Evidences of Cenozoic strike-slip dislocations of the red river fault system in Paleozoic carbonate...
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ISSN 18197140, Russian Journal of Pacific Geology, 2014, Vol. 8, No. 3, pp. 163–176. © Pleiades Publishing, Ltd., 2014. Original Russian Text © S.A. Kasatkin, V.V. Golozubov, Phung Van Phach, Le Duc Anh, 2014, published in Tikhookeanskaya Geologiya, 2014, Vol. 33, No. 3, pp. 14–28. 163 INTRODUCTION The Red River Fault System (RRFS) is one of the major fault structures in Southeast Asia. It extends from Tibet to the coast of South China Sea for a dis tance of about 1000 km, separating the Indochina and South China blocks (Fig. 1). On the territory of North Vietnam, the RRFS is represented by a series of sub parallel faults, which are traced along the Red River valley from the Vietnam–China boundary up to the mouth of river for a distance of about 200 km at a width of 20–50 km. Within the Ha Noi Trough, from the Viet Tri City to the southeast, all the faults are overlapped by PlioceneQuaternary deltaic sediments of the Red River. In turn, the Ha Noi Trough is a northwestern wedge shape ending of the Cenozoic sedimentary Red River Basin. In the southeastern direction, this basin is traced in the South China Sea, along the coast of Viet nam, for a distance of about 500 km at a width of up to 200 km (Fig. 2a). The formation of the Red River Basin, it’s filling, and subsequent deformations were limited by strikeslip displacements along the RRFS, which bound the basin and complicate its structure [17]. Given that the IndoEurasian plate collision con tinues up to the present time, strikeslip dislocations within the RRFS need a close comprehensive exami nation [9, 10, 12, 13, 15, 19, etc.]. In particular, it was established that the sense of strikeslip displacements along faults changed during their tectonic evolution. In the Oligocene–Early Miocene (32–16 Ma), there were sinistral displacements with amplitudes of more than 500 km according to some estimates [10]. Many researchers suggest that these displacements were associated with the beginning of the IndoEurasian plate collision (Fig. 1a) and clockwise rotation of the Indosinian block [8, 19, etc.]. These processes have been accompanied by the formation of synfault gra bens and halfgrabens (including the Red River Basin) filled by predominantly continental, partly deltaic, and rarely littoral deposits, the thickness of which sometimes exceeds 15000 m [5, 11, 13, 17]. The strikeslip displacements and the filling of grabens were nonuniform processes, as evidenced from the seismograms, which recorded repeated episodes of folded deformations accompanied by erosion and for mation of angular unconformities [13, 17] (Fig. 2b). It is assumed that there was a slow down of the tectonic activity within the RRFS during the period from 16 to 5 Ma [10], and, already, dextral strikeslip displace ments with an amplitude varying from 20 to 57 km, according to various estimates, became predominant in the PlioceneQuaternary (5–0 Ma) [4, 10, 18]. Over the last 5 million years, a new portion of terrige Evidences of Cenozoic StrikeSlip Dislocations of the Red River Fault System in Paleozoic Carbonate Strata of Cat Ba Island (Northern Vietnam) S. A. Kasatkin a , V. V. Golozubov a , Phung Van Phach b , Le Duc Anh b a Far East Geological Institute, Far East Branch, Russian Academy of Sciences, pr. 100 let Vladivostoku 159, Vladivostok, 690022 Russia email: [email protected] b Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology, Hanoi, Vietnam email: [email protected] Received September 1, 2013 Abstract—The structural researches of carbonate strata in the northeastern segment of the framework of the Red River Fault System (Cat Ba Island, Northern Vietnam) has been carried out. It was found that weakly deformed carbonate strata are cut by NWtrending (300–310°) strikeslip faults. Development of plicative and disjunctive dislocations occurred along predominantly sinistral strikeslip fault zones formed as a result of ENE regional compression (80°) during the Oligocene–Miocene phase of deformation. Late dislocations confined to the Pliocene–Quaternary phase of deformation (NNW regional compression 330–350°), are relatively less developed. Seismic monitoring data show that both plicative and disjunctive dislocations have continued to the present. Keywords: Cenozoic strikeslip dislocations, Paleozoic carbonate strata, the Red River Fault System, Cat Ba Island, Northern Vietnam DOI: 10.1134/S1819714014030051
Transcript of Evidences of Cenozoic strike-slip dislocations of the red river fault system in Paleozoic carbonate...
ISSN 1819�7140, Russian Journal of Pacific Geology, 2014, Vol. 8, No. 3, pp. 163–176. © Pleiades Publishing, Ltd., 2014.Original Russian Text © S.A. Kasatkin, V.V. Golozubov, Phung Van Phach, Le Duc Anh, 2014, published in Tikhookeanskaya Geologiya, 2014, Vol. 33, No. 3, pp. 14–28.
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INTRODUCTION
The Red River Fault System (RRFS) is one of themajor fault structures in Southeast Asia. It extendsfrom Tibet to the coast of South China Sea for a dis�tance of about 1000 km, separating the Indochina andSouth China blocks (Fig. 1). On the territory of NorthVietnam, the RRFS is represented by a series of sub�parallel faults, which are traced along the Red Rivervalley from the Vietnam–China boundary up to themouth of river for a distance of about 200 km at a widthof 20–50 km. Within the Ha Noi Trough, from the VietTri City to the southeast, all the faults are overlappedby Pliocene�Quaternary deltaic sediments of the RedRiver. In turn, the Ha Noi Trough is a northwesternwedge shape ending of the Cenozoic sedimentary RedRiver Basin. In the southeastern direction, this basin istraced in the South China Sea, along the coast of Viet�nam, for a distance of about 500 km at a width of up to200 km (Fig. 2a). The formation of the Red RiverBasin, it’s filling, and subsequent deformations werelimited by strike�slip displacements along the RRFS,which bound the basin and complicate its structure[17].
Given that the Indo�Eurasian plate collision con�tinues up to the present time, strike�slip dislocationswithin the RRFS need a close comprehensive exami�
nation [9, 10, 12, 13, 15, 19, etc.]. In particular, it wasestablished that the sense of strike�slip displacementsalong faults changed during their tectonic evolution.In the Oligocene–Early Miocene (32–16 Ma), therewere sinistral displacements with amplitudes of morethan 500 km according to some estimates [10]. Manyresearchers suggest that these displacements wereassociated with the beginning of the Indo�Eurasianplate collision (Fig. 1a) and clockwise rotation of theIndosinian block [8, 19, etc.]. These processes havebeen accompanied by the formation of syn�fault gra�bens and half�grabens (including the Red River Basin)filled by predominantly continental, partly deltaic,and rarely littoral deposits, the thickness of whichsometimes exceeds 15000 m [5, 11, 13, 17]. Thestrike�slip displacements and the filling of grabenswere nonuniform processes, as evidenced from theseismograms, which recorded repeated episodes offolded deformations accompanied by erosion and for�mation of angular unconformities [13, 17] (Fig. 2b). Itis assumed that there was a slow down of the tectonicactivity within the RRFS during the period from 16 to5 Ma [10], and, already, dextral strike�slip displace�ments with an amplitude varying from 20 to 57 km,according to various estimates, became predominantin the Pliocene�Quaternary (5–0 Ma) [4, 10, 18].Over the last 5 million years, a new portion of terrige�
Evidences of Cenozoic Strike�Slip Dislocations of the Red River Fault System in Paleozoic Carbonate Strata of Cat Ba Island
(Northern Vietnam)S. A. Kasatkina, V. V. Golozubova, Phung Van Phachb, Le Duc Anhb
a Far East Geological Institute, Far East Branch, Russian Academy of Sciences,pr. 100 let Vladivostoku 159, Vladivostok, 690022 Russia
e�mail: [email protected] Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology, Hanoi, Vietnam
e�mail: [email protected] September 1, 2013
Abstract—The structural researches of carbonate strata in the northeastern segment of the framework of theRed River Fault System (Cat Ba Island, Northern Vietnam) has been carried out. It was found that weaklydeformed carbonate strata are cut by NW�trending (300–310°) strike�slip faults. Development of plicativeand disjunctive dislocations occurred along predominantly sinistral strike�slip fault zones formed as a resultof ENE regional compression (80°) during the Oligocene–Miocene phase of deformation. Late dislocationsconfined to the Pliocene–Quaternary phase of deformation (NNW regional compression 330–350°), arerelatively less developed. Seismic monitoring data show that both plicative and disjunctive dislocations havecontinued to the present.
Keywords: Cenozoic strike�slip dislocations, Paleozoic carbonate strata, the Red River Fault System, Cat BaIsland, Northern Vietnam
DOI: 10.1134/S1819714014030051
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nous deposits accumulated in the Red River Basinreaching a thickness of more 700 m. These strata liehorizontally and overlap Miocene sediments with anangular unconformity [13, 17] (Fig. 2b).
The RRFS is seismically active to the present dayand clearly visible in satellite images [2, 4]. Dextralstrike�slip displacements are identified by a kinematicanalysis of the earthquakes focal mechanisms [2] andreconstructed based on the separation of the geomor�phological boundaries, such as alluvial fans, terracesabove the flood plain, etc. [4, 18, 21], and the recentmovements of geoblocks are traced according GPSobservations [7, 20].
Due to the fact that the faults in the mouth of theRed River are almost completely overlapped by the HaNoi Trough sediments (Fig. 2), the character of thetectonic movements on the fault planes cannot bedirectly studied. An exception is the northeasternframework of the RRFS in the area of Ha Long Bayand Cat Ba Island [6, 14] (Figs. 2, 3). The researchgoal was to study the structure and to recognize thestages of the above mentioned deformations in thePaleozoic carbonate strata on Cat Ba Island. Thepresent article is based on the results of our investiga�tions.
The base of the visible columnar section consists ofUpper Devonian–Lower Carboniferous deposits (PhoHan Formation, thickness of 400–650 m) representedby laminated limestone with intercalations of cherty
limestone, marl, and black�grey banded shale. Theyare outcrop in the middle of anticlinal structures of thecentral and southwestern parts of the island (Fig. 3).
The overlying Lower Carboniferous strata (Cat BaFormation, thickness of 450 m) is made of alternationof black�grey, oolitic, and cherty limestone with a fewlayers of siltstone in the lower part.
The uppermost part of the section of the Permian–Carboniferous carbonate strata (Bac Son Formation,thickness of 750 m) is composed mainly of light grey,thick�bedded to massive limestone, oolitic limestonewith lenses of cherty limestone.
Most of our field works were carried out within theCat Ba fault zone along the coastal and roadside out�crops on the southern edge of Cat Ba Island (Fig. 4).Numerous measurements of the structural (bedding,fractures) and kinematic (striation, tectonic steps,etc.) elements with interpretation, when it was possi�ble, of the type of displacement, as well as measure�ments of the orientations of the fracture zones and cal�cite veins, were made. The results of the field measure�ments were processed by V.P. Utkin’s method [3] usingspecialized software (StereoNett 2.46). All the dataobtained were put on the Wulff net (upper hemi�sphere) displaying isolines of the distribution of thepoles of the bedding planes and fractures.
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RRFSIndia
90° 100° 110° 120°
40°
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ab 1
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Fig. 1. Trajectories of the maximum compressive stress within the Indochina Peninsula during the Oligocene (a) and at the presenttime (b) (after [8], modified).(1) trajectories of the maximum compressive stress is directly related to the Indo�Eurasian plate collision (a) and its far�fieldeffects (b); (2) faults and directions of displacement (arrows); (3) zone of continental collision; (4) subduction zone; (5) extensionstructures; (6) spreading zones; (7) current position of the land; (RRFS) Red River Fault System.
BorneoIsland
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RESULTS
A characteristic feature of the studied area is theheterogeneous distribution of the dislocations. Weaklyfolded carbonate strata are in alternation with NW�trending subparallel bands 500–800 meters wide,
where the beds become steeply dipping (up to vertical)and they are often complicated by numerous subinter�layer fractures and microfolding. According to ourdata, the character and type of dislocations are evi�dence that these bands correspond to strike�slip fault
RRFS
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Fig. 2. Geological scheme of the southeastern flank of the Red River Fault System (a) and a seismic profile across the Red RiverBasin (b) (after [13], modified and added to).(1) Pliocene�Quaternary sedimentary rocks; (2) Red River Basin (within the occurrence of Pre�Pliocene deposits); (3) Pre�Pliocene sedimentary rocks, surfaces of angular unconformities (dashed lines), and their age datings (numerals—million years);(4) Pre�Pliocene terrigenous formations; (5) faults: established (a) and inferred (b); (6) seismic profile; (RRFS) Red River FaultSystem.
Viet TriViet Tri
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zones (K1, K2, and K3) (Fig. 4) that are elements ofthe Cat Ba fault zone (Fig. 3) recognized on the basisof previous works [6, 14].
The main belts of folding and fractures (Fig. 5)constructed against the set and orientation of minorstructural forms at different observation pointsrevealed at the analysis of the primary diagrams (notgiven in the text) show a genetic relationship betweenthe relevant systems and the stability of their develop�ments.
Plicative Dislocations
The apparent simplicity of the folded structure ofthe carbonate strata of Cat Ba Island is illustrated inthe diagram (Fig. 5a) as the belts with the axes of NW�SE direction. However, a comprehensive statistical
analysis of the spatial (including zonal) distribution ofthe bedding orientations in a complex with field obser�vations allowed us to distinguish the following systemsof folds.
System I is represented by NNW�trending gentlefolds (dip angles of 0–20°). In the diagram (Fig. 5a), itis expressed by two maximums, which are combinedinto the belt with a gentle axis (dip azimuth of 350°,dip angle of 5°). These maximums correspond to thebackground structural elements of the regional folding(Figs. 6a, 6d). The spatial relationship between theregional NNW strike of the gentle folds (350°) and themain direction of the Cat Ba fault zone (NW 305°)form an angle of 45° in plan representing the structuralparageneses formed under sinistral displacements atENE (80°) compression (Fig. 5a). These deformationscan be apparently considered as primary ones, which
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Fig. 3. Geological map of the Cat Ba Island and vicinity (after [6] with additions)(1–6) sedimentary rocks: (1) Quaternary alluvium; (2) Upper Triassic, Norian and Rhaetian Stages: sandstone, siltstone, mud�stone, coal; (3) Upper Permian, Bai Chay Formation: alternation of chert and sandstone, intercalations of cherty limestone; (4)Carboniferous–Permian, Bac Son Formation: light grey, thick�bedded to massive limestone, with rare lenses of cherty limestone;(5) Lower Carboniferous, Cat Ba Formation: black�grey, oolitic and cherty limestone with a few layers of siltstone; (6) UpperDevonian–Lower Carboniferous, Pho Han Formation: laminated limestone, marl, and black�grey banded shale; (7) faults:established (a) and inferred (b); (8) inferred boundaries of the Cat Ba fault zone; (9–10) directions of displacement along thefaults (9) and the orientation of the regional stress field (10) during the Oligocene–Early Miocene; (11–12) directions of dis�placement along faults (11) and the orientation of the regional stress field (12) during the Pliocene–Quaternary.
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32–16 Ma
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were followed by the development of the strike�slipfault zones. This stress field is sometimes realized asdeformations of longitudinal compression in the formof subinterlayer corrugation of calcite veinlets in gentledipping strata (Fig. 7a).
System II is represented by folding elements thatform maximums with dip azimuths of NE 30–60° andSW 235–260° and dip angles of 30–50°. These foldingelements are combined into the belt with a NW�dip�ping axis (325°) at a dip angle of 15° (Fig. 5a). These
Fig. 4. Geologic�structural map of the southeastern part of Cat Ba Island (after [6])(1–3) sedimentary rocks: (1) Carboniferous–Permian, Bac Son Formation: light grey, thick�bedded to massive limestone withrare lenses of cherty limestone; (2) Lower Carboniferous, Cat Ba Formation: black�grey, oolitic and cherty limestone with a fewlayers of siltstone; (3) Upper Devonian–Lower Carboniferous, Pho Han Formation: laminated limestone, marl, and black�greybanded shale; (4) faults; (5) strike�slip fault zones; (6–7) directions of displacement (6) and orientation of the stress field (7) dur�ing the Oligocene–Early Miocene; (8) bedding attitude; (9) number of the figure in the text and the direction of the exposition(arrow).
Cat Ba CityCat Ba City
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(a)
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Fig. 5. Total diagrams of the orientations of the bedding (a) and fractures (b) in the carbonate strata of the southeastern part ofCat Ba Island.Diagrams (stereographic projections to the upper hemisphere, Wulff net) showing the one�percent isolines of the distribution ofthe poles of the bedding and fractures; the equators of the bedding and fractures (great circle arcs) and their axes (points); theroman numerals indicate the systems of folds and fractures; the dotted line indicates the general strike of the Cat Ba Fault Zone;the arrows indicate the orientations of the compression and displacement directions along faults during the Oligocene–EarlyMiocene (black arrows) and Pliocene–Quaternary (gray arrows);(σ1–1) orientation of the regional compression; (σ1–1n) orientation of the compression in the strike�slip fault zones; (σ1–2)recent orientation of the compression; (N) the number of measurements.
(а)
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10° 40°
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(c) (d)0 1 m
Fig. 6. Examples of limestone bedding.Gentle (a, d) background (system I) inclined (c) in the area of the dynamic influence of strike�slip fault zones (system II) andsteeply dipping (b) within strike�slip fault zones (system III); the numerals indicate the bedding attitude: the azimuth (numerator)and angle (denominator) of dipping. The localities of the observation points are shown in Fig. 4.
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folded structures (Fig. 6) with a width from tens to afew hundred meters are traced along the external bor�ders of the strike�slip zones. At this, close to theboundary of the strike�slip zone, the dip angles of thebedding become steeper (Figs. 8b, 8c), which isdirectly associated with an increase in the intensity ofthe dynamic influence of the syn�fault dislocations.
System III is represented by steeply dipping (60–90°) folds, which are represented in the diagram(Fig. 5a) by maximums with NE (20–50°) and SW(190–230°) dip angles. The axes of the belts, combin�ing the folds of this system, are gentle (0–10°) dippingin the NW and SE directions (290–310°) (Fig. 5a).Field observations have shown that isoclinal folds withsteeply dipping bedding planes are usually commonwithin strike�slip fault zones (Figs. 6b, 7b).
The regular distribution of the I, II, and III systemsof folds is evidence that gentle regional NNW�trend�ing (350°) folds (system I) were transformed into steep(system II) to isoclinal (system III) folds with thechange of the strike from NNW to NW (300–310°)due to the development of sinistral strike�slip disloca�tions and the formation of strike�slip zones. The zonaldistribution of the NW�trending bedding within thestrike�slip fault zones suggests folding under NE (35°)compression (σ1–1n) (Fig. 5a). The subsequent trans�verse compression of the steeply dipping strata of lam�inated limestone sometimes resulted in the origin of enechelon thrust�type calcite veinlets within flexuralbends (Fig. 9). At the same time, axonoclinal (withsteeply dipping hinges) folds formed as a result of sin�
50°
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40°
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Corrugated calcite einlets
Bedding
Plan viewN
(c) (d)
Fig. 7. Elements of folded deformations in limestone.Formation of subinterlayer corrugated calcite veinlets in gentle dipping carbonate strata under the influence of longitudinal com�pression (A); steep dipping laminated limestone in strike�slip fault zone (B) and its S�shaped flexure at sinistral displacement (C);interlayer sliding in the central part of the strike�slip fault zone (D); the arrows indicate the displacement directions; the numeralsindicate the bedding attitude: the azimuth (numerator) and angle (denominator) of the dipping. The localities of the observationpoints are shown in Fig. 4.
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30°
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60° 20°
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Fig. 8. Characteristic types of dislocations in the field of the dynamic influence along the southwestern boundary of the strike�slipfault zone (K1) at the sinistral displacements. A general view (a); the decrease of the dynamic influence of the syn�fault disloca�tions caused changes in the bedding dip angles from steep (b) to gentle (c); en echelon calcite veinlets formed due to interlayersinistral displacements (d); normal fault (e), the numerals indicate the attitude of the normal fault: the azimuth (numerator) andangle (denominator) of dipping; structural interpretation of the fault zone (f); the dashed lines indicate the bedding; the arrowsindicate the displacement directions. The localities of the observation points are shown in Fig. 4.
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istral displacements were observed (close to theboundaries of strike�slip zones (Fig. 7b).
Thus, the influence of the NE (35°) compression(σ1–1n) revealed within the strike�slip fault zones is anormal component of the regional ENE (80°) stressfield. At this, the normal component could be gener�ated at sinistral displacement along the boundary
faults simultaneously with the tangential (strike�slip)component (σ1–1n) (Fig. 10).
System IV of folds is less developed and shown inthe diagram (Fig. 5a) as the small maximums with dip�ping of beds in the northern and southern direction(20–50°). Bedding of the orientation was observed farfrom the strike�slip zones. We can assume that the for�
(a)
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En echeloncalcite veinlets
Flexural
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Fig. 9. Evidence of transverse compression (σ1–1n) in the strike�slip fault zone (K2) in steeply dipping laminated limestone asen echelon thrust�type calcite veinlets in the flexural bend of the strata. A general view (a) and a fragment (b); the arrows indicatethe displacement directions; the dotted lines indicate the boundaries of the en echelon structure. The location of the observationpoint are shown in Fig. 4.
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mation of these ductile deformations was superim�posed and occurred under the influence of NNW�trending (350°) compression (σ1–2) (Fig. 5a).
Disjunctive Dislocations and Their Kinematics
The results of the comparative analysis of the orien�tations of the bedding and fractures (Fig. 5) show thatthe fracture systems I, II, and III are predominantlysubinterlayer. These include, for example, NNW�dip�ping fractures of the IV system. The fractures of theV system are mainly oblique cutting the bedding andfolding.
The overlap of the fracture systems with foldedones is primarily due to the rheological properties ofthe carbonate strata (Pho Han and Cat Ba Forma�tions) composed of laminated limestone. At the real�ization of the stress fields, the bedding planes of lami�nated limestone, marl, and banded shale can bepotential fault planes. At the same time, Zh.S. Erzha�nov (1) notes that {under bending, when the load doesnot exceed 70% of the failure load, solid rocks (silt�stone, mudstone, sandstone, and limestone) indicate acreep behavior. Consequently, the threshold creep inthese rocks may reach several dozens of kg/cm2.Among the rocks under consideration, limestone arecharacterized by a specific creep behavior; the creep isattenuated 10–20 times faster comparing to otherrocks. This behavior seems to be explained by the high
angle of the internal friction and the high viscosity,which is typical of calcite, the main rockforming min�eral of limestone. In other words, disjunctive disloca�tions in limestone are more contrasting, and the tracesof tectonic movements (slickenlines, striations, pri�mary and accretionary steps defined by calcite) areoften visible on fracture surfaces (including interlayerones) (Fig. 11). This allows us to establish the types ofdisplacements with a high degree of reliability. Itshould also be noted that most of faults, along withintensively dislocated (up to isoclinal) folds, are wide�spread within strike�slip fault zones and the area oftheir dynamic influence.
Thus, the subinterlayer fractures in the I, II, and IIIsystems at a general NW (290–330°) strike have dipangles from gentle (20°) to nearly vertical. The regulardistribution of the maximums and the elongation ofthe isolines enable one to combine these systems intobelts with gently dipping axes (up to 30°) to the NWand SE directions (Fig. 5b). Owing to the analysis ofthe fault plane surfaces, the next structural compo�nents were revealed: reverse faults and thrusts; sinis�tral; and, rarely, dextral displacements, as well as nor�mal faults, along which there are fractured zones andsubinterlayer caverns.
The multidirectional kinematic characteristics offractures of the I, II, and III systems, in our opinion,represent the dislocation paragenesis caused by thesubsequent process of development of plicative and
σ1–1
σ1–1n σ1 –1t
N
W E
S
Fold axes direction Fold axes direction
Strike�slip fault zone
of the strike�slipffault zones (300–310°)
of the regionalfolding (350°)
σ1–1 σ1–1
σ1–1n
σ1–1n
σ1–1nσ
1 –1tσ1–1
III
III
III
σ1–1n σ1 –1t
σ1–1 σ1–1
σ1–1n
σ1–1n
σ1–1nσ
1 –1t
Fig. 10. Model of the development of folded deformations in the strike�slip fault zones and beyond their boundaries.(σ1–1) orientation of the regional compression and its components realized in the strike�slip fault zones: the normal (σ1–1n) andtangential (σ1–1t); the dashed lines indicate the boundary faults and the displacement directions along them (arrows); theRoman numerals indicate the systems of folds. The explanations are in the text.
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EVIDENCES OF CENOZOIC STRIKE�SLIP DISLOCATIONS 173
disjunctive dislocations due to sinistral displacementsalong strike�slip zones (Fig. 12).
As noted above, the formation of isoclinal folds(Fig. 10) could be possible under normal compression(σ1–1n). The subsequent compression, when itreaches the limiting state, can be realized in the formof interlayer reverse fault movements (Fig. 11a) and enechelon thrust structures (Fig. 9). These dislocationsoccurred simultaneously with sinistral displacements,evidence of which was found in the area of thedynamic influence of the boundary faults (Figs. 7b, 8g,11b).
The combination of reverse fault and sinistralstrike�slip displacements, along with intense folding,resulted in the uplifting and formation of the “palmtree” structure [16]. However, given the significantwidth (a few hundred meters) between the boundaryfaults, this structure is proposed as a transpressionalstrike�slip zone, where the dynamic influence of thenormal compression (σ1–1n) is attenuated fromboundary faults (sinistral strike�slip faults) to the cen�ter of the zone (Fig. 12). As a result, in the backgroundof the general uplift in the central part of the transpres�sional shear zone, creep structures and normal faults
(a) (b)
(c) (d)
60°
Slickenlines
BeddingNormal
Strike�slipfault
fault
Fig. 11. Evidences of tectonic displacements in limestone.The accretionary steps defined by calcite at the reverse fault (a) and normal fault (b) displacements; the slickenlines are at thesinistral strike�slip fault (c); strike�slip and fault striations on the fault plane surface (d); the arrows indicate the displacementdirections. The localities of the observation points are shown in Fig. 4.
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Formation of the
7C8D
7D
7A
σ1–1
σ1–1n
σ
1 –1t
N
W E
S
σ1–1n
σ1 –1t
σ1–1
9
“palm tree” structurein the strike�slip zone
σ1–1n
σ
1 –1t
σ1–1n
σ1 –1t
Fig. 12. Scheme of the dislocation parageneses in the “palm tree” structure of a transpressional strike�slip fault zone at sinistraldisplacement as evidenced from the laminated limestone of Cat Ba Island. (σ1–1) orientation of the regional compression and its components realized in the strike�slip fault zones: the normal (σ1–n) andtangential (σ1–1t); the dashed lines indicate the boundary faults and the displacement directions along them (arrows); dottedlines show bedding in the strike�slip fault zone; digits in the circles are numbers of figures in the paper and its locations. The expla�nations are in the text.
formed (Figs. 7d, 11b), as well as the transformation ofstrike�slip faults into normal faults (Fig. 11d).
Sometimes, traces of superimposed differently ori�ented (including strike�slip) displacements with a dex�tral component are observed on the surfaces of NW�trending fractures (systems II and III). This seems tobe evidence of a NNW (Pliocene–Quaternary) stressfield (σ1–2) activity.
The sublatitudinal and submeridional fractures ofsystems IV and V, respectively, have dip angles of 50–90° (Fig. 5b). The detailed kinematical analysis showsthat, beyond the strike�slip fault zones, the fault planesof system IV are sinistral and those of system V aredextral. Relative to the NW�trending folded structure,these systems were conjugated and they were active atthe folding stage under the influence of NE compres�sion (σ1–1 to σ1–1n).
On the surfaces of the submeridional fractures (sys�tem V), there are also traces of sinistral displacementsthat are conjugated with dextral displacements alongthe NW fractures (systems II and III), and they are instructural paragenesis with sublatitudinal folds (sys�tem IV). Although these deformations are less devel�oped, in general, they are likely to reflect the influenceof late (Pliocene–Quaternary) NNW compression(σ1–2) (Fig. 5).
EVIDENCES OF RECENT STRIKE�SLIP DISLOCATIONS
The data obtained over the last few decades [2]show high seismic activity along the Red River FaultSystem at depths of up to 33 km in the northwesterndirection. The methods of the modern seismic moni�toring allow us to very reliably decipher the earthquakefocal mechanisms with magnitudes of more than5 Mw (Fig. 13). At this, it is possible to reconstruct theorientations of the main nodal planes of the seismicfaults with the identified direction (type) of the dis�placements, the spatial positions of principal stressaxes (compression, extension and middle axes) at theearthquake focus. In order to evaluate the recent geo�dynamic environment, special attention was paid tothe distribution of the earthquakes with subhorizontal(0–20°) sinistral and dextral displacements along thefault plane, as well as steeply dipping (70–90°) dis�placements, among which only normal faults wereidentified (Fig. 13a). Despite the apparent random�ness in the distribution of the earthquake epicenters,the set of strikes of the sinistral (NE 10–30°) and dex�tral (NW 285–320°) displacements forms a conjugatesystem (Fig. 13b), and the compression axes for thesefault planes are oriented predominantly in the NNW(330–350°) direction (Fig. 13c).
Thus, the recent dislocations revealed by the struc�tural analysis of the Paleozoic carbonate strata of Cat
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EVIDENCES OF CENOZOIC STRIKE�SLIP DISLOCATIONS 175
Ba Island apparently correspond to the Pliocene–Quaternary stage, and these were caused by the far�field effects of the recent NNW (330–350°) stress fieldinitiated by the Indo�Eurasian plate collision(Fig. 1b).
CONCLUSIONS
(1) The plicative and disjunctive dislocations in thecarbonate strata of Cat Ba Island in the segment of thenortheastern framework of the Red River Fault Systemoccurred mainly due to the ENE (80°) regional stressfield. This is evidenced by the predominant sinistraldisplacements along the NW (300–310°) strike�slipzones under local NE (35°) compression. Taking intoaccount the results of the previous investigations, onecan assume that dislocations of mainly the Early Oli�gocene–Miocene stage are developed in the studiedarea.
(2) The sinistral displacements along the strike�slipfault zones (several hundred meters in width) in CatBa Island led to a general uplift of intensively foldedcarbonate strata and the formation of the “palm tree”structure. At the same time, the development of theprocesses of sliding and formation of normal faults inthe central part of the “palm tree” structure are causedby the weakening of the normal NE (35°) compressionfrom the boundary faults (sinistral strike�slip faults) tothe axial areas of the strike�slip fault zones.
(3) The late dislocations of the Pliocene–Quater�nary stage occurring due to the NNW (330–350°)regional compression are very less developed inCat Ba Island. They are fragmentary and representedby: (a) gentle sublatitudinal folding at a distance fromthe strike�slip fault zones and (b) sinistral and dextraldisplacements along the meridional and northwesternfaults, respectively. Judging by the seismic monitoringdata, these dislocations have continued to the presentday.
030
60
90
120
150180
210
240
270
300
3300
30
60
90
120
150180
210
240
270
300
330
60
3060 30 60
30
60
030
60
90
120
150180
210
240
270
300
330
(a) (b)
(c)
RRFS
Ha Noi
Fig. 3
100 km
Hainan Island
SOUTH CHINASEA
σ1–2
σ1–2
N
W
S
E
100° 102° 104° 106° 108° 110° 112° 114°
30°
28°
26°
24°
22°
20°
18°
Dextral faults (N = 16)Sinistral faults (N = 32)Normal faults (N = 9)
Dextralfaults (N = 16)
Sinistral faults (N = 32)
Stress axes (N = 48)
Fig. 13. Distribution of earthquake epicenters with deciphered focal mechanisms (M ≥ 5) recorded over the period of 1977–2010(based on the data of the United States Geological Survey National Earthquake Information Center (2)).(a) stereogram of the orientations of the poles of the principal nodal planes of the earthquakes with the dominant strike�slip faultand normal fault displacements (Wulff net, upper hemisphere); (b) rose diagram of the sinistral and dextral faults; (c) rose dia�gram of the compression axes for the strike�slip displacements; the arrows indicate the displacement directions; (σ1–2) the aver�age orientation of the recent stress field; (N) the number of earthquakes; (RRFS) Red River Fault System. The explanations arein the text.
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(4) Owing to their rheological properties, lami�nated limestone are very favorable for developing awide range of tectonic dislocations, which rightly indi�cate as stages as reorientations of the stress fields.Accordingly, they can be applicable for the most reli�able geodynamic reconstructions.
ACKNOWLEDGMENTS
We are grateful to Prof. V.G. Khomich for detailededitorial corrections of the manuscript and comments,which allowed us to essentially improve it; toDr. A.N. Mitrokhin for discussions of the results; andto T.M. Mikhailik for assistance in preparing the fig�ures.
This work was supported by the Program of theFEB RAS, projects nos. 12�III�A�08�147 and 12�I�0�ONZ�07.
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Recommended for publishing by A.N. Didenko
Translated by D. Voroschuk
