© 2005 Lavoisier SAS. All rights reserved.

Geodinamica Acta 18/3-4 (2005) 333–342

Determination of horizontal extension from fissure-ridge travertines: a case study from the Denizli Basin, southwestern Turkey

Erhan Altunel *, Volkan KarabacakEski‚ehir Osmangazi University, Faculty of Engineering, Department of Geological Engineering, Eski‚ehir, Turkey.

Abstract

Travertine deposits reflect some aspects of the regional tectonics because of the close association between travertine deposits andactive fractures, that later of which provide conduits along which travertine-depositing waters may rise. Fissure-ridge travertines formabove extensional fissures which are located in the hanging walls of normal faults, in step-over zones between fault segments, or in active(or recently active) volcanic provinces. Numerous active and inactive fissure-ridge travertines are located in the hanging walls of normalfaults in the Denizli Basin. A typical fissure-ridge comprises a central fissure along its long axis and flanking bedded travertines dippingaway from the fissure. Central fissures of travertine ridges have been dilating since the initiation of the fissures. Samples from both themargins and centres of banded travertine deposits were dated by Th/U methods in order to determine dilation rates. Individual fissureshave been dilating at average rates of between 0.008 and 0.1 mm yr–1 during travertine deposition, and ~ 0.001 and 0.007 mm yr–1 aftercessation of travertine deposition. There is a noticable decrease in dilation rate from west to east in the Denizli Basin, and this decreasein dilation rate may be related to decrease in overall extension in southwest Turkey, which decreases eastward.

© 2005 Lavoisier SAS. All rights reserved.

Keywords: Extension; Normal fault; Travertine; Denizli Basin; Southwest Turkey

1. Introduction

Southwestern Turkey is an area of active continentalextension (Fig. 1) and this region has been dominated byextensional deformation since stretching began, sometimebetween the Early Miocene and Pliocene (e.g. [1-26]). Awide variety of techniques, such as seismic, geodetic, geo-logical and geomorphological, have been employed todetermine extension values for either the whole area or partsof it. For example, Le Pichon and Angelier [3] estimated anextension rate in the Aegean Sea area using arguments basedon the length and probable age of the subdacted slab beneaththe Hellenic trench. Averaged over ~ 13 Ma, their estimatedextension rate of 30-40 mm yr–1 is comparable to that ofJackson [6], who determined an extension rate of about 30-38 mm yr–1 for the same area from the spatial distribution of

seismic moment tensors. Le Pichon and Angelier [3] arguedthat the eastern margin of the province is expanding at a rateof 20 mm yr–1, which is less than that which they estimatedfor the Aegean Sea area as a whole. Eyido…an [27] summedseismic moment tensors for normal faulting earthquakes thatoccurred in southwestern Turkey between 1943 and 1983,and concluded that the extension rate is about 13.5 mm yr–1,significantly less than that in the Aegean Sea area.McClusky et al. [28] estimated an extension rate of approx-imately 30 mm yr–1 in the central Aegean from globalpositioning system (GPS) receivers. Extension rate is great-est beneath the central Aegean Sea, and it decreaseseastward beneath Turkey [1, 3, 22, 28-32]. In a simple butrevealing plot of normal fault heave along section linesacross parts of southwestern Turkey, Westaway [30] showedthat the eastward decrease in overall extension is linear.

* Corresponding author.E-mail address: ealtunel@ogu.edu.tr (E. Altunel)

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Regional extension has given rise to a distributed horstand graben topography that characterises most of southwest-ern Turkey (Fig. 1). This active continental extension alsoproduces some local near-surface extension fractures in theregion. For example, numerous active and inactive fissure-ridge travertines are located in the Denizli Basin, southwest-ern Turkey (Fig. 2). These fissure ridges contain a centralfissure along their long axes, and Altunel and Hancock [33,34] and Hancock et al. [35] showed that central fissureswithin Late Quaternary fissure-ridge travertines strike atright angles to contemporary directions of horizontal exten-sion. Thus, fissure-ridge travertines may be used forestimating horizontal stretching caused by continental exten-sion. This paper presents field characteristics and isotopicages of travertine-ridge central fissures in the Denizli Basin,and considers the implications of the obtained results forestimation of horizontal stretching.

2. Tectonic and geological setting

The E-W-trending Büyük Menderes Graben meets theNW-SE-trending Gediz Graben near the eastern limit of thesouthwestern Turkey extensional province (Fig. 1). At thejunction of these grabens, there is a Neogene-Quaternarybasin called, by Westaway [29, 30], the Denizli Basin. The

Denizli Basin is limited by topographic escarpments: to thesouth by a NE-facing one about 2000-m-high, and to thenorth by a SW-facing one roughly 700-m-high; these escarp-ments have been interpreted by Koçyi…it [36], ¥aro…lu et al.[37] and Westaway [29, 30] as expressions of active normalfaults. The principal active normal faults in the rapidlyextending area of southwestern Turkey generally strike E-W, but grabens locally trending NW-SE and NE-SW alsooccur (e.g. [8, 10, 12, 14, 30, 38]. The extensional neotec-tonic phase of southwestern Turkey has been the subject ofmuch debate (e.g. [1-6, 8-23, 25, 39-41]). However, it isagreed that regional extension has given rise to a distributedhorst and graben topography that characterises most ofsouthwestern Turkey.

There are four main rock associations in the DenizliBasin. They are: (1) pre-Neogene basement metamorphicrocks, including dominant marbles and subordinate schistsof the Palaeozoic Menderes Massif (cf. [41-43] and refer-ences therein); (2) Neogene terrigenous clastic sedimentaryrocks, together with associated massive and shelly lime-stones; (3) Quaternary fluvial and colluvial sediments; and(4) Quaternary travertines. Contacts between these rockassociations around the Denizli Basin are mainly uncon-formities, but the contacts between the basementmetamorphic rocks and Neogene rocks, and between Neo-gene rocks and Quaternary materials, are faulted, with

Fig. 1 Simplified tectonic map of Turkey showing the major neotectonic structures of Turkey (from Bozkurt [10]).

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normal downthrows to the south on the northern side, and tothe north on the southern side of the basin (Fig. 2).

The Quaternary part of the basin extends NW-SE but nearits eastern end, it is oriented approximately E-W (Fig. 2).The NW-SE-trending and actively filling basin is boundedby Neogene rocks both on its southwestern and northeasternsides (Fig. 2). The average elevation of about 350 m abruptlydrops by about 100 m to the flat surface of the basin on itssouthwestern side. The northeastern margin of the flat-floored basin is a roughly 100-m-high, SW-sloping escarp-ment, rising from the valley floor at about 250 m above sealevel. According to ¥aro…lu et al. [37] and Westaway [30],these escarpments in the south and north reflect locations ofactive normal faults.

The actively subsiding, flat-floored Denizli Basin gentlyrises to the southeast until the southeastern end of the basinwhere it is bounded by a gentle W-facing slope that risesfrom about 300 m to 400 m above sea level. To the east ofthis W-facing slope, there is an E-W-trending subbasin

(Fig. 2). The south side of this subbasin is bounded by ahigh, N-facing escarpment which exposes a more than 500-m-long and 50-m-high striated fault surface in limestones.Westward, this fault passes into an area of uplifted Neogenesediments [29], the scarp dying out about 1 km farther west.To the north of this subbasin, a steep S-facing slope risesfrom the nearly flat floor at about 450 m, to 700 m above sealevel. The eastern end of the northern margin of the basin isbounded by a SW-facing normal fault which separates Neo-gene clastic sediments from pre-Neogene limestones;however, the fault is not exposed toward the west, probablyhaving been buried by travertines [44].

Figure 2 shows the distribution of fissure-ridge traver-tines in the Denizli Basin. Active and inactive fissure-ridgetravertines are located on the hanging wall of the Pamukkalerange-front fault at the northern margin of the basin. Fissure-ridge travertines near the eastern end of the basin (aroundKocaba‚) are located on the flat floor of the basin and areinactive.

Fig. 2 Sketch map of the geology of the Denizli Basin. (from Altunel and Hancock [33]).

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3. Fissure-ridge travertines

3.1. Introduction

Fissure-ridge travertines are deposited by hot waters issu-ing from springs within fissures that cut both underlyingbedrocks and overlying travertines. Travertine is precipatedon both fissure walls (fissure travertine) and on the groundsurface (bedded travertines) [44]. Fissure-ridge travertinesform above extensional fissures which are located in thehanging walls of normal faults [33], in step-over zonesbetween fault segments [45], or in active (or recently active)volcanic provinces [46]. In the Denizli Basin, fissure-ridgetravertines are located in the hanging walls of normal faults(Fig. 2). Figure 3a shows a typical fissure ridge comprisingflanking bedded travertines dipping away from the fissure,and a nearly vertical and slightly sinuous (in plan) centralfissure, partly filled by fissure travertine.

Bedded travertines form as a result of degassing duringthe flow of surface waters away from the source fissure.Thus, the sloping sides of ridges adjacent to fissures are

gradually erected [47, 48]. The formation of fissure traver-tine involves the transport of carbonates in hot-watersolutions from a source region to the zone of deposition inthe fissure. Figure 3b shows a typical fissure travertine occu-pying a nearly vertical fissure. Such travertines are generallybanded, and parallel or subparallel to fissure walls. Thick-nesses of individual travertine bands range from a fewmillimetres to a few centimetres. Bands are accreted on fis-sure margins and grow towards fissure centres and, thus,fissure travertine is younger in the centres of fissures than attheir margins [44].

3.2. Pamukkale travertine-filled fissures

Numerous active and inactive fissure-ridge travertines arelocated in a NW-trending zone to the northwest ofPamukkale (Fig. 4a). Central fissures, partially filled bybanded travertine, roughly follow ridge crests and trend in avariety of directions, but mainly E-W and NNW-SSE(Fig. 4a). N-S- and NE-SW-trending ridges also occur, butare relatively small and few in number.

Fig. 3 (a) An inactive fissure ridge with a central fissure along its crest(arrows). Bedded travertine dipping gently away from the central fissure.(b) Banded travertine in the central fissure of a ridge. The total width of thefissure is about 50 cm at the surface and increases with depth. Dashed whitelines show the margin of the fissure. (c) An irregular central fissure. Themargins of the fissure match as a result of dilation normal to the fissuretrend.

b

a

c

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The distinctive characteristic of the Pamukkale centralfissures is that they are open (Fig. 3c). The widths of theopenings vary, ranging at the surface from about 2 cm to afew metres. The widths of central fissures are at a maximumnear the mid-points (in plan) of the ridges, but decreasetowards both ends [34, 35]. Thicknesses of these fissure tra-vertines also vary in width. At the surface, fissure travertinesare generally a few millimetres to 2 cm in width on active oruneroded ridges. However, where ridges have been eroded,fissure travertine widths at the surface are up to severalmetres. The thicknesses of fissure travertine are at a maxi-mum near the mid-points (in plan), and decrease towardsboth ends.

3.3. Kocaba‚ travertine filled fissures

The Kocaba‚ fissure-ridge travertines are mainly locatedon the level surface of the eastern part of the Denizli Basin(Fig. 4b). They are inactive and, although some fissures arenot well-exposed, most ridges have central fissures alongtheir crests. Their long axes mainly trend NW-SE, but a fewE-W-trending ridges also occur (Fig. 4b). These fissuresmainly contain vertical banded travertines and, unlike thoseof the Pamukkale area, are less open. The width of fissuretravertines ranges from about 20 cm to a few metres at the

surface. Like the Pamukkale fissures, they are at their maxi-mum widths close to their mid-points, and each fissurebecomes narrower towards its ends.

3.4. Th/U isotopic ages of fissure travertines

According to Sturchio ([49], p. 631), the 230Th/234U dise-quilibrium method, using a-spectrometry techniques, hasbeen shown to be useful for dating travertines generally inthe age range from ~ 5 to 350 ka (reviewed by Schwarcz[50]). In the present study, fissure travertines were dated bythe Th/U dating method in a laboratory of the Department ofGeography, University of Bristol. The following assump-tions were made during dating: (1) the entire carbonatesample crystallises immediately from the same solution; (2)there is no 230Th incorporated in the crystal lattice on depo-sition, and correction for detrital 230Th is generally needed ifthe 230Th/232Th ratio is less than 20; (3) the system shouldremain closed to the migration of uranium and thorium afterdeposition. Samples were collected from fissure travertinesbecause they grew from fissure margins towards fissure cen-tres, and are thus younger at the centres of the fissures thanat their margins (Fig. 5). Figure 4 shows locations ofsamples, and numerical results of the dating are set out inTable 1.

Fig. 4 Maps showing locations of samples collected for uranium-series age determination. (a) Pamukkale area, (b) Kocaba‚ area.

a

b

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Fig. 5 Schematic cross-section of an eroded fissure ridge showing locations of sampled parts. Solid horizontal line shows location of the core, anddetails of the banded travertine are given below in scale.

Table 1Th/U disequilibrium age determinations from fissure travertines in the Denizli Basin, western Turkey.

Sample No. U (mg/g) 234U/238U 230Th/234U 230Th/232Th Age, ka

1m1c

0.128 ± .0020.035 ± .001

1.27 ± .0151.26 ± .031

0.418 ± .0150.266 ± .013

413.121 ± 347.47690.050 ± 47.482

57.4 ± 3.633.2 ± 2.5

2m2c

0.461 ± .0120.418 ± .009

1.15 ± .0211.19 ± .017

0.487 ± .0160.401 ± .010

33.319 ± 2.89174.407 ± 7.714

71.1 ± 4.054.8 ± 2.2

3m3c

0.087 ± .0010.080 ± .001

1.38 ± .0181.44 ± .017

0.241 ± .0070.194 ± .006

44.034 ± 7.1760.194 ± .006

29.5 ± 1.421.5 ± 1.0*

4m4c

0.086 ± .0030.063 ± .001

1.36 ± .0371.33 ± .022

0.870 ± .0330.610 ± .014

23.153 ± 1.46571.502 ± 7.466

188 ± 2797.1 ± 5

5m5c

0.213 ± .0070.182 ± .003

1.38 ± .0401.36 ± .012

0.712 ± .0320.613 ± .012

24.093 ± 2.580122.869 ± 12.796

124.3 ± 1597.5 ± 5.0

6c 0.020 ± .001 1.31 ± .040 0.273 ± .020 18.274 ± 4.709 34.9 ± 4.0

7m7c

0.032 ± .0010.008 ± .001

1.29 ± .0381.05 ± .099

0.659 ± .0241.10 ± .08

11.927 ± 1.0912.776 ± .1322

> 400105 ± 9.8

Notes: Uncertainties are one standard deviation (1σ) derived from counting statistics. Samples 1-6 are from Pamukkale fissure travertines, 7m and 7c are from Kocaba‚ fissuretravertines. m is from margin, c is from centre of fissure deposit.* corrected age.

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4. Discussion

The travertines of the Denizli Basin are located at the junc-tion of the E-trending Büyük Menderes and NW-trendingGediz grabens, two of the most active structures in westernTurkey. As Figure 4 shows, fissures roughly follow ridgecrests, trending in a variety of directions but mainly betweenNW-SE and E-W. Fissures following ridge crests have beeninterpreted by Altunel and Hancock [33, 34] and Hancock etal. [35] as extension fractures. As is well known (e.g. [51]),extension fractures form perpendicular to the direction ofstretching and, thus, it is possible to infer that the direction ofhorizontal extension is oriented between NE-SW and N-S inthe Denizli Basin. The NE-SW and N-S extension directionsmirror those of the two dominant directions of extensionresponsible for the opening the NW-trending Gediz and W-trending Büyük Menderes grabens, respectively. Althoughthe Denizli Basin is at the eastern end of the Büyük MenderesGraben, the northern boundary fault of the basin is an exten-sion of the NW-trending Gediz Graben at its southeastern end(Fig. 2). Therefore, the dominant NW-SE trend of the fissuressuggests that the influence of the Gediz Graben stress field isgreater than that of the Büyük Menderes Graben. The local-ised N-S extension direction presumably reflects the localinfluence of the Büyük Menderes Graben.

The width of the fissure travertines in the central fissuresvaries from a few centimetres in active or uneroded ridges toseveral metres in eroded ridges. In addition, inactive ridgesare open to varying extents along their long axes (Fig. 3c).Furthermore, detailed examination of banded travertinesshows that some secondary voids or veins develop within themassive, banded travertines (Fig. 5). These observationsindicate that the central fissures have been widening sincethe initiation of the fissures. Both field relationships and iso-topic ages of travertines from the margins and centres ofthese fissure deposits (Table 1) indicate that the fissures areolder along their margins than at their centres; thus, it seemsclear that the obtained ages are reliable. The time of cessa-tion of travertine deposition may be obtained by datingtravertine at the centre of a fissure. By contrast, travertine atthe margins of fissures at the surface can provide only anapproximate older age for the ridge because fissures becomewider and their margins older with depth [34, 35, 44, 52].Despite this drawback, fissure dilation rates during and aftertravertine deposition can be determined.

A provisional estimate of average dilation rates may beobtained, assuming that the deposition rate on opposing fis-sure walls is equal to the rate of fissure dilation. On the basisof this assumption, an average dilation rate during deposi-tion may be determined by dividing the total thickness oftravertine deposited in a given time interval by the length ofthat time interval. Furthermore, because the central fissuresof ridges are open, an average dilation rate after depositionmay also be estimated by knowing the fissure width and theamount of time elapsed since travertine was last deposited.The results of such calculations are given in Table 2.

Figure 6 plots averaged dilation rates for the Pamukkaleand Kocaba‚ areas. It is possible from dated samples to recog-nize that dilation rates were greater during travertinedeposition than after deposition stopped. For example, indi-vidual fissures have been dilating at average rates of between0.008 and 0.1 mm yr–1 during travertine deposition, and~ 0.001 and 0.07 mm yr–1 after travertine deposition ceased(Table 2). Considering this difference in dilation rates duringand after travertine deposition, it may be concluded thatextension is not linear. It is also notable that the rate duringtravertine deposition in the Kocaba‚ area is at least an order ofmagnitude less than at Pamukkale. This decrease in dilationrate may be related to decrease in total extension from west toeast, as noted by previous workers (e.g. [1, 3, 22, 28-32]).

Westaway [30] estimated total extension from heave val-ues of normal faults across different parts of the DenizliBasin (Fig. 7a), and showed that it is about 4,000 m at thewestern end of the basin, 2,200 m in the Pamukkale area, andabout 1000 near the eastern end of the Denizli Basin. West-away’s [30] view was that at least half of this extension hasoccurred since about 4 Ma and, thus, extension rates of0.5 mm yr–1, 0.3 mm yr–1 and 0.2 mm yr–1, respectively,apply along these section lines.

The ridge-crest fissures of the Denizli Basin are ideal phe-nomena for estimating the contribution to horizontalstretching made by extension fracturing. Although every fis-sure-ridge has not been dated, a time averaged extension rateacross different parts of the Denizli Basin can be estimated,knowing total fissure widths (i.e., total dilation) and usingavailable dates. Total horizontal extension can be roughlyestimated as ~ 120 ± 20 m in the western part (about 5 km Wof Pamukkale), 78 ± 22 m near the centre (about 3 km W ofPamukkale), and about 45 ± 5 m in the eastern part (inPamukkale) of the basin. Extension values for this part of thebasin are 0.6 mm yr–1 at the western end of the basin,0.39 mm yr–1 near the centre and 0.23 mm yr–1 acrossPamukkale, determined from fissures. Assuming that theonset of deposition was about 400 ka in the Kocaba‚ area,the amount of extension was about 10 m over 400 ka in that

Table 2Average dilation rates during and after travertine deposition at Pamukkaleand Kocaba‚ (see Figure 4 for locations).

Sample No. w (mm) v (mm) dd (mm yr–1) ad (mm yr–1)

1 450 2,150 0.0375 ± 0.007 0.065 ± 0.005

2 800 1,800 0.094 ± 0.03 0.033 ± 0.001

3 400 1,500 0.10 ± 0.02 0.071 ± 0.003

4 1,600 850 0.035 ± 0.01 0.009 ± 0.0004

5 1,500 2,000 0.111 ± 0.03 0.020 ± 0.001

6 nm 3,600 – 0.102 ± 0.012

7 2,600 ~ 150 < 0.008 < 0.001

w: width of banded travertine, v: width of void, dd: dilation during deposition, ad:dilation after deposition, nm: not measured.

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area; that is, a rate of 0.025 mm yr–1. In Table 3, the esti-mated extension rates from ridge-crest fissures investigatedduring this study are compared to those derived from heavevalues by Westaway [30].

The central fissures are open both in the Pamukkale andKocaba‚ areas, but the Kocaba‚ fissures are less open. Tak-ing into account this observation and the extension ratesdetermined both in this study and by Westaway [30], themost significant feature is the eastward decrease in extensionover a distance of 40 km along the Denizli Basin. The east-ward decrease in horizontal extension, as determined fromridge-crest fissures in the Denizli Basin, allows the predic-tion that extension should die out about 30 km east of thePamukkale area (Fig. 7b), a notion that accords well withWestaway’s [30] proposal that total extension dies out nearthe eastern end of the Denizli Basin.

5. Conclusions

Fissure-ridge travertines are ideal phenomena for estimat-ing the contribution of extension fracturing to horizontalstretching. Knowing total fissure widths and the ages of thefissures, it is possible to estimate extension rates in an area.In the present study, although every fissure ridge in the Den-izli Basin was not dated, a time-averaged extension rate wasestimated using available dates. Extension rates across dif-ferent parts of the Denizli Basin were determined fromfissure-ridge travertines as 0.6 mm yr–1 near the western endof the basin, 0.39 mm yr–1 near the centre, 0.23 mm yr–1

across Pamukkale, and about 0.025 mm yr–1 near the easternend of the Denizli Basin. Extension rates estimated fromridge-crest fissures investigated during this study accordwell with those derived from the heave values of Westaway[30].

Acknowledgements

A major part of this paper was a part of E. Altunel`sPh.D. thesis, completed in Bristol University (UK) underthe supervision of Prof. P.L. Hancock, with a scholarshipfrom the Turkish Ministry of National Education. U-seriesdating was carried out in Peter Smart`s laboratory in theGeography Department, University of Bristol, during E.Altunel`s Ph.D. programme. Horizontal core from bandedtravertine (Fig. 5) was taken and examined by a USGS team

Fig. 6 Variations of fissure dilation rates within travertines of the Denizli Basin. (a) Location of the Pamukkale and Kocaba‚ areas, (b) and (c) mapsshowing averaged dilation rates of selected ridge-crest fissures in the Pamukkale and Kocaba‚ areas, respectively.

cb

a

Table 3Estimated extension rates across different parts of the Denizli Basin.

Subareafrom normal fault heave

(Westaway [30])(mm yr–1)

from central fissures(this study)(mm yr–1)

~ 20 km W of Pamukkale 0.5

~ 5 km W of Pamukkale 0.6 ± 0.1

~ 3 km W of Pamukkale 0.39 ± 0.11

Pamukkale 0.3 0.23 ± 0.03

~ 15 km E of Pamukkale 0.2

~ 20 km E of Pamukkale 0.025

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leaded by Dr. Ike Winograd. Field observations werereviewed and improved in 2001, and field trips were fundedby Osmangazi University. We would like to thank JamesJackson and an anonymous referee for their reviews of thepaper. We are grateful to Prof. Erdin Bozkurt for invitingthis contribution.

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a b

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