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Late Neogene structural inversion around the northern

Gulf of Tonkin, Vietnam: Effects from right-lateral

displacement across the Red River fault zone

Michael B. W. Fyhn1 and Phung V. Phach2

1Geological Survey of Denmark and Greenland, Copenhagen, Denmark, 2 Institute of Marine Geology and Geophysics,

Hanoi, Vietnam

AbstractContinental extrusion may take up much of the deformation involved in continental collisions.Major strike-slip zones accommodate the relative extrusion displacement and transfer deformation away

from the collision front. The Red River fault zone (RRFZ) accommodated left- and right-lateral displacements

when Indochina and South China were extruded during the Indian-Eurasian collision. The northern Song

Hong basin onshore and offshore in the Gulf of Tonkin delineates the direct extension of the RRFZ and thus

records detailed information on the collision-induced continental extrusion. We assess the rapidly evolving

kinematics of the fault zone buried within the basin based on seismic analysis. Contrary to previous

studies, we do not identify indications for latest Miocene left-lateral motion across the RRFZ. We tentatively

consider the shift from left- to right-lateral motion to have occurred already during the middle Late

Miocene as indicatedby inversionof NE-SW-striking faults in theBach Long Vi area. Right-lateral displacemen

terminated around the end of the Miocene in the Song Hong basin. However, continued inversion in the Bach

Long Vi area and NNW-SSE-striking normal faulting suggests a stress regime compatible with right-lateral

motion across the onshore part of the RRFZ continuing to the present. Inversion around the Bach Long Vi

Island may have accommodated up to a few kilometers of right-lateral displacement between the Indochina

and South China blocks. Comparable NE-SW-striking fault zones onshore may have accommodated a larger

fraction of theright-lateralslip across theRRFZ, thusaccountingfor therestrictedtransfer of lateraldisplacemen

to the offshore basins.

1. Introduction

Escape tectonics may take up much of the deformation associated with continental collisions [ Burke and

Sengr, 1984;Tapponnier et al., 1986]. During the process, major crustal blocks are squeezed away from the

collision zones across major strike-slip faults, thus transferring collision-related deformation away from the

suture zone [Sengr et al., 1985;Tapponnier et al., 1986;Leloup et al., 2001]. Escape tectonics may evolve

rapidly during the progress of continental collisions. This is recorded in changing deformation styles along

the transform edges of the extruded crustal blocks. Resolving of exact timing and extent of these processes

tends to be highly intricate.

Although controversial, much of the deformation linked with the collision of India and Eurasia is proposed to

have been taken up by lateral escape of the Indochina, South China andNorth China blocks [e.g.,Tapponnier et al.

1982, 1986;Leloup et al., 2001]. The RRFZ constitutes one of the primary strike-slip fault zones that accommodate

the extrusion of the Indochina and South China blocks away from the Himalayan collision front (Figure 1).The RRFZ stretches from eastern Tibet to the extensional basins underlying the Gulf of Tonkin offshore

northern Vietnam (Figure 1). Signicant mid-Cenozoic to Recent lateral extrusion of Indochina and South

China is recorded within the fault zone [Leloup et al., 1995, 2001]. The RRFZ initially took up left-lateral

displacement between the Indochina and South China blocks [e.g., Leloup et al., 1995]. During the latter half

of the Neogene, the RRFZ reversed and became right-lateral due to the progression of the India-Eurasia

collision [Allen et al., 1984;Replumaz et al., 2001;Schoenbohm et al., 2006;Trinh et al., 2012;Zuchiewicz et al.

2013]; however, the timing and nature of this change in deformation is not well-constrained by the outcropping

part of the fault zone.

The Red River delta and the Gulf of Tonkin are underlain by Cainozoic extensional basins outlining the direc

continuation of the RRFZ(Figure 1) [Tapponnier et al., 1986; Rangin et al., 1995]. The changing phases of latera

FYHN AND PHACH 2015. American Geophysical Union. All Rights Reserved. 290

PUBLICATIONS

Tectonics

RESEARCH ARTICLE10.1002/2014TC003674

Key Points:

Right-lateral slip across the Red River

fault zonelikelybeganin theL Miocene

Right-lateral slip drove structural

inversion of the Gulf of Tonkin

rift systems

Lateral shearing is taken up by

shortening across existing

NE-SW-striking faults

Correspondence to:

M. B. W. Fyhn,

[email protected]

Citation:

Fyhn, M. B. W., and P. V. Phach (2015),

Late Neogene structural inversion around

the northern Gulf of Tonkin, Vietnam:

Effects from right-lateral displacement

across the Red River fault zone, Tectonics,

33, 290312, doi:10.1002/2014TC003674.

Received 8 JUL 2014

Accepted 10 JAN 2015

Accepted article online 14 JAN 2015

Published online 21 FEB 2015
http://publications.agu.org/journals/http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1944-9194http://dx.doi.org/10.1002/2014TC003674http://dx.doi.org/10.1002/2014TC003674http://dx.doi.org/10.1002/2014TC003674http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1944-9194http://publications.agu.org/journals/

7/23/2019 Fyhn and Phac, 2015

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deformation in the RRFZ are therefore recorded by the complex buried structures and stratigraphy in these

basins [Rangin et al., 1995], and the rapid Cainozoic deposition allows for a detailed chronologic resolution of

the tectonic development.

This study documents Late Neogene tectonic inversion and uplift taking place in the northern central part of

the Song Hong basin (Chinese name: Yinggehai basin) and in a corridor connecting the Beibuwan and

the Song Hong basins. Thendings are based on roughly 20,000 km 2-D seismic data tied to exploration wells

andcomplemented by sparker seismic data, as well as outcrop information and core data from the Bach Long

Vi Island (Figure 2). The spatial and temporal variation in deformation are discussed in a regional context, and

a Late Neogene tectonic model is proposed in which Late Miocene

Recent extrusion-related right-lateralmotion between South China and Indochina is taken up by compression across NESW-trending structural

lineaments like the Bach Long Vi inversion zone and comparable fault zones onshore northern Vietnam.

2. Geological Setting

2.1. Late Neogene Activity in the Red River Fault Zone

The RRFZ outlines a conspicuous topographic lineament traceable for roughly 900 km between the Eastern

Himalaya and the Hanoi trough in Vietnam (northern onshore part of the Song Hong basin) (Figure 1).

The fault zone continues for another ~1000 km within the offshore basins anking the Vietnamese margin

(Figure 1) [Fyhn et al., 2009a, 2009b]. The fault zone accommodated substantial left-lateral displacement

between the Indochina and South China blocks especially during Oligocene time [Leloup et al., 1995]. Based

Figure 1.Simplied structural outline of Cenozoic basins and selected fault zones onshore and offshore Indochina and

southernmost China adapted afterSchoenbohm et al. [2004] andFyhn et al. [2009a, and references therein]. Small insert

map illustrates the main structural framework in greater East Asia. Box indicates the position of study area and Figure 2.

AS = Ailao Shan metamorphic core complex, BLV = Bach Long Vi Island, DNCV= Dai Nui Con Voi metamorphic core

complex, EVBFZ= East Vietnam Boundary fault zone, MPSZ= Mai Ping shear zone, TPSZ = Three Pagodas shear zone, and

XXFS = Xianshuihe-Xiaojiang fault systems.

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on extrusion tectonic models, Tapponnier et al. [1982] predicted a Neogene slip reversal across the fault zone

dueto thenorthwardprogression of the Indian indenter into Eurasia. The expected reversal has subsequently

been veried by right-lateral river offsets observed across the fault zone taken to imply Pliocenesub-Recen

right-lateral motion across the fault zone [Allen et al., 1984;Replumaz et al., 2001;Schoenbohm et al., 2006;Trinh et al., 2012;Zuchiewicz et al., 2013].

Total right-lateral offset estimates range from approximately 5 km to more than 50 km, with more recent

estimates of ~25 and ~40 km [Replumaz et al., 2001 andSchoenbohm et al., 2006, respectively] (Table 1).

Displacement assessments are based on the reconstruction of fault-offset geomorphology as well as of

laterally displaced lithologic units. These methods are not without complications since (1) the right-lateral

offset followed a left-lateral offset an order of magnitude larger potentially rendering assessment of offset

lithologic units difcult and (2) matching offset tributaries may be difcult in the continuously and rapidly

developing drainage network associated with the humid tropical climate and the developing topography

along the RRFZ.

The onset of right-lateral motion along the RRFZ is poorly constrained. Left-lateral motion throughout the

Miocene as suggested byRangin et al. [1995] would restrict right-lateral motion to the PliocenePleistocene

On the other hand,Schoenbohm et al. [2006] tentatively suggested a Late Miocene onset.Replumaz et al.[2001] argued for an onset no later than the earliest Pliocene. BothSchoenbohm et al. [2006] andReplumaz

et al. [2001] proposed a long-term slip-rate of ~5 mm/yr or greater, whereas Allen et al. [1984] suggested a

smaller offset and thus a lower long-term slip-rate of ~23 mm/yr (but possibly up to 5 mm/yr) (Table 1).

The present relief along the RRFZ decreases toward the southeast. This complicates assessment of the

right-lateral offset based on offset geomorphological markers in northern Vietnam northwest of Hanoi

where the RRFZ becomes buried underneath modern sediments in the Red River delta (Song Hong delta).

Zuchiewicz et al. [2013] based on geomorphic analysis of the north Vietnamese part of the RRFZ, recently

estimated the Quaternary offset to be ~14 km with a long-term slip-rate of ~5.57.8 mm/yr across the RRFZ

Similarly,Trinh et al. [2012] suggested a slip-rate of 8 5 mm/yr for the past 150 Kyr based on offset of Late

Pleistocene alluvial fans, tributaries, and river valleys along the north Vietnamese part of the RRFZ.

Figure 2.Structural outline of the northern Song Hong basin emphasizing the main fault system conning Paleogene

syn-rift depressions and structural highs. The two doted areas denote Late Neogene inversion zones discussed in the

text. Blue lines delineate the available 2-D seismic database with red stippled lines indicating the position of documented

seismic examples. CFZ= Chay fault zone, SLFZ = Song Lo fault zone, and VNFZ = Vinh Ninh fault zone.

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GPS measurements document a distinct

modern east-southeast-ward displacement

of South China and Indochina. The signal

is less clear on the extent of differential

motion between the South China andIndochina blocks and, by inference, the

present right-lateral displacement taking

place across the RRFZ [Zhao et al., 1993;Zhao

1995;Cong and Feigl, 1999; Michel et al., 2001

Iwakuni et al., 2004;Simons et al., 2007;

To et al., 2013]. Some studies suggest

~13 mm/yr right-lateral motion across the

fault zone based on super-regional Asian GPS

networks [Simons et al., 2007;Zhang et al.,

2013] (Table 1), while others consider the

RRFZ as a now passive lineament [Chen et al.

2000; Shen etal., 2005]. Across the Vietnamese

portion of the RRFZ, To et al. [2013] was not

able to pick outan offset signicantly different

from 0 at a 95% condence level based on a

denser local GPS network, but a consistent

sense of right-lateral displacement across the

RRFZ was suggested by their data with a

possible yearly slip-rate compatible to that

proposed bySimons et al. [2007] (~2 mm/yr).

The long-term slip-rates generally tend to be

higher compared to the calculated short

term slip-rates across the RRFZ. This has been

ascribed to either a fairly recent decrease

in slip-rates or the build-up of strain andinstantaneous fault ruptures accompanied by

substantial earthquakes recurring in hundreds

or even thousands years intervals [Allen et al.

1984;Cong and Feigl, 1999;Replumaz et al.,

2001;Schoenbohm et al., 2006].

Large earthquakes have not been documented

directly along the trace of the RRFZ in recen

history [Allen et al., 1984;Lap, 1988; Cong

and Feigl, 1999; Nguyen et al., 2012;H.-H.

Huang et al., 2013]. The largest shocks are

mainly associated with left-lateral slip along

the NESW to NS-striking fault systemsnorth and south of the RRFZ (Dien Bien Phu

fault system in Vietnam and Laos and the

Xianshuihe-Xiaojiang fault system in China)

(Figure 1). Minor earthquakes along the

anks of the RRFZ in Vietnam have been

recorded. Their focal mechanisms suggest

strike-slip faulting and general northsouth

compression compatible with right-lateral

offset across the NWSE-striking fault zone

[Cong and Feigl, 1999].Table

1.

CompilationofExistingRight-LateralDisplacementEstimatesAcrosstheRedRiverFaultZone

Reference

Total

Offset(RightLateral)

Duration

SlipRate

StudyArea

Metho

d

Schoenbohm

etal.[2005]

>40km

LateMioceneRecent

(~8My)

5mm/yr

Yunnan

Displacedgeologic

almarkersand

offsetgeomo

rphology

ZuchiewiczandCuong[2009]

N.A.

LatePleistocene

(mayhaveinitiatedearlier)

5.5

7.8mm/yr

No

rthernVietnam

Offsetgeomo

rphology

Allenetal.[1984]

56km

Pleistocene

(mayhaveinitiatedearlier)

2

3mm/yr

(possiblyupto5mm/yr)

Yunnan

Offsetgeomo

rphology

Replumazetal.[2001]

25km

PlioceneRecent

5mm/yr

YunnanandNorthernVietnam

Offsetgeomo

rphology

Trinhetal.[2012]

(1.2

km

since150KA)

N.A.

85mm

No

rthernVietnam

Offsetgeomo

rphology

Zuchiewiczetal.[2013]

(~14km

during

thePleistocene)

N.A.

5.57.8mm/yr

No

rthernVietnam

Offsetgeomo

rphology

CongandFeigl[1999]

N.A.

N.A.

15mm/yr

No

rthernVietnam

Geodeticmeasureme

ntincludingGPS

Simonsetal.[2007]

N.A.

N.A.

2mm/yr

SEAsia

Geodeticmeasureme

ntincludingGPS

Iwakunietal.[2004]

N.A.

N.A.

Insignicant

(below

GPSdetectionlimit)

Indochina/SEAsia

Geodeticmeasureme

ntincludingGPS

Micheletal.[2001]

N.A.

N.A.

Below

GPSdetection

limit(2km) thickness of the Upper Neogene along the south-eastern ank of the Bach Long

Vi inversion zone documents fairly rapid subsidence and deposition in this area (Figure 5). Comparable to

that along the anks of the central northern Song Hong basin, subsidence may have been enhanced by the

loading and exural bending associated with the adjacent Bach Long Vi inversion uplift.

Inversion-related deformation and internal thickness variations of the Upper Neogene succession document

that inversion starting during the Late Miocene and continued into the PliocenePleistocene (Figures 6, 12

Figure 14.Gross-depositional facies map based on seismic interpretation from part of the Upper Miocene in the area west

of the Bach Long Vi Island. Depositionwas governed by contemporaneous inversion as well as the structural style inherite

after Paleogene rifting.

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and 15). Inversional deformation of

Upper Pleistocene and Holocene strata

indicated by shallow seismic data

suggests that inversion continues to the

present (Figure 16).

Analysis of the Upper Neogene seismic

facies indicates that deposition along

theanks of the Bach Long Vi area was

strongly inuenced by the inversion.

Along the south-eastern ank of the

inversion zone, the south-eastward

prograding clinoforms are interpreted

as marine deltaic units building out from

the inversion uplift (Figure 13). The

unconformities capping the clinoform

topsets are in places interlayered by

sets of stacked channels several tens

of milliseconds (TWT) deep which is

interpreted to reect periods of

relative uplift followed by uvial/alluvia

deposition signifying periods of

regression. This pattern suggest a highly

dynamic Late Miocene depositional

environment characterized by pulsed

uplift along the inversion trend and

rapid subsidence and sedimentation in

the east with deposition guided by

uplift and erosion along the Bach Long

Vi inversion trend. The aerially restricted

occurrence of the unconformities

favours local tectonic uplifts rather than

glacio-eustatic sea-level uctuations,

for example, as forcing mechanism of

these Late Miocene regressions.

Along the south-western inversion

strand, the Upper Miocene prograding

clinoforms are similarly interpreted as

deltaic foresets (Figure 14). Basinward

from the foresets, the bottomsets cut

by incisions are interpreted as fairly

deep-marine strata cut by turbidite

channels. In turn, the amalgamation

of the topsets into a single strong

soft-kick reector is interpreted as a thin

coal-dominated succession deposited on

a Late Miocene delta plain. The platform

buildup capping a basement high is

interpreted as a carbonate platform.

Clinoforms prograded basinward during

the Miocene and were guided by the

Bach Long Vi inversion zone, supporting

Late Miocene inversion activity. Along

the southern inversion strand, the

0

0.5

1.0

1.5

2.5

2.0

TWT(sec.)

Thickness variation due to inversion

Acoustic basement

Palaeogene syn-rift

Plio. Pleistocene

M. U. Miocene

U. Miocene

2 km

0

2 km

0

0.5

1.0

1.5

TWT(

sec.)

0.5

1.0

1.5

TWT(sec.)

2 km

0

0.5

1.0

1.5

TWT(sec.)

2 km

0

0.5

1.0

1.5

TWT(sec.)

2 km

0NW SE

0.5

1.0

1.5

2.5

2.0

TWT(sec.)

Figure 15.(a) Seismic transect and (b) redrawnstratigraphic section from

the the southwestern ank of the Bach Long Vi inversion zone. (cf)

Thickness variations a cross inversion zones document inversion taking

place since the Late Miocene, which is emphasized by back-stripping

(disregarding burial compaction).

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Seafloor

m

ultiple

10

BachLo

ngViIsland

BachLongViIsland

~3kmW E

30

50TWTmsec

70

10

U. Pleistocene Holocene Fault

30

50TWTmsec

70

2 km

Eocene OligoceneWater

Figure 16.Sparker seismic lines from conjugated sides of the Bach Long Vi Island resolving the upper few tens of meters of section. The shallow seismic data

document inversion-related deformation of latest PleistoceneHolocene sediments around the island and thus document the inversion continuing to the presen

along the Bach Long Vi trend.

0

0.5

1.0

1.5

2.5

2.0

TWT(sec.)

0

SW NE

NNW-trending normal faults

Near-basePliocene

Periodoflocalmax.inversion

0.5

1.0

1.5

2.5

2.0

TWT(sec.)

2 km

Acoustic basement

Palaeogene syn-rift

U. Miocene

Plio. Pleistocene

Figure 17. In the southern part of the Bach Long Vi inversion trend, Late Neogene inversion seems to have peaked

already during the Late Miocene. During the PliocenePleistocene minor NNWSSE-striking normal faulting affected this

area as well.

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PliocenePleistocene decrease in inversion-related thickness variations and deformation suggest a decrease in

inversion during the latter Neogene in this area.

At the same time, the formation of NNWSSE-striking subtle extensional faults in the south-eastern part of

the Bach Long Vi trend may indicate a slight change in the regional stress regime (Figure 8). The prevailing

position of the NNWSSE-striking subtle extensional faults above the fairly at crest of inversion structurestogether with their deep rooting suggests a regional tectonic rather than a local gravity induced origin for

the extension.

The subtle increase in maturity from top to bottom in the core well from Bach Long Vi Island suggests a

normal to low geothermal gradient at the time of maximum maturation (~maximum burial), in which case

a maturity ofRo0.39% andTmax of 431C may correspond to an approximate burial depth in the order of

~12 km. This is compatible with the level of sandstone diagenisis of the cored Paleogene succession.

This suggests that apart from the observed ~1.5 km of Late Neogene relative inversion uplift, as much

as 12 km of additional uplift and erosion may have occurred across the central part of the inversion zone.

However, the regional stratigraphic outline suggests that the maximum burial in the Bach Long Vi area

occurred already before the end of the Oligocene, and that part of the uplift and denudation took place

immediately thereafter, thus preceding the Late Neogene. It is not possible to assess how much of the

additional roughly 1

2 km uplift occurred during the Late Neogene.

5. Discussion

5.1. Evolving Late Neogene Stress Pattern and Strike-Slip Deformation

Lying at the direct continuation of the RRFZ, the highly dynamic Late Neogene development of the northern

Gulf of Tonkin testies to the rapidly shifting nature of continental extrusion. Intense deformation and high

depositional rates make the northern Gulf of Tonkin excellently suited for studying South China and Indochinas

escape away from the Indian-Eurasian suture zone and makes the region well-suited for investigating the

dynamic extrusion tectonic mechanisms in general.

Since Late Neogene deformation in the region is virtually restricted to reactivation of older rift structures,

hints of the approximate stress pattern affecting the region at a given period of time cannot be deduced

solely from deformation across single faults but is rather provided by (1) the combination of fault trendsactive at the given period of time, (2) the combination of inactive fault trends, (3) the nature of deformation

(compressive or dilative), (4) the relative magnitude of deformation taking place across individual fault

trends, and (5) potential indicators of strike-slip motions, e.g., the presence ofower structures.

The inferred stress patterns indicated in Figure 8 are average stress patterns covering the roughly 30,000 km2

study area throughout longer periods of time corresponding to the analyzed stratigraphic intervals. The

stress regimes may therefore have deviated from this average more locally andduring more restricted stages

During Middle and early Late Miocene time in the central northern Song Hong basin, the roughly EW-striking

extensional faults together with the NWSE-striking compressional faults delineating prominent ower

structures suggest a left-lateral deformation pattern associated with roughly NS extensional and EW

compressional stresses (Figure 8a).

During the second half of Late Miocene time, the compression affecting the NW-trending lineaments within

the central part of the northern Song Hong basin, together with the coeval compression across the centralNE-verging Bach Long Vi inversion zone, suggests a stress regime dominated by both roughly NESW

and NWSE compression (Figure 8b). This is in accordance with the halt of normal faulting across the centra

Song Hong basin prior to this period.

This stress regime does not provide unequivocal information as to the overall displacement sense across the

offshore continuation of the RRFZ. Lateral displacement is not revealed by consistent releasing/restraining bend

geometries or other deformation features. This could be taken to indicate the absence of strike-slip across

the RRFZ and a prevalence of pure thrusting taking place across the offshore part of the RRFZ. However, the

geometric outline of the two inversion zones could be explained by right-lateral motion across the RRFZ in

a pure shear model (and a left-lateral component across the Bach Long Vi inversion zone). This would be

comparable to the modern relationships between the right-lateral onshore RRFZ and left-lateral Dien Bien Phu

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and the Xianshulhe-Xiaojiang fault systems. Furthermore, this would be in accordance with the timing

considerations from the onshore part of the fault zone [Schoenbohm et al., 2006].

A Late Miocene strike-slip inversion is thus tentatively proposed in this study, with right-lateral displacement

prevailing from around middle Late Miocene time. This pushes back the strike-slip inversion in the

offshore realm by a few million years as compared to previous estimates [Rangin et al., 1995]. The availablebiostratigraphic data in the area from exploration wells do not allow a detailed subdivision of the Upper

Miocene and thus assignment of the precise age of inversion (Vietnam Petroleum Institute in house reports)

A rough estimate of the onset age of inversion may be deduced if more or less uniform long-term subsidence

and depositional rates are assumed since the start of inversion for the depocenter south of the Bach Long Vi

inversion trend and further assuming a paleobathymetry/-topography at the start of inversion and at the

earliest Pliocene comparable to the present. The age of the PliocenePleistocene succession is fairly well

constrained. Based on the above assumptions, the thickness of the Upper Miocene succession deposited

during the inversion may be compatible to a roughly 34 My long depositional period. Inversion may thus

have commenced around roughly 810 Ma. Concurrent estimates are reached for the Upper Miocene

depocenters along the eastern ank of the central northern Song Hong basin. However, the mechanism

inuencing subsidence across these depocenters seems to have changed after the Late Miocene, and the

later estimate could be more coincidental.

The lack of faulting since the end of the Miocene in the central northern Song Hong basin precludes

PliocenePleistocene right-lateral motion across the basin. On the other hand, inversion continued in the Bach

Long Vi area during the the PliocenePleistocene, although apparently in the west at a more moderate scale

than during the Late Miocene. Together with the NNWSSE-striking normal faulting in the southern part of the

Bach Long Vi inversion zone, this suggest a prevailing PliocenePleistocene stress pattern dominated by

roughly NNWSSE-compression and more subtle WSWENE extension. This is compatible with right-lateral

motion across NWSE-striking faults like the RRFZ and is consistent with the modern stress regime indicated by

fault plane solutions and GPS analysis (Figure 8c) [Cong and Feigl, 1999;Simons et al., 2007].

The gradually evolving Neogene inversions in the area around the northern Song Hong basin seem to reect

the increasing compressional extrusion forces coming to dominate the latter Neogene in the area. We

speculate that the compression is related to the increasing effects of the South China block extrusion and the

northward indentation of the Indian continent. During Miocene time, the extrusion of the South China block

accelerated and the escape of the Indochina block diminished, causing convergent forces between the

two blocks to dominate, in contrast to the former strike-slip dominance [ Replumaz and Tapponnier, 2003]. As

theescape rate of the South China block eventually exceeded theIndochinese rate during the latest Miocene

the relative motion across the RRFZ changed to right-lateral [Tapponnier et al., 1982].

5.2. Lateral Offset and the Take-up of Right-Lateral Displacement

The disconnected nature of faults within the middle and upper Upper Miocene does not indicate a Late

Miocene right-lateral offset greater than a few kilometers in the Song Hong basin (Figures 5 and 6). Furthermore

the lack of PliocenePleistocene faulting in the central northern Song Hong basin precludes any direct

right-lateral displacement across the basin in the latter period (Figure 7). Extrusion-related lateral displacement

must therefore have been taken up withinand/or northwest of thestudy area. The Bach Long Vi zone may have

acted to accommodate lateral shearing and a fraction of the differential motion between the South China

and Indochina blocks during the Late Neogene inversion. Inversion across the Bach Long Vi zone continuedduring the PliocenePleistocene when right-lateral shearing had ceased in the Song Hong basin. This suggests

that part of the PliocenePleistocene extrusion of the South China block observed onshore may have been

accommodated within the Bach Long Vi inversion. However, the extent of Late Neogene shortening taking

place across the inversion zone may only have accommodated a relative displacement in the order of 25 km

Onshore deformation zones must therefore have accommodated most of the relative motion between the

South China and the Indochina blocks when considering onshore RRFZ-displacement estimates around 25 to

40 km [e.g.,Replumaz et al., 2001;Schoenbohm et al., 2006].

There is no hard-link between the Bach Long Vi inversion zone and the RRFZ being active during the Late

Neogene. Consequently, the offshore right-lateral displacement of the South China block relative to the

Indochina block is considered to have been modest.

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Compatible with the orientation of the Bach Long Vi inversion zone, a broad belt of NE SW-striking faults

exist onshore in northern Vietnam and extend into China (Figure 5). Shortening may have taken place across

the onshore belt by analogy to the Late Neogene shortening across the parallel Bach Long Vi inversion

zone. The onshore fault belt may thus have accommodated relative displacement between the Indochina

and the South China blocks since the Late Miocene. This could explain the absence of lateral offset across the

Song Hong basin during a period of well documented right-lateral motion across the RRFZ immediately to

the northwest.

Similar to in the Beibuwan basin, Pubellier et al. [2003] interpreted the onshore NESW-striking fault system to

have accommodated left-lateral extrusion-related motion during the Paleogene by extension. This suggests

that the onshore fault system behaved as a crustal weakness zone prone to being reactivated by Late

Neogene extrusion tectonics.

5.3. Exhumation of the RRFZ and Displacement Rates

Based on geomorphological offsets across the onshore RRFZ, cumulative right-lateral displacement estimate

~25 km have been proposed [Allen et al., 1984;Replumaz et al., 2001;Schoenbohm et al., 2006;Trinh et al.

2012;Zuchiewicz et al., 2013]. Major incision along the Red River and its tributaries has been interpreted as

PliocenePleistocene in age, occurring in response to thesuper-regional Tibetanplateau uplift [e.g., Replumaz

et al., 2001;Schoenbohm et al., 2004, 2005, 2006].

However, our data suggest an earlier onset of uplift and denudation along a narrow belt at least including the

southern part of the RRFZ. The domed Middle to Late Miocene inversion zone in the central northern Song

Hong basin lies in direct continuation of the Dai Nui Con Voi metamorphic core complex outlining the

southern part of the RRFZ (Figure 5). In the northern, data covered part of the Song Hong basin (Figure 2);

several hundred meters of exhumation occurred regionally during the Middle to Late Miocene. We speculate

that the style and trend of the basinal inversion makes a similar contemporary uplift event plausible

across the Dai Nui Con Voi metamorphic core complex. Onset of uplift and denudation along part of the RRFZ

may thus precede the PliocenePleistocene plateau growth.

Apatite Fission Track (AFT) analyses of the Dai Nui Con Voi metamorphic core complex yield fully reset

ages between 18 and 30 Myr [Maluski et al., 2001;Viola and Anczkiewicz, 2008], which suggest a Late

OligoceneEarly Miocene cooling event and most likely in the range of 23 km of Neogene exhumation to

the present surface level without specifying potential periods of pulsed uplift. AFT analysis from the Aliao

Shan metamorphic core complex on the other hand yield ages around 1014 Myr [Bergman et al., 1997]

which suggest a MiddleLate Miocene cooling event and further may indicate uplift and exhumation in the

range of 23 km to the present surface level. This is overlapping with the timing and style of inversion in

the northern central Song Hong basin. However, a potential genetic link between the Miocene inversion in

the northern Song Hong basin and the coeval exhumation of the Aliao Shan metamorphic core complex

remains speculative.

If uplift along part of the RRFZ took place already during the Middle to Late Miocene, the present

geomorphological pattern along the Red River may have initiated before the Pliocene. This calls for caution in

the assessment of the long-term right-lateral slip rates across the fault zone. A PliocenePleistocene lateral

offsetof ~25 km would require an average slip rate around 5 mm/yr. If the uplift commencedalready during the

Middle to Late Miocene time, average slip rates may in fact be less than half. We note that this seems more

compatible with measured modern slip-rates [e.g. ,Simons et al., 2007] (Table 1).

Furthermore, if uplift started already during Middleearly Late Miocene time, the RRFZ would have been in a

left-lateral slip regime during the initial exhumation phase. This may further complicate quanti cation of the

total offset based on geomorphology.

6. Conclusion

1. Extrusion of both Indochina and South China controlled basin development in the northern Gulf of

Tonkin, making the area ideal for studying the mechanisms behind continental escape tectonics. The Late

Neogene progression of the continental escape tectonics caused basin inversion and reversals of the

RRFZ strike-slip regime.

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2. There is no indication of left-lateral motion during the latest Miocene in the Song Hong basin. In contrast

inversion in the Bach Long Vi area is tentatively interpreted to reect Late Miocene onset of right-latera

displacement across the RRFZ.

3. In the central northern Song Hong basin, the termination of faulting after the Miocene preclude

PliocenePleistocene strike-slip movement across the basin. In contrast, continued inversion across the

NESW-trending Bach Long Vi inversion zone together with moderate NWSE-striking extensional faulting

suggests a stress regime compatible with continued right-lateral shearing across the onshore RRFZ.

4. Late Neogene deposition was strongly inuenced by the ongoing inversion. The proto-Red River was

diverted by uplift to a prevailing path along the northeastern ank of the Song Hong basin. Similarly,

the depositional pattern along the Bach Long Vi inversion zone was inuenced by alternating periods of

uplift and subsidence and progradation away from or along the uplifted inversion zone guided by the

overall structural trend.

5. Inversion within the Bach Long Vi zone may have accommodated up to a few kilometers of right-latera

displacement between the Indochina and South China blocks. The amount of compression and the lack

of a Late Neogene hard-link to the onshore RRFZ preclude accommodation of a larger offset within the

inversion zone. However, comparable fault zones located immediately onshore may have accommodated a

larger fraction of the right-lateral motion across the RRFZ, thus accounting for the restricted transfer of

motion to offshore.6. The Middle to Late Miocene inversion uplift zone within the central northern part of the Song Hong basin

lies in direct continuation of the Dai Nui Con Voi and Ailao Shan metamorphic core complexes, and a

related Middle Neogene uplift of the core complexes is speculated. This predates other suggestions for

their late-stage uplift and consequently calls for caution in the evaluation of right-lateral slip-rates and

total displacement based on geomorphological analysis.

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Acknowledgments

The study is an outcome of the ongoing

joint Vietnamese-Danish research

program termed the ENRECA Project

(ENhanced REsearch CApacity building)

sponsored by the Danish Ministry of

Foreign Affairs and supported by

PetroVietnam and Vietnam Petroleum

Institute. Vietnam Petroleum Institute

and PetroVietnam made data available

to the project group and are thanked

together with GEUS for permission to

publish. Center of Marine Geology and

Resources, Hanoi, provided shallow

seismic data. Data are propriety data

owned by PetroVietnam and cannot be

released according to Vietnamese law.

The ENRECA team has contributed

with stimulating discussions, and L.H.

Nielsenis thankedfor commentingon an

earlier version of the manuscript.

J. Halskov is acknowledged for the

gure artwork. The constructive reviews

by A. Replumaz and L. Schoenbohm are

highly appreciated.

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