Critical Taper Wedge Research Articles

The Himalayan Collisional Orogeny: A Metamorphic Perspective

AbstractThis paper introduces how crustal thickening controls the growth of the Himalaya by summarizing the P‐T‐t evolution of the Himalayan metamorphic core. The Himalayan orogeny was divided into three stages. Stage 60–40 Ma: The Himalayan crust thickened to ∼40 km through Barrovian‐type metamorphism (15–25 °C/km), and the Himalaya rose from <0 to ∼1000 m. Stage 40–16 Ma: The crust gradually thickened to 60–70 km, resulting in abundant high‐grade metamorphism and anatexis (peak‐P, 15–25 °C/km; peak‐T, >30 °C/km). The three sub‐sheets in the Himalayan metamorphic core extruded southward sequentially through imbricate thrusts of the Eo‐Himalayan thrust, High Himalayan thrust, and Main Central thrust, and the Himalaya rose to ≥5,000 m. Stage 16–0 Ma: the mountain roots underwent localized delamination, causing asthenospheric upwelling and overprinting of the lower crust by ultra‐high‐temperature metamorphism (30–50 °C/km), and the Himalaya reached the present elevation of ∼6,000 m. Underplating and imbricate thrusting dominated the Himalaya' growth and topographic rise, conforming to the critical taper wedge model. Localized delamination of mountain roots facilitated further topographic rise. Future Himalayan metamorphic studies should focus on extreme metamorphism and major collisional events, contact metamorphism and rare metal mineralization, metamorphic decarbonation and the carbon cycle in collisional belts.

The impact of erosion on fault segmentation in thrust belts: Insights from thermochronology and fluvial shear stress analysis (southern Longmen Shan, eastern Tibet)

Along the southern Longmen Shan thrust belt in eastern Tibet, the Dayi subsegment stands out as it remained unruptured in both the 2008 Wenchuan and the 2013 Lushan earthquakes. Whether localized weak materials or differential erosion caused this fault segmentation is unclear. According to the critical-taper wedge theory, these two mechanisms would generate distinct morphotectonic features and erosional distributions, such as a recess vs. a salient structure at the range front, and higher vs. lower erosion rate than that of the adjacent areas along strike in the hinterland, respectively. We integrated two independent methods to evaluate these mechanisms. We obtained 10 apatite fission track and two apatite (U-Th)/He dates from the Dayi subsegment, which yield ages of ~3–44 Ma, and deduced an average erosion rate of 0.5–0.6 mm/yr and 0.2–0.4 mm/yr since ~6–8 Ma in the hanging-wall and footwall of the Shuangshi-Dachuan Fault, respectively. We also calculated the fluvial shear stress and erosion rate at 708 sites along five rivers that flow across the faults in the southern Longmen Shan. These two methods yield consistent mean erosion rates, implying an exhumational steady state since at least 5 Ma. Intriguingly, a systematic, heterogeneous pattern of erosion is found on a 3-by-3 grid along and across the Dayi subsegment. In the hinterland, the erosion rate in the Dayi subsegment is lower than that of the adjacent areas along strike, whereas at the range front, the erosion rate in the Dayi subsegment is greater. The spatial patterns of erosion and topography suggest that differential erosion plays a major, but not exclusive, role in regulating the fault segmentation and earthquakes in the Longmen Shan. Moreover, the Dayi subsegment could have acted as a barrier during the recent great earthquakes, posing substantial seismic potential to this region.

Growth of a thrust fault array in space and time: An example from the deep-water Niger delta

The temporal and spatial evolution of thrust fault arrays is currently poorly understood, and marine fold and thrust belts at the toe of passive margin gravitational systems, imaged by commercial 3D seismic reflection datasets, afford a unique opportunity to investigate this problem in three dimensions. Using an extensive 3D seismic data set and age data, the total cumulative strain (shortening) and interval strain rates have been calculated for 11 thrust-related folds mapped in the toe-thrust region of the southern lobe of the Niger Delta. For the first time, the sequence of thrust nucleation, propagation and linkage through time at a scale of 10 s km both along and across strike is documented. Short thrust segments had nucleated throughout the entire study area by 15 Ma. They then grew largely by lateral growth and linkage, increasing the fault trace length and generating asymmetric strain-distance plots, for the first 50% of their history. Thereafter, growth continued by shortening, with minimal along strike increase in fault length. Changes in shortening-distance data between adjacent structures across strike suggest that the change in growth mode occurred once the thrusts had linked in 3D through the common underlying detachment. Over the entire thrust array the strain rate varies through time, starting slowly (<200 m/Ma), then increasing between 9.5 and 3.7 Ma (200–400 m/Ma) before slowing down in the last ~ 4 Ma (<150 m/Ma). The variation in strain rate is attributed to a change in boundary conditions of the gravitational system. An increase in sediment supply to the delta occurred in the late Miocene-Pliocene, driving higher shortening rates in the toe area. A subsequent reduction in sediment supply in the last ~4 Ma led to a reduction in deformation rate and the cessation of activity on a number of the thrusts. Predictions of the critical taper wedge model are used to explain the near-synchronous growth of the entire thrust array over the last 15 Ma. Because sedimentation acts to lower the surface slope, the wedge can only continue to deform if shortening occurs over a wide area allowing the surface slope to build up. These new results suggest that models of piggyback fault propagation are not appropriate for deep-water fold and thrust belts.

Role of erosion in creating thrust recesses in a critical-taper wedge: An example from Eastern Tibet

Mechanical models and analog experiments show that erosion plays an important role in controlling along-strike variations in thrust-belt deformation. Despite this fundamental insight, natural examples of focused erosion causing local variations of thrust-belt geometry, such as the formation of salients and recesses, remain underappreciated. In this study, we address this issue by integrating theoretical model predictions with structural, morphological, and erosion-pattern analyses along the thrust system defining the eastern margin of the Tibetan Plateau. Our work shows a nearly constant surface slope across and an overall cylindrical fault-related fold geometry along the thrust belt. These phenomena can be explained by the development of a critically-tapered thrust wedge along the plateau margin. A prominent recess exists at the intersection between the transverse Minjiang River and the range-bounding thrust. There, the surface trace displays a much higher curvature than the rest of the thrust belt and the local fold geometry is conical rather than cylindrical. We infer a warped passive-roof duplex under the recess. We build a classification diagram for recesses developing in critical-taper wedges, in which six independent variables lead to three morphotectonic scenarios regarding the effects of mass influx, mass efflux, and intrinsic properties of a thrust wedge. Formation of the Dujiangyan recess and its spatial correlation to the Minjiang River can be best attributed to localized fluvial erosion during forward propagation of the thrust wedge in eastern Tibet. This study sets a type example for map-view curves in thrust belts undergoing deformation-erosion interactions.

Characterizing anatexis in the Greater Himalayan Sequence (Kumaun, NW India) in terms of pressure, temperature, time and deformation

We applied field observations combined with P-T pseudosection modelling, zircon U-Pb geochronology and bulk rock geochemistry along the Kali River Valley, Kumaun Himalaya to understand conditions of peak metamorphism and partial melting of the Greater Himalayan Sequence (GHS) along with spatiotemporal relationship between anatexis and fault activation. The southern tectonic boundary of GHS or the Main Central Thrust (MCT) is marked on the basis of structural, metamorphic and chronological evidences. Outcrop-scale observations suggest generation of partial melt at the base of the MCT. This partial melt migrated to higher structural levels and finally emplaced as tourmaline bearing leucogranite in the northern tectonic boundary of the GHS, which is marked by the South Tibetan Detachment Zone (STDZ). P-T pseudosection modelling shows that GHS have experienced muscovite dehydration melting at 9.2–9.8 kbar and 720°–725 °C, 8.4–8.7 kbar and 700°-710 °C, 7.8–8.4 kbar and 700–720 °C respectively at its lower, lower-middle and middle structural levels. Zircon U-Pb geochronology suggests that the GHS underwent suprasolidus peak metamorphism and post-peak anatexis during a time span of ~26–22 Ma at the base of the MCT and ~32–27 Ma at the middle structural level. The MCT is at least ~22 Ma old, being synkinematic to the partial melting event that took place at its base. Diachronous and brief episodes of partial melting and absence of sillimanite zone at the base of the GHS help us envisage a ‘critical taper wedge’ scenario, where partial melt weakened the overlying Himalayan wedge and triggered gravity collapse that formed the STDZ.

Wedge geometry, frictional properties and interseismic coupling of the Java megathrust

The mechanical interaction between rocks at fault zones is a key element for understanding how earthquakes nucleate and propagate. Therefore, estimating frictional properties along fault planes allows us to infer the degree of elastic strain accumulation throughout the seismic cycle. The Java subduction zone is an active plate boundary where high seismic activity has long been documented. However, very little is known about the seismogenic processes of the megathrust, especially its shallowest portion where onshore geodetic networks are insensitive to recover the pattern of elastic strain. Here, we use the geometry of the offshore accretionary prism to infer frictional properties along the Java subduction zone, using Coulomb critical taper theory. We show that large portions of the inner wedge in the eastern part of the Java subduction megathrust are in a critical state, where the wedge is on the verge of failure everywhere. We identify four clusters with an internal coefficient of friction μint of ∼ 0.8 and hydrostatic pore pressure within the wedge. The average effective coefficient of friction ranges between 0.3 and 0.4, reflecting a strong décollement. Our results also show that the aftershock sequence of the 1994 Mw 7.9 earthquake halted adjacent to a critical segment of the wedge, suggesting that critical taper wedge areas in the eastern Java subduction interface may behave as a permanent barrier to large earthquake rupture. In contrast, in western Java topographic slope and slab dip profiles suggest that the wedge is mechanically stable, i.e deformation is restricted to sliding along the décollement, and likely to coincide with a seismogenic portion of the megathrust. We discuss the seismic hazard implications and highlight the importance of considering the segmentation of the Java subduction zone when assessing the seismic hazard of this region.

Crustal velocity structure and earthquake processes of Garhwal-Kumaun Himalaya: Constraints from regional waveform inversion and array beam modeling

In order to understand present day earthquake kinematics at the Indian plate boundary, we analyse seismic broadband data recorded between 2007 and 2015 by the regional network in the Garhwal-Kumaun region, northwest Himalaya. We first estimate a local 1-D velocity model for the computation of reliable Green's functions, based on 2837 P-wave and 2680 S-wave arrivals from 251 well located earthquakes. The resulting 1-D crustal structure yields a 4-layer velocity model down to the depths of 20km. A fifth homogeneous layer extends down to 46km, constraining the Moho using travel-time distance curve method. We then employ a multistep moment tensor (MT) inversion algorithm to infer seismic moment tensors of 11 moderate earthquakes with Mw magnitude in the range 4.0–5.0. The method provides a fast MT inversion for future monitoring of local seismicity, since Green's functions database has been prepared. To further support the moment tensor solutions, we additionally model P phase beams at seismic arrays at teleseismic distances. The MT inversion result reveals the presence of dominant thrust fault kinematics persisting along the Himalayan belt. Shallow low and high angle thrust faulting is the dominating mechanism in the Garhwal-Kumaun Himalaya. The centroid depths for these moderate earthquakes are shallow between 1 and 12km. The beam modeling result confirm hypocentral depth estimates between 1 and 7km. The updated seismicity, constrained source mechanism and depth results indicate typical setting of duplexes above the mid crustal ramp where slip is confirmed along out-of-sequence thrusting. The involvement of Tons thrust sheet in out-of-sequence thrusting indicate Tons thrust to be the principal active thrust at shallow depth in the Himalayan region. Our results thus support the critical taper wedge theory, where we infer the microseismicity cluster as a result of intense activity within the Lesser Himalayan Duplex (LHD) system.

Fluid overpressures and strength of the sedimentary upper crust

The classic crustal strength-depth profile based on rock mechanics predicts a brittle strength σ1−σ3=κ(ρ¯gz−Pf) that increases linearly with depth as a consequence of [1] the intrinsic brittle pressure dependence κ plus [2] an assumption of hydrostatic pore-fluid pressure, Pf = ρwgz. Many deep borehole stress data agree with a critical state of failure of this form. In contrast, fluid pressures greater than hydrostatic ρ¯gz>Pf>ρwgz are normally observed in clastic continental margins and shale-rich mountain belts. Therefore we explore the predicted shapes of strength-depth profiles using data from overpressured regions, especially those dominated by the widespread disequilibrium-compaction mechanism, in which fluid pressures are hydrostatic above the fluid-retention depth zFRD and overpressured below, increasing parallel to the lithostatic gradient ρ¯gz. Both brittle crustal strength and frictional fault strength below the zFRD must be constant with depth because effective stress (ρ¯gz−Pf) is constant, in contrast with the classic linearly increasing profile. Borehole stress and fluid-pressure measurements in several overpressured deforming continental margins agree with this constant-strength prediction, with the same pressure-dependence κ as the overlying hydrostatic strata. The role of zFRD in critical-taper wedge mechanics and jointing is illustrated. The constant-strength approximation is more appropriate for overpressured crust than classic linearly increasing models.

Structural Setting of the 2008 Mw 7.9 Wenchuan, China, Earthquake

The 2008 M w 7.9 Wenchuan earthquake ruptured an imbricate thrust system in the Longmen Shan range, which forms the eastern boundary of the Tibetan Plateau. We use seismic reflection data and surface geology to construct geologic cross sections and a three-dimensional (3D) fault model in the range front. The rupture was complex, with slip on both the steeply dipping Beichuan fault and the more shallowly dipping Pengguan fault, which appear to merge at depth. In contrast, only the Beichuan fault ruptured in the north, with the transition near a lateral fault offset. Thus, the earthquake involved multiple thrust splays and breached a significant segment boundary. To investigate the tectonic setting of the earthquake, we extend our previous measurements of crustal shortening (Hubbard and Shaw, 2009) into the interior of the range and show that shortening correlates with topography. This suggests that upper-crustal shortening can produce the high topography of the Longmen Shan without calling on other uplift mechanisms. Based on this understanding, we model the thrust belt as a critical taper wedge, to assess what rock and fault strengths are required to explain the topographic slopes within the range front and basin. The low taper within the basin is consistent with a weak detachment ( δ f ≈0.1), while a slightly stronger ( δ f ≈0.3–0.4) detachment is necessary to explain the higher taper of the range front. This difference may relate to the localization of the detachment within a Triassic evaporite sequence within the basin, while the faults beneath the range front are rooted in Precambrian metasedimentary or igneous rocks. Critical taper theory implies that shortening in the range front where the 2008 Wenchuan earthquake occurred is many times greater than in the basin, consistent with our shortening measurements. We examine the implications of these findings for seismic hazard assessment in the region and in other active mountain belts around the world.

Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

[1] The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non-coaxial shear with a top-to-the-south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non-coaxial shear with a top-to-the-north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post-date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust.

Evaluation of the orogenic belt hypothesis for the formation of the Thaumasia Highlands, Mars

Thrust faults along the southern margin of the Thaumasia Highlands, Mars, comprise a set of discrete subparallel south verging structures. Previous work proposed that the faults collectively define a narrow Earth‐like orogenic belt composed of a deformed lithospheric section translating southward through Tharsis above a regional décollement. This hypothesis is tested here by comparing topographic slopes across the Thaumasia Highlands to the values required by thrust faulting in a critical taper wedge setting. In general, topographic slopes across the Thaumasia Highlands, 0.418–1.017°, are too shallow for the Martian lithosphere to have deformed as a critical taper wedge, assuming reasonable values for pore fluid pressure ratios and frictional strengths of the wedge material and putative décollement. Combined with the absence of a mechanism to provide the necessary values of horizontal shortening (hundreds of km), nearly lithostatic pore fluid pressure ratios, and nearly frictionless décollements over regional scales on Mars that would be necessary for lithospheric translation as a wedge, the results are consistent with thrust fault deformation across Mars that is not integrated at depth into subhorizontal structures, but which function instead as discrete structures.

Interaction between critical wedge geometry and sediment supply in a deep-water fold belt

Research Article| February 01, 2007 Interaction between critical wedge geometry and sediment supply in a deep-water fold belt C.K. Morley C.K. Morley 1272 Baan Yosawaadi, 7, Pahonyohothin, Bangkok 10500, Thailand Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (2): 139–142. https://doi.org/10.1130/G22921A.1 Article history received: 21 Apr 2006 rev-recd: 13 Sep 2006 accepted: 25 Sep 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C.K. Morley; Interaction between critical wedge geometry and sediment supply in a deep-water fold belt. Geology 2007;; 35 (2): 139–142. doi: https://doi.org/10.1130/G22921A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The late Miocene–Holocene deep-water fold-and-thrust belt of offshore northwestern Borneo displays one of the most acute angles recorded for a submarine critical-taper wedge. The very low surface (1°–2.5°) and basal detachment (2°–5°) dips require basal pore-fluid pressures near lithostatic pressure for the wedge to propagate at such angles. In the deep-water environment, sedimentary processes act against the wedge attaining critical taper. In places of lower sedimentation rates, the fold-and-thrust belt extends to the base of the slope and dips ∼1.5°–2.5°. In areas of the highest sedimentation rates (Baram Delta), the slope has a lower angle (1°) and the slope continues beyond the limit of the fold-and-thrust belt. Slope morphology changes along the margin from concave up (relatively low sedimentation rate, low shortening rate), to convex up (relatively high sedimentation rate, low shortening rate), to stepped (relatively low sedimentation rate, high shortening rate). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles

We use critical-taper wedge mechanics theory to show that the Niger Delta toe-thrust system deforms above a very weak basal detachment induced by high pore-fluid pressure. The Niger Delta exhibits similar rock properties but an anomalously low taper (sum of the bathymetric slope and dip of the basal detachment) compared with most orogenic fold belts. This low taper implies that the Niger Delta has a very weak basal detachment, which we interpret to reflect elevated pore-fluid pressure ( 0.90) within the Akata Formation, a prodelta marine shale that contains the basal detachment horizon. The weak basal detachment zone has a significant influence on the structural styles in the deep-water Niger Delta fold belts. The overpressured and, thereby, weak Akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe-thrust structures, leading to the development of shear fault-bend folds and detachment anticlines that form the main structural trap types in the deep-water fold belts. Moreover, the low taper shape leads to the widespread development of backthrust zones, as well as the presence of large, relatively undeformed regions that separate the deep-water fold and thrust belts. This study expands the use of critical-taper wedge mechanics concepts to passive-margin settings, while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep-water Niger Delta.

Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography

We use 110 000 small earthquakes to locate and map active faults in three dimensions within the Taiwan arc-continent collision. The structure is dominated by a nearly horizontal band of small earthquakes at ∼10 km depth that is interpreted to seismically illuminate the main detachment zone of the mountain belt, with other illuminated fault zones abutting the detachment zone. The zone steepens below eastern Taiwan to 30°–90° and reaches depths of 30–60 km. The three-dimensional shape of the detachment zone in relation to topography allows a new test of critical-taper wedge mechanics and suggests that the reversal of topographic slope across Taiwan is controlled by the shape of the detachment.

ABSTRACT: Modeling the compressive toe of the Niger Delta as a critical taper wedge

ABSTRACT: Modeling the compressive toe of the Niger Delta as a critical taper wedge

Qu'est-ce que le front des orogènes? L'exemple de l'orogène pyrénéen

The concept of 'orogenic front' is discussed theoretically and on the basis of the Pyrenees example. Depending on the geological criterion, a morphologic front (mountain front), a thrust-wedge front, a reactivation front and a deformation front can be distinguished. The classical, critical thrust wedge made of cover material, moving above a shallow décollement, is considered as the upper part of a larger, steady-state wedge above a shallow basal décollement connected to a deeper one (lower crust?) and where both sedimentary and basement units are involved: its front corresponds to both the mountain front and the thrust-wedge front, that is the front of the allochthonous nappes. Transmission of orogenic stresses (which decrease exponentially with increasing distance to the belt), responsible for intraplate deformations, especially inversion of preexisting extensional features, over a distance greater than 1 000 km away from the belt, require both a relatively rigid lithosphère and a crustal decoupling far in the foreland. The outermost reactivated and/or inverted structure thus locates the reactivation front, which limits a low-slope and low-strength foreland area that is not yet at steady-state and therefore cannot be discussed in terms of critical taper wedge. An outermost deformation front, limiting the part of the foreland where orogenic stresses have only been recorded by microstructures without significant displacements, can also be considered.

Critical taper wedge mechanics of fold‐and‐thrust belts on Venus: Initial results from Magellan

Fold‐and‐thrust belts exist on Venus at the margins of crystal blocks such as plateaus, tesserae, and coronae and as ridge belts within the plains. These fold belts display a number of key features that are consistent with their formation by critical taper wedge mechanics, a mechanics that is well known for fold‐and‐thrust belts and accretionary wedges on Earth. For example, an analysis of fold geometry at the toe of the Artemis Chasma fold belt indicates fault‐bend folding above a regionally extensive decollement horizon at a depth of about 1.5 km. Near‐surface deformation on Venus is interpreted to be brittle and is anticipated to be dominated by cohesive strength in the upper 1–2 km. Critical taper wedge mechanics under anticipated Venus conditions suggests that brittle wedges should have maximum surface slopes in the range 10–20°, which is similar to some estimated slopes in the steep parts of the fold belts. The low taper toes of fold belts may be cohesion‐dominated toes on either brittle or plastic decollement horizons. Once the base of the wedge undergoes the brittle‐plastic transition, the surface slope is expected to flatten to near horizontal, in qualitative agreement with many topographic profiles of fold‐and‐thrust belts on Venus. The estimated depth of the brittle‐plastic transition is uncertain based on rock mechanics data but is expected to be close enough to the surface to be affected by the atmospheric‐temperature gradient. The relief of fold belts (measured between the toe of the wedge and the flat crest) displays a remarkable linear dependence on absolute elevation (Figure 15), ranging from 6 km for Maxwell Montes at an elevation of +10 km to a few hundred meters at the lowest planetary elevations (0 to −2 km). This remarkable phenomenon appears to reflect an absolute elevation dependence of the depth of the brittle‐plastic transition, possibly controlled by an isostatic coupling of elevation, lithospheric thickness, and geothermal gradient.