Wedge Strength Research Articles

Accretion Cycles, Structural Evolution, and Thrust Activity in Accretionary Wedges With Various Décollement Configurations: Insights From Sandbox Analog Modeling

AbstractThe architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation‐based kinematics, to delineate the wedge growth history with four décollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded décollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high‐friction patch in the basal décollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low‐angle, long‐lived forethrust and multiple cycles of frontal accretion on short‐lived forethrusts at the deformation front. Our findings suggest that décollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of ∼105 yr in natural accretionary systems. This result should be considered when investigating modern subduction zones.

Convergence Velocity Controls on the Structural Evolution of Orogens

AbstractThe collision of continental crust results in the formation of orogenic wedges to accommodate convergence. We use 2D thermomechanical models to address the growth of viscoplastic orogenic wedges, focusing on the structural evolution over time. We find that when the shear stress at the base of the crust is below the yield stress then viscous behavior dictates the evolution, with the wedge structure and geometry influenced by the convergence velocity. In contrast, when the shear stress is equal to or above the yield stress across the entire wedge then plastic behavior controls the evolution, with the structural style and geometry independent of convergence velocity. The models highlight the controls of the basal décollement rheology on the deformation style of the orogenic wedge. We find that for increasing crustal thickness, the velocity required to transition from viscous‐to‐viscoplastic and then to plastic‐dominated wedges increases. We determine empirically how the viscous deformation influences wedge strength and estimate an effective friction coefficient based on the geometry of the entire wedge. The models allow inferences on the dominant deformation mechanism currently accommodating convergence in the Zagros, Himalayas, and European Alps when estimating the effective friction coefficient from the geometry, showing the relevant role of rate‐dependent viscous deformation in each orogenic wedge.

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

AbstractStudies of recent destructive megathrust earthquakes and tsunamis along subduction margins in Japan, Sumatra, and Chile have linked forearc morphology and structure to megathrust behavior. This connection is based on the idea that spatial variations in the frictional behavior of the megathrust influence the tectono-morphological evolution of the upper plate. Here we present a comprehensive examination of the tectonic geomorphology, outer wedge taper, and structural vergence along the marine forearc of the Cascadia subduction zone (offshore northwestern North America). The goal is to better understand geologic controls on outer wedge strength and segmentation at spatial scales equivalent to rupture lengths of large earthquakes (≥M 6.7), and to examine potential linkages with shallow megathrust behavior.We use cross-margin profiles, spaced 25 km apart, to characterize along-strike variation in outer wedge width, steepness, and structural vergence (measured between the toe and the outer arc high). The width of the outer wedge varies between 17 and 93 km, and the steepness ranges from 0.9° to 6.5°. Hierarchical cluster analysis of outer wedge width and steepness reveals four distinct regions that also display unique patterns of structural vergence and shape of the wedge: Vancouver Island, British Columbia, Canada (average width, linear wedge, seaward and mixed vergence); Washington, USA (higher width, concave wedge, landward and mixed vergence); northern and central Oregon, USA (average width, linear and convex wedge, mixed and seaward vergence); and southern Oregon and northern California, USA (lower width, convex wedge, seaward and mixed vergence). Variability in outer wedge morphology and structure is broadly associated with along-strike megathrust segmentation inferred from differences in oceanic asthenospheric velocities, patterns of episodic tremor and slow slip, GPS models of plate locking, and the distribution of seismicity near the plate interface. In more detail, our results appear to delineate the extent, geometry, and lithology of dynamic and static backstops along the margin. Varying backstop configurations along the Cascadia margin are interpreted to represent material-strength contrasts within the wedge that appear to regulate the along- and across-strike taper and structural vergence in the outer wedge. We argue that the morphotectonic variability in the outer wedge may reflect spatial variations in shallow megathrust behavior occurring over roughly the last few million years. Comparing outer wedge taper along the Cascadia margin to a global compilation suggests that observations in the global catalog are not accurately representing the range of heterogeneity within individual margins and highlights the need for detailed margin-wide morphotectonic analyses of subduction zones worldwide.

Assessment of vertebral wedge strength using cancellous textural properties derived from digital tomosynthesis and density properties from dual energy X-ray absorptiometry and high resolution computed tomography

The purpose of this study was to examine the potential of digital tomosynthesis (DTS) derived cancellous bone textural measures to predict vertebral strength under conditions simulating a wedge fracture. 40 vertebral bodies (T6, T8, T11, and L3 levels) from 5 male and 5 female cadaveric donors were utilized. The specimens were scanned using dual energy X-ray absorptiometry (DXA) and high resolution computed tomography (HRCT) to obtain measures of bone mineral density (BMD) and content (BMC), and DTS to obtain measures of bone texture. Using a custom loading apparatus designed to deliver a nonuniform displacement resulting in a wedge deformity similar to those observed clinically, the specimens were loaded to fracture and their fracture strength was recorded. Mixed model regressions were used to determine the associations between wedge strength and DTS derived textural variables, alone and in the presence of BMD or BMC information. DTS derived fractal, lacunarity and mean intercept length variables correlated with wedge strength, and individually explained up to 53% variability. DTS derived textural variables, notably fractal dimension and lacunarity, contributed to multiple regression models of wedge strength independently from BMC and BMD. The model from a scan orientation transverse to the spine axis and in the anterior-posterior view resulted in highest explanatory capability (R2adj = 0.91), with a scan orientation parallel to the spine axis and in the lateral view offering an alternative (R2adj = 0.88). In conclusion, DTS can be used to examine cancellous texture relevant to vertebral wedge strength, and potentially complement BMD in assessment of vertebral fracture risk.

Reconciling regional continuity with local variability in structure, uplift and exhumation of the Timor orogen

Along-strike variations in orogenic development can be difficult to constrain. Resulting assumptions projecting similarity or variability along strike can lead to erroneous conclusions at the orogen scale. Young orogens provide opportunities to document limits of along-strike projection and test factors that may control lateral variability. Here we present new constraints on the history of uplift, exhumation and shortening in the Timor orogen from West Timor, Indonesia. Structural mapping documents a foreland thrust stack of Jurassic-Miocene Australian margin strata and a hinterland antiformal stack of Permo-Triassic Australian continental units duplexed below Banda Arc lithosphere. Biostratigraphy within the piggyback Central Basin reveals earliest deepwater synorogenic deposition at 5.57–5.53Ma, uplift from lower to middle bathyal depths at 3.35–2.58Ma, and uplift from middle to upper bathyal depths at 2.58–1.30Ma. Hinterland Permo-Triassic strata yield apatite (U-Th)/He ages of 0.33–2.76Ma, apatite fission track ages of 2.19–3.53Ma and partially reset zircon (U-Th)/He ages. These thermochronology ages are youngest or most strongly reset in the center of the antiformal stack and yield modeled exhumation rates of 0.45–3.31km/Myr. A balanced cross section reveals a minimum of 300km of shortening including 210km of Australian continental subduction below the Banda forearc. When compared to published results from Timor-Leste, these data show that the timing of initial collision, synorogenic basin uplift, and total shortening amount were broadly similar across the island. Therefore, despite along-strike changes in orogen morphology and significant small-scale spatial variability in deformation, first-order structural similarity dominates at large scales. We suggest that along-strike variations in orogen morphology are due to changes in the distribution of deformation within the orogen driven by differences in backstop strength, internal wedge strength and basal décollement friction as well as the presence of the wedge-top Central Basin in West Timor.

Non-linear critical taper model and determination of accretionary wedge strength

The critical taper model has been widely used to evaluate the strength contrast between the wedge and the basal detachment of fold-and-thrust belts and accretionary wedges. However, determination of the strength parameters using the traditional critical taper model, which adopts the Mohr–Coulomb failure criterion, is difficult, if not impossible. In this study, we propose a modified critical taper model that incorporates the non-linear Hoek–Brown failure criterion. The parameters in the proposed critical Hoek–Brown wedge CHBW model can be directly evaluated via field investigations and laboratory tests. Meanwhile, the wedge strength is a function of the wedge thickness, which is oriented from stress non-linearity. The fold-and-thrust belt in western central Taiwan was used as an example to validate the proposed model. The determined wedge strength was 0.86 using a representative wedge thickness of 5.3km; this was close to the inferred value of 0.6 from the critical taper. Interestingly, a concave topographic relief is predicted as a result of the wedge thickness dependency of the wedge strength, even if the wedge is composed of homogeneous materials and if the strength of the detachment is uniform. This study demonstrates that the influence of wedge strength on the critical taper angle can be quantified by the spatial distribution of strength variables and by the consideration of the wedge thickness dependency of wedge strength.

The effects of artificial asperities on crack closing behavior in quenched and tempered medium carbon steel

To better understand the effect of an asperity on crack closure behavior, K-CMOD relations were examined using artificial asperity/wedge, inserted into the fatigue crack in a three point bending specimen made of a hardened medium carbon steel. Experimental results revealed that the unloading phase of the K vs. CMOD curve exhibited a concave shape if soft artificial asperity (Al alloy) was inserted, signifying acceleration in the CMOD decrease at zero applied load. This was mainly related to elastic and plastic deformation in the wedge material during the unloading process. On the contrary, the linear unloading portion of K vs. CMOD was obtained as hard asperity (high carbon steel) was employed, which specified deceleration in the CMOD decrease at zero applied load, where the only elastic deformation in the asperity was affected. From their unloading curves, the severity of crack closure or ΔKeff value was found to be related to the strength of the asperity material. The values of ΔKeff were examined in two different ways, e.g., (i) the remote displacement method and (ii) the adjusted compliance ratio method (ACR). The ΔKeff value, measured using both approaches, decreases with increasing wedge strength, such as hardness and yield strength. The rate of reduction in ΔKeff was, however, changed depending on the manner of ΔKeff examination, in which the ΔKeff decreased at a higher rate for the compliance ratio method and at a lower rate for the remote displacement method. The reason for this is discussed in the present work.

Grey cast iron cast in shell and sand moulds: relationship between tensile and wedge strengths

The tensile strength St of grey cast iron can be calculated from measurement of wedge strength Sc by using a linear relation between both variables: St = ASc-B. The coefficients A and B depend on the casting process, the geometry of wedge penetration tool and specimen, and also on the device used to perform the wedge penetration test. Low alloyed grey cast irons of FL15, FL20, FL25 and FL30 basic materials have been cast in shell and sand moulds, at two different pouring temperatures and with and without inoculation. These changes in the process produce large variations in the structure and mechanical strength of the materials. Although these variations are important, the relationship between tensile and wedge strengths can be well described by an equation of the above mentioned type with a unique pair of values for coefficients A and B. IJCMR/453

The impact of copper contamination on the quality of the second wire bonding process using X-ray photoelectron spectroscopy method

A lot of wedge bonding failures were observed on the leadframe (LF) type A due to non-stick on lead (NSOL) during the second wire bonding process of TQFP package. The copper ion contamination from the plating process was identified as one of the key factors that attributed to the NSOL failures. Surface analyses were performed in terms of X-ray photoelectron spectroscopy on the surface of LF type A. It was found that as received silver-plated surface of the copper LF type A was contaminated by the copper. After the copper contamination was solved in the plating process, the design of experiment was implemented for the verification of the influence of copper contamination on the quality of the second bonding process. It was confirmed that copper contamination dramatically reduced the strength of wedge bonding. The wedge pull test showed that NSOL failures were not observed after the copper ion contamination in the plating process was well controlled.

Strike-parallel variations in clay minerals and fault vergence in the Cascadia subduction zone

Clay minerals probably affect the zonation of mechanical properties within a thick unit of abyssal-plain deposits as they enter the Cascadia subduction zone. Landward- vergent thrust faults develop above a deeper decollement because smectite-rich mudrocks within that corridor release more water during clay dehydration, which in turn elevates pore pressure and reduces basal shear stress relative to wedge strength. Conversely, di- lution of smectite by illite and chlorite increases the frictional coefficient, and fluid over- pressure should drop where smectite dehydration is volumetrically reduced. Thus, thrust faults within chlorite-rich segments of the margin are seaward vergent.

Mechanics of Machining With Chamfered Tools

Chamfered cutting tools are used in high speed machining of hardened steels due to their wedge strength. An analytic model is proposed to investigate the influence of chamfer angle and cutting conditions on the cutting forces and temperature. The model is based on the tool geometry, cutting conditions, steady state temperature in the shear and chip-rake face contact zones, strain, strain rate, and the corresponding flow stress of the work material. With the aid of a slip line field model, the cutting and friction energy in the primary, secondary and chamfer zones are evaluated. By applying the minimum energy principle to total energy, the shear angle in the primary deformation zone is estimated. The corresponding shear strain, strain rate and flow stresses are identified. The model leads to the prediction of cutting forces and temperature produced in three deformation zones. The model is experimentally verified by high-speed orthogonal cutting tests applied to P20 mold steel using ISO S10 carbide and CBN cutting tools. It is shown that the analytic model is quite useful in selecting optimal chamfer angle and cutting speed which gives the minimum tool wear and relatively lower cutting forces. [S1087-1357(00)00204-5]

A thin orogenic wedge upon thick foreland lithosphere and the missing foreland basin

Along the Caledonian front in central Scandinavia, the expected peripheral or pro-foreland basin is neither physically present nor are there any significant traces in the sedimentary record. In order to explain and quantify this situation, the authors assess the major geometric and mechanical constraints on the Caledonian orogenic wedge and model the orogenic load and its influence on the foreland lithosphere of Baltica. Geologic and geophysical data show a strong foreland lithosphere with a flexural parameter (α) of approximately 100 km. The shape of the orogenic wedge and its critical taper angle are dependent mainly on basal friction and wedge strength. In the external part organic-rich black shales provide a low-friction horizon both at the basal detachment surface and within the wedge itself. The more internal part of the wedge is composed of metamorphic and crystalline rocks, which cooled and strengthened prior to thrusting. As a result, the external part of the wedge had a lower strength and a smaller critical taper angle than its internal part, so the orogenic load is upward concave. Modelling of the effect of such a load on the Baltica lithosphere shows a very small depression in front of the load (2 km). The flexural depression produced by the main part of the orogenic load is filled up by the thickening thrust-and-fold belt, so that there is little space left for a foreland basin. These results imply that the missing foreland basin in front of the central Scandinavian Caledonides is not due to subsequent erosion, but is a primary feature.

Control of the casting process of grey cast iron by means of the wedge-penetration test

The graph of force versus penetration-depth from a wedge-penetration test gives information on the structure of the phases of the material under test. The simplicity of this test makes it ideal for on-line quality control of cast pieces. When strength is not satisfactory, knowledge about this structure permits a fast correction of the solidification conditions, increasing production capacity. Finally, it is proposed to adopt wedge strength as a standard, instead of converting this strength to tensile strength.

History of the Sevier orogenic wedge in terms of critical taper models, northeast Utah and southwest Wyoming

Problems with applying critical taper models to ancient orogenic wedges are overcome in the Late Cretaceous–late Paleocene Sevier orogenic wedge in Utah and Wyoming by a symptomatic approach in which wedge behavior is inferred from the time-space distribution of thrust faulting, erosion, and synorogenic sediment accumulation in association with the orogenic wedge. In turn, the causes of wedge behavior are interpreted in terms of features that are known about the Sevier wedge, such as the locations of major decollements and the lithologic constituents of major thrust sheets. From Coniacian through late Paleocene time (∼35 m.y.), basement and cover rocks in the Sevier wedge were shortened by ∼100 km in three major and one minor events. An overall eastward progression of thrusting was punctuated by several episodes of out-of-sequence and hinterland vergent thrusting. Over the long term, wedge taper was controlled by internal wedge strength, initial taper, and changes in the durability of the upper surface of the wedge and its internal lithostratigraphy. Critical taper was maintained by the offsetting effects of basement duplexing in the rear of the wedge and forward imbrication at the front of the wedge. While the basal decollement was mainly in basement rocks, the wedge was highly tapered and thrust spacing was relatively close (∼15 km). Beginning ca. 75 Ma, the basal decollement ramped upward and eastward into weak Cambrian shale and Jurassic salt, thrust spacing increased to 25–45 km, and wedge taper decreased drastically. The highly tapered rear part of the wedge carried strong basement rocks above moderately weak mylonitic and cataclastic decollements, whereas the frontal, lower-taper part of the wedge carried sedimentary rocks above extremely weak (salt and shale) decollements. Through time, internal strength of the wedge decreased as it incorporated an increasing proportion of sedimentary rocks; the thrust belt was able to advance eastward in spite of its decreasing internal strength and decreasing taper because the basal decollement of the wedge also became weaker through time, both by strain softening (beneath the rear of the wedge) and by propagating along extremely weak rocks (beneath the front of the wedge). Provenance data from associated synorogenic conglomerates indicate that the durability of the upper surface of the wedge probably increased over the long-term history of the Sevier wedge, as an increasing proportion of the erosional surface became occupied by resistant Precambrian quartzite and basement rocks. This would have had the effect of decreasing the rate of erosion through time and maintaining a steeper surface slope, which would have helped the wedge to remain at critical to supercritical taper. The Sevier wedge also exhibits several shorter-term ( 10 km, and (3) was subjected to regional erosion (marked by major unconformities) that ultimately caught up with tectonic thickening and caused the wedge to stall. This cyclicity is attributed to the need for wedges to become supercritical before they can propagate forward over large distances and a lag-time between wedge thickening (and uplift) and wedge erosion. Erosion eventually catches up to the rate of uplift, the wedge stalls, and the locus of deformation shifts to the rear of the wedge in order to rebuild taper. This analysis suggests that orogenic wedges continually deform and “find a way” to maintain taper sufficient to allow forward propagation as long as a sufficient push from the rear exists. The actual taper value may change substantially through time as lithostratigraphy, internal strength, upper surface durability, and presence/absence of weak potential stress guides conspire to maintain a critical taper.

Constraints on friction and stress in the Taiwan fold-and-thrust belt from heat flow and geochronology

Research Article| February 01, 1990 Constraints on friction and stress in the Taiwan fold-and-thrust belt from heat flow and geochronology Terence D. Barr; Terence D. Barr 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar F. A. Dahlen F. A. Dahlen 1Department of Geological and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544 Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (2): 111–115. https://doi.org/10.1130/0091-7613(1990)018<0111:COFASI>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Terence D. Barr, F. A. Dahlen; Constraints on friction and stress in the Taiwan fold-and-thrust belt from heat flow and geochronology. Geology 1990;; 18 (2): 111–115. doi: https://doi.org/10.1130/0091-7613(1990)018<0111:COFASI>2.3.CO;2 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 In the thin-kinned approximation, both the critical taper of an active fold-and-thrust belt and the rate of brittle frictional heat generation depend on only two uncertain parameters: the effective coefficient of friction on the basal decollement fault, μb*= (1-λb)μb and the effective Coulomb wedge strength,μ* = (1-λ)sin ϕ/(1-sin ϕ). An increase in μb* increases the critical taper, whereas an increase in μ* decreases the taper; the basal and internal heat generation and therefore the surface heat flow are, in contrast, increased by an increase in either μb* or μ*. We exploit this difference in sensitivity to constrain μb* and μ* in the well-studied active fold-and-thrust belt of Taiwan. The measured surface heat flow and known geometry of the Taiwan wedge are best fit by an effective coefficient of basal friction μb* = 0.16 ±0.06. If the basal pore fluid:lithostatic pressure ratio is everywhere similar to that measured in several oil wells near the deformation front, then the coefficient of sliding friction on the Taiwan decollement fault is μb = 0.50 ±0.20. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Mechanics of fold‐and‐thrust belts and accretionary wedges: Cohesive Coulomb Theory

A critically tapered fold‐and‐thrust belt or submarine accretionary wedge is one that is on the verge of Coulomb failure everywhere, including its base where frictional sliding along a decollement is assumed to be occurring. Cohesion within a wedge can add significantly to the overall strength near the toe; the effect of this is to decrease the near‐toe taper, leading to a critical topographic profile that is concave upward if the decollement is planar. We obtain an approximate self‐consistent solution for the state of stress within a thin‐skinned cohesive critical Coulomb wedge, and determine the relationship between the wedge taper and its strength and basal friction. The theory is then applied to the presently deforming fold‐and‐thrust belt of western Taiwan. Fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step‐up angles of thrust faults from the basal decollement are used to constrain the Taiwan wedge strength parameters. An attractive assertion fully consistent with all the observations is that the mechanics of fold‐and‐thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory. In particular, if Byerlee's law µb = 0.85 is adopted as the coefficient of sliding friction on the base, we find a coefficient of internal friction µ = 0.9–1.0 in the wedge and a wedge cohesion So = 5–20 MPa. Other solutions having strengths and ambient stresses up to 4 times lower than this can also, however, satisfy the data.