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Palaeostress magnitudes in the Khao Khwang fold-thrust belt, new insights into thetectonic evolution of the Indosinian orogeny in central ThailandFrancesco Arboit a, ⁎, Khalid Amrouch b, Christopher Morley c, Alan S. Collins a, Rosalind King a

a Centre for Tectonic Resources and Exploration (TRaX), Department of Earth Sciences, The University of Adelaide, SA 5005, Australiab The Australian School of Petroleum, The University of Adelaide, SA 5005, Australiac Chiang Mai University, 239 Huaykaew Road, Tumbol Suthep Amphur Muang, Chiang Mai, Thailand

A R T I C L E I N F O

Article history:Received 5 January 2016Received in revised form 10 January2017Accepted 12 January 2017Available online xxx

Keywords:TectonicsCalcite twinsPalaeo differential stressThrust beltsCentral ThailandIndosinian orogeny

A B S T R A C T

Using the Khao Khwang fold–thrust belt in central Thailand as case study, we handled data from calcite twinning analy-sis in order to propose the quantification of the effective principal palaeostresses magnitudes since the onset of the earlystages of the Indosinian orogeny. We also combined the differential stress estimates from mechanically-induced calcitetwins with geochronological data in order to constrain the timing of the palaeoburial depth and subsequent uplift byfolding within the Khao Khwang fold–thrust belt. The proposed mechanical scenario is based on the time-constrainedkinematic sequence of fracturing, faulting, and folding in the strata of the carbonate formations of the Saraburi Group.Cross-checking the data on the palaeostress orientations, regimes, and differential stress magnitudes with rock mechanicsanalysis; we provided data on the principal stress magnitudes for each tectonic stage that developed during the Indosinianorogeny. Further, 40Ar/39Ar and U-Pb geochronological data allowed to place reliable constraints on the amount and rateof vertical uplift of the carbonate formations of the Saraburi Group.

© 2016 Published by Elsevier Ltd.

1. Introduction

The state of stress in rocks is generally anisotropic and is repre-sented by the orientations and magnitudes of the principal axes of thestress ellipsoid. In positive compression, the longest axis is the ellip-soid's major stress (σ1), the intermediate axis is the intermediate stress(σ2), and the shortest axis is the minimum stress (σ3) (Jaeger and Cook,1969; Sibson, 1977; Price and Cosgrove, 1990). The distribution ofthe modern-day stress-field states (e.g. Zoback, 1992; Sandiford et al.,2004), as well as the palaeostress orientation and differential stressvalues (Lacombe et al., 1992; Lacombe and Laurent, 1996; Lacombe,2001; Lacombe et al., 2007; Lacombe, 2007; Amrouch et al., 2010a),have been deeply investigated in the last decades. However, quan-titative estimates of effective palaeostress magnitudes through geo-logical time are difficult to make and have been well studied onlyin the last years (Lacombe et al., 1996; Lacombe, 2001; Lacombeet al., 2009; Amrouch et al., 2011; Choi, 2013; Choi et al., 2013;Kulikowski et al., 2016). Nevertheless, estimates of the palaeostressstates from rocks affected by tectonic events are of fundamental im-portance for addressing unsettled problems such as the mechanicalbehaviour of geological materials and deciphering various tectonicmechanisms, from those related to plate motions at a large scaleto those causing jointing and faulting or even microstructures at asmaller scale (Lacombe, 2007; Amrouch et al., 2010a, 2010b). Forthese purposes, several analytical methodologies

⁎ Corresponding author.Email address: [email protected] (F. Arboit)

were developed in the last decades (Etchecopar et al., 1981; Angelieret al., 1982; Armijo et al., 1982; Gephart and Forsyth, 1984; Michael,1984; Carey-Gailhardis and Mercier, 1987; Reches, 1987; Angelier,1990; Gephart, 1990; Marrett and Almandinger, 1990; Will andPowel, 1991; Yin and Ranalli, 1993). The dislocation creep withincalcite crystals is an important stress indicator, and this is possi-ble because the quantification of palaeostresses can be numericallyexpressed using calcite twinning (Lacombe, 2001; Amrouch et al.,2011). Here, following the analysis in Arboit et al. (2015), we use cal-cite twinning analysis using Etchecopar Method (Etchecopar, 1984)to provide estimates of maximum differential stress (Lacombe, 2007;Amrouch et al., 2010a). This contribution is aimed at presenting andconstraining the effective principal stress magnitudes during the In-dosinian orogeny, in order to better understand the complicate geody-namic evolution of the Khao Khwang fold-thrust belt since the MidPermian in central Thailand.

2. Tectonic framework

The Khao Khwang fold-thrust belt (KKFTB) (Fig. 1) lies in theSaraburi Province in central Thailand, and it is tectonically locatedon the SW margin of the Indochina Block (Bunopas, 1982; Metcalfe,2011; Morley et al., 2013). It is bounded to the north and to theeast by the Khorat Plateau, which trends NW-SE, and to the southby the Cenozoic Mae Ping strike-slip fault (Morley, 2007; Morley etal., 2013). This region has undergone a complex geological history,which mainly developed during the Indosinian tectonic event, this ischaracterized by two different subduction and collision episodes that

http://dx.doi.org/10.1016/j.tecto.2017.01.0080040-1951/© 2016 Published by Elsevier Ltd.

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Fig. 1. a) Geological map of the Khao Khwang fold and thrust belt, (modified from Warren et al., 2014); 1 stereonet of poles to bedding for the region, 2 stereonet of poles to mainthrusts along Highway 21. b) N-S oriented regional cross-section H-H′ through the southern portion of the Saraburi Region, the Khao Khwang fold-and-thrust belt (see a for location).Stereonets on the sides of b are of the stress tensors representing each stress stage, and are deduced from the analysis of the calcite twin.

covered the period from ca. 260 to 200 Ma (Late Permian to Late Tri-assic) (Sone and Metcalfe, 2008; Morley, 2007; Morley et al., 2013;Arboit et al., 2015). The resulting tectonic belts and suture zones inThailand have a dominant N-S trend. The Indosinian orogeny in Thai-land has been considered to involve two stages of collision during theTriassic-Early Jurassic between the three main terranes, which, fromeast to west, are: Indochina, Sukhothai and Sibumasu (Fig. 1a) (Soneand Metcalfe, 2008; Metcalfe, 2013). Additionally, beyond Thailand,to the NE, coeval Triassic collision also occurred between both theIndochina/South China Blocks (Cai and Zhang, 2009) and the South/North China Cratons (Dong et al., 2015).

The Sukhothai Terrane is believed to have been a volcanic arcthat rifted away from the south-western margin of Indochina in theEarly Permian, as consequence of rollback above the subducting Pa-leo-Tethys, and opening of the back-arc basin between the volcanicarc (Sukhothai) and the Indochina Terrane (Sone and Metcalfe, 2008).However, the geodynamic evolution on the southwestern margin ofthe Indochina terrane has been poorly understood. The first tectonic

event on the Indochina margin has been recently constrained as result-ing from the latest Permian (255– 4 Ma; Arboit et al., 2016a) collisionas the Sukhothai Terrane re-amalgamated with Indochina, with the re-lated closure of the Permian back-arc basin (Metcalfe, 2005; Sone andMetcalfe, 2008; Metcalfe, 2013). Subsequently, in the Late Triassic,during the late stages of the Indosinian collision, the Sibumasu Ter-rane is thought to have collided with the now combined Indochina/Sukhothai Terrane, causing the complete closure of the Paleo-Tethysin this region. The Indosinian orogeny has usually been thought as re-sulting from the collisions between these two strongly linear terranesand Indochina. However, despite the common N-S trend of the su-ture zones (Nan-Sra Kaeo and Changning-Menglian S.Z.) (Fig. 1a)between the blocks involved in the collision, in some areas the tec-tonic trend diverges from simply N-S to NW-SE and E-W trends.The most prominent of these regions is the Saraburi region (Morleyet al., 2013; Arboit et al., 2014). One explanation for the differenttrend is that the belt was rotated from a N-S direction to a moreE-W orientation by motion along the NW-SE trending Cenozoic Mae

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Ping Fault Zone; Tapponnier et al. (1986) proposed sinistral displace-ment of about 150 km on this fault. However, even after restoring thisoffset, and applying a relative clockwise rotation of about 25°–30°(Charusiri et al., 2006; Cung and Geissman, 2013; Singsoupho et al.,2014) to the northern side of the fault, the boundary does not restoreto a N-S orientation (Anchuela et al., 2012; Mochales et al., 2012;Morley et al., 2013). Alternatively, the deflection may be due to theoriginal orientation of the continental margins, or possibly due to apoorly documented intra-Indochina suture, that strikes east-west, andlies close to the southern margin of the Khorat Plateau (Hutchison,1975; Morley et al., 2013). The principal stress orientation of each tec-tonic event that took place within the KKFTB during the Indosinianorogeny has been recently constrained (Arboit et al., 2015), and re-lies on the determination of the paleostress orientations by comput-erised inversion of calcite twin data (Etchecopar, 1984), this techniquehas proven to be suitable for identifying geologically superimposedstress regimes in a polyphase tectonic setting (Lacombe et al., 1990,1992).The oldest system of conjugated fractures is correlated with rift-ing of the Sukhothai Terrane off the Indochina margin, and is limitedto the southern part of the KKFTB, while the second set of conjugatefractures is the more widespread throughout the Saraburi region; andmost likely took place during initial Indosinian compressional stress(LPS event) and was probably coeval with the Sukhothai-Indochinacollision (Arboit et al., 2015). This set of fractures in the southern por-tion of the KKFTB seems to have formed under the same stress tensorthat is responsible for the formation of conjugate sets of newly formedreverse faults that strike parallel to the major anticlines in the area. Themajority of the folds in the southern portion of the KKFTB developedin response to thrusting as fault bend, detachment, and fault propaga-tion folds (Morley et al., 2013; Arboit et al., 2014); hence, thrustingis interpreted as coeval with the growth of the major anticlines. Thethird stage of fractures developed during the Indosinian event with ageneral low angle and striking 200° to 240° appear to be almost syn-chronous to, or to have formed just after, the main LPS event (Fig. 1b).The subsequent stress tensor is associated with a stage of fold-tighten-ing (Warren et al., 2015; Arboit et al., 2015). This stage is character-ized by the reactivation of fracture set-2, which was developed duringthe main LPS event, and by the newly formed fractures striking 070°to 090° mainly detected on the hinge zones parallel to the fold axis,and probably developed in response to the fold-tightening. The fifthstage of fracturing is the latest brittle event that seems to have affectedthe southern portion of the KKFTB, it is marked by strike-slip faults,tail-cracks associated with shear veins and the reactivation of reversefaults previously emplaced during the main LPS event (Fig. 1).

3. Methodologies

The palaeostresses that affected central Thailand since the MidPermian were correlated to the stress that developed mainly during theIndosinian event (Morley et al., 2013; Arboit et al., 2015). The ori-entations and regimes were determined at both macroscopic and mi-croscopic scales, we carried out a study from fractures, faults, andcalcite twinning analysis using the Calcite Stress Inversion Tech-nique (CSIT) (Etchecopar, 1984; Lacombe and Laurent, 1992, 1996;Lacombe, 2001; Amrouch et al., 2010a)

3.1. Calcite twin analysis

In terms of deformation, an individual e-twin can be considered asa zone of perfect simple shear resulting from the slide of the position

of the atoms in the crystal lattice along a plane (e); which for un-metamorphosed calcite are ei:{01 2}. Twin gliding in calcite requiresa resolved shear stress (RSS) that exceeds the yield stress value fortwinning (τs) of 10 ± 4 MPa in order to develop (Turner et al., 1954;Lacombe and Laurent, 1996; Ferrill, 1998; Laurent et al., 2000;Lacombe, 2007, Lacombe, 2010; Amrouch, 2010). Resolved shearstress is the component of stress that is aligned with the twinning di-rection. The yield stress value has a very small sensitivity to temper-ature and confining pressure but depends mainly on grain size and in-ternal twinning strain (Tullis, 1980; Rowe and Rutter, 1990; Lacombeet al., 2007).

3.1.1. Palaeodeviatoric stressThe inversion of calcite twin data yields the four parameters of

the reduced stress tensor, as well as a non-dimensional differentialstress. The analysis of both the twinned and untwinned planes leads di-rectly to the simultaneous computation of principal stress orientations(with: σ1-maximum principal stress; σ2-intermediate principal stress;σ3-minimum principal stress) and stresses (Tourneret and Laurent,1990; Lacombe and Laurent, 1992; Amrouch et al., 2010a), which alsoyields data on the ellipsoid shape ratio Φ = (σ2 − σ3)/(σ1 − σ3), andthe peak differential stress (σ1 − σ3) (with σ1 ≥ σ2 ≥ σ3 as a compres-sive stresses, positive in value). The tensor solution is calculated as anormalized reduced stress tensor, such that (σ1 − σ3), and is scaled to[(σ1 − σ3)* = 1].

Lacombe (2001) brought to light the approximation of using a con-stant value of CRSS for determining the scalar (σ1 − σ3), which doesnot take in consideration the grain-size dependence of twinning. How-ever, we proceeded with a constant CRSS value since the homoge-neous grain-size (Table 2) within all the analysed samples. Thus, tak-ing in consideration the strain hardening of the crystals after the defor-mations, once the four parameters of the reduced stress tensor were de-termined the calculation proceeded under the assumption of a constantCRSS (τa = 10 ± 4 MPa), and a fifth scalar parameter such as the dif-ferential stress magnitudes were determined as follows (Etchecopar,1984; Lacombe and Laurent, 1996; Laurent et al., 2000):

where τa′ is the smallest resolved shear stress applied on the twinnedplanes accounted for by the stress tensor and therefore the normalizedvalue of the CRSS when (σ1 − σ3) is scaled to 1. Occurrence of a verylow number of untwinned planes (< 10–15% of the whole data set)due, for instance, to polyphase tectonism may lead to underestimatedτa′ values and hence to overestimated (σ1 − σ3) values. The existenceof such a bias towards high values when the number of untwinnedplanes is not sufficient has led to discard two samples (T020, T029)where high differential stress values (double than its expected valuesbased on sample T019b) were obtained. Since the values of both thereduced stress tensors and the scalar (σ1 − σ3) are known for each tec-tonic phase (Arboit et al., 2015), we quantified the deviatoric stresstensor following (Lacombe, 2001):

with the parameters of the deviatoric stress tensor that can hence

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be determined as (Lacombe, 2001):

However, following Eqs. (1), (2) one parameter is still missingin order to define the complete stress tensor (e.g. Lacombe, 2001;Amrouch, 2010a). This parameter corresponds to the isotropic compo-nent that can't be determined using calcite twin data only, since twin-ning does not depend upon isotropic stress. Hence, the palaeostressorientations and regimes were here combined with rock mechanicsdata; these data were successively plotted as a Mohr circle in orderto extrapolate the effective principal stress magnitudes (Lacombe andLaurent, 1996; Lacombe et al., 2007, 2009; Amrouch et al., 2011).The values of the differential stresses (Table 1) were estimated fromcalcite twin analyses (Arboit et al., 2015). For each specific tectonicevent, the quantification of the complete stress tensor consists in de-termining the effective values of the vertical stress (σveff), σe1, σe2,and σe3, required for consistency between newly formed faulting/frac-turing (Mohr-Coulomb criterion), and sliding on pre-existing planes(Byerlee's friction law; Byerlee, 1978) (Lacombe and Laurent, 1992;Lacombe, 2001; Amrouch et al., 2011).

The mechanical properties of the limestones that crop out in thesouthern and eastern portion of the KKFTB (Khao Khad formationsand Saraburi marble) were here used to describe the intrinsic failureenvelope of the Khao Khad Formation (Fig. 2a) (Tepnarong, 2001).The objective of the triaxial compressive strength tests were to deter-mine the compressive strengths of Saraburi limestones under variousconfining pressures. The sample preparation and test procedure fol-lowed the applicable ASTM (ASTM D2664-86) and ISRM suggestedmethod (Brown, 1981). A total of 5 specimens have been tested un-der various confining pressures. The length/diameter of the specimenequals 2.0. The samples underwent compression under confining pres-sures of 1.7, 3.4, 6.9, 13.8 and 20.7 MPa, the deviatoric stresses nec-essary to obtain failure within the samples were calculated at each step(Table 2).

Table 1Experimental determination of the brittle strength of the Saraburi limestone formations.Sizes of the samples during the Brazilian test: diameter = 22.5, 38.5, 54.0, 67.4 mm,length/diameter = 0.5.

Saraburi limestone and marble

Diameter Length D/LLoad atfailure

Confiningpressure

Axial stress atfailure

(mm) (mm) ratio (kN) (MPa) (MPa)

See capt. Seecapt.

0.5 0 0 − 8.5

53.9 100.7 0.5 174 1.7 76.253.9 100.8 0.5 250 3.4 109.554.1 100.1 0.5 274 6.9 119.854 102.8 0.5 284 13.8 124.454 100.3 0.5 386 20.7 169.1

Fig. 2. a) Crack development curve (CDC) (red circles) from rock mechanic tests. b)Mohr circle representation of the orientations of planes explained by methodology ap-plied in this manuscript, which consist on finding the values of the principal stresses σ1,σ2 and σ3 required for consistency between newly formed faulting/fracturing (β), fric-tional sliding along pre-existing planes (α), and differential stresses estimated from cal-cite twinning. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

4. Palaeostress conditions

Bergerat (1987), and Angelier (1989) proposed that palaeostresscould be estimated based on determining the most easily derivablestress value, which is the vertical stress (σv). This value is controlledby the thickness of the overburden -in our case 2 km-, the average den-sity of the rock column (ρ = 2500 kg/m3) under hydrostatic fluid pres-sure conditions, and gravitational acceleration (g = 9.82 ms− 2). Withan effective vertical stress ~ 30 MPa ± 6 MPa (based on the uncer-tainties between burial thickness, rock density and non-coaxiality ofthe high dip stress and the gravity). Jaeger and Cook (1969), Byerlee(1970, 1978) and Sibson (1994) demonstrated that at intermediatepressures (5 < σn < 200 MPa; and τ = 50 MPa + 0.6 σn) rock type doesnot influence friction coefficient values in the failure criteria for reac-tivation of pre-existing shear surfaces. Additionally, the initial surfaceroughness has little effect on friction values and the failure criteria forpre-existing shear surfaces can be approximated by Eq. (3):

where τ, μk and σn are respectively the shear stress, the coefficientof static friction and the normal stress. Palaeostress analysis of reac-tivated faults is based on the kinematic theory of reactivation for a

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Table 2Parameters and data set used in the construction of the Mohr's circles to quantify effective principal stress magnitudes. EXT: extensional; PC: pure compressive; SS: strike-slip. Insquare brackets values obtained from samples with insufficient untwinned planes.

Stressstage

Samplenumber

Bedding (DipDir - Dip)

Vein (DipDir - Dip)

Stressratio (Φ)

Differential stress(σ1 − σ3) (MPa)

Grain size(μm)

CRSS(MPa)

Stressregime

Magnitude ofσ3 (MPa)



SI T020 174/14 068/88 0.6 84 ± 17 150–170 10 ± 4 EXT [− 2] [42] [78]SI T029 352/39 235/80 0.4 76 ± 15 150–170 10 ± 4 EXT ″ ″ ″SI T019b 160/40 251/72 0.3 37 ± 7 150–170 10 ± 4 EXT − 5 8 38SII T029b 352/39 358/82 0.4 58 ± 12 150–170 10 ± 4 PC − 4 26 54SII T083b 175/59 HOST 0.9 35 ± 7 150–170 10 ± 4 SS − 4 ± 30 40SII T018vb 160/40 138/25 0.8 38 ± 7 150–170 10 ± 4 SS ″ ″ ″SIII T011b 178/60 HOST 0.5 66 ± 13 150–170 10 ± 4 SS 13 21 87SIII T019 160/40 322/71 0.2 62 ± 12 150–170 10 ± 4 PC ″ ″ ″SIII T018v 160/40 322/71 0.5 68 ± 13 150–170 10 ± 4 SS ″ ″ ″SIII T011 178/60 HOST 0.5 72 ± 15 150–170 10 ± 4 SS ″ ″ ″SIV T081 175/59 140/78 0.3 71 ± 14 150–170 10 ± 4 PC 12 16 66SIV T082 175/59 217/75 0.1 55 ± 11 150–170 10 ± 4 PC ″ ″ ″SIV T080b 175/59 255/52 0.8 38 ± 7 150–170 10 ± 4 SS 12 55 66SV T081b 175/59 281/42 0.4 51 ± 10 150–170 10 ± 4 SS 46 30 72SV T082b 175/59 261/49 0.3 46 ± 9 150–170 10 ± 4 SS ″ ″ ″SV T083 175/59 045/87 0.6 52 ± 11 150–170 10 ± 4 PC ″ ″ ″SV T080 175/59 337/85 0.7 70 ± 14 150–170 10 ± 4 PC ″ ″ ″

given state of stress. This theory predicts: (I) the favourable orienta-tions of the fault planes for reactivation, and (II) the direction alongwhich slip is likely to occur on a given fault surface (Wallace, 1951;Bott, 1959). Therefore, the methodology applied in this manuscriptconsists on finding the values of the principal stresses σ1, σ2 and σ3 re-quired for consistency between newly formed faulting/fracturing (β),frictional sliding along pre-existing planes (α), and differential stressesestimated from calcite twinning (Fig. 2b).

4.1. Fracture sets

The interpretation of the data collected within the KKFTB wasbased on the statistical analysis of fracture orientations and their rel-ative ages (Arboit et al., 2015). The analysis of these fractures wascarried out both with a field based study at the meso-scale, and at themicro-scale in 27 thin sections. The Permian carbonate of the KhaoKhad Formation contains a great variety of fractures, many filled withcalcite, which developed during the Indosinian Orogeny (Warren etal., 2014; Hansberry et al., 2015). The term “fractures” is here usedin a general sense to refer to either (Type I) opening-, (Type II & III)sliding-, or closing-mode displacement discontinuities along surfaces.We differentiate five major regionally systematic phases of deforma-tion, composed of 12 fracture sets – classified from AI to BX. Thefracture sets form at both low and high angles to bedding (NE-SWand NW-SE), and when the fractures occur near to inter-layered shale,it is difficult to identify clear patterns in the fracture orientationsthroughout the KKFTB, or, at a smaller scale, to follow them over thesame layer, because of intensive dissolution (burial pressure-solution),which significantly affected the thickness of the bedding. In these ar-eas, the relationship to local and regional structures is uncertain as aresult of these conditions. Field evidence also shows that the carbon-ate layers of the Khao Khad Formation are often interlayered with thinshale layers. In these cases, veins and joints developed within a singlebed and joint/vein spacing increases with the increasing bed thickness.

4.2. Stage 1

The samples yielding the effective principal stress tensor relatedto the first tectonic stage of the Indosinian orogeny represent an ex-tensional event, and is well constrained by the conjugated newlyformed combined mode I and mode II veins characterized by a shear

ing and an opening during this stage (Fig. 3a). After unfolding calcu-lations (Arboit et al., 2015) the veins are almost normal to the bed-ding with a θ angle of about 25°. Two stress tensors out of the threethat provided data consistent with extensional setting have an insuf-ficient number of untwinned planes and this might be the reason forthe anomalously high differential stress values (Rocher et al., 2004).In the few samples with insufficient untwinned planes (values be-tween square brackets, Table 2), the calculated differential stress val-ues appear to be overestimated. Hence, these high differential stresseshave not been taken in consideration, and for Stage 1, only the low-est differential stress value with a magnitude of 43 MPa is taken inaccount in the following observations. The geometrical representationof the maximum differential stress (σe1 − σe3 = 43 MPa) is tangent tothe crack development curve, with effective principal stress values be-ing σe1 (σe1 = σveff) = ~ 38 MPa and σe3 = ~−5 MPa. The value of thestress ratio “Φ” drives σ2 towards values of ~ 8 MPa. The Mohr circleis tangent at the point representing the two sets of conjugate fracturesSet-AI (65°/163°) and Set-AII (60°/350°), which lies at an angle β of~ 22° to σ1 (Fig. 3a).

4.3. Stage 2

The second tectonic stage is interpreted to be the main compres-sive tectonic event in SE Asia during the Early Triassic and corre-sponds to the formation of a new set of faults (pure compressivestress regime) and to two new fracture sets (strike-slip stress regime):Set-AIII (80°/050°) and Set-AIV (90°/080°) (Arboit et al., 2015). Thestress tensors detected with calcite twin analysis describe both the de-formations observed in the field, both with a horizontal σ1 axis ori-ented NE-SW that trends perpendicular to the mean fold hinge ori-entation. The emplacement during the Early Triassic throughout theKKFTB of newly formed conjugate fractures and of newly formed re-verse faults containing the σ2 axis and at an angle β of 20° to 28° to theσ1 axis requires the Mohr circle to be tangent to the CDC (Fig. 3b1).Differential stress in compressive conditions (sample T029b, Table 2)reaches the value of ~ 58 MPa with the effective principal stress val-ues σe1 = 54 MPa and σe2 = 27 MPa and (σveff) σe3 = − 5 (Fig. 3b1).Taking into account the estimated value of the expected vertical stressin hydrostatic conditions, the drop of the vertical stress value by over~ 30 MPa indicates an important increase of fluid overpressure duringthis compressive phase of the Stage 2.

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Fig. 3. Kinematic and mechanical scenario of development of the Khao Khwang fold-thrust belt. For each stage are shown: sketches representing the microstructures formed, Mohrcircles and the relative stress regime (all data coming from the Khao Khad Formation, within the southern part of the KKFTB). Mohr circles construction corresponding to eachtectonic stage within the KKFTB since the beginning of the Indosinian orogeny (Mid-Permian, Stage 1), Early Triassic (Stage 2), to the latest stage of the Indosinian orogeny (Stage4, latest Triassic), until the latest deformation that affected the KKFTB in the Cenozoic (Stage 5), (Modified from Arboit et al., 2015). For details on these kinematic and mechanicalscenarios, see Tables 1 and 2.

A second strike-slip phase was recorded during this stage. Withtwo strike slip regime tensors, presenting a very similar differentialstress respectively of ~ 35 MPa (sample T083b, Table 2) and~ 38 MPa (Sample T018vb, Table 2). These two samples have anearly identical value of the stress ellipsoid shape ratio (σ2 − σ3)/(σ1 − σ3) (Table 2) that leads to similar values of vertical effectivestress for both tensors (σveff) σe2 = ~ 35 MPa. The values of the σ2 − σ3differential stress obtained after CSIT calculation are the same forboth the stress tensors (~ 30 MPa) with only the maximum horizontalstress (σe1) being higher in the pure compressive tensor. In both thegeometrical construction the Mohr circles corresponding to the twodifferential stresses present a similar minimum principal stress, withthe pure compressive stage (T029b) σe3 = − 3 MPa and the strike-slip(T083b and T018vb) σe3 = − 4 MPa.

4.4. Stage 3

The third stress event caused the left lateral reactivation of thepre-existing set of fractures formed during the extensional phase(Stage 1), and the LPS (Stage 2) under strike-slip regime, these reac-tivated veins are often associated with wing cracks and Riedel arrays(R and R′) when reactivated under a strike-slip regime. The only purecompressive stress tensor yields a quite low stress ratio (Φ = 0.2) thatmight permit σ2 − σ3 stress permutation. The reactivated strike-slipfractures sets are vertical and contain the principal stress σ2. The Mohrcircle representing the stress tensors in Stage 3 is tangent to the re-activation friction curve (τ = 0.85 σn), with the fractures being reac-tivated under a strike-slip regime. The differential stress has same

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value in all the stress tensors (~ 66 MPa) and the absolute valuesof the effective principal stresses are: σe1 = ~ 78 MPa, (σveff)σe2 = ~ 12 MPa, and σe3 = ~ 46 MPa.

4.5. Stage 4

The fourth stress event is characterized by two compressive andone strike-slip stress tensors, the latter of which have a similar attitudeto the strike-slip tensors of Stage 3. The differential stress value of thisstress tensor (~ 38 MPa) was high enough to allow right lateral shearreactivation of the pre-existing set of strike-slip fractures formed dur-ing Stage 1 and Stage 2. The two compressive stress tensors yield rel-atively low stress ratios (Φ = 0.1, 0.35), and values of high differentialstress (~ 65 MPa) that reactivated some of the major north-verging re-verse faults that created the most significant fault-related folds withinthe KKFTB during the LPS. The Mohr circle representing the com-pressive regime is tangent to the reactivation friction curve at ~ 45° (αangle) that represents the reactivated thrusts throughout the KKFTB.The principal effective stresses are: σe1 = ~ 77 MPa, σe2 = ~ 18 MPa,and (σveff) σe3 = ~ 12 MPa. The strike-slip stress tensor is associatedwith reactivation of fractures with an average θ angle of ~ 25°. Consis-tently, the Mohr circle representing the reactivation of these fracturesis tangent to the friction curve at an angle α of the same amount andpresent effective stress values of σe1 = ~ 49 MPa, σe3 = ~ 11 MPa, and(σveff) σe2 = ~ 43 MPa (Fig. 3d).

4.6. Stage 5

This stage is associated with the peak differential stress (σe1 − σe3)recorded by the calcite twins during the tectonic stage that has beeninterpreted as the only one not related to the Indosinian event. Bothfractures and calcite twins recorded a consistent post-folding rela-tionship. This stage is represented by two strike–slip and two com-pressive stress tensors. The stress ratios of the latter are lower (aver-age Φ = 0.36) than those of the strike-slip tensors (average Φ = 0.66).Arboit et al. (2015) pointed out that several fractures and faults under-went a post-tilting reactivation with a high angle between the reacti-vated planes and the maximum horizontal stress σ1 (Fig. 3e). For thesake of simplicity, we consider that all the strike-slip fractures werereactivated during Stage 5 with an angle α = ~ 25°, while the angleof the reactivated reverse faults is around ~ 47° (Fig. 3e). The Mohrcircle representing the pure compressive regime is tangent to the re-activation friction curve, with the pure compressive regime havinghigher differential stress (~ 61 MPa), whereas the strike-slip regimeshows a lower differential stress, quite similar to the values of theprevious strike-slip stress tensors (~ 47 MPa). The effective principalstress values ranges from ~ 75 to ~ 60 MPa (σe1), with quite similarσe3 = ~ 12 MPa (Fig. 3e).

5. Discussion

5.1. Uncertainties

Before discussing the quantification of the stress magnitude, wewill first review few underlying factors that are necessary to definethe complete stress tensors. There are several criteria and inaccura-cies that have to be taken in account for the final palaeostress quan-tification. For instance: 1) the values of differential stress detectedwith calcite twin analysis suffer of a level of uncertainties of ~ 20%(Lacombe, 2001). However, the accuracy of the quantification of thedifferential stress values might have increased after the statistical cal

ibration between the critical resolved shear stress, twinning strain andgrain size (Lacombe et al., 2009; Amrouch, 2010b). 2) It was not pos-sible to construct the CDC directly from the samples that were usedto extrapolate the stress tensors responsible for the deformation of theKKFTB. Therefore, uncertainties might be induced by using a proxyCDC from the literature (Tepnarong, 2001), in order to constraint theposition of the Mohr circles. 3) One principal stress is considered tobe close to the vertical position. When it is not the case that will lowerthe value of the vertical stress. 4) The Mohr circles, representing thefractures sets at each tectonic stage, are assumed to be representativeonly if the fractures were coeval, either if newly formed or the resultsof reactivation of pre-existing discontinuities (André et al., 2001).

As consequence of these considerations, the values obtainedshould be considered in the order of magnitudes rather than absolutevalues (Lacombe, 2001; Lacombe et al., 2009; Amrouch et al., 2011).However, these are the only available record for the uppermost crustalpalaeostresses in central Thailand from the onset of the Indosinian de-formation. Further, the validity of the methodology has already beenshown by Lacombe (2001) and Amrouch et al. (2010a, 2011).

5.2. Consistency of the of palaeostress results from calcite twins withThailand regional tectonics

The relationship between the evolution of tectonic regimes, theirstress patterns and the development of complex systems of fractur-ing, within a tectonic complex zone such as a fold and thrust belt, hasbeen observed and explained by several authors (Lacombe et al., 2007;Amrouch, 2010; Vitale et al., 2012; Tavani et al., 2015). The same re-lationship was observed within the KKFTB, where each stress state,revealed by the twinning analysis, has been correlated to a specific tec-tonic event. However, we need to take in consideration that the sam-ples used for calculating the differential stress were collected within afold-thrust belt, and might correspond to a local value (e.g. high stressconcentrations related to asperities along major thrust in the area); andconsequently, might not be indicative of the far field stress conditions.However, the tight stress-strain relationships observed in Arboit et al.(2015) indicate that the palaeostress tensors can be related to regionaltectonic events.

All the calculated maximum differential stress values (Table 2) fitreasonably with a bracket of values between 30 MPa and 65 MPa.The highest values, calculated in samples deformed during compres-sive regimes present similar magnitudes of those calculated in foldand thrust belts around the world, such as in the south AquitaineBasin (Rocher et al., 2000), in the Zagros (Lacombe et al., 2007), andin the Taiwan foothills (Lacombe et al., 1996; Lacombe, 2001) andin the Apennines (Beaudoin et al., 2016), among others. These val-ues are higher than the average values in a common intraplate set-ting (Lacombe et al., 1996; Rocher et al., 2004) and possibly adequatefor an active collisional setting. Samples with insufficient untwinnedplanes can give overestimated differential stress values (Rocher et al.,2004). In the two samples with insufficient untwinned planes (samplesT020 and T029, Table 2), the calculated differential stress values ap-pear to be overestimated; hence, these samples are not taken in con-sideration.

5.2.1. Tectonic scenario

5.2.1.1. Stage 1The formation of pre-folding extensional veins can be related to

the overburden stress associated with burial, predating, or coeval

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with, formation of few bedding-parallel stylolites. However, the inter-pretation of the extensional stage is based on one single stress tensor,therefore it is possible to imply that the validity of the results is re-stricted to the southern KKFTB and is not representative of a regionalscale. All these fractures and bedding-parallel stylolites have a morerestricted occurrence than the sets of fractures that developed duringlater stages. The relatively narrow strike of the fracture sets that tookplace during Stage 1 implies a uniform distribution of minimum stressσ3 and a constant position through time of the vertical stress σ1. Weproposed that the sets of veins developed during a Permian extensionalphase on the southwestern margin of the Indochina terrane, before themain LPS event. This was possibly in response to the N-S orientedflexure of the foreland in front of the advancing thrust sheets, contem-porary with burial, and possibly under high fluid pressures (Arboit etal., 2015).

5.2.1.2. Stage 2The second tectonic stage is connected to a major N-S oriented col-

lision. This compressional stage is consistent with the direction of tec-tonic transport within the KKFTB (Morley et al., 2013; Arboit et al.,2014), supporting the regional significance of the stress tensors, rep-resented by strike-slip and compressive stress tensors. These tensorsare linked to the formation of both strike-slip fractures, and major re-verse faults. They were mostly recorded within the N-E striking veinsand present lower differential stress values than the compression stresstensors that were recorded in the N-S striking veins. The strong de-crease of the σe1 during/after folding (Table 2, Fig. 3) could be inter-preted as a drop of the fluid pressure related to the development of thebending-related reverse faults throughout the KKFTB. These fracturesincreased the vertical permeability of the entire Khao Khad Forma-tion, possibly including the clastic Permian Sap Bon Formation, andthis might have caused a subsequent drop of the fluid pressure.

5.2.1.3. Stage 3Stage 3 is represented by three strike-slip and one compressive

stress tensors. The strike-slip stress tensors calculated on the cal-cite veins present distinctive differential stress values that resemblethe magnitude of the average compressive stages (~ 65 MPa). Theseanomalously high strike-slip differential stress values allow us to ad-vance the hypothesis where Stage 3 might correspond to a compres-sional tectonic stage; however, the high stress ratio does not justifythis idea (Table 2). Also, it must be taken into consideration that thehigh fracturing and faulting activity in Stage 2 released fluid pressure.This drop in fluid pressure may have shifted the Stage 3 Mohr circletowards higher effective stress values (Fig. 3c).

5.2.1.4. Stage 4The macro deformation associated with Stage 4 is associated with

the peak differential stress (σ1 − σ3) recorded by calcite twinning justbefore the reactivation of E-W striking thrusts, the tightening ofpre-existing fault-propagation folds and reactivation of the fracturesdeveloped during the earlier events. During the latest Triassic, thehalf-graben basins in the Khorat Plateau (NE Thailand) containing theKuchinarai Group (Late Triassic) ceased to subside and some werestructurally inverted. The degree of deformation associated with thisevent is considerably less than deformation associated with the ear-lier Indosinian I event (Booth and Sattarayak, 2011). However, thedifferential stress that was necessary to reactivate the fractures in thepost-folding stress states was in the order of 50–60 MPa. This mightalso be caused by the complex polyphase pre-folding deformation thatinduced high levels of strain hardening in the crystal lattices, and this

had a remarkable effect on the yield stress value of the post-fold-ing events (Stage 4; Stage 5). These post-folding differential stressesrange in the order of few MPa (Table 2), and might be a consequenceof the more homogeneous tectonic framework in central Thailand afterthe collision of Sibumasu with the amalgamated Sukhothai-Indochinaterrane during the Late Triassic ~ 220 Ma (Sone and Metcalfe, 2008;Morley et al., 2013; Metcalfe, 2013; Arboit et al., 2015; Ng et al.,2015; Arboit et al., 2015).

:

Price and Cosgrove (1990) demonstrated that from Eq. (4), us-ing a coefficient k = 4.0, that corresponds to a coefficient of friction(μ) of 0.75, at a given depth, the ratios of differential stress requiredfor reaching shear conditions are higher for (a) compressive condi-tions, (b) intermediate for strike-slip and lower for (c) extensional sys-tems. This partially agrees with the differential stress values calcu-lated within the KKFTB, except for the anomalous differential stressvalues in Stage 3. Indeed, the average differential stress values ofthe pure compressive tectonic stages have higher magnitudes (aver-age ~ 65 MPa) than those of strike-slip regimes (average ~ 50 MPa).Sibson (1977) assumed that if the angle (θ) between the plane of thefault and the principal stress (σ1) increased, the magnitude of the stresstensor necessary to reactivate a fault plane has to be higher. This prin-ciple is reflected on the differential stresses that acted on the KKFTB,where the magnitudes increased from Stage 2 to Stage 5 (Table 2) be-cause of the tilting of the bedding, and the consequent slanting of thefaults, after the main folding event (Arboit et al., 2015).

5.3. Relationship between palaeostress, tectonic framework andpalaeoburial in central Thailand from the Mid-Permian

The values of differential stress are independent of the fluid pres-sure (Rowe and Rutter, 1990), and we can use this evidence in orderto prove that the values of differential stress calculated herein are themaximum values reached during the deformational cycles undergoneby the calcite grains during the same tectonic event. Lacombe (2001,2007), and Lacombe et al. (2009) demonstrated the relationship be-tween increasing values of differential stresses with depth. It is thenpossible to infer that the main shortening event was coeval with theonset of the folding, and hence the stress tensor related to the mainfolding event was probably recorded by twinning at the time of themaximum burial, right before uplift.

After CSIT calculation we obtained differential stress for Stage1 of ~ 43 MPa, with effective stress as σe3 = ~− 5 MPa andσe1 = ~ 38 MPa (Fig. 3a). These values imply a possible maximumburial depth of about 1.9 km; however, the stratigraphic thicknessat which the LPS occurred in the KKFTB is not well established.Nonetheless, Ueno and Charoentitirat (2011) suggested similar bur-ial depths for the Khao Khad Formation during the Early Triassic(1800 m). Stock et al. (1985) and Morris et al. (1996) modelled thatwithin an extensional setting σ2 is 50–70% of σ1 and σ3 is 20–30%of σ1, in such case the effective stresses would be σe3 = ~ 10 MPa,σe2 = ~ 30 MPa, and σe1 = ~ 55 MPa. Consequently the depth of bur-ial would be ~ 2 km, and we might consider these palaeoburial es-timates as maximum values (Lacombe, 2007; Lacombe et al., 2009,Amrouch et al., 2011). Hansberry et al. (2015) reported illite crys-tallinity palaeo-temperatures of ca. 160–220 °C for elsewhere in theKKFTB. Noting that the high ‘temperatures’ may represent strain in

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Fig. 4. Modified from Arboit et al. (). Palaeo-geographic schematic reconstructions of SE Asia based on the GPlate software reconstruction (contour and rotation file in Supplemen-tary data). The cross-sections show the tectonic development of the Indochina, Sukothai and Sibumasu blocks during the Early Permian to the Late Triassic; resulting in the LPS andfold-tightening deformations, within the southern portion of the KKFTB.

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duced crystallinity, the lower values are broadly consistent withdepths estimated by this study, assuming relatively high geothermalgradients consistent with coeval volcanism present in the region(Arboit et al., 2016b).

The active folding within the KKFTB started when the flexureof the foreland basin shut off and collision between Sukhothai andIndochina began in the Early Triassic. The Mid Permian structuralregime was, therefore, likely dominated by foreland flexure (Sone andMetcalfe, 2008; Morley et al., 2013) or extension due to back-arc rift-ing, so that burial stylolites and the extensional veins in the forelandprobably developed during the Mid to Late Permian.

Thrusting/folding in the KKFTB began during the Early Triassic,with maximum shortening probably at the time of the Sukhothai-In-dochina collision. After Morley et al. (2013), and Arboit et al. (2014) itis possible to link the formation of the main anticlines to the first majorfolding episode, with possibly a later reactivation coeval with a phaseof fold tightening that occurred during the Late Triassic after the Sibu-masu-Indochina collision. Depending on the structural evolution dur-ing the Triassic, the maximum burial might have been reached at dif-ferent times through the Triassic, depending on the position within thefold-thrust belt. However, it is possible to imply that the main foldingevent occurred while the KKFTB reached its maximum burial depthin the Early Triassic.

Maximum depositional ages of syntectonic sediments in the fore-deep (251 ± 3 Ma) and foreland (205 ± 6 Ma) constrain the age ofcontractional basin formation (Arboit et al., 2016a, 2016b), whereas208 ± 4 Ma 40Ar/39Ar ages on orogenic illite collected on a reacti-vated thrust plane of the KKFTB (Hansberry et al., 2015) directlydates thrusting within the orogen. In addition, 255 ± 6 Ma, 224 ± 2 Ma40Ar/39Ar crystallization ages of biotite and muscovite from andesiticdykes that intruded the core of a fault-propagation fold and propa-gated passively on a thrust of the KKFTB (Arboit et al., 2016b) (Fig.4), provide good constraints on the timing of the several palaeostressstates that deformed the KKFTB. As a result, the limestones of theKKFTB were buried in the Early Triassic at depths of ~ 2 km (Uenoand Charoentitirat, 2011). These data show that it is possible to de-rive the maximum value of synfolding erosion from the Early-MidTriassic. The value is poorly constrained without any low temperaturechronological data. However, assuming that the U-Pb geochronolog-ical data from the intrusive dykes provide a good constraint on thetiming of folding, it is then possible to calculate a mean rate of ex-humation of the sedimentary Permian and Early Triassic cover of theKKFTB in the range 0.05–0.07 mm/yr.

6. Conclusions

Quantitative estimates of crustal stress and strength are central tomany problems of rock mechanics. This paper presents a first at-tempt at comparing and combining palaeostress magnitude data to-gether with geomechanical data. This analysis was performed in orderto constrain:

- the magnitude of the stresses that affected the sedimentary layersof the KKFTB in central Thailand during the Triassic Indosinianorogeny;

- Compressional settings that affected the KKFTB during the In-dosinian orogeny recorded the highest values of differential stress(± 65 MPa), while extensional settings the lowest values(± 38 MPa);

- feasible estimates of average exhumation rates that affected the rockformations of the KKFTB, which are in line with thicknesses es-timated by previous analyses (Ueno and Charoentitirat, 2011) and

palaeo-temperatures estimated by illite crystallinity data (Hansberryet al., 2015).

Hence, this combination of stress data brings useful information onthe strength and mechanical behaviour of the upper continental crustin central Thailand since the Mid Permian, and might be considered asvaluable inputs in numerical models of the geodynamic evolution ofcentral Thailand.

Acknowledgements

This work was funded by Australian Research Council DiscoveryProject #DP 120101460. ASC is funded by Australian ResearchCouncil grant #FT120100340. This is a contribution to IGCP pro-jects #589 (Development of the Asian Tethyan Realm) and #628 (TheGondwana Map) and we gratefully acknowledge all the funding orga-nizations. This publication forms TRaX Record #xxx.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.tecto.2017.01.008.

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