tectonically related. Therefore we have also simulated the stresses induced in Io's crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses. If this mechanism is responsible for some of Io's mountains, it could also explain the common association of mountains with paterae.

Evolutionary model for the Taiwan collision based on physical modelling

2-D and 3-D physical modelling of lithospheric convergence in the Luzon-Taiwan-Ryukyu region is performed with properly scaled laboratory models. The lithospheric model consists of two parts, continental (the Asian Plate, AP) and oceanic (the Philippine Sea Plate, PSP). The oceanic lithosphere has one layer, while the continental lithosphere includes both mantle and crustal layers. The continental margin is covered by sediments. A low-viscosity asthenosphere underlies the lithosphere. The opposing Luzon and Ryukyu subduction zones are initiated by inclined cuts made within the PSP. The subduction/collision is driven by a piston. Pre-collisional intraoceanic subduction along the Luzon and Ryukyu boundaries results in the formation of a transform zone between them, with two tear faults at the ends. The PSP undergoes strong compression along this zone. Subduction of the Chinese margin under the Luzon boundary further increases the compression. Compressive stresses reach the yield limit of the PSP in the arc area, which is a weak zone in the experiments. The plate fails at the western side of the arc along an eastward dipping fault, the Longitudinal Valley Fault. Underthrusting of the frontal wedge of the PSP along this fault results in the closure of the fore arc basin and is then blocked. The PSP fails at the opposite side of the Luzon arc along the westward dipping fault. The failure releases lithospheric compression in this region and results in the initiation of southward-propagating subduction of the PSP under northeastern Taiwan. The incipient subduction zone becomes part of the southeastward-retreating Ryukyu subduction zone, which allows the Okinawa back arc rift to propagate into Taiwan. The Taiwan collision thus includes the following succession of major processes over time, or from south to north: (1) an E-W shortening of the PSP in the Luzon arc; (2) a failure of this plate at the western side of the arc and the formation of the eastward-dipping Longitudinal Valley Fault (the transient plate boundary); (3) a closure of the fore arc basin and a rapid uplift of the orogen; (4) a failure of the PSP at the eastern side of the Luzon arc partly overthrusting the orogen, and the initiation of westward (WN-ward) subduction of the PSP; (5) and finally ‘back arc’ rifting in the rear of this incipient subduction zone (i.e. in northern Taiwan). All these processes commence with some delay with respect to the preceding ones and propagate southwards.

Regional Tertiary Exhumation in and around the United Kingdom

Abstract Sonic velocities in the Upper and Middle Chalk, the Bunter Sandstone and the Bunter Shale were used independently to quantify apparent exhumation (amount of missing section) in the UK Southern North Sea (SNS). Apparent exhumation results derived from these units are statistically similar. The consistency of results from chalks, shales, and sandstones in the UK SNS, and in other areas (Inner Moray Firth and Celtic Sea/South-Western Approaches), suggests that, at a formation and regional scale, burial-depth is the primary control on velocity (and porosity) in these units, and validates the use of lithologies other than shale in maximum burial-depth determination. The consistency of apparent exhumation results from units of Early Triassic to Late Cretaceous age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that maximum Mesozoic-Cenozoic burial-depth was attained prior to Tertiary exhumation. The analysis of sonic velocities, apatite fission tracks, vitrinite reflectance, normalized drilling rate and clay mineral diagenesis all suggest that there was a regional component of approximately 1 km of exhumation during Tertiary times, which affected structurally uninverted areas in and around the UK. The widely-recognized inversion-related exhumation is superimposed on regional Tertiary exhumation. Regional Tertiary exhumation need not have been contemporaneous with structural inversion. Further refinement beyond Tertiary age is not possible from the sonic velocity data. Regional exhumation, however, is most likely to be associated with regional Palaeocene or Oligocene/Miocene unconformities. The evidence for regional exhumation implies that there was a pre-exhumational phase of burial (during which the eroded sedimentary rocks were deposited). Exhumation and prior burial associated with the regional Tertiary event need to be incorporated in aspects of basin analysis such as sediment decompaction and diagenetic and maturation modelling, and can, for example, account for the maturity of the Carboniferous of the onshore East Midlands Shelf, and the Lias exposed on the north coast of Somerset. The regional, Tertiary tectonic uplift associated with exhumation must have had a thick-skinned origin, and, in areas where there is no evidence for significant Tertiary igneous activity, a decoupled, two-layer model of lithospheric compression is invoked to account for the subsidence/uplift patterns of both inverted and uninverted areas.

Quantification of Tertiary exhumation from sonic velocity data, Celtic Sea/South-Western Approaches

Abstract Sonic velocities from the Danian Chalk, the Upper Cretaceous Chalk, the Lower Cretaceous Greensand/Gault Clay, and the Triassic Mercia Mudstone were used to quantify apparent exhumation (i.e. amount of missing section) in the Celtic Sea/South-Western Approaches (southern Irish, south-western United Kingdom, and north-western French continental shelves). Analysis of all four of these stratigraphic units yield similar results. Since it is unlikely that sedimentological and/or diagenetic processes are responsible for similar amounts of overcompaction in the chalks, shaly sandstones and shales analysed, burial at depth greater than that currently observed is the most likely cause of overcompaction (anomalously high velocities). The use of lithologies other than shales in maximum burial-depth/exhumation studies is thus validated. The consistency of results from units of Triassic to Danian age suggests that Tertiary exhumation was of sufficiently great magnitude to mask any earlier Mesozoic periods of exhumation, and that the maximum Mesozoic-Cenozoic burial-depth in the Celtic Sea/South-Western Approaches was attained prior to Tertiary exhumation. The recognized inversion axes of the Brittany Basin, South-West Channel Basin and North Celtic Sea Basin are highs in apparent exhumation, however highs in apparent exhumation of approximately 1 km also occur in the uninverted St. Mary’s and Plymouth Bay Basins, and on the margins of the Armorican and Cornubian Platforms and the Pembroke Ridge. Tertiary exhumation was regional and not restricted to inversion axes, and thus had a thick-skinned origin. Heterogeneous lithospheric compression is the geodynamic model preferred to account for the regional exhumation of the Celtic Sea/South-Western Approaches. The identification of regional exhumation of approximately 1 km has important implications for modelling thermal maturation in the Celtic Sea/South-Western Approaches, with source rocks previously believed to be immature shown to be potentially mature for oil generation. When corrected for exhumation, the post-rift sequence in the Celtic Sea/South-Western Approaches area shows relatively little localization over the syn-rift Jurassic-Lower Cretaceous sequence, suggesting that during basin formation, the lower lithosphere, thermal re-equilibration of which controls post-rift subsidence, did not experience localized thinning on the scale of the syn-rift basins.

Hunting for azimuthal anisotropy beneath the Pacific Ocean region

Lateral variations in azimuthal anisotropy cause significant waveform anomalies in long‐period surface waves (ƒ &lt; 15 mHz, T ≳ 70 s), as a result of coupling between fundamental branch Rayleigh and Love waves. These anomalies, termed “quasi‐Love” (QL) waves, have elliptical polarization, arrive slightly behind the Love wave but prior to the Rayleigh wave, and are observed on many propagation paths in the Pacific Ocean region. Our observations of quasi‐Love waves indicate the existence of strong lateral anisotropic gradients in the western Pacific Ocean, in particular, near Hawaii and seaward of the Tonga‐Kermadec, Kurile and Marianas‐Izu‐Bonin subduction zones. A lack of quasi‐Love generation beneath the Phillipine Sea suggests weak azimuthal anisotropy in the region. In the southwest Pacific Ocean, the long‐period quasi‐Love waveforms recorded at station SNZO (South Karori, New Zealand) can be fit well with a simple anisotropic Earth model with a 90° rotation in the orientation of the fast P velocity axis near the boundary between the Indo‐Australian and the Pacific plates. In this model, 6% P wave velocity anisotropy from the Moho to 210 km depth, with a NNE‐SSW fast direction (roughly parallel to the plate boundary), extends 500–1000 km east of the Tonga‐Kermadec trench, where it shifts to a WNW‐ESE orientation (parallel to fracture zones in the southernmost Pacific plate). This solution is nonunique, however, as a similar lateral variation of 2% S wave anisotropy at asthenospheric depths (100–300 km) generates similar waveform anomalies. We attribute the laterally varying anisotropy to either (1) flow variations in the asthenosphere, consistent with lateral shear detected in deep Tonga‐Kermadec seismicity, (2) variations in fossil spreading direction in the Cretaceous long normal polarity interval, (3) the disturbance of fossil anisotropy caused by the passage of the Louisville Ridge hotspot, and (4) lithospheric compression associated with continental collision along the Alpine Fault. The apparent location of the Love‐to‐Rayleigh “scatterer” favors options 1 and 3. Gradients in azimuthal anisotropy are inferred in the northwestern Pacific, associated with one or more of these mechanisms. A quasi‐Love wave observed on a path from Alaska to PPT (Papeete, Tahiti) is bandlimited to ƒ &gt; 8 mHz, suggesting lateral variations with wavelength twice the spacing of fracture zones in the north central Pacific.

A two‐layer lithospheric compressional model for the tertiary uplift of the southern United Kingdom

Reflection seismic records show that many of the Mesozoic sedimentary basins of the southern United Kingdom were compressed and uplifted (inverted) during the Late Cretaceous‐Tertiary. Recent quantitative studies based on apatite fission track analysis and Chalk porosity data show that uplift and erosion, although concentrated in the inverted basins, was regional, and not limited to the recognized areas of crustal shortening (inversion). A two‐layer, or heterogeneous model of lithospheric compression, similar to two‐layer models of lithospheric extension, may account for this pattern of uplift. Upper‐lithospheric shortening is accommodated by reverse movement on major, reactivated, extensional detachments, causing inversion of pre‐existing basins. Pure shear shortening of the lower lithosphere is decoupled, and may be laterally displaced, from that in the upper lithosphere. Lower lithospheric shortening and thickening displaced from that in the upper lithosphere causes initial subsidence due to the submersion of cold, dense lithosphere into the surrounding asthenosphere. Subsequent thermal re‐equilibration of the lower lithosphere may have generated the uplift without attendant crustal shortening witnessed by apatite fission track and Chalk porosity data.

Mechanical strength of extended continental lithosphere: Constraints from the Western Rift System, East Africa

Although regional isostasy generally is associated with continental lithospheric compression and foreland basin formation, local isostatic compensation commonly is assumed in models of extensional basin formation. The assumption of negligible lithospheric strength during rifting often is justified on the basis of: (1) high heat flow and temperatures produced by elevating the lithosphere ‐ asthenosphere boundary and (2) fracturing of the crust and lithosphere by normal faults. By modeling the development of rift basins within the Western rift system of East Africa and their associated free air gravity anomalies, we assess the role of basin‐producing normal faults in modifying the flexural strength of extended lithosphere. Heat flow and seismicity data from the East African plateau region indicate that the Western rift system located on the western side of the plateau developed in old, cold continental lithosphere. These relatively narrow (40–70 km wide), but deep, basins are bounded along one side by high‐angle border faults that penetrate to lower crustal levels, as indicated by seismicity data. Along the length of the Western rift system, depth to pre‐rift basement and rift flank topography vary between basins from 1 to 8 km and from 1 to 2 km respectively, with deeper basins generally correlating with higher flanks. Comparison of model predictions with topography and free air gravity profiles reveals that the basin depth and the flank height in the majority of the Western rift basins studied can be explained simply by small heaves (3–10 km) across the border fault and with significant flexural strength of the lithosphere maintained during extension. Where both observed basin depth and flank height could not be reproduced, basins were located adjacent to eruptive volcanic centers active in Miocene‐Recent times. In these areas, basin depth, rift flank elevation, and free air gravity anomaly may be modified by magmatic underplating of the crust. Estimates of effective elastic thickness Te obtained from our forward models vary between 17 and 38 km. These Te estimates are consistent with previous values obtained through the wavenumber domain correlation of Bouguer gravity and topography data. We find that the lithosphere beneath East Africa maintains significant flexural strength in extension despite localized, but intense, fracturing of the crust by normal faults that penetrate to lower crustal levels. Therefore the assumption of local isostasy (Te ∼ 0) appears invalid within the youthful Western rift system.

Lithospheric compression flanking spreading axes: A reappraisal

It has been suggested that descending return flows in the mantle beneath the flanks of spreading oceanic ridges produce horizontal compressional stress, which can give rise locally to compressional tectonics. This hypothesis has been advanced in order to account for early subduction or accretion of ophiolites. In the vicinity of present spreading axes, this hypothesis can explain geodetic and geophysical evidence for horizontal contraction and elastic compression. It also fits structural data suggesting embryonic compressional tectonics, and may provide a clue to the explanation of other geological features such as the Great Escarpment at some continental borderlands. From the theoretical point of view, descending flows in the mantle and lithospheric compression relatively close to spreading axes can be predicted by some models of convective spreading. This hypothesis can reduce several independent phenomena to a single process.

A tectonic redefinition of the Southern Rocky Mountains

Abstract The Southern Rocky Mountains of the Western United States, physiographically defined and described by N.M. Fenneman nearly 60 yrs ago, are tectonically redefined and extended. They are shown to constitute the crestal range of a mammoth, continental, arch-like feature here named the Alvarado ridge. Its axis trends south from Casper, Wyoming at least as far south as El Paso, Texas, beyond which the ridge begins to lose morphological identity. Maximum elevations along the crest of the ridge exceed 4.2 km. The summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km, falling to elevations of less than 400 m. Modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out. A major sediment-filled axial graben exists over the southern two-thirds of the ridge, but equivalent parts of it farther north were stripped of their Neogene fill by erosion in the headwaters areas of the Colorado and North Platte rivers. The maximum elevation of earlier Laramide mountains in this area has been estimated to have been no more than 2 km in Colorado, half that of the present range. Related topography was nearly obliterated by erosion prior to late Eocene time. Lateral stream planation produced a southeast-sloping, major late Eocene erosion surface across the region that had what was probably an isostatically adjusted, average maximum elevation of less than 900 m. Present day elevations and relief on the ridge crest are the result of steep crestal normal faulting, pronounced block uplift, and regional arching, with extensional strain limited to an axial corridor less than 200 km wide. This episode of mountain building began in middle Miocene time (17 to 12 Ma), culminating in latest Miocene and early Pliocene time, between 7 and 4 Ma ago. Debris from the newly elevated range (the Southern Rocky Mountains, sensu lato) was shed along the full length of the Neogene Alvarado ridge down parts of both rises, as well as into its medial graben. The uplifted and tilted east rise (the Great Plains ramp) has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle Miocene to earliest Pliocene time. Its fluviatile, lacustrine and eolian sediments are undergoing dissection today, but a sufficient portion remains to define the unmodified profile of the east rise, as well as the original length of the ridge. It is coincidentally similar in configuration and scale to the west rise of the mid-Atlantic ridge. The topography of the crestal mountain ranges on both structures is also remarkably similar. The Southern Rocky Mountains are as high as the Alps, but are not the product of lithospheric compression and crustal thickening. Rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge.

Numerical models of subduction and forearc deformation

Finite element models for shallow subduction produce realistic behaviour for a wide variety of mechanical strength and density distributions. Characteristic displacements are found to occur even without a discrete low-strength megathrust if there is a high-density subducted plate to localize lithospheric compression. A high-density plate is itself unnecessary in the presence of a low-strength megathrust and regional compression. Successful finite element models produce an outer arc at the top of the trench slope, and forearc basin with geometry characteristic of natural analogues. These structural features occur by upward inelastic bending of the lithospheric wedge overlying the megathrust. This mechanically unstable behaviour may dissipate significant energy and cause the megathrust to migrate continuously by accretion, tectonic erosion, or abandonment and reinitiation farther offshore. Upward bending in the overriding plate is promoted by low megathrust dip, low megathrust shear strength, and high horizontal compression in the overriding plate.

Venus banded terrain: Tectonic models for band formation and their relationship to lithospheric thermal structure

Recent radar images of the mountains of Ishtar Terra, Venus, at approximately 3‐km radar resolution show a series of linear bands of alternatively greater and lesser backscatter; the bands are generally aligned with the topographic strike of the mountain ranges. We test the hypothesis that these band features are tectonic in origin, the product either of folding during lithospheric compression or of block faulting during lithospheric extension. Tested models for folding include uniform elastic and viscous plates overlying in viscid substrates, layered plates, and a viscous half space in which viscosity decreases exponentially with depth. The uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on Venus, about 15 to 20 km, if the elastic or high‐viscosity layer is, at most, a few kilometers in thickness, though the required compressive stresses are several kilobars in magnitude. For the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness, and similar results hold. Layered elastic or viscous plates, however, can fold at the required wavelength with subkilobar stresses and, therefore, are favored over uniform plate models. Tested extensional models include graben and horst formation in a surface elastic‐brittle layer, imbricate normal faulting, and necking of a surficial layer that is everywhere in a state of extensional failure. The spacing between adjacent extensional features can match the spacing between bands on Venus if the surficial brittle layer is no more than a few kilometers in thickness. Both compressional and extensional models for band formation are therefore consistent with radar images of Venus mountain ranges at the image resolution presently available. Additional evidence, however, including the linearity and continuity of individual bands, the relationship between band trends and topographic contours in some regions near the ends of mountain ranges, and the very different manifestations of lithospheric extension in other highland areas of Venus, leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of Ishtar Terra as a result of horizontal compression of the Venus lithosphere. Independent arguments on the thermal structure of the Venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of Venus is approximately 1–10 km thick. We therefore suggest that the surficial layer of elastic‐brittle or high‐viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the Venus highlands at the time of band formation.