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 Department of Geological Sciences, Chiang Mai University, Chiang Mai 50200, Thailandb 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

Accepted 26 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 Unpublished seismic reflection data across the East Andaman Basin supports the presence of continental crust under the basin that thins drastically westwards towards the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) inthe 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 and the Alcock and Sewell rises as being oceanic crust, 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 prevailing view at the time that the Andaman spreading centre opened during the Middle Miocene (following Curray et 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 largeCenozoic 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. Curray et 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), Rangin et 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

j o u rn a l h o mep ag e : www.e l se v i e r.c o m/ lo ca t e / j seaes

C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 447

are oceanic crust-type magnetic anomalies interpreted (Raju et al.,2004).

The possibility that spreading was episodic has previously been discussed by Curray et al. (1979) and Curray (2005) based on sed- imentological arguments. However, they ultimately favoured a continual spreading model. This study re-examines the published geophysical data, 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 sedimentary geometries are incompatible with continual sea floor spreading since 4.0 Ma. This in turn raises questions about 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 shelf area wherein places seismic reflection 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 across the central trough from Raju et al. (2004). There are 3 sedimentary packages: (1) a deformed lower 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 geographically limited than unit 2. The lower layer is not dis- cussed here because it 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 central trough, the problem for the contin- ual spreading model of explaining the presence and geometry of layers 2 and 3 remains.The long-term spreading rate for the Central Basin is estimated between 3.0–3.8 cm/yr (Diehl et al., 2013). Raju et al. (2004) inter- preted initial slow spreading rates of 1.6 cm/yr beginning around

2. Sedimentation in the East Andaman Sea

The Salween and Ayeyarwady rivers have input a considerablevolume of sediment onto the Gulf of Mottama margin from the Late Miocene ( 7 Ma?) to the present day (Morley, 2013). Deposi- tion is focused in a N–S trending synclinal trough that follows the strike of two major strands of the Sagaing Fault (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

MyanmarAI

Central Andaman Basin

Cenozoic sedimentarybasin

Late Miocene-Recent sediment input from A) Ayerwaddy River, and B) Salween River

AR = Alcock RiseSR = Sewell RiseSSG = South Sagaing FaultMB = Mergui BasinNSB = North Sumatra BasinSREBF = Sewell Rise East

Bounding Fault

Oceanic (west)/continental crust(east) boundary from Curray (2005).

Gulf of Martaban

15°

?CocoBasin

Andaman- Nicobar Islands

?

AR

AxialTrough

10°

Shelf- slope breakArea of

highest quality magnetic anomalies

SR Western limit of probable Oligocene(possibly including Late Eocene)sedimentary rocks of East AndamanBasin observed on seismic reflection data

A’AMF

SouthSagaing Fault

??

?

1

?

West AndamanFault zone

??

?

?Sumatra- Andaman Trench

NorthSumatra Basin

SumatranFault zone5°

91° 95° 99°Sumatra

Mergui Fault EA’

W0 A

West Andaman Fault Sewell Rise SREBF

South Sagaing Fault (SSG)Palaeogene-early Middle Miocene-RecentMiddle Miocene

N end ofMergui Ridge7

B East Andaman Basin

Fig. 1. (A) Regional map of the Andaman Sea region and surrounding areas. (B) Regional cross-section through the southern-central Andaman Sea based on 2D seismic

reflection data (modified from Srisuriyon and Morley (2014)).

TW

TT

(s)

E.

And

aman

Bas

inM

ergu

i R

idge

448 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456

0.5-1 kmMyanmar

1-2 km

2-3 km

15°3-4 km

4-5 kmAndaman-NicobarIslands

5-6 kmSagaingFault

6-7 km

AIndia 7-8 km

8+ km

10°

S

West AndamanFault zone Me

Thailand91° 95°

Fig. 2. Regional isopach 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 about1.8 cm/yr occurring on the Sagaing Fault, and the rest of the motion along diffuse structures further to the west. Hence the 3.0–3.8 cm/ 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 2000 m/s seismic inter- val velocity), but is missing approximately 250 m thickness of layer3, which is present at point A. This situation conflicts with 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 improbable sedimen- 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 of50–70% compaction towards the base of the sequence) would not change the relative differences in rates.

2.1. Rapid sedimentation model of Curray (2005)

The discussion of Fig. 4 is based on a very general interpretation,where the general sediment thickness and distribution is the issue. Detailed interpretation of packages across the seismic data 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 discussing a 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 Trough during 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 funneling sediments reworked from the shelf into the deepwater area. Fig. 6 is a line drawing from seismic data taken from Curray (2005), added to the original figure is shad- ing indicated how Curray’s model 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 sequence that lies SSE of ridge C, a high amplitude, tabular sequence, and a lower ponded sequence that

Mer

gui R

idge

C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 449

15°

14°

13°

12°

Figs. 4,5

Central11°

East

Andaman

10°

Fig. 8B

9°

93° 94° 95° 96° 97°

Fig. 3. Geometry and 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 sediment4.0

34.4

24.8

5.2

5.6

base of sediment24 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 significantonlap 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-dipping faults (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 sequenceand the crest of the high at C0 is eroded and unconformably sealed 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 Curray model (Fig. 6). One of the clearest

Wes

t A

ndam

an F

ault

? 10 km B

3

2

1

A

applicable

High area Sagaing FaultZone

bathymetry

Late Miocene- Recent sediment

Faults associated with Central Basin

Spreading centre

of Curray(2005)

Alcock Rise

Area

for Fig.

8A TroughF

ig. 7

AndamanCentral

Basin

Basin

Sewell Rise

Shelf-slope break

450 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456

SSE

Central Trough NNW4.0

4.4

4.8

5.2?

Lower ponded sequence High amplitude tabular sequence Upper ponded sequence

Fig. 5. Detailed line 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

4Sewell

5

10 km Older 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 Curray et al. (1979) and Curray (2005).

parts of the seismic image in Raju et al. (2004) is the abrupt thin-ning of the upper ponded sequence onto the high at C, there seems no plausible way to significantly thicken 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 ponded sequence cannot entirely 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 seismic reflection data (down to 11 s) east of the Alcock Rise, while Figs. 1B and 8B shows the basin east of the Sewell Rise. The patterns in the basin are similar, above an impor- tant transition zone of intra-Middle Miocene age 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 Miocene event. It can be seen from Figs. 1B, 2 and 8 that the post Middle Miocene event 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 Miocene and Pliocene times 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 clastic deposition rates in the Gulf of Martaban, sediment input from the north via the Salween and Ayeyerwady rivers dra- matically increased 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

Seismic reflection data across the Central Andaman Trough wasacquired 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 crosses the central trough accompanied by a cap- tion stating ‘‘the top of the oceanic crust is reached at 6.0 (seconds) TWTT, below 1000 m of sediment’’. This line demonstrates that 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. Assuming the 3.8 cm/yr spreading rate determined by Raju et al. (2004) it can be seen that the width of the seismic line repre- sents over 0.5 my of spreading, yet there is no thinning of sediment towards the spreading centre during this time. Note that in the dis- cussion of Fig. 4 in Section 2 above, sediment thickness of 600 m close to the spreading centre 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 theCentral 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 spreading rate of 3.0–3.8 cm/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 Central Basin are the moredistal 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

TW

TT

(s.)

TW

TT

(s

.)

Central Trough

A B Ponded basin C C’ Ponded basin B’

Rise

C C’

?

? I II III

10 km

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 heave3.8 cm/yr = 20,000 years

Fig. 7. Depth converted line drawing of the Central Trough from 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 0Middle Miocene event2

4

6

8

10

12

14

16

18

20

22

Inferred mantle shear zonewestern side of seismic data from lower crust

20 kmpresence of fault planes or shear zones

Paleogene-EarlyMiocene syn-rift depositsMantle Continental crust Middle Miocene-Recent deposits

N S

BMiddle Miocene- Recent

3

4 Middle Miocene event

Middle Miocene- Oligocene

5

6

7UpperCrust?

8LowerCrust?

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 intrusionsaccount 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 Andaman Sea, and as described for the Red Sea by Almalki et al. (2014), or (2) the crust is attenuated

continental crust to transitional continental-oceanic crust, whereigneous activity is beginning to take over from crustal extension (e.g. Suguta Valley Kenya, Tongue et al., 1992; northern Ethiopian Rift, Keir et al., 2006; Ebinger et al., 2010). In the episodic spreading model during periods when seafloor spreading is inactive, extension is accommodated in adjacent areas of thinned continental crust.

Dep

th (

km)

Tw

o w

ay tr

avel

tim

e (s

.)

Dep

th (

km)

Sediment

5 km Oceanic (?) crust

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 passing towards thecentral 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 anomalieswould 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 (Diehl et 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 3.0–3.8 cm/yr is actually only applicable to the Central Basin area if it is a spreading centre. If the region is 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; Rangin et al., 2013), rather than being focused entirely on the Central Basin. Consequently the revised interpreta- tion of the significance of the central trough supports suspicions articulated previously by Bertrand and Rangin (2003) and Rangin et al. (2013) that the Sagaing Fault 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 lowand extension has only recently become reactivated (in the

Fig. 9. Crustal thickness 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.

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 fromflows associated with volcanic activity, 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 associated with 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 granulitic crust forming the lower crust, and crustal thinning being focused on ductile, felsic middle crust (following the 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 Promprated et al. (2003).

The problem with the data discussed above is that much of 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. Alcock and Sewell rises

The discussion regarding the nature of the Central Basin wouldbe 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 remains uncertain whether the rises rep- resent unusually thick (magmatically underplated) back-arc oce- anic crust, hyper-thinned continental crust, or possibly island arc crust. For the latter case the surface morphology and crustal thick- ness are similar to the Kyushu-Palau Ridge, which lies in a back-arc setting in the Philippines Sea (Nishizawa et 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 evidence that led Curray (2005) to favour the back-arc oceanic crust interpretation were: (1) seismic refrac- tion data that indicated the presence of high-velocity crust, and (2) the presence of 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 6.38–6.45 km/s two way travel time (TWTT) in the And- aman Central Basin, 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 Philippine Sea area oceanic crust has velocities between 6.8 and 7.2 km/s TWTT (Nishizawa et al.,2011). Mean velocities for continental crust excluding sedimentary rocks for NW Europe are around 6.4–6.6 km/s TWTT (Kelly et al.,200

7).Subsequent to 2005, there has been considerable unpublished oil industry seismic reflection data gathered in the area between the eastern margins of the rises, and the Mergui Ridge. These data show the presence of a thick extensionally faulted sequence beneath the Middle Miocene event (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 possibility that 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-continent boundary defined by Curray (2005) is actu- ally the narrow necking zone 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’s ocean-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 volcanics interbedded 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 (Radhakrishna et al., 2008), do not comfortably fit with the inter- pretation of crust created at a back-arc spreading centre (although anomalously thick oceanic crust or island arc crust can be invoked). (4) Seismic reflection data across the Sewell Rise shows scattered small triangular cones (probably volcanic edifices), some onlapped

6. Crustal thickness from gravity inversion

Crustal thickness has been derived from gravity inversion usingthe 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 gravity anomaly signal from bathymetry, sediment thickness, and for the elevated geothermal gradient 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 another area of very thin crust (<5 km) that could indicate this region is underlain by oceanic crust. Crustal thick- ness alone however, cannot distinguish between thin continental or oceanic crust, so on the basis of gravity inversion results 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-ridge running 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 volcanic crust 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.

454 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456

continental crust (and hence the continent-ocean boundary (COB)).This is done independently of magnetic-anomaly information, which can be difficult to interpret and sometimes is misleading.

When the predicted crustal thickness due to volcanic addition is subtracted from the total crustal-basement thickness the resulting map shows the residual thickness of the continental crust (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 oceanicregion (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 interpretation of the gravity data in the region east of the central oceanic segment (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 typically be interpreted as oceanic crust (area X Fig. 9B). However, the texture of the crust is the similar to Alcock and Sewell Rises and if it were oceanic it would require either sig- nificantly asymmetric spreading or multiple ridge jumps, in order to account for the asymmetry observed across the ridge in its pres- ent-day location. Hence, by looking at the free-air gravity data and ridge location the 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

EABS

EAB = East Andaman Basin

MB = Mergui Basin

NSB = North Sumatra Basin

A = Alcock Rise

S = Sewell Rise

11

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.

C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456 455

progressively thinning continental crust westwards towards theAlcock and Sewell Rises. The inset in Fig. 9B shows the likely distri- bution of oceanic crust.

part of a sequence of Late Miocene-Recent sediments that infill theEast 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 Central Basin (Figs. 4 and 5) is not one of sediments younging from each side towards the basin axis. Instead the data shows phases of ponded sediments and tab- ular sediments that fit with episodic extension, and sedimentation across a basin that had attained its maximum width prior to, not during sedimentation.

7. Discussion

Inversion of gravity data for crustal thickness (Fig. 9B) indicatesthat 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 buried by 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 oceanic crust segment to match with the sedimentation history, then it is most reasonable to assume a similar age for the Central Basin 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 Andaman Sea modi- fied from Srisuriyon and Morley (2014) to reflect the timing of the Central Basin 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 influenced the location of transform faults that offset the 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 (Sagaing Fault). It is proposed that these overlapping seg- ments could transfer displacement between the faults and through the Alcock Rise area, and that seafloor spreading was abandoned during the Late Miocene and 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 that600 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 frequency events are mostly related to Milan- kovich-scale sea level changes (cyclicity at 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 frequently reworked from the shelf into the deepwater over an extended period of time, rather than in a single major event.The general pattern of sedimentation indicates that the influx of sediments from the north began during the Late Miocene, and are

8. Conclusion

The presence and 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

51 10 years). An alternative model that the central trough isequivalent to magmatically active narrow troughs in attenuated continental crust for transitional continental-oceanic crust such as those found in the northern Ethiopian Rift, or the Suguta Valley in Kenya was investigated. However, gravity modelling supports the existing interpretations that the Central Basin is composed of oceanic crust. Conversely, the adjacent regions of the Alcock and Sewell Rises, which Curray (2005) interpreted as oceanic crust, are here concluded to be composed of highly extended continental crust. Our results also suggest that the East Andaman basin is underlain by highly attenuated 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 by4 + km of sediment, which strongly suggests the crust is older than the Pliocene-Recent.

The results have implications for: (1) the way incipient spread- ing centres can develop (i.e. extension is not inevitably abandoned 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-Recent spreading 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 Sagaing Fault only accommodates half that motion, and that deformation is widely distributed across Myanmar (see review in Rangin et 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 resolve the problem of trying to accommodate 3.8 cm/yr motion on structures confined to eastern central Myanmar (i.e. in the vicinity of the Sag- aing Fault).

Acknowledgments

An older version of the manuscript benefitted from constructivereviews by an anonymous referee, Joe Curray and 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 disagrees with the interpretation presented here, and still favours Pliocene-Recent seafloor spreading. Larry Lawver is also thanked for helpful

456 C.K. Morley, A. Alvey / Journal of Asian Earth Sciences 98 (2015) 446–456

correspondence regarding the magnetics data, although, any errorsin the manuscript associated with these data are naturally ours, not his. We would also like to thank Badley Geoscience Ltd. for use of crustal thickness images from their OCTek Asia-Pacific report.

Miller, K.G., Kominz, M.A., Browning, J.V., Wright, J.D., Mountain, G.S., Katz, M.E., Sugarman, P.J., Cramer, B.S., Christie-Blick, N., Pekar, S.F., 2005. The Phanerozoic Record of Global Sea-Level Change. Science 310, 1293–1298.Mohn, G., Manatschal, G., Beltrando, M., Masini, E., Kusznir, N., 2012. Necking of continental crust in magma-poor rifted margins: evidence from the fossil Alpine Tethys margins. Tectonics 31. http://dx.doi.org/10.1029/2011TC002961.Morley, C.K., 2012. Late Cretaceous-Early Palaeogene tectonic development of SE Asia. Earth Sci. Rev. 115, 37–75.Morley, C.K., 2013. Discussion of tectonic models for Cenozoic strike-slip fault- affected continental margins of mainland SE Asia. J. Asian Earth Sci. 76, 137–151.Morley, C.K., in press. Cenozoic Structural Evolution of the Eastern Andaman Sea: Evolution from an Extensional to a Sheared Margin. Geological Society of London, Special Publication.Nielsen, C., Chamot-Rooke, N., Rangin, C.The Andaman Cruise Team, 2004. From partial to full strain partitioning along the Indo-Burmese hyper-oblique subduction. Mar. Geol. 209, 303–327.Nishizawa, A., Kaneda, K., Katagiri, Y., Kasahara, J., 2007. Variation in crustal structure along the Kyushu-Palau Ridge at 15–21 N on the Philippine Sea plate based on seismic refraction profiles. Earth Planet. Space 59, e17–e20.Nishizawa, A., Kandea, K., Oikawa, M., 2011. Backarc basin oceanic crust and uppermost mantle seismic velocity structure of the Shikoku Basin, south of Japan. Earth Planet. Space 63, 151–155.Promprated, P., Taylor, L.A., Neal, C.R., 2003. Petrochemistry of mafic granulite xenoliths from the Chantaburi basaltic field: implications for the nature of the Lower Crust beneath Thailand. Int. Geol. Rev. 45, 383–406.Radhakrishna, M., Lasitha, S., Mukhopadhyay, M., 2008. Seismicity, gravity anomalies and lithospheric structure of the Andaman arc, NE Indian Ocean. Tectonophysics 460, 248–262.

Raju, K.A.K., Ramprasad, T., Rao, B.R., Varghese, J., 2004. New insights into thetectonic evolution of the Andaman Basin, Northeast Indian Ocean. Earth Planet. Sci. Lett. 221, 145–162.Rangin, C., Maurin, T., Masson, F., 2013. Combined effects of Eurasia/Sunda oblique convergence and East-Tibetan crustal flow on the active tectonics of Burma. J. Asian Earth Sci. 76, 185–194.Reston, T.J., Ranero, C.R., 2011. The 3-D geometry of detachment faulting at mid- ocean ridges. Geochem. Geophys. Geosyst. 12. http://dx.doi.org/10.1029/2011GC003666.Roy, T.K., Chopra, N.N., 1987. Wrench faulting in Andaman forearc basin, India. Proc.Offshore Technol. Conf. 19, 393–404.Sandwell, D.T., Smith, W.H.F., 2009. Global marine gravity from retracked Geosat and ERS-1 altimetry: ridge segmentation versus spreading rate. J. Geophys. Res.114, B01411. http://dx.doi.org/10.1029/2008JB006008.Scaif, G., Billings, A., 2010. Andaman Basin. GEOExPro, 7 (5). <http://www.geoexpro.com/articles/2010/05/andaman-basin>.Smith, D.K., Cann, J.R., Escartin, J., 2006. Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge. Nature422, 440–443.Smith, D.K., Escartin, J., Schouten, H., 2012. Active long-lived faults emerging along slow-spreading mid-ocean ridges. Oceanography 25, 94–99.

Srisuriyon, K., Morley, C.K., 2014. Pull-apart development at overlapping fault tips:Oblique rifting of a Cenozoic continental margin, northern Mergui Basin, Andaman Sea. Geosphere 10, 80–106. http://dx.doi.org/10.1130/GES00926.1.

Tongue, J.A., Maguire, P.K.H., Burton, P., 1992. Seismicity distribution fromtemporary earthquake recording networks in Kenya. Tectonophysics 204, 71–79.White, R.S., McKenzie, D.P., 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research94, 7685–7729.

References

Almalki, K.A., Bettes, P.G., Ailleres, L., 2014. Episodic sea-floor spreading in theSouthern Red Sea. Tectonophysics 617, 140–149.

Alvey, A., Kusznir, N.J., Torsvik, T.H., Gaina, C., 2008. Quantifying continental overlap in plate reconstruction models for the North Atlantic using continental extension estimates from gravity inversion. Earth Planet. Sci. Lett. 274, 310–321.Bertrand, G., Rangin, C., 2003. Tectonics of the western margin of the Shan plateau (central Myanmar): implication for the India-Indochina oblique convergence since the Oligocene. J. Asian Earth Sci. 21, 1139–1157.Buck, W.R., Lavier, L.L., Collins, J.A., 2004. Modes of faulting at mid-ocean ridges.Nature 434, 719–723.Chamot-Rooke, N., Rangin, C., 2000. Andaman Cruise Website. <http://www.geologie.ens.fr/Andaman/Pages> (02.14).Chappell, A.R., Kusznir, N.J., 2008. Three-dimensional gravity inversion for Moho depth at rifted continental margins incorporating a lithosphere thermal gravity anomaly correction. Geophys. J. Int.. http://dx.doi.org/10.1111/j.1365-246X.2008.03803.x.Cowie, L., Kusznir, N.J., 2012. Mapping crustal thickness and oceanic lithosphere distribution in the Eastern Mediterranean using gravity inversion. Petrol. Geosci. 18, 373–380.Curray, J.R., 2005. Tectonics of the Andaman Sea region. J. Asian Earth Sci. 25, 187–232.Curray, J.R., Moore, D.G., Lawver, L.A., Emmel, F.J., Raitt, R.W., 1979. Tectonics of the Andaman Sea and Burma. In: Watkins, J., Montadert, L., Dickenson, P.W. (Eds.), Geological and Geophysical Investigations of Continental Margins, vol. 29. American Association of Petroleum Geologists Memoir, pp. 189–198.Diehl, T., Waldhauser, F., Cochran, J.R., Raju, K.A., Seeber, L., Schaff, D., Engdahl, E.R.,2013. Back-arc extension in the Andaman Sea: tectonic and magmatic processes imaged by high-precision teleseismic double-difference earthquake relocation. J. Geophys. Res. 118, 1–19. http://dx.doi.org/10.1002/jgrb.50192.Ebinger, C., Ayele, A., Keir, D., Rowland, J., Yirgun, G., Wright, T., Belachew, M., Hamling, I., 2010. Length and timescales of rift faulting and magma intrusion: the Afar rifting cycle from 2005 to present. Annu. Rev. Earth Planet. Sci. 38, 439–466.Hall, R., 2002. Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations. J. Asian Earth Sci. 20, 353–434.Keir, D., Ebinger, C.J., Stuart, G.W., Daly, E., Ayele, A., 2006. Strain accommodation by magmatism and faulting as rifting proceeds to breakup: seismicity of the northern Ethiopian rift. J. Geophys. Res. 111. http://dx.doi.org/10.1029/2005JB003748.Kelly, A., England, R.W., Maguire, P.K.H., 2007. A crustal seismic velocity model for the UK, Ireland and surrounding seas. Geophys. J. Int.. http://dx.doi.org/

10.1111/j.1365-246X.2007.03569.Maurin, T., Rangin, C., 2009. Structure and kinematics of the Indo-Burmese Wedge:recent and fast growth of the outer wedge. Tectonics 28. http://dx.doi.org/10.1029/2008TC002276.