Trishear Fault-propagation Research Articles

Application of trishear fault-propagation folding to active reverse faults: examples from the Dalong Fault, Gansu Province, NW China

Determining accurate fault slip rates at 1 ka to 1 Ma timescales requires well-constrained palinspastic reconstructions of dateable geomorphic and/or geologic markers. Although general kinematic models have been developed to simultaneously reconstruct both bedrock (e.g. bedding and fault attitudes) and neotectonic markers (e.g. strath terraces) along active strike-slip and thrust faults, it is not clear if these models can also account for deformation along steeply dipping (>45°) reverse faults. To address this problem, we have investigated the active, ∼50° dipping, Dalong reverse fault system. This ∼40-km-long fault system forms part of the Aksai restraining stepover along the active, left-slip Altyn Tagh Fault in northwestern China. Our geometric and kinematic analyses show that conventional fault-bend fold models cannot satisfy the steeply-dipping fault geometry we observe in the bedrock record. Likewise, standard fault-propagation fold models fail to match our measurements of a set of fluvial terraces. However, by expanding the trishear model of fault-propagation folding to track both bedrock and neotectonic markers, we are able to match both sets of records. In particular, we have developed trishear kinematic models for two sites (Liuchenzi and Qingyazi) using the numerical modeling program, Fault/Fold v.5.0. This work indicates that an important implication of active trishear fault-propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace. Thus, to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall, terrace profiles across active reverse faults must extend 1–2 km on either side of this zone of deformation.

Pseudo 3-D modeling of trishear fault-propagation folding

Basement structures, to which trishear fault-propagation models have most successfully been applied, are commonly three-dimensional folds formed at the tip-line of a fault. We present here a ‘pseudo-3D’ trishear model in which various parameters are permitted to vary along strike and oblique-slip can be modeled. These variations may be combined in an infinite number of ways, facilitating the simulation of many real structures. A thrust changing from blind to emergent can be produced by a change in the slip or propagation-to-slip (P/S) along strike. Folds with forelimbs changing from overturned to upright along strike can be modeled either by changing the slip, P/S or trishear angle. Also some minor folds perpendicular or oblique to the main structure can result from changes in the trishear angle or fault angle along the strike. Models including growth strata show that it is practically impossible to distinguish between growth and pre-growth strata using the map patterns. As a field test, we have modeled the oblique slip East Kaibab monocline, demonstrating a good fit between the field observations and model predictions.

Geometry, numerical models and revised slip rate for the Reelfoot fault and trishear fault-propagation fold, New Madrid seismic zone

The geometry and structural relief of late Holocene sediments folded across the Reelfoot scarp are characterized with trench excavations, shallow borings, a digital elevation model (DEM) of topography and Reelfoot Lake (RL) bathymetry. Results suggest the scarp forms by heterogeneous shear in the forelimb of a fault-propagation fold. Seismic data and the accompanying structural analysis support the argument that the Reelfoot fault does not offset the surface, and that the Reelfoot scarp is a fold limb. Structural relief measured across the Reelfoot scarp is slightly higher than previous determinations and varies from 7 to 11m along the western shoreline of RL. Numerical models based on trishear kinematics constrain growth of the Reelfoot monocline and indicate that the underlying Reelfoot thrust fault is steeply dipping. The shallowest portion of the central Reelfoot thrust segment dips 75° where the fault tip has propagated upward from 911m to its current location 465m beneath the surface. The models indicate that the shallowest section of the southern thrust segment in the Reelfoot thrust system dips even more steeply at 80° but has propagated upward only a few tens of meters to its present location 1016m beneath the surface. The thrust is flatter at deeper levels based on the location of earthquake hypocenters. Strain across the Reelfoot scarp is partitioned into two or three separate fold scarps that collectively record a late Holocene fault slip rate of 3.9±0.1mm/yr. The slip rate is based on 9m of structural relief, the 2290±60ybp age of folded sediment and a 75° dip for the fault. This implies a rate of horizontal contraction in the seismic zone of ∼1.0mm/yr. This rate is strongly dependent on the dip of the thrust calculated by the structural analysis. The numerical models argue for reactivation of faults that previously offset Paleozoic strata.

Abstract: Numerical Analyses of Trishear Fault-propagation foldsImplications for Geometry, Growth Strata, Strain Evolution, and Fracturing 

Abstract: Numerical Analyses of Trishear Fault-propagation foldsImplications for Geometry, Growth Strata, Strain Evolution, and Fracturing 

Numerical modeling of trishear fault propagation folding

In contrast to kink band migration modeling methods, trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges. Modeled fold hinges tighten and converge downward, within a triangular zone of distributed deformation which is focused on the fault tip. Such features have been reported from field studies and are also seen in analogue models of compressional deformation. However, apart from its initial application to Laramide folds, little quantitative work has been undertaken on trishear fault propagation folding in other settings. In addition, no study has been undertaken into the growth strata which might be associated with such structures. This paper uses an equivalent velocity description of the geometric model of trishear, together with models of erosion and sedimentation, to investigate trishear fault propagation folding of both pregrowth and growth strata. The trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement. The models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata. When total slip on a structure is high, the model predicts overturned pregrowth and growth strata. During the initial stages of deformation, beds in the forelimb thicken but later thin when they become steep or overturned. The effect of variations in fault propagation to slip ratios on two‐dimensional finite strain in the models is assessed by the use of initially circular strain markers. High fault propagation to slip (p/s) ratios lead to narrow zones of high finite strain, while lower p/s ratios lead to more ductile deformation and broader zones of high strain. In all cases, hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault. Modeled structures are compared with natural examples.

Trishear fault-propagation folding

Previous models of fault-propagation folding used kink-band geometries to approximate folding in front of propagating thrusts. However, kink-band kinematics cannot replicate the curved fold surfaces and complex strain patterns in natural and experimental fault-propagation folds, which also occur in front of steeper reverse and normal faults. Fault-propagation fold hinges tighten and converge downward, forming a triangular zone of penetrative deformation focused on the tip of the propagating fault. The downward convergence of deformation in fault-propagation folds can be modeled as triangular shear zones. Trishear, here defined as distributed, strain-compatible shear in a triangular (in profile) shear zone, provides an alternate kinematic model for fault-propagation folds. Trishear is analogous to simple shear in a tabular shear zone except that area balance in a triangular shear zone requires curved displacement oblique to the fault slip direction. Incremental computer models of trishear folding can replicate many geometric features of fault-propagation folds, including variably curved fold hinges, downward-tightening fold surfaces, heterogeneous strains, and multiple fault-propagation trajectories.

Electrolytic Medication with Special Reference to Ionic Chemistry

Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the décollement, to accommodate WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.