ACTIVE TECTONICS AND QUATERNARY BASIN FORMATION ALONG THE ĐIỆN BIÊN PHỦ FAULT ZONE, NORTHWESTERN VIETNAM

ACTIVE TECTONICS AND QUATERNARY BASIN FORMATION ALONG THE ĐIỆN BIÊN PHỦ FAULT ZONE, NORTHWESTERN VIỆT NAM

KUANG-YIN LAI¹, YUE-GAU CHEN¹, LING-HO CHUNG¹ , DOÃN ĐÌNH LÂM²

¹Institute of Geosciences, National Taiwan University, P.O. Box 13-318, Taipei 106, Taiwan
²Institute of Geological Sciences, VAST, Hà Nội,Việt Nam

Abstract: The Điện Biên Phủ (DBP) fault zone is one of the most seismically active fault systems in the Indochina, extending over a distance of some 150 km from the border between Yunnan, China and Việt Nam through the NW Việt Nam into Laos. Active sinistral movement is indicated by clear geomorphic features, moderate seismic activity in a NNE-striking zone paralleling the fault zone and focal solutions. By analogy to the Cenozoic tectonic evolution of Indochina related to the Indian indentation, we propose that the Tertiary propagating extrusion block transformed to intraplate extension and took place in northwestern Việt Nam. The Quaternary basins along the DBP fault zone are in line with the inferred extension style of deformation with active faults.

Mapping of active fault segments along the DBP fault zone using field outcrops, thickness maps of Quaternary deposits, seismological data, and geomorphological features seen in the SRTM, ASTER images and derived digital elevation models shows that virtually all active faults are reactivated structures sub-parallel to chronostratigraphic boundary. Our observations demonstrate three largest basins with different kinematics in different fault segments. In the northern part of the DBP fault zone active faulting defines a small-scale pull-apart structure with an active opening rhomboidal Quaternary basin (2.5 km×1.5 km). Toward south a bit the fault pattern changes to multi parallel sinistral or sinistral-normal faults, forming a narrow subsiding weak zone (10 km×0.5 km) filled with Quaternary deposits. In the southern part, however, the major DBP fault shifts the direction from NNE to NE where a branched sinistral-normal fault with N-striking controls a half graben basin (17 km×5 km) filled with Quaternary deposits about 200 m in depth above the late Neogene olivine basalt. The late Neogene basalt of age ca. 5 Ma seems being displaced 10 km by the branched sinistral-normal fault, giving a rate of ca. 2 mm/yr by left-lateral strike-slip since volcanic eruption. The dynamic model of each Quaternary basin, however, needs further other information to establish.

I. INTRODUCTION

The more than 150 km long Điện Biên Phủ (DBP) fault zone is one of the most conspicuous crustal structures in Indochina extending from south Yunnan through northwest Việt Nam into Laos, probably connecting the Nan (Nan-Uttaradit) suture in Thailand and approaching the Gulf of Thailand [3, 13] (Fig. 1). The fault system was developed during Cenozoic northeast-directed movement of Indian plate resulting in the southeastward Indochina extrusion. Compared to the recently dextral Red River fault zone as the boundary fault of extrusion, the sinistral DBP fault zone is probably conjugate fault (Zuchiewicz et al., 2004).

The DBP fault zone corresponds to one of the most seismically active zone in northwestern Việt Nam in Indochina, proving its continued activity. Additional long-known data indicative for active deformation along the fault zone come from the satellite images, topographic maps, and in the field [10]. These data, however, has so far not been integrated into a coherent model of Quaternary and active tectonics in the area. The only attempt that we are aware of comes from Zuchiewicz et al. (2004) who used OSL-SAR dating results of Quaternary sediments to develop a record of young tectonic processes.

We therefore try to assess recent kinematics of the Điện Biên Phủ fault system by drawing analogies to the Cenozoic tectonic evolution of Indochina related to the Indian indentation and by integrating field outcrops, seismological data, geomorphological data and sedimentary data from Quaternary deposits. From this integrated approach we derive information on the location, geometry and kinematics of the active Điện Biên Phủ fault system, and on the tectonic control of Quaternary basin formations. Finally, we discuss a possible model for regional kinematics of fault system by reviewing relevant tectonic, seismological, geomorphological, and geodetic data sets.

II. CENOZOIC TECTONIC EVOLUTION

The Indo-Asian collision and the subsequent northward Indian indentation have resulted in the extrusion tectonics in the eastern part of Asian continent [e. g. 5, 9, 10]. The Indochina Peninsula is composed of the Indochina Block, Shan-Thai Block, and the southern portion of the South China Block (Fig. 1).

South of the Red River fault, the Indochina Block is highly deformed in the north [7], but behaves approximately like a rigid block in the south [4]. The Cenozoic deformational history of the eastern Indo-Asian collision zone may be divided into three stages: (1) Eocene-Oligocene (~40-24 Ma) transpression in eastern Tibet, (2) Early-Middle Miocene (24-17 Ma) transtension in eastern Tibet, and (3) Late Neogene-Quaternary east-west extension widespread in eastern Tibet and Indochina [14 and references therein]. The currently right-lateral Red River fault previously accommodated left-lateral shear between ~30 and 17 Ma [3]. While several authors [2, 15] suggest that the southeastern Red River fault zone experienced regional extension between 34 and 25 Ma. The east-west contraction in the north and transpression in the south along N- and NW-striking strike-slip fault systems along the eastern margin of Tibet gave way to transtension in the latest Oligocene. Transtension was replaced by east-west and northwest-southeast extension in the Late Neogene. The extension is manifested by north-striking active normal faults and associated extensional basins in the north and south of the active right-lateral Red River fault in eastern Tibet and Indochina [2, 3]. Recent Global Positioning system (GPS) data proposed that the present clockwise rotation of crust in southeastern Tibet corresponds geologically to a region of left-slip faults like Xianshuihe-Xiaojang fault and Điện Biên Phủ fault which appear to have accommodated clockwise rotation for at least the past 2-4 Ma and possibly the past 6-8 Ma [6, 15, 16], whereas other GPS data of Southeast Asia proposed that Indochina along with the western and central part of Indonesia constitutes a stable tectonic block moving approximately east with respect to Eurasia and no apparent differential motion between South China block [1, 4].

The structural evolution of the Red River fault zone has been dealt with several authors [cf. 3 and references therein], while the structural development of DBP fault zone is still under reconstruction. The pre-Pliocene history of the DBP fault zone is remained unknown due to lacking of Tertiary strata for age constraints. Although the character of Quaternary basins along the DBP fault zone are far from unequivocal, the DBP fault zone is undoubtedly under extensional tectonic environment in rigid Indochina block.

Figure 2. Up: Shaded relief topographic map of northwestern Việt Nam derived from SRTM (see location in Fig. 1). Down: Coloured relief topographic map of northwestern Việt Nam derived from SRTM (see location in Fig. 1). Red lines are the mapped faults in DBP fault zone. Back circles are earthquakes with magnitude over 4 from National Earthquake Information Center of USGS. Chăn Nưa, Lai Châu, Mường Lay, and Điện Biên Phủ are four segments of DBP fault zone

3. Major segments of the Điện Biên Phủ fault zone

Superimposed on the slightly arcuate geometry of DBP fault zone are several strands. The strand with dilatational step is releasing bend and may represents pull-apart basin, while with contractional step is restraining bend and may represents divide of linear valley. We have used these second-order geometric bends to divide the DBP fault zone into 4 segments. Each segment bears the name of a major town or townlet along the segment. These include Chăn Nưa, Lai Châu, Mường Lay, and Điện Biên Phủ (Fig. 2). In sections 3.3.1-3.3.4, we describe each segment, beginning in the north. Each description focuses on the geomorphic expression of the segment and its termination.

a. Chăn Nưa segment (22.12°N to 22.37°N). Clear dilatational step demarcates the southern termination of this 27-km-long segment of DBP fault zone. The predominately linear sinistral fault cuts the hillside paralleling the river and makes the straight river into meander loop flowing around the southern portion of rhomboidal pull-apart basin in the southern termination. According to the world earthquake data from the National Earthquake Information Center (NEIC), there were two light earthquakes occurring at the southern termination in 1996 and 2001.

b. Lai Châu segment (21.88°N to 22.12°N). This 26-km-long segment runs southward from the north of the cross-intersection of Sông Đà river and its tributaries to the contractional step in the southern end of the narrow submeridional basin, which is bounded by predominately multi-linear sinistral and sinistral-normal faults. The recent earthquakes show moderate seismic activity around this segment.

c. Mường Lay segment (21.60°N to 21.88°N). This is a predominately straight 31-km-long segment of DBP fault zone. It consists of restraining bends in both northern and southern end, and exhibits perfect linear valley and obvious deflected river. However, this segment is under low seismic activity in recent decades.

d. Điện Biên Phủ segment (21.26°N to 21.60°N). This segment shifts the DBP fault zone to NE-striking and has a N-S trending branch sinistral-normal fault through the biggest basin along the DBP fault zone. The southern end of this segment is approaching the boundary between Việt Nam and Laos. This segment is also associated with the strongest seismic activity along the DBP fault zone.

4. Other related structures

In northwestern Việt Nam active faults are parallel or sub-parallel to the Red River fault zone with NW-SE striking except for DBP fault zone. These faults may be the conjugate faults of DBP fault zone.

IV. QUATERNARY AND RECENT KINEMATICS

1. Active faulting and Quaternary basin in the DBP fault zone

The Quaternary sinistral and sinistral-normal active faults and their recent kinematics in northwestern Việt Nam are assessed using the mapping results of faults in DBP fault zone and combining the associated seismological data, field data, thickness maps of Quaternary deposits, and geomorphological features like offset and deflected drainage, and appearance of numerous shuttle ridges. Additionally, the normal component is certified by preserved triangular facets and tributary hanging valleys. The distribution of seismicity in northwestern Việt Nam highlights a zone of active deformation along the DBP fault zone. The majority of focal plane solutions are indicative for sinistral faulting on N- to NE- striking strike-slip faults. Along DBP fault zone there are three obvious Quaternary basins, i.e. Chăn Nưa, Lai Châu and DBP basin, within different fault segments and with different kinematics.

2. The Chăn Nưa pull-apart basin

The kinematic model of strike-slip releasing bend is strongly supported by faults mapped in the Chăn Nưa segment. There, active N-striking sinistral on the east and west, as well as NW-striking normal faults on the NE and SW delimit an opening rhomboid-shaped Quaternary basin. The basin is referred to as Chăn Nưa basin and the basin size is about 2.5 km×1.5 km. Structural and gemorphological data support a pull-apart origin of the Chăn Nưa basin with Quaternary subsidence occurring between left-stepping N-striking faults of DBP fault system. (Fig. 3)

Figure 3. Coloured relief map
of Chăn Nưa basin and surrounding area

3. Quaternary basin related to normal faulting

a. Lai Châu basin (Fig. 4). Toward south a bit of the Chăn Nưa basin, multi parallel sinistral and sinistral-normal faults form another submeridional Quaternary basin of 10 km long and 0.5 km wide subsiding weak zone. Two kinematic model interpretations can be taken into account. One model is the same as the origin of Chăn Nưa basin, but with long releasing bend or multi-connecting small ones. The other model is the main fault with 0.5 km wide flower structure delimiting the basin boundary. Considering the topographic cross-section, the major normal faulting probably occurred in west-dipping branch in the eastern side of the basin.

Figure 4. Coloured relief map
of Lai Châu basin and its surrounding area

b. Điện Biên Phủ basin (Fig. 5). The DBP basin, ca. 17 km long and 5 km wide, is passed through by a N-striking branch of major NE-striking DBP fault. This basin is the largest of three basins along DBP fault zone. The shallow seismic profile shows the west-dipping normal fault right beneath the central part of the basin [11] and the thickness map of the Quaternary deposits derived from the well data proves the active normal faulting (Zuchiewicz et al., 2004 and references therein). Also the topographic cross-section shows flat plain in the western part of the basin and gently west-tilting in the eastern part which indicates the minor parallel normal faulting in the east side of the basin. The deepest depocenter is a nearly 130-m-thick series of alternating sands, silts, clays and gravel of Quaternary fluvial and lacustrine sequence which overlies more than 20-m-thick basalt intrusion that was K-Ar-dated as 5.20±0.34 Ma (Zuchiewicz et al., 2004 and references therein). Thus the kinematic model for the half graben DBP basin is active normal faulting, probably starting from Late Neogene.

4. Slip rate

By using optical stimulated luminescence/single aliquot regenerative dose technique dating results of displaced terraces and alluvial fans, Zuchiewicz et al. (2004) proposed that sinistral and sinistral-normal faults in the southern portion of DBP fault zone reveal minimum rates of left-lateral strike-slip ranging from 0.6-2 mm/yr in Holocene and 0.5-3.8 mm/yr in Pleistocene. In the north of the DBP basin, the intrusion basalt which was Ar-Ar-dated ass 4.8±0.1Ma [2] can be considered as the same origin of the basalt lying on the bottom of the basin. Therefore, the 10-km-long displaced intrusion basalt can be estimate rate of left-lateral strike-slip ca. 2 mm/yr since the volcanic eruption in Late Neogene, the rate of which is compatible with that in Pleistocene and Holocene. More precise estimation, however, is difficult due to poor age control and inexact offset calculation.

V. DISCUSSIONS AND CONCLUSIONS

1. Quaternary tectonics and basin formation along the ĐBP fault zone

Structural data show that neotectonics of the ĐBP fault zone is characterized by N- to NE-striking reactivate boundary faults of pre-existing suture zone. Active faulting is related to the formation of major Quaternary basins and active fault kinematics include strike-slip faulting with releasing bend as documented by the 2.5 km×1.5 km rhomboidal Chăn Nưa pull-apart, and normal faulting of flower structure and half graben as documented by the 10 km×0.5 km N-elongated narrow basin in Lai Châu and the largest 17 km×5 km basin in DBP respectively. Timing of the Quaternary deformations is not well constrained and presently has to rely on few published age data. An estimate for the maximum age of E-directed extension active sinistral-normal faulting along the DBP fault zone comes from displaced Late Neogene basalt. Age constraints for younger (Pleistocene and Holocene) sinistral faulting include displaced terraces and alluvial fans (Zuchiewicz et al., 2004). In sum, data indicate that reactivation of the DBP fault zone commenced in the Late Neogene and proceeded to form Quaternary basins.

2. Regional kinematics

The regional extent of the active DBP fault system is indicated by geomorphic features and moderate seismic activity [8, 17] in the northwestern Việt Nam. Epicenters line up to a submeridian seismically active zone and focal plane solutions are compatible with sinistral strike-slip motion along the fault. Hypocenter depth, however, is not well constrained due to lacking of delicate velocity model. By analogy to the Cenozoic tectonic evolution of Indochina we propose that the intraplate extension occurred on the Tertiary extrusion block in Northwestern Việt Nam. This model is confirmed by the formation of Quaternary basins which are created by active sinistral and sinistral-normal faulting with E-directed extension along DBP fault zone. Also the recent GPS measurements indicate that the Indochina block is moving eastward with respect to Eurasia and the DBP fault zone probably accommodates the current extrusion of clockwise rotation. The kinematic model for each Quaternary basin formations has been reconstructed; however, lacking good age control and precise offset we need more information to establish the regional dynamic model.

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