C.K. Morley a,⇑, A. Alvey b a Department of Geological Sciences, Chiang Mai University, Chiang Mai 50200, Thailand b Badley Geoscience Ltd., North Beck House, North Beck Lane, Hundelby, Spilsby, Lincolnshire PE23 5NB, UK

1. Journal of Asian Earth Sciences 98 (2015) 446–456
Is spreading prolonged, episodic or incipient in the Andaman Sea?
Evidence from deepwater sedimentation
C.K. Morley a,⇑, A. Alvey b
a
Departmentof Geological Sciences, Chiang Mai University, Chiang Mai 50200, Thailand
b
Badley Geoscience Ltd., North Beck House, North Beck Lane, Hundelby, Spilsby, Lincolnshire PE23 5NB, UK
a r t i c l e i n f o a b s t r a c t
Article history:
Received 14 July 2014
Received in revised form 12 November 2014
Accepted26 November 2014
Available online 9 December 2014
The Central Andaman Basin (CAB) is generally accepted to be a site of continuous sea floor spreading
since the Early Pliocene ( 4.0 Ma). The adjacent Alcock and Sewell Rises, and part of the East Andaman
basin have been interpreted as probable Miocene oceanic crust. Published seismic lines across the eastern
half of the spreading centre show that 100’s m thickness of sediment are present right up to the central
trough. The central trough margins are faulted, uplifted and tilted away from the central trough. The
youngest sediment is ponded and onlaps the tilted central trough margin, while older faulted sediment
lies within the trough. Such a configuration is incompatible with continuous spreading. Instead, either
spreading in the central basin was episodic, probably comprising a Late Miocene–Early Pliocene phase
of spreading, followed by extension accommodated in the Alcock and Sewell rise area (by faulting and
dike intrusion), and then a recent (Quaternary) return to spreading in the central trough; or the central
trough marks an incipient spreading centre in hyper-thinned continental (or possibly island arc) crust. To
resolve these possibilities regional satellite gravity data was inverted to determine crustal type and thick-
ness. The results indicate the CAB is oceanic crust, however the adjacent regions of the Alcock and Sewell
Rises and the East Andaman Basin are extended continental crust. These regions were able to undergo
extension before seafloor spreading, and when seafloor spreading ceased. Unpublished seismic reflection
data acrossthe East Andaman Basin supports the presence of continental crust under the basin that thins
drastically westwards towards the spreading centre. Episodic seafloor spreading fits with GPS data
onshore that indicate the differential motion of India with respect to SE Asia is accommodated on widely
distributed structures that lie between the trench and the Sagaing Fault.
2014 Elsevier Ltd. All rights reserved.
Keywords: Back
arc Andaman Sea
Spreading centre
Strike-slip
Turbidites
1. Introduction and the nature of the crust (continental, oceanic,transitional) in
the central part of the Andaman Sea, particularly the Alcock and
Sewell rises (e.g. Morley, 2012; Srisuriyon and Morley, 2014;
Fig. 1). Curray (2005) interprets much of the East Andaman Basin
andthe Alcock and Sewell rises as being oceaniccrust, with a prob-
ably Early Miocene age of formation. The ENE–WSW trending
region between the Alcock and Sewell rises (Central Basin) is gen-
erally accepted to be composed of back-arc oceanic crust (e.g.
Curray et al., 1979; Raju et al., 2004; Curray, 2005; Diehl et al.,
2013). Consequently Hall (2002) in his regional plate reconstruc-
tions showed the prevailingview at the time that the Andaman
spreading centre opened during the Middle Miocene (following
Currayet al., 1979). However, the geophysical study of the Central
Basin by Raju et al. (2004), including re-appraisal of the magnetic
data indicated the basin has formed by continual spreading from
about 4.0 Ma to the present. This interpretation is supported by
Curray (2005). However, the eastern half of the Central Basin is
covered by a blanket of sediment, and only in the western part
The Andaman Sea has long been recognized as a very large
Cenozoic pull-apart basin formed by dextral shear between the
right-stepping Sumatra-West Andaman-Sagaing fault systems
(Fig. 1) within a back-arc setting (e.g. Currayet al., 1979; Curray,
2005). The highly oblique orientation of the Andaman-Sumatra
subduction zone to the northwards motion of the India Plate is
responsible for the structural configuration (see reviews in
Curray (2005), Nielsen et al., (2004), Ranginet al. (2013)). A num-
ber of regional geophysical surveys have been conducted in the
Andaman Sea that have defined the general tectonic setting (e.g.
Curray et al., 1979; Curray, 2005; Chamot-Rooke and Rangin,
2000; Raju et al., 2004). However, questions remain about the
extent of the spreading centre, whether it is a spreading centre,
⇑ Corresponding author.
E-mail address: chrissmorley@gmail.com (C.K. Morley).
http://dx.doi.org/10.1016/j.jseaes.2014.11.033
1367-9120/ 2014 Elsevier Ltd. All rights reserved.
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jseaes

2. C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 447
are oceaniccrust-type magnetic anomalies interpreted (Raju et al.,
2004).
The possibilitythat spreading was episodic has previously been
discussed by Currayet al. (1979) and Curray (2005) based on sed-
imentological arguments. However, they ultimately favoured a
continual spreading model. This study re-examines the published
geophysicaldata, particularly seismic reflection data, over the east-
ern part of the Central Basin (published in Chamot-Rooke and
Rangin (2000), Raju et al. (2004) and Curray (2005), and suggests
that the sedimentarygeometriesare incompatible with continual
sea floor spreading since 4.0 Ma. This in turn raisesquestionsabout
the nature of the crust in the Central Basin, Alcock and Sewell
Rises, and the East Andaman Basin, which are addressed by dis-
cussing seismic reflection data, and new gravity modelling of the
Andaman Sea region following the methods described by
Chappell and Kusznir (2008), Alvey et al. (2008) and Cowie and
Kusznir (2012). Alternative tectonic scenarios are discussed.
this sediment has accumulated in the present day shelfarea where
in places seismicreflection data shows in excess of 8 km of section.
Some of the sediment has moved off the shelf to be deposited in
the deepwater East Andaman Basin (Figs. 1–3). The ENE–WSW
Central Andaman Basin intersects the western margin of the East
Andaman Basin, and hence there is a pathway for sediment to
enter the eastern side of the Central Andaman Basin.
Seismic reflection data across the Central Basin has been pre-
sented by Raju et al. (2004), Curray (2005) and Chamot-Rooke
and Rangin (2000). Fig. 4 is a line drawing of the single channel
seismic line acrossthe central trough fromRaju et al. (2004). There
are 3 sedimentary packages:(1) a deformedlower unit that is iden-
tified on the NW part of the line, (2) a well layered package, that
appears to be of fairly uniform thickness across the entire section
and (3) a ponded sediment unit that is the youngest unit, but is
more geographicallylimited than unit 2. The lower layer is not dis-
cussed here becauseit is not well imaged,but if it is present all the
way across the section then it only compounds the problem
described for layers 2 and 3 below. Even if layer 1 thins and
pinches out towards the centraltrough,the problem for the contin-
ual spreading model of explaining the presence and geometry of
layers 2 and 3 remains.
The long-term spreadingrate for the CentralBasin is estimated
between 3.0–3.8cm/yr (Diehlet al., 2013). Raju et al. (2004) inter-
preted initial slow spreading rates of 1.6 cm/yr beginningaround
2. Sedimentation in the East Andaman Sea
The Salween and Ayeyarwady rivers have input a considerable
volume of sediment onto the Gulf of Mottamamargin from the
Late Miocene ( 7 Ma?) to the present day (Morley, 2013). Deposi-
tion is focused in a N–S trending synclinaltrough that followsthe
strike of two major strands of the SagaingFault (Fig. 1). Much of
A Area of oceanic or hyper-
extended continental crust
Oceanic crust or hyper-
extended continental crust
Central Basin
Shan Scarp
area Thailand
B
Myanmar
A
I
Central Andaman Basin
Cenozoic sedimentary
basin
Late Miocene-Recent sediment
input from A) Ayerwaddy River,
and B) Salween River
AR = Alcock Rise
SR = Sewell Rise
SSG = South Sagaing Fault
MB = Mergui Basin
NSB = North Sumatra Basin
SREBF = Sewell Rise East
Bounding Fault
Oceanic (west)/continental crust
(east) boundary from Curray (2005).
Gulf of Martaban
15°
?
Coco
Basin
Andaman-
Nicobar
Islands ?
AR
Axial
Trough
10°
Shelf-
slope
breakArea of
highest
quality
magnetic
anomalies
SR Western limit of probable Oligocene
(possibly including Late Eocene)
sedimentary rocks of East Andaman
Basin observed on seismic reflection data
A’A
MF
South
Sagaing Fault
?
?
?
1
?
West Andaman
Fault zone
? ?
?
?Sumatra-
Andaman
Trench
North
Sumatra Basin
Sumatran
Fault zone5°
91° 95° 99°
Sumatra
Mergui Fault E
A’
W
0 A
West Andaman Fault Sewell Rise
SREBF
South Sagaing Fault (SSG)
Palaeogene-early Middle Miocene-Recent
Middle Miocene
N end of
Mergui Ridge7
B East Andaman Basin
Fig. 1. (A) Regional map of the Andaman Sea region and surrounding areas. (B) Regionalcross-section through the southern-central Andaman Sea based on 2D seismic
reflection data (modified from Srisuriyon and Morley (2014)).
TWTT
(s)

3. 448 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456
0.5-1 km
Myanmar
1-2 km
2-3 km
15°
3-4 km
4-5 km
Andaman-
Nicobar
Islands
5-6 kmSagaing
Fault
6-7 km
A
India 7-8 km
8+ km
10°
S
West Andaman
Fault zone Me
Thailand91° 95°
Fig. 2. Regionalisopach map of Middle Miocene to Recent sediments in the East Andaman Basin based on seismic reflection data.
4 Ma, that increased to 3.8 cm/yr from anomaly 2 to the present
(i.e. the past ~2 my). Maurin and Rangin (2009) and Rangin et al.
(2013) discuss how the 3.5 cm/yr motion of India relative to Sun-
daland east of the Sagaing Fault is distributed, with about
1.8 cm/yr occurringon the SagaingFault, andthe rest of the motion
along diffusestructures further to the west. Hencethe 3.0–3.8cm/
yr spreading rate estimates do not fit with the distribution of
deformation cited above. However, for the purposes of discussing
the Pliocene-Recent spreading model, the spreading rates deter-
mined by Raju et al. (2004) will be used.
Assuming a symmetric spreading rate of 1.9 cm/yr in each
direction away from the central trough, point which lies 24 km
from the spreading centre (location A, Fig. 4) should be composed
of crust formed 1.3 my ago. Point B is 6 km from the spreading
centre and should be composed of crust with an approximate age
of 0.32 Ma (6 km formed at 1.9 cm/yr). However, the sediment
overlying point B comprises approximately the same thickness of
layer 2 as does point A ( 600 m, assuming 2000m/s seismic inter-
val velocity),but is missing approximately 250 m thicknessof layer
3, which is present at point A. This situation conflictswith contin-
ual spreading where the base of the sediment overlying oceanic
crust must young towards the spreading centre, since it can be
no older than the time of formation of the crust. The thickness of
layers 3 and 2 combined has decreased from 850 m to 600 m, while
the possible time span of the units has changed from 1.3 Ma, to
0.32 Ma. Consequently even if sediment correlations across the
faults are wrong, this information implies an improbablesedimen-
tation rate increase from 0.65 mm/yr to 1.87 mm/yr in a distance
of 18 km. While such changes in sedimentation rate can be
justified when passing from uplifted regions,with thinned section,
into basinal areas (layer 3 for example in Fig. 4), or passing away
from a point source of sediments, this is not the case for layer 2
in Fig. 4. The sediments have very low dips, reflection packages
are sub-horizontal and the depositional situation appears to be
laterally very similar.
The compaction trends for the sediments are unknown, conse-
quently present day thicknesses are used since the relative differ-
ences are important to the argument and multiplying both
numbers by similar compaction values (probably in the range of
50–70% compaction towards the base of the sequence) would not
change the relative differencesin rates.
2.1. Rapid sedimentation model of Curray (2005)
The discussionof Fig. 4 is based on a very general interpretation,
where the generalsediment thicknessand distribution is the issue.
Detailedinterpretation of packagesacrossthe seismicdata is more
problematic due to resolution, and the general problems of corre-
lating horizons across faults on a single line interpretation. Despite
this problem it is worthwhile discussinga more detailed interpre-
tation of the line, which is presented in Fig. 5, and how it impacts
the model Curray(2005), which provides an explanation for how
thick sediment can accumulate in the Central Troughduring active
spreading. Curray (2005) and Curray (pers. comm. 2014) require
that young, rapid deposition during low-stands was able to fill
the central trough with turbidite deposits.The central trough acted
as a broad channel for funnelingsediments reworked from the
shelf into the deepwater area. Fig. 6 is a line drawing from seismic
data taken from Curray(2005), added to the originalfigure is shad-
ing indicated how Curray’smodel would work, with the youngest
sediment in the Central Trough onlapping the older sediments
forming a tilted fault block morphology marked by locations C
and C0
. In this model the sediments forming the ponded basins
and the central trough are all of the same age, and are younger
than the deformation which formed the tilted fault blocks.
Interpretation of the seismic line in Raju et al. (2004; Fig. 5)
shows that the interpretation required by Curray (2005), (Fig. 6)
does not work in detail. Four reflection packages are highlighted
in Fig. 5, an upper ponded sequencethat lies SSE of ridge C, a high
amplitude, tabular sequence, and a lower ponded sequence that

4. C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 449
15°
14°
13°
12°
Figs. 4,5
Central
11°
East
Andaman
10°
Fig. 8B
9°
93° 94° 95° 96° 97°
Fig. 3. Geometryand key elements of the Central Andaman Basin and East Andaman Basin. The strike-slip fault geometry on eastern side of the basin and linkage with the
Sagaing Fault is based on Diehl et al. (2013). This interpretation contrasts with the spreading centre geometry of Curray (2005), which is superimposed. The figure is also
based on data in Raju et al. (2004) and Morley (2013).
NW Central trough SEPonded sediment
4.0
34.4
24.8
5.2
5.6
base of sediment
24 km 6 km
Fig. 4. Line drawing interpretation of seismic line across the Central Andaman Basin, and putative spreading centre (original seismic line in Raju et al. (2004)). See Fig. 3 for
location.
shows subtle expansion SSE of C, and which shows significant
onlap onto tilted fault block C0
. Below the lower ponded sequence
is a less well imaged package that is broken up by faults, and is
possibly dominated by expansion into SSE-dippingfaults (I, II,
and III, Fig. 5). The interpretation in Fig. 5 shows the timing of
faulting (I and III) affecting high blocks C and C0
respective, is dif-
ferent. Fault I affects the youngest ponded sediments and offsets
the seafloor. While fault III affects the older ponded sequence
and the crest of the highat C0
is eroded and unconformablysealed
by the upper ponded sequence (a minor, later reactivation also
causes the fault to propagated to the seafloor).The implication of
the interpretation is that there is no abrupt change in the age of
the sediments passing from tilted fault block C to the Central
Trough as required by the Curraymodel (Fig. 6). One of the clearest
WestAndamanFault
? 10 km
B
3
2
1
A
applicable
High area Sagaing Fault
Zone
bathymetry
Late Miocene-
Recent sediment
Faults associated
with Central Basin
Spreading
centre
of Curray
(2005)
Alcock Rise
Area
for Fig.
8A Trough
Fig. 7
Andaman
Central Basin
Basin
Sewell Rise
Shelf-slope
break

5. 450 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456
SS
E
Central Trough NN
W4.0
4.4
4.8
5.2
?
Lower ponded sequence High amplitude tabular sequence Upper ponded sequence
Fig. 5. Detailedline drawing and interpretation of seismic line (same as Fig. 4) across the Central Andaman Basin, and putative spreading centre (original seismic line in Raju
et al. (2004)). It this figure the suggested extent of 3 depositional sequences is highlighted. C, C0
location of ridges following labelling of Curray (2005). I, II, III inferred or
observed faults discussed in the text. See Fig. 3 for location.
SE T 66-67N NW
3
4
Sewell
5
10 kmOlder sediments Younger sediments
Fig. 6. Cross-section across the Andaman Central Basin, redrawn from Curray (2005), showing the required distribution of relative sediment ages needed to explain the
Pliocene-Recent spreading model. A, B, B0
, C, C0
are locations of paired spreading-related structures identified by Currayet al. (1979) and Curray (2005).
parts of the seismic image in Raju et al. (2004) is the abrupt thin-
ning of the upper ponded sequenceonto the high at C, there seems
no plausible way to significantlythicken the unit into the Central
Trough. The sequences that underlie the upper ponded sequence
(south of C) must continue into the Central Trough.Consequently
the upper pondedsequencecannotentirely fill the trough between
locations C and C0
(following the Curray model in Fig. 6).
summarizes the overall crustal geometry and depositional pack-
ages that are seen on 2D seismicreflectiondata (down to 11 s) east
of the Alcock Rise, while Figs. 1B and 8B showsthe basineast of the
Sewell Rise. The patterns in the basin are similar, above an impor-
tant transition zone of intra-Middle Mioceneage a phase of Oligo-
cene–Miocene extension and strike-slip faulting ceases (Fig. 8B).
The cessation of activity occurred in deep water, so while there is
a cessation of faulting and tectonic activity there is no time gap
in the depositional record, hence the term event is used instead
of unconformity. In the southern part of the basin most of the
extensional activity ceases after the Middle Mioceneevent. It can
be seen from Figs. 1B, 2 and 8 that the post MiddleMioceneevent
section expands westwards, and is controlled by fault activity on
the eastern margin of the Alcock and Sewell rises. In the Mergui
Basin seismic reflection data shows westwards prograding Late
Miocene clinoforms indicating that Peninsular Thailand was as
sediment source at that time (Srisuriyon and Morley, 2014). How-
ever, around the latest Mioceneand Pliocenetimes the importance
of an easterly sediment source diminished considerable. Con-
versely in Myanmar most of the sediment supplied from the Shan
Scarp area and through the eastern Himalayan syntaxis was
trapped in the Central Basin and little reached the coast during
the Early Miocene to early Late Miocene. Following this time of
very low clasticdeposition rates in the Gulf of Martaban, sediment
input from the north via the Salween and Ayeyerwady rivers dra-
maticallyincreased into the northern Gulf of Mottama. Hence the
main depocentre today is seen in the northern part of the offshore
basin (Fig. 2) where it attains a thickness in excess of 8 km.
2.2. Andaman Cruise MD116 Marion-Dufresne II seismic line
Seismicreflection data across the Central Andaman Trough was
acquired during the Andaman Cruise MD116 Marion-Dufresne II
(Chamot-Rooke et al., 2000, http://www.geologie.ens.fr/And-
aman/Pages/index_rapport.html). In the online report a seismic
line is show that crossesthe central trough accompaniedby a cap-
tion stating ‘‘the top of the oceaniccrust is reachedat 6.0 (seconds)
TWTT, below 1000 m of sediment’’. This line demonstratesthat
right at the deepest part of the central trough there is 1000 m of
sediment, where in the spreading model oceanic crust should be
forming present day. A depth converted, true-scale line drawing
of the seismic line, which can be found at http://www.geolo-
gie.ens.fr/Andaman/Pages/index_rapport.html is presented in
Fig. 7. Assumingthe 3.8 cm/yr spreading rate determined by Raju
et al. (2004) it can be seen that the width of the seismicline repre-
sents over 0.5 my of spreading, yet there is no thinningof sediment
towards the spreadingcentre duringthis time. Note that in the dis-
cussion of Fig. 4 in Section 2 above, sediment thickness of 600 m
close to the spreadingcentre was used, which is conservative with
respect to the 1 km thickness described from the Andaman Cruise
MD116 data, which lies further to the NE and closer to the sedi-
ment source. 4. What is the Central Andaman Basin?
The pattern of sedimentation discussed above indicates the
Central Andaman Basin is not a spreading centre that has been
active for the last 4 my. It is, however, a region of active tectonics
characterized by extensional earthquakes, (e.g. Raju et al., 2004;
Diehl et al., 2013). Earthquake activity indicates that only 10% of
the long-term spreadingrate of 3.0–3.8cm/yr is accounted for by
extensional faulting,and modelling of the earthquake swarms sug-
gests the presence of intrusive dyke activity (Diehl et al., 2013).
3. Sedimentation in the East Andaman Basin
The sediments that have reached the CentralBasin are the more
distal deposits of the East Andaman Basin. Fig. 2 is a regional iso-
pach map of Middle Miocene-Recent sediments in the East And-
aman Basin based on seismic reflection data. Fig. 8A is a
schematic cross-section through the East Andaman Basin that
TWTT
(s.)
TWTT
(s.)
Central Trough
A B Ponded basin C C’ Ponded basin B’
Rise
C C’
?
?
I II III
10 km

6. C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 451
Age of oceanic crust following
model of Raju et al. (2004)0.5 Ma 0.25 Ma 0 Ma 0.25 Ma 0.5 Ma
NW SE
3.0
4.0
5.0
800 m heave
3.8 cm/yr = 20,000 years
Fig. 7. Depth converted line drawing of the Central Troughfrom the Andaman Seismic line shown on the Andaman Cruise website (http://www.geologie.ens.fr/Andaman/
Pages/index_rapport.html). See Fig. 3 for location.
N-S seismic line
(B,, below)
W South Sagaing Fault (?) E
A 0
MiddleMiocene event2
4
6
8
10
12
14
16
18
20
22
Inferred mantle shear zone
western side of seismic data from lower crust
20 kmpresence of fault planes or shear zones
Paleogene-Early
Miocene syn-rift deposits
Mantle Continental crust MiddleMiocene-Recent deposits
N S
B
Middle
Miocene-
Recent
3
4 Middle Miocene
event
Middle
Miocene-
Oligocene
5
6
7
Upper
Crust?
8
Lower
Crust?
Moho?9
5 km
Fig. 8. (A) Summary cross-section showing the main characteristics of the crustal structure on based on industry 2D seismic lines across the East Andaman Basin, east of the
Alcock Ridge. (B) N–S 3D seismic line from the Thailand deepwater area showing the presence of highly reflective, thin lower crust. The equivalent location of the line on the
E–W section is shown in A (modified from Morley (in press).
Consequently these authors concluded that igneous intrusions
account for 90% of current extension. The recent fault and dyke
activity can be explained in two ways: (1) spreading has been
episodic and has relatively recently been reactivated (as discussed
by Curray(2005) for the AndamanSea, and as described for the Red
Sea by Almalki et al. (2014), or (2) the crust is attenuated
continental crust to transitional continental-oceaniccrust, where
igneous activity is beginning to take over from crustal extension
(e.g. Suguta Valley Kenya, Tongueet al., 1992; northern Ethiopian
Rift, Keir et al., 2006; Ebingeret al., 2010). In the episodicspreading
modelduringperiods when seafloorspreadingis inactive,extension
is accommodated in adjacent areas of thinned continental crust.
Depth(km)Twowaytraveltime(s.)
Depth(km)
Sediment
5 km Oceanic (?) crust

7. 452 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456
In Fig. 4 if layer 1 thins and pinches out passingtowards the
central trough (which cannot be demonstrated from the existing
data) then this geometry would support the idea of episodic
spreading, with layer 1 representing the initial spreading phase,
layer 2 the quiescent phase, and layer 3 the onset of reactiva-
tion. In the spreading model the magnetic anomalies present
in the western part of the basin can still represent oceanic crust,
but the age of the anomalies would have to be older than pro-
posed in Raju et al. (2004) and are most likely of Middle to Late
Miocene age. However, the quality of the magnetic anomalies is
low. Raju et al. (2004) described the absence of lineated anom-
alies in much of the Central Basin, which they noted occurred in
other examples of young oceanic crust as a consequence of a
variety of mechanisms that mask or destroy magnetic intensity
(e.g. sediment blanketing, faulting, igneous activity, hydrother-
mal activity). Only in one segment (their segments B and part
of A, outlined in Fig. 1) did Raju et al. (2004) find more linear
magnetic anomalies, but this segment is merely 60 km wide.
Apparently the magnetic anomalies do not show the characteris-
tics of episodic activity (Curray per. comm., 2014). However, if
virtually all the spreading occurred during the earlier stage,
order of 10,000s years to 100, 000s years) then the anomalies
would not show indications of episodicity.
The absence of extensive linear magnetic anomalies, the evi-
dence of sedimentation across the active central trough, the lack
of seismic activity to support a spreading rate of 3.0–3.8 cm/yr
(Diehlet al., 2013), are arguments for the attenuated continental
crust model. In this model displacement transfer of strike-slip
motion occurs partially by hard linkage of the strike-slip faults
via the central trough,but also some displacement transfer occurs
by soft linkage across overlapping strike-slip faults. Additionally
motion of 3.0–3.8cm/yr is actually only applicable to the Central
Basin area if it is a spreading centre. If the regionis highly attenu-
ated crust then the relative motion of the India Plate with respect
to SE Asia can be accommodated on more geographically diffuse
structures (such as oblique motion in the trench area, Maurin
and Rangin,2009; Ranginet al., 2013), rather than being focused
entirely on the CentralBasin. Consequentlythe revised interpreta-
tion of the significanceof the central trough supports suspicions
articulated previously by Bertrand and Rangin (2003) and Rangin
et al. (2013) that the SagaingFault is probably a relatively young
feature (initiating in the Late Miocene or Pliocene) and that total
displacement on the fault zone is most likely towards the low
and extension has only recently become reactivated (in the
Fig. 9. Crustalthickness from gravity inversion using a lithosphere thermal gravity anomaly correction overlain onto shaded relief free-air gravity anomaly, (A) total crustal
basement thickness, (B) residual continental crustal thickness i.e. total crustal basement thickness minus thickness of the new volcanic crust produced during rifting and
breakup. A = Alcock Rise, CB = Central Basin, S = Sewell Rise.

8. C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 453
end of the estimates (i.e. 100 km) rather than the high end
( 400 km).
by sediment,that can explain the dredged basalts as coming from
flows associated with volcanicactivity,not an organized spreading
centre. (5) The Alcock and Sewell rises are extensively affected by
normal faults with a wide range of displacement values, fault ori-
entations and degrees of rotation. Some high-relief (up to 1.7 km)
fault blocks are present (e.g. Scaif and Billings, 2010). Although
oceanic crust is highly faulted it is generally related to processes
around the spreading centre, which either form low-offset faults
that dip towards the rift axis, or low-angle, large-displacement
detachment faults (generally associatedwith low spreading rates),
relief on fault blocks is generally <1 km, (e.g. Buck et al., 2004;
Smith et al., 2006, 2012; Reston and Ranero, 2011). The style of
faulting indicates a continental or island arc crust interpretation
is most likely. (6) The Invisible Ridge, which lies on the western
side of the rises, is interpreted to be continental crust (Roy and
Chopra, 1987). (7) The presence of high velocity crust can be
explained by the presence of mafic granuliticcrust formingthe
lower crust, and crustal thinning being focused on ductile, felsic
middle crust (followingthe model of Mohn et al., 2012), or from
island arc crust. The presence of mafic lower crust in western Thai-
land has been documented from xenoliths in Cenozoic basalts by
Prompratedet al. (2003).
The problem with the data discussedaboveis that muchof it is
held by various companies and government bodies related to the
oil industry and it cannot be published. An independent way to
address the issue of crustal type and thickness is modelling of
gravity data, which is discussed below.
5. Alcockand Sewell rises
The discussionregarding the nature of the CentralBasin would
be considerably simplified if the nature of the crust forming the
Alcock and Sewell Rises (Fig. 2) were well established. However,
this is not the case, and it remainsuncertainwhether the risesrep-
resent unusually thick (magmatically underplated) back-arc oce-
anic crust, hyper-thinned continental crust, or possibly island arc
crust. For the latter case the surfacemorphologyand crustal thick-
ness are similar to the Kyushu-PalauRidge, which lies in a back-arc
setting in the PhilippinesSea (Nishizawaet al., 2007, 2011).
Curray (2005 his Figs. 20 and 21) shows that between 23 Ma
and 15 Ma the rises developed as a result of an early stage of
back-arc spreading between the Mergui Ridge, and the Invisible
Ridge. Two key pieces of evidencethat led Curray (2005) to favour
the back-arcoceaniccrust interpretation were: (1) seismic refrac-
tion data that indicatedthe presenceof high-velocity crust, and (2)
the presenceof Early Miocene thoelitic basalt from dredge samples
that could indicate basalts formed at a back-arc spreading centre.
Curray (2005) identified oceanic crustal layers with velocities of
around 6.38–6.45km/s two way travel time (TWTT) in the And-
aman CentralBasin, and about 6.7 km/s TWTT in the East Andaman
Basin. However, these velocities seem slow for typical back-arc
oceanic crust, for example for the PhilippineSea area oceaniccrust
has velocities between 6.8 and 7.2 km/s TWTT (Nishizawa et al.,
2011). Mean velocities for continental crust excludingsedimentary
rocks for NW Europe are around 6.4–6.6km/s TWTT (Kelly et al.,
2007).
Subsequent to 2005, there has been considerable unpublished
oil industry seismic reflection data gathered in the area between
the eastern marginsof the rises, and the MerguiRidge. These data
show the presence of a thick extensionally faulted sequence
beneath the Middle Mioceneevent (Fig. 8). This deeper sequence
is several kilometres thick,and is probably of Oligocene–Early Mio-
cene age (Morley, in press). The data eliminates the possibilitythat
the rises formed during the Early Miocene as suggested by Curray
(2005).
Srisuriyon and Morley (2014), and Morley(in press) argue that:
(1) the ocean-continentboundarydefinedby Curray(2005) is actu-
ally the narrow neckingzone between 25 and 30 km thick conti-
nental crust, and <15 km thick continental crust, and the mapped
boundary coincides with a major strike-slip fault (South Sagaing
Fault; Fig. 8A). (2) On 11 s two way travel time seismic reflection
data, the crust can be seen to thin across Curray’socean-continent
boundary,but there is no change in character to the crustal reflec-
tivity, and the Moho reflection is similar on both sides of the
‘boundary’. The probable lower crust is characterized by high
amplitude anastomosing discontinuous reflections, which are
present on some on 2D data, and are very prominent features of
3D seismic reflection data in the East Andaman Basin offshore
Thailand (Fig. 8B, Morley, in press). An alternative interpretation
for the high amplitude reflections is that they represent igneous
rocks,either volcanicsinterbedded with sediments, or sills, within
the upper crust. However, regional correlation with 2D seismic
data further north, is more consistent with a reflective lower crust
interpretation. (3) The maximum crustal thicknesses of the Alcock
and Sewell rises (13–18 km) modelled from gravity data
(Radhakrishnaet al., 2008), do not comfortably fit with the inter-
pretation of crust createdat a back-arcspreading centre (although
anomalously thickoceaniccrust or island arc crust can be invoked).
(4) Seismicreflection data across the Sewell Rise shows scattered
small triangular cones (probably volcanicedifices), some onlapped
6. Crustalthickness from gravityinversion
Crustalthickness has been derived from gravity inversion using
the methods described in Alvey et al. (2008) and Cowie and
Kusznir (2012). The available regional gravity data for the And-
aman Sea is the satellite-derived free-air gravity of Sandwell and
Smith (2009). The gravity inversion method includes corrections
for the negative gravityanomaly signalfrom bathymetry,sediment
thickness,and for the elevated geothermalgradient due to stretch-
ing and thinning during rifting and breakup of continental litho-
sphere, which is particularly important when looking at regions
with a young (<65 Ma) breakup/rifting age (Alvey et al., 2008).
Fig. 9A shows the predicted crustal thickness for the Andaman
Sea region overlaid onto a map of shaded-relief free-air gravity
(Sandwell and Smith, 2009). By looking at the colour on Fig. 9A
we can see there is a region of thinner crust (blues1
and greens)
surrounded by thicker (definitely) continental crust (yellow to reds
and greys). The crust within the Indian Ocean is predicted to be
unrealistically thin, as a result of the inversion parameters being
tuned to the age of the Andaman Sea and not the Indian Ocean.
Within the Andaman region of thinner crust (which includes Alcock
and Sewell Rises) there is anotherarea of very thin crust(<5 km) that
could indicatethis region is underlain by oceanic crust. Crustalthick-
ness alone however, cannot distinguish between thin continental or
oceanic crust, so on the basis of gravityinversionresults alone this
area could be either. By superimposing the shaded-relief free-air
gravity data onto the crustal thickness we can illustrate additional
tectonic information which suggests that there is an oceanic spread-
ing-ridgerunning through the area of thin crust, supporting the idea
that this area maybe oceanic.
The gravity inversion method also includes a correction for the
amount of new volcaniccrust produced during adiabatic decom-
pression melting as described by White and McKenzie (1989),
which allows the result to predict the location of oceanic and
1
For interpretation of colour in Fig. 9, the reader is referred to the web version of
this article.

9. 454 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456
continentalcrust (and hencethe continent-oceanboundary(COB)).
This is done independently of magnetic-anomaly information,
which can be difficult to interpret and sometimes is misleading.
Whenthe predictedcrustal thicknessdue to volcanicaddition is
subtracted from the total crustal-basement thicknessthe resulting
map shows the residualthicknessof the continentalcrust (Fig. 9B).
In Fig. 9B there are regions that are white and define a total
absence of continental crust, implying that the crust is oceanic
and 7 km or less thick. This supports the initial observation that
the very thin crust (bisected by an apparent oceanic spreading-
ridge) is oceanic.
Fig. 9B shows the remaining region of thin crust (see in Fig 9A)
is attenuated continental crust, meaning that the Alcock and
Sewell Rises are underlain by continental crust. This conclusion is
supported by the raw free-air gravity data, which show a different
character to that observed within the oceanic crustal region. The
fault-blocks striking parallel to the rifted margin of the oceanic
region (i.e. classical rifted margin geometry). Such tilted fault
blocks are present on proprietary industry seismic reflection data
and published data (Curray,2005; Scaif and Billings, 2010) crossing
the rises.
There is a complication to the interpretationof the gravitydata
in the regioneast of the central oceanicsegment (northeast of Sew-
ell Rise and southeast of Alcock Rise, within the East Andaman
Basin Fig. 9A). The crustal thicknesses predicted (Fig. 9A) are
7 km, which would typicallybe interpreted as oceanic crust (area
X Fig. 9B). However, the texture of the crustis the similarto Alcock
and Sewell Rises and if it were oceanicit would require either sig-
nificantly asymmetric spreading or multiple ridge jumps, in order
to account for the asymmetry observedacrossthe ridgein its pres-
ent-day location. Hence,by looking at the free-airgravitydata and
ridgelocationthe crust is probably continental. This interpretation,
free-air gravity shows what appears to be a region of tilted matches the seismic data (Fig. 8) very well, which shows
Sagaing Fault Sagaing Fault
Land
Marine post-rift
basin
Marine syn-rift basin
Fore-arc basin
east of Andaman-
Nicobar Islands
Back-arc
oceanic crust 1
Continental
rift basin
Marine, no major basin
shallow water <1 km) Late Miocene Middle Miocene
Shan Scarp Fault Shan Scarp Fault
Marine, no major basin
deepwater
Extension/transtension red
arrows approximate
minimum horizontal stress
direction
Inversion/compression blue
arrows approximate
maximum horizontal stress
direction
A
Schematic swarms of minor
normal faults
EA
BS
EAB = East Andaman
Basin
MB = Mergui Basin
NSB = North Sumatra
Basin
A = Alcock Rise
S = Sewell Rise
1
1
MB
NSB
Late OligoceneEarly Miocene
Fig. 10. Palaeogeographic reconstruction of the Andaman Sea region from the Late Oligocene to Late Miocene, illustrating the key changes in tectonic and structural
development of the basin, considerably modified from Srisuriyon and Morley (2014), to reflect the re-interpretation of the region discussed in this paper.

10. C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 455
progressively thinning continental crust westwards towards the
Alcock and Sewell Rises. The inset in Fig. 9B showsthe likely distri-
bution of oceanic crust.
part of a sequence of Late Miocene-Recentsediments that infill the
East Andaman Basin during its post-extensional phase. The sedi-
ments progressively thin to the south along the basin axis
(Fig. 2). The pattern of infill of the CentralBasin (Figs. 4 and 5) is
not one of sediments younging from each side towards the basin
axis. Instead the data shows phases of pondedsedimentsand tab-
ular sedimentsthat fit with episodicextension, and sedimentation
across a basin that had attained its maximum width prior to, not
during sedimentation.
7. Discussion
Inversion of gravitydata for crustal thickness (Fig. 9B) indicates
that oceanic crust is present both in the Central Basin and in a
small region on the NE side of the Alcock Rise, in the location of
a small trough segment previously identified by Curray (2005).
This small trough is buriedby over 4 km of sediment (Fig. 2). Hence
while a short period of rapid deposition has been argued to explain
young spreading in the Central Basin (Curray, 2005) it does not
seem feasible to apply the same argument to the NE segment. If
a Middle or Late Miocene age is assumed for the NE oceaniccrust
segment to match with the sedimentation history, then it is most
reasonable to assume a similar age for the CentralBasin too. Other-
wise a difficulty arises in explaining why two small oceanic seg-
ments adjacent to each other would be significantly different in
age.
A palaeogeographic reconstruction of the AndamanSea modi-
fied from Srisuriyon and Morley (2014) to reflect the timing of
the CentralBasin spreading centre and limit of oceanic crust dis-
cussed in this paper is provided in Fig. 10. The extensional history
is complex with WNW–ESE oriented extension in the Oligocene,
which evolved into more NNW–SSE oriented extension during
the Early Miocene (Srisuriyon and Morley, 2014; Fig. 10). Hence
the earlier extension history probably defined the N–S trending
trough and region of thinned continental crust on the east side of
the Alcock and Sewell Rises. This continental crust was extremely
attenuated in places (<10 km thickness). Then as the extension
direction rotated the ENE–WSW extensional trends developed,
and the spreading centre cut across the old N–S trending struc-
tures, some of which appear to have acted as transform faults or
influencedthe locationof transform faults that offsetthe spreading
centre (Fig. 10). The Middle Miocene event in the East Andaman
Basin and the abrupt cessation of NNW–SSE oriented extension
direction (ENE–WSW trending faults) are interpreted here as the
time when continental extension ceased,and was replaced by sea-
floor spreading in the Central Andaman Basin.
The geographically limited area of sea floor spreading was
bounded by strike-slip faults to the west (East Andaman Fault)
and east (SagaingFault). It is proposed that these overlappingseg-
ments couldtransfer displacementbetween the faultsand through
the Alcock Rise area, and that seafloor spreading was abandoned
duringthe Late Mioceneand has only become renewed in the cen-
tral trough. Whether this truly marks renewed seafloor spreading,
or just tectonic reactivation of old features in uncertain.
The Pliocene-Recent spreading centre model requires that
600 m or more of sediment was rapidly deposited during one or
several very young lowstand events. However, sea-level fluctua-
tions in the range of 10’s to 100 + m documented for the Neogene
occur consistently as high frequency events (Miller et al., 2005),
although there is an overall lowering in sea level during the Plio-
Pleistocene. The high frequencyevents are mostly related to Milan-
kovich-scale sea level changes (cyclicityat 19/23, 41 and 100 ky,
Miller et al., 2005). Consequently it is considered here that it is
unrealistic to select a single or a few, young low-stand events to
transport the 600 m of sediment observed in the axial trough.
The high frequency of significant low-stand events suggests that
sediment would be frequentlyreworked from the shelf into the
deepwater over an extended period of time, rather than in a single
major event.
The generalpattern of sedimentation indicates that the influxof
sediments from the north began during the Late Miocene, and are
8. Conclusion
The presenceand internal geometry of 100’s m thickness of sed-
iment in the eastern half of the Andaman Central Basin, and the
young development of central trough-style faulting indicates that
the model for continual seafloor spreading since about 4.0 Ma
(Raju et al., 2004; Curray, 2005) is incorrect. The Central Basin is
interpreted to have formed by episodic spreading (perhaps Mid-
dle-Late Miocene spreading, a period of quiescence, followed by
renewed extension in the order of the last several 1 10 to 4
5
1 10 years). An alternative model that the central trough is
equivalent to magmatically active narrow troughs in attenuated
continental crust for transitional continental-oceanic crust such
as those found in the northern EthiopianRift, or the Suguta Valley
in Kenya was investigated. However, gravity modelling supports
the existinginterpretations that the CentralBasin is composed of
oceanic crust. Conversely, the adjacent regions of the Alcock and
Sewell Rises, which Curray (2005) interpreted as oceanic crust,
are here concludedto be composed of highlyextended continental
crust. Our results also suggest that the East Andaman basin is
underlain by highlyattenuated continental crust in a narrow neck-
ing zone. However, at the northern end of the basin,east of a trans-
form fault bounding the eastern side of the Alcock Rise, gravity
inversion modelling supports the presence of a small segment of
oceanic crust (Fig. 9B). Here the oceanic crust is overlain by
4 + km of sediment, which strongly suggests the crust is older than
the Pliocene-Recent.
The results have implications for: (1) the way incipientspread-
ing centres can develop (i.e. extension is not inevitablyabandoned
in the adjacent areas of continental crust once seafloor spreading
begins), and (2) the ways in which sheared back-arc margins
develop. The model where the Pliocene-Recentspreading centre
accommodates all of the 3.8 cm/yr northwards motion of India
with respect to the SE Asia east of the Sagaing Fault (e.g. Raju
et al., 2004; Curray, 2005; Diehl et al., 2013) conflicts with obser-
vations from GPS data that the SagaingFault only accommodates
half that motion,and that deformationis widely distributed across
Myanmar (see review in Ranginet al., 2013). The conclusions of
this study that the seafloor spreading is mostly Miocene in age,
and that the trough is the result of recent re-activation (with an
unknown extension rate) fit with the GPS data, and resolvethe
problem of trying to accommodate 3.8 cm/yr motion on structures
confined to eastern central Myanmar(i.e. in the vicinityof the Sag-
aing Fault).
Acknowledgments
An older version of the manuscript benefitted fromconstructive
reviews by an anonymous referee, Joe Currayand Tony Barber. This
manuscript benefitted from constructive reviews by Claude Rangin
and Joe Curray. I would also like to thank Joe Curray for extensive
correspondence on this issue, that was very helpful to the manu-
script,while at the same time pointing out that he disagreeswith
the interpretation presented here,and still favours Pliocene-Recent
seafloor spreading. Larry Lawver is also thanked for helpful

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