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Tectonophysics 379 (2004) 183–198

Striated and pitted pebbles as paleostress markers: an example from

the central transect of the Betic Cordillera (SE Spain)

Patricia Ruano*, Jesus Galindo-Zaldıvar

Departamento de Geodinamica, Universidad de Granada, Fuentenueva s/n, 18071 Granada, Spain

Received 19 May 2003; accepted 3 November 2003

Abstract

Striated and pitted pebbles provide scarce structures that preserve information on the stresses that their host rocks have

undergone. This information can be obtained by the measurement of a large number of microfaults with striae and solution

marks within a small rock volume. For non-rotational deformation, the statistical procedures for microfault analysis provide a

valid tool for determining the overprinting of successive stress ellipsoids, including their axial ratios and the orientations of the

main axes. The trends of compressions obtained from striae can be compared with the determinations from the pole of pebble

solution pits. However, in complex tectonics settings, the solution pits of several deformation phases are mixed and only striae

analysis allows overprinted paleostresses to be accurately distinguished. The analysis of several pebbles from the same outcrop,

including five from moderately complex settings, allows determination of the homogeneity of the paleostresses at outcrop scale,

the detection of redeposited pebbles, and supports the results of microtectonic analysis for large areas. Solution mark

distributions on pebbles depend on the burial and tectonic stresses. Conglomerates from shallow levels, such as those from

Quaternary fluvial terraces, only record horizontal compressional solution marks because the minimum vertical stress needed to

develop these structures are not reached by burial.

In the central Betic Cordillera, striated and pitted pebbles are composed of carbonate surrounded by a matrix containing

siliciclastic elements. The study of several outcrops located across a transect of the Cordillera shows a change in the recent stress

field. While conglomerates near the Internal–External zone boundary show extensional stresses that may be related to the uplift

of the Cordillera since Tortonian times, the outcrops located in the External Zone and up to the mountain front indicate the

existence of horizontal NW–SE and NE–SW compressions related to prolate ellipsoids. These two compression directions,

which affect conglomerates up to the Quaternary in the same outcrop, may be produced by a local permutation of stress axes,

which in general indicates NW–SE compression related to the Eurasia–Africa plate boundary convergence, but which locally

may switch to an orthogonal compression.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Striated pebbles; Pitted pebbles; Paleostress determination; Betic cordillera; Stress permutation

0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tecto.2003.11.001

* Corresponding author. Tel: +34-958243351; fax: +34-

958248527.

E-mail address: pruano@ugr.es (P. Ruano).

1. Introduction

Striae, solution pits, and polished surfaces on

pebbles, in spite of their low abundance, have been

recognized since the end of the 19th century (Werner,

1802; Hughes, 1895). Such features have generally

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198184

been grouped under the collective heading of striated

pebbles, because they generally develop together and

until the last quarter of the 20th century, work was

mainly focussed on their origin (Judson and Barks,

1961). Although there are many processes that may

Fig. 1. (a) Geological setting of the Betic-Rif Cordillera. (b) Location of stu

(A), Manzanil (M), Trinchera (T), Tocon (TO), Pinos (P), Trevenque (TR

produce striae, such as wind or water currents, an

origin due to tectonic motion or compaction has been

suggested for most examples. Polished surfaces and

striae can form as a consequence of the relative

motion of small sized abrasive matrix particles against

died outcrops in the central transect of the Betic Cordillera. Alhama

), Alcala (AL) and Jaen (J).

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 185

the pebble surface. The degree of polish is higher on

planar surfaces, as well as in hard pebbles in contact

with fine grained matrix (Clifton, 1965).

Solution marks are found on the contact between

carbonate pebbles and result from pressure solution

processes (Sorby, 1863; Trurnit, 1968). The develop-

ment of solution pits in carbonate pebbles requires a

minimum pressure of 10 MPa (McEwen, 1978).

Photoelastic experiments indicate that the stress

increases above the mean stress in the contacts among

neighbouring pebbles (e.g. Stromgard, 1973; Ramsay

and Lisle, 2001), and this could facilitate pressure

solution. Additionally, the siliciclastic matrix particles

may create solution pits in the pebble surfaces (Este-

vez et al., 1982). The trend of the maximum com-

pression can be determined from solution marks that

coincide with the pole of convergence of the striae on

pebbles (Sanz de Galdeano and Estevez, 1981;

Schrader, 1988). These structures have been used in

regional studies, such as in the Nice Arch area

(Campredon et al., 1977) and within the Betic Cor-

dillera (Sanz de Galdeano, 1980; Estevez et al., 1982).

The surface of the pebbles represents a weak

discontinuity that aids brittle deformation. Faults that

affect conglomerate layers have segments that are

preferentially located along pebble surfaces and can

be used for paleostress determinations (Petit et al.,

1985; Hippolyte, 2001). In these cases, the striae in

the pebbles are restricted to the surfaces that are in

contact with the fault. The study of the striae orienta-

tions on pebble surfaces has been mainly developed

from a theoretical point of view (Schrader, 1988;

Taboada, 1993). Although, at first glance, striae all

around a pebble can be seen as forced, being incom-

patible with Bott’s equation, when matrix has a brittle

behaviour, theoretical models have been based on the

Bott (1959) equation and consider a single stress state

that affects a pebble of spherical shape. In non-

rotational deformation settings, elements of symmetry

for striae orientations coincide with the main axes of

stress and strain ellipsoids and the striae pattern varies

relative to the axial ratio of the stress ellipsoids

(Schrader, 1988; Taboada, 1993). However, these

authors did not develop practical applications for

paleostress determinations. The only extensive at-

tempt to determine paleostress ellipsoids from striated

pebbles was done by Rodrıguez-Pascua and de Vice-

nte (1998) in conglomerates from the Iberian Moun-

tain range (NE Spain) and this was unsuccessful due

to the scarce amount of measured data and hetero-

geneities that produce poor analytical solutions.

The main aim of this contribution is to analyze, in

practice, the suitability of striated and pitted pebbles

developed in non-rotational deformation fields for

determining paleostresses in moderately complex

regions and discuss the problems that arise. A second

goal of this study is to present new data on recent

paleostresses across the Betic Cordillera that may

contribute to understanding of the mechanisms of

deformation that have been active during its recent

evolution.

Striated pebbles have been found in several outcrops

located across a central transect through the Betic

Cordillera (Fig. 1). The Betic and Rif Cordillera are

Alpine mountain chains located in the western Medi-

terranean at the Eurasian–African plate boundary.

In this region, there has been recent NW–SE conver-

gence of about 4 mm/year (DeMets et al., 1990). The

Betic Cordillera shows a complex evolution that

includes post-Tortonian uplift related to normal

faulting in the Internal Zone and mainly NW–SE

compression, in the External Zone (Galindo-Zaldıvar

et al., 1993; Ruano et al., in press), as in the

foreland (Jabaloy et al., 2003). The present-day

stress field reconstructed from earthquake focal

mechanisms indicates complex heterogeneous

stresses, including simultaneous extension and com-

pression in neighbouring areas (Galindo-Zaldıvar et

al., 1999; Ruano et al., 2000).

2. Geological setting of striated and pitted pebble

outcrops

The Betic Cordillera is formed by Internal and

External zones, which are differentiated by the degree

of metamorphism and the age of the rocks (Fig. 1a).

The Internal Zone is formed by the superposition of

several metamorphic complexes extending back to the

Paleozoic. ‘‘Flysch’’ units crop-out between the Inter-

nal and External zones. The External Zone is formed

by mainly carbonate rocks of Mesozoic and Cenozoic

age grouped in several tectonic units.

In order to determine the recent stress field across

the chosen transect of the Betic Cordillera (Fig. 1),

conglomerates belonging to Neogene Basins, with

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198186

ages ranging from Burdigalian up to the Quaternary

have been examined to see if they contain striated

pebbles. We have studied conglomerates from differ-

ent tectonic settings, ranging from near the Internal–

External zone boundary to the foreland basin. These

conglomerates have undergone a variable burial and

tectonic history, which allows one to check the meth-

odology. All the striated pebbles are carbonates,

mainly limestone, and in general, they show a low

degree of cementation.

The southernmost outcrops are located in the

Granada depression (Fig. 1b), which was filled by

Miocene and Quaternary detrital rocks, marls, calcar-

enites and lacustrine limestones. This depression may

be considered as a half-graben bounded to the north

and east by normal faults (Ruano et al., 2000).

The Trevenque outcrop (Fig. 1b) is located in

Tortonian conglomerates interlayered with marine

calcarenites belonging to the basal levels of the

Granada Basin fill and is located within the Internal

Zone of the Betic Cordillera. It is a pebble-supported

conglomerate, with bedding dipping at a low angle to

the northwest, and the pebbles consist mainly of

marble. The matrix consists of carbonate and silici-

clastic components. About 15 pebbles from this out-

crop have been observed in detail and show similar

structures. Marine Tortonian rocks cover the Exter-

nal–Internal zone contact and reach heights of more

than 1500 m above sea level, providing evidence for

fast and recent uplift of the Cordillera. This conglom-

erate has been buried by nearly 1000 m.

The Alhama de Granada outcrop, where the bed-

ding is horizontal, consists of Late Tortonian–Messi-

nian conglomerates and is located near the contact

between External and Internal zones of the Cordillera.

In this outcrop of mud-supported conglomerates, 12

pebbles were selected for striation analysis. Taking

into account their stratigraphic position, the maximum

burial undergone by these rocks was several hundreds

of meters but less than in the Trevenque outcrop.

Several outcrops (Manzanil, Trinchera, Tocon and

Pinos; Fig. 1b) are located within Plio-Quaternary

conglomerates in alluvial fans and fluvial terraces on

the northern border of the Granada depression. Sanz

de Galdeano (1980) and Estevez et al. (1982) first

identified the Tocon and Pinos outcrops. All these

outcrops lie near a major fault, that has a normal slip

component and forms the northern boundary of the

Granada depression. The load supported by these

rocks was very low, with a maximum burial of several

tens of meters. These rocks consist of pebble- and

matrix-supported conglomerates and the bedding is

subhorizontal, except in the Pinos outcrop, where dips

of 37j towards the ESE are preserved. The number of

studied pebbles ranges from five to six in each

outcrop, except in Manzanil, where measurements

from eight different pebbles have been integrated.

The Alcala la Real depression is a complex syn-

form filled by Neogene and Quaternary rocks located

on rocks from the External Zone. Striated pebbles

have been found in a pebble-supported conglomerate

of Late Burdigalian–Early Langhian age, with bed-

ding dipping 22j towards the NNE and that has

undergone a maximum burial about 400 m. Only

one pebble was studied due to the scarcity of striae.

The Jaen outcrop, in the front of the mountain

range, is located in a fluvial Late Messinian to

Pliocene conglomerate, where the bedding is subhor-

izontal. It is difficult to estimate the maximum burial

that may reach a hundred of meters. Five pebbles have

been analyzed from this mud-supported conglomerate.

The present-day mountain front does not seem to be

very active, because it has a high sinuosity and recent

deformations are not intense (Ruano et al., in press).

3. Surface structures on pebbles and paleostresses

The pebble surfaces from these outcrops are pol-

ished and contain solution pits and striae (Figs. 2 and

3). The degree of polish varies independently of the

tectonic context. In general, the degree of polish is

higher in hard pebbles surrounded by a fine grained

matrix. Solution pits were identified on most of the

pebbles (Fig. 2b), formed by the interaction of differ-

ent pebbles or small size matrix particles of the

conglomerate, whose surface distribution, quantity,

shape and size varied from pebble to pebble and

between outcrops. Generally, the solution pits are

approximately concentrated on opposed poles of the

pebble (Fig. 2b), suggesting an axis parallel to the

maximum compressive stress (Estevez et al., 1976;

Campredon et al., 1977). In extreme cases, like in the

Trevenque outcrop, rounded solution marks occur

along the whole surface of the pebble (Fig. 2c) The

size of these marks is very variable, with a maximum

Fig. 2. Polish and solution marks on pebbles. (a) Polish surfaces. (b) Solution pits in opposite poles of the pebble. (c) Solution pits over all the

pebble surface (Trevenque outcrop). (d) Example of rounded solution pits. (e) Example of elongated solution pits. (f) Carbonate deposit on

pebble surface.

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 187

around 20-mm diameter and 5-mm depth. The shapes

of the solution marks are also very variable, from

rounded to ellipsoidal (Fig. 2d and e), and appear to

involve transitions to stylolites. Some pebbles show

accumulation of carbonate cement on surfaces per-

pendicular to the solution marks, and reveal the

orientation of the extensional axis (Fig. 2f).

Striae and grooves on pebbles (Fig. 3) have a

maximum length of 30 mm, and generally are less

than 15 mm. While some of the grooves are very

narrow (0.1 mm), others show a variable width that

increases with depth, and the particle that formed the

groove, which is generally quartz, may be preserved at

one end (Fig. 3a). Most of the grooves are rectilinear,

Fig. 3. Striae on pebble surfaces. (a) Striae with insoluble particle at striae end. (b) Striae with ‘V’ shape evidencing two stages of deformation.

(c) Striae with ‘M’ shape, formed by several deformation stages. (d) Convergent striae on pebble surface. (e) Parallel striae on a flat surface of a

pebble. (f) Curved striae on a flat surface of a pebble.

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198188

although curved and angular trajectories have been

also observed (Fig. 3b and c), with ‘V’ and ‘M’

morphologies. In general, the grooves on pebble

surface have a centrifugal trajectory starting from

the solution marks (Fig. 3d).

Behrens and Wurster (1972) proposed a field

procedure to measure striations for spherical pebbles

that considers the sphere center. However, the meth-

odology that has proven most useful for irregular

pebbles consists of the collection of oriented pebbles

from the field, and, after cleaning, repositioning of the

pebbles in laboratory into their field orientation on

plasticine. Once oriented, the tangent planes to the

striated surfaces and the striation trends on the upper

Fig. 4. Striae and tangent plane orientation measurement on pebble surface. (a) After the relocation of the pebble, a transparent flat piece is

placed tangent to the pebble surface over the striae. (b) Orientation of the tangent plane is measured with a compass. (c) The pitch of the striae is

determined with the help of a rectilinear reference line (parallel to the striae) on the transparent flat piece.

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 189

face of the pebble can directly be measured with a

compass helped with a transparent flat rigid piece

(Fig. 4), and the microfault regime can be determined

from the groove asymmetry. After, the pebble is

inverted by rotating it 180j about a horizontal axis,

Fig. 5. Example of striae analysis in pebble Trinc-1 with Etchecopar et al

1988) methods. The determined principal stresses are similar because the

ellipsoid, the odd axis) and in this case, the values of r2 and r3 are close anEtchecopar’s Method; (b) Search Grid Method; (c) tangent plane pole

stereographic projection, lower hemisphere.

the underneath face can also be measured. All meas-

urements are labelled and referred to the field orien-

tation. This procedure enables one to measure the

striae with the best possible illumination. The mea-

surement of the axis connecting the solution marks on

. (1981) and Search Grid (Galindo-Zaldıvar and Gonzalez-Lodeiro,

y are prolate and they have the same orientation of r1 (in prolate

d their orientations have no significance. (a) Paleostress results with

s with striae and solution pit axis (star) on the studied pebble,

Table 1

Paleostress ellipsoids determined in each pebble

Pebble Phase Fit R r1 r2 r3 Assign. Data (N) Total Solution pits

ALHA-1 1 G 0.4 N321jE/78j N82jE/06j N173jE/10j S 25 (3) 29 N335jE/80jALHA-2 1 R 0.21 N254jE/63j N0jE/8j N94jE/26j S 17 (2) 21

ALHA-3 1 G 0.28 N305jE/86j N152jE/04j N62jE/02j S 23 (1) 26 N307jE/75jALHA-4 1 B 0.96 N333jE/54j N238jE/04j N145jE/36j S 14 (6) 20

2 B 0.42 N215jE/32j N14jE/56j N119jE/10j S 5 (1)

ALHA-5 1 G 0.16 N199jE/42j N342jE/42j N90jE/19j S 16 (1) 18 N224jE/35jALHA-6 1 G 0.61 N245jE/18j N115jE/63j N341jE/19j S 17 (5) 23 N224jE/35j

2 G 0.53 N185jE/14j N299jE/58j N88jE/28j 14

0.47 N88jE/28j N299jE/58j N185jE/14jALHA-7 1 R 0.27 N280jE/66j N105jE/24j N14jE/02j S 21 (3) 27 N275jE/65jALHA-8 1 G 0.5 N279jE/63j N86jE/26j N178jE/05j S 29 (3) 35 N225jE/58jALHA-9 1 G 0.64 N303jE/69j N74jE/14j N168jE/16j S 34 (7) 50 sub-vertical

2 B 0.01 N132jE/46j N312jE/44j N42jE/00j S 9

ALHA-10 1 G 0.39 N192jE/83j N68jE/04j N338jE/06j S 19 (5) 27

2 G 0.67 N6jE/50j N221jE/35j N118jE/18j S 10

ALHA-11 1 G 0.3 N231jE/74j N231jE/74j N128jE/04j S 37 (13) 52 N337jE/72j2,3 B – – – – – –

ALHA-12 1 G 0.56 N260jE/65j N33jE/18j N129jE/17j MP 38 (33) 76

0.44 N129jE/17j N33jE/18j N260jE/65j2 G 0.06 N273jE/63j N37jE/16j N133jE/21j MP 31 (12)

0.94 N133jE/21j N37jE/16j N273jE/63j3,4 FD – – – – – –

TOC-1 1 G 0.23 N224jE/04j N316jE/28j N127jE/62j MPF 22 (21) 46 NE–SW

0.77 N127jE/62j N316jE/28j N224jE/04j2 R 0.2 N200jE/67j N318jE/11j N52jE/20j MPF

0.8 N52jE/20j N318jE/11j N200jE/67j 16 (8)

3 R 0.39 N81jE/64j N222jE/21j N318jE/15j MPF

0.61 N318jE/15j N222jE/21j N81jE/64j 9 (2)

TOC-2 1 G 0.06 N65jE/38j N299jE/37j N182jE/30j S 40 (34) 84 N79jE/25j2, 3, 4 B – – – – – –

TOC-3 1 R-B 0.4 N212jE/08j N321jE/57j N119jE/22j MPF 20 (18) 38 NE–SW

0.6 N119jE/22j N321jE/57j N212jE/08j2 R 0.09 N232jE/66j N2jE/16j N9jE/77j MP 13 (6)

0.91 N97jE/17j N2jE/16j N232jE/66j3 B – – – – – –

TOC-4 1 R 0.3 N242jE/02j N333jE/30j N149jE/60j MP 21(13) 38 N67jE/10j0.7 N149jE/60j N333jE/30j N242jE/02j

2 B – – – – – –

TOC-5 1 G 0.07 N242jE/10j N148jE/22j N355jE/66j MPF 47 (44) 106

(several pebbles) 0.93 N355jE/66j N148jE/22j N242jE/10j2 R 0.18 N268jE/16j N13jE/42j N162jE/44j MPF 29 (24)

0.82 N162jE/44j N13jE/42j N268jE/16j3 R 0.22 N198jE/04j N293jE/54j N105jE/36j MPF 21 (10)

0.78 N105jE/36j N293jE/54j N198jE/04j4 B – – – – – –

TRINC-1 1 G 0.01 N309jE/27j N48jE/18j N168jE/57j S 31 (5) 41 N298jE/10j2 R 0.22 N292jE/32j N39jE/24j N158jE/48j S 20

TRINC-2 1 G 0.5 N168jE/15j N7jE/74j N260jE/05j MPF 18 (6) 25

0.5 N260jE/05j N7jE/74j N168jE/15j2 B – – – – – –

TRINC-3 1 G 0.31 N134jE/14j N226jE/08j N345jE/74j MP 19 (8) 28

0.69 N345jE/74j N226jE/08j N134jE/14j2 FD-B – – – – – –

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198190

Table 1 (continued)

Pebble Phase Fit R r1 r2 r3 Assign. Data (N) Total Solution pits

TRINC-4 1 G 0.11 N320jE/05j N27jE/54j N227jE/36j S 17 (7) 26 N108jE/18j2 G 0.21 N133jE/12j N227jE/18j N11jE/68j MP 12 (1)

0.79 N11jE/68j N227jE/18j N133jE/12jTRINC-5 1 G 0.24 N234jE/10j N113jE/71j N327jE/16j MP 41 (27) 78 N220jE/14j

0.76 N32jE/76j N113jE/71j N234jE/10j2 R 0.7 N159jE/72j N288jE/12j N20jE/14j MP 32 (6)

0.3 N20jE/14j N288jE/12j N159jE/72j3 R 0.4 N248jE/10j N346jE/37j N145jE/51j MP 24 (1)

0.6 N145jE/51j N346jE/37j N248jE/10jMANZANIL 1 R 0.03 N158jE/25j N61jE/14j N305jE/61j MP 9 (2) 11

0.97 N305jE/61j N61jE/14j N158jE/25jPP-1 1 G 0.42 N273jE/43j N82jE/46j N177jE/06j MP 32 (58) 92

0.58 N17jE/76j N82jE/46j N273jE/43j2 R-B 0.42 N316jE/34j N69jE/30j N190jE/42j MP 33 (29)

0.58 N190jE/42j N69jE/30j N316jE/34j3 R-B 0.52 N219jE/70j N124jE/02j N33jE/20j MP 25 (17)

0.48 N33jE/20j N124jE/02j N219jE/70j4 G 0.9 N230jE/04j N134jE/58j N322jE/32j S 23(11)

5, 6 FD – – – – – –

PP-2 1 G 0 N135jE/34j N298jE/55j N40jE/08j MP 41 (33) 81 N130jE/43j1 N40jE/08j N298jE/55j N135jE/34j

2 R-B 0.07 N111jE/19j N207jE/18j N338jE/63j S 29 (17)

3 R-B 0.69 N54jE/78j N246jE/12j N156jE/02j S 26 (5)

4 FD – – – – – –

PP-3 1 G 0.01 N122jE/46j N310jE/44j N216jE/04j MPF 55 (34) 103

0.99 N216jE/04j N310jE/44j N122jE/46j2 G 0.98 N38jE/02j N335jE/56j N159jE/34j S 33 (14)

3, 4 FD – – – – – –

PP-4 1 G 0.53 N103jE/56j N202jE/06j N296jE/34j S 20 (13) 38

2 R-B 0.1 N101jE/38j N294jE/52j N196jE/06j S 17 (3)

PP-5 1 R 0.44 N164jE/03j N259jE/58j N72jE/32j MP 29 (25) 55

0.56 N72jE/32j N259jE/58j N164jE/03j2 G 0.76 N104jE/42j N288jE/48j N196jE/02j S 22 (11)

3 R 0.19 N178jE/19j N76jE/32j N293jE/52j S 16 (3)

PP-6 1 R-B 0.82 N5jE/30j N262jE/21j N143jE/52j S 30 (31) 68

2 R-B 0.06 N26jE/48j N248jE/34j N143jE/22j S 22 (14)

3 R-B 0.39 N133jE/08j N230jE/39j N34jE/50j S 23 (4)

4 FD – – – – – –

A-1 1 G 0.07 N311jE/03j N217jE/54j N44jE/36j MP 34 (17) 55 N125jE/55j0.93 N44jE/36j N217jE/54j N311jE/03j

2 G 0.97 N21jE/34j N194jE/56j N289jE/03j MP 26 (7)

0.03 N289jE/03j N194jE/56j N21jE/34j3 FD – – – – – –

JAEN-1 1 R 0.62 N216jE/10j N357jE/77j N125jE/08j MP 30 (12) 43 N243jE/48j0.38 N125jE/08j N357jE/77j N216jE/10j

2 B 0.17 N263jE/30j N60jE/58j N167jE/10j S 18 (3)

JAEN-2 1 G 0.24 N216jE/01j N306jE/84j N126jE/06j MP 21 (12) 33 N220jE/10j0.76 N126jE/06j N306jE/84j N216jE/01j

2 R 0.18 N318jE/24j N113jE/64j N224jE/10j S 15 (5)

3 B – – – – – –

JAEN-3 1 R 0.34 N230jE/02j N90jE/88j N320jE/01j S 39 (31) 74 N43jE/21j2 R 0.07 N68jE/12j N336jE/08j N214jE/76j S 29 (14)

3,4 FD – – – – – –

(continued on next page)

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 191

Table 1 (continued)

Pebble Phase Fit R r1 r2 r3 Assign. Data (N) Total Solution pits

JAEN-4 1 R-B 0.6 N317jE/10j N169jE/78j N48jE/06j MPF 25 (18) 45 N184jE/14j0.4 N48jE/06j N169jE/78j N317jE/10j

2 B – – – – – –

JAEN-5 1 G 0.37 N224jE/02j N123jE/80j N314jE/10j S 32 (35) 68 N19jE/58j2 R 0.32 N300jE/26j N30jE/00j N120jE/64j MP 19 (18)

0.68 N120jE/64j N30jE/00j N300jE/26j3 R 0.17 N66jE/04j N246jE/86j N336jE/01j S 30 (7)

4 FD – – – – – –

Only the main solutions are included. Location of the outcrops in Fig. 1. R, axial ratio ((r2� r3)/(r1� r3)); G, good fit; R, regular fit; B, bad fit;FD, few data; S, sure ellipsoid; MP, more probable ellipsoid; MPF, more probable by field relations; N, non-assigned data.

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198192

the two opposite poles of the pebbles is not exact

because they cover a rounded area. This measurement

may have an error of up to 20j.Paleostress from striated and pitted pebbles may be

determined using the orientation of solution pits and

striae, by qualitative or quantitative analysis. The

location of solution pits and the pole of convergence

of the striae may indicate the r1 orientation (Fig. 3d),

but do not give insights about the axial ratio of the

stress ellipsoid (R=(r2� r3)/(r1� r3)). In addition,

when the entire pebble surface is covered by solution

pits, like in the Trevenque outcrop (Figs. 1 and 2d), it

is only possible to determine that deformation was

constrictional and the orientation of the main axis of

stress ellipsoid cannot be measured. However, axis

orientations and axial ratios may be determined by a

statistical analysis of the striae. In practice, the most

accurate determinations of paleostress need to take

into account structures on pebbles ranging from 5 to

20 cm in diameter in order to do a readily obtainable

measurement. Although the best pebble shapes to use

are those that are rounded, in order to fit the models

proposed by Taboada (1993) and Schrader (1988),

who consider pebble as spheres, irregular shapes also

can produce good results.

Bott (1959) working with microfaults and Taboada

(1993) using a theoretical analysis on striated pebbles

suggested that the orientation of the striae at each

point on the striated surface is parallel to the orienta-

tion of maximum shear stress. If it is so, then, each

stress ellipsoid should be associated with it a typical

pattern of striae on a pebble (Taboada, 1993). Non-

coaxial deformation, involving rotation of a pebble,

theoretically ought to produce a typical hairspring

shaped striae. Striated pebbles of the Betic Cordillera,

however, typically show patterns suggesting that there

has been no pebble rotation and this allows one to

determine the paleostresses. Additionally, the fact that

planar surfaces of the pebble generally contain striae

that are parallel to one another suggests that the

stresses are homogeneous at least at the scale of a

pebble (Fig. 3e), allowing them to be determined

accurately. On one pebble, divergent striae occur on

a plane surface (Fig. 3f), possibly as consequence of

heterogeneous flux in the matrix.

The methods of Etchecopar et al. (1981) and

Galindo-Zaldıvar and Gonzalez-Lodeiro (1988; called

Search Grid method) were initially designed for striae

analysis of microfaults. These methods provide data

on the main axis orientations and the axial ratios of the

overprinted stress ellipsoids, provided the striae on

microfaults are parallel to the maximum shear stress

(Bott, 1959) and that the stress field is homogeneous

in an isotropic body. Both methods try to justify as

many as possible of the measured striae with the

minimum number of overprinted stress ellipsoids.

The main differences between the methods lie in the

procedure used to determine the ellipsoids. The Etch-

ecopar et al. (1981) method is done with a Montecarlo

and later iterative search using only faults from a

known regime. The method of Galindo-Zaldıvar and

Gonzalez-Lodeiro (1988) uses a systematic search on

a grid and involves striae from both known and

unknown regimes. These methods have been applied

for paleostress determinations on striated pebbles (e.g.

Fig. 5), while considering the pebble surface as a

reactivated microfault. The analysis of several repre-

sentative pebbles from each outcrop, generally about

five, allows one to confirm whether stresses are

homogeneous at outcrop scale if the stresses obtained

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 193

in several of the pebbles are similar. A complete

paleostress determination is made on each single

pebble. Several determinations from several pebbles

can be compared at the same locality. If they are

consistent, the confidence in the results is far greater

than with faults.

The results obtained from eight selected pebbles

with enough determination of fault regimes are very

close when the Etchecopar et al. (1981) and Galindo-

Fig. 6. Main stresses determined from the analysis of striated

pebbles in the central transect of the Betic Cordillera. The size of the

arrows indicates the importance of extension. Outcrop initials as in

Fig. 1.

Zaldıvar and Gonzalez-Lodeiro (1988; Search Grid

method) are applied (e.g., example in Fig. 5). Because

most of the pebbles are striated, and the fault regime

has not been determined for all striae, we have used

the Search Grid method to consider these data.

The stress ellipsoids obtained from the pebbles for

each outcrop are summarized in Table 1. In most of

the outcrops, the stresses determined for individual

pebbles are similar for the same outcrop, suggesting

that stresses were generally homogeneous, at least at

outcrop scale. The compression axes determined by

solution pits are in agreement with orientations estab-

lished by striation analysis. However, comparison of

the results from each outcrop reveals a heterogeneous

distribution of paleostress across the transect of the

Cordillera. Over the complete transect moving north-

wards, stresses change from radial to ENE–WSW

directed extension near the Internal–External zone

boundary to N–S oblate stress extension in the

northern Granada depression, where there is also

evidence of prolate NW–SE and NE–SW compres-

sional deformations in Plio-Quaternary rocks; to fi-

nally the External Zone and the mountain front, which

show evidence of both prolate NW–SE and NE–SW

compressive stress ellipsoids (Fig. 6).

4. Discussion

4.1. Striae and solution pit development

The development of striated and pitted pebbles is a

consequence of both the lithology and the tectonic

evolution. In the studied outcrops, siliciclastic par-

ticles of the matrix cause striations and pitting mainly

on pebbles composed of carbonate. In addition, stria-

tions and pitting can be developed on the contacts

between pebbles, particularly in clast-supported con-

glomerates like those in the Trevenque outcrop. The

presence of encrusted insoluble particles at the end of

asymmetric striae on pebble surfaces (Fig. 3a) sug-

gests that both solution and abrasion were active

during the development of these structures. Carbonate

deposits are found on pebble surfaces orthogonal to

the pitted ones indicating that dissolution of surfaces

that have undergone compression has occurred with

precipitation on surfaces of extension. This mecha-

nism of deformation is similar to that involved in the

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198194

development of stylolites and tension gashes. In

practice, it is easier to observe striae and pits (indi-

cating r1) than the cement accumulations (showing

r3), because when the pebble is extracted from the

outcrop, it usually breaks along the contact between

the cement and the pebble surface and any zones of

calcite precipitation are lost. However, the trajectories

of the striae appear to be independent of the mecha-

nism of striation development, depending only on the

tectonic evolution.

The distribution of solution structures on pebble

surfaces is variable in different outcrops. In carbonate

pebbles, a minimum pressure of 10 MPa is needed to

develop solution structures (McEwen, 1978). This

pressure is reached with a load of 300–400 m of

sedimentary rocks with mean densities around 2.5 to

2.6 g/cm3, when there is no confining pore fluid

pressure. The Trevenque, Alhama de Granada, Alcala

la Real and probably Jaen outcrops have probably

reached these conditions. The Trevenque outcrop

shows solution marks over the whole of the pebble

surface and constitutes an extreme example indicating

that this minimum pressure has been overpassed in all

directions. However, the development of striated peb-

bles in conglomerates of fluvial terraces that have

never been subjected to a high load, such as the

outcrop located in the northern part of the Depression

of Granada (Manzanil, Trichera, Tocon and Pinos),

indicates that compressional settings may develop

striated pebbles at shallow levels. In all these out-

crops, horizontal compression may have been also

amplified at the contact between pebbles and with the

insoluble particles of the matrix such that it exceeded

the minimum pressure for solution mark development

that has not been reached due to the vertical burial

stress.

4.2. Paleostress determination

Avigorous debate on the possibility of determining

paleostresses from structural observations has devel-

oped in recent years. Paleostresses cannot be directly

measured in the field, but their consequences are

certainly observed. Bott (1959) argued that the max-

imum shear stress on a plane is parallel to the striae

and this is considered, for most paleostress determi-

nation methods, to be the key assumption from which

the stress direction can be inferred. For the accurate

determination of paleostresses, the structures mea-

sured should not have undergone rotation. In addition,

it is well known that stresses are greatly perturbed

near major faults. Short slip during a deformation

involving little rotation of the pebbles provides the

most favourable setting for paleostress determination,

together with the presence of a large number of

different oriented striae.

Striated and pitted pebbles have been considered a

good marker for the determination of the trend of

maximum compression (Sanz de Galdeano and Este-

vez, 1981) and also, theoretically, as an appropriate

structure to determine paleostress axial ratios

(Schrader, 1988; Taboada, 1993) in single stage

deformations. However, previous attempts to use

striae orientation for paleostress determination

(Rodrıguez-Pascua and de Vicente, 1998), using the

Right Dihedra (Angelier and Mechler, 1977), Slip

Model (Reches, 1983; De Vicente, 1988), Stress

inversion (Reches, 1987; Reches et al., 1992) and

the Delvaux (1993) methods have resulted in very

inaccurate determinations, and this structure is con-

sidered unsuitable for paleostress determinations

from striae.

However, striated pebbles are a suitable structure

for the acquisition of tens, and even hundreds of striae

measures on a curved surface over a very small rock

volume that approaches the conditions necessary for

paleostress determination. Taking into account the

short slip on surfaces, each small area of the striated

surface may be considered, in practice, as a small flat

microfault. Striae can be considered in these small

segments as rectilinear, although they describe curved

trajectories if all striae on the pebble surface are

considered. Fault planes are regarded in this approach

as planes tangential to the pebble surfaces at the

location of each striae. In all the studied outcrops,

stable paleostresses solutions, including the orienta-

tion of the main axes of the stress ellipsoid and the

axial ratios, have been found using the Etchecopar et

al. (1981) and the Search Grid methods, based on Bott

(1959) analysis (Figs. 5 and 6; Table 1) on individual

pebbles. These outcrops are affected by polyphase

deformation because ‘M’ and ‘V’ striae shapes (Fig.

3b and c) have been found indicating two overprinted

deformation stages. ‘M’ striae probably formed by

contiguous ‘V’ shapes. In these cases, rectilinear seg-

ments of ‘M’ and ‘V’ striae have been considered for

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 195

paleostress determination. With respect to pebble

shape, in the more general irregular pebbles, a higher

representation of the flat surfaces is found in diagrams

of striae orientation, but no differences have been

found from stress determinations.

In practice, it is necessary to keep in mind that a

large number of striae are considered from each

pebble, and, therefore, a relatively high number of

stress ellipsoids are required to explain the entirety of

the grooves. Those that explain the greatest number of

striae, determined in the first stages of the analysis, are

considered to have the most tectonic significance.

Those that explain the least number of striae are

considered to be a consequence of measurement errors

or small stress perturbations. To be certain of the

validity of the paleostress determination, at least five

pebbles from each outcrop should be studied to check

the consistency of the results. Our work has shown

this number produces reproducible results in several

pebbles from the same outcrop. This check detects the

presence of any resedimented pebbles, which would

of course give anomalous paleostress determinations.

4.3. Recent paleostresses in Betic Cordillera

The outcrops, of Late Miocene to Quaternary age

in the central transect of the Betic Cordillera, enable

us to contribute to the knowledge of the recent stress

fields and the tectonic evolution of this region. Micro-

fault analyses combined with the activity of recent

structures indicate that while the Internal Zone of the

Betic Cordillera has undergone extensional stresses,

the External Zone is dominated by NW–SE compres-

sion (Galindo-Zaldıvar et al., 1993). Paleostresses for

the southernmost portion of the External Zone and in

the contact with the Internal Zone (outcrop Alhama de

Granada) are extensional and could have been active

during the exhumation of the conglomerate, with

stress ellipsoids of different axial ratios. This is

compatible with the extension observed in other

sectors of the southern Granada depression, like the

Padul–Niguelas fault (Alfaro et al., 2001) and the

eastern prolongation of the normal Zafarraya fault,

southwest of the studied outcrop (Fig. 1). The Tre-

venque outcrop, made up of Tortonian marine sedi-

ments located in the Internal Zone, provides evidence

for deformation as a result of burial, because it

represents the exhumed deep deposits of the Granada

depression filling. Deformation is a consequence of

hydrostatic stresses that are larger than deviatoric

stresses at these depths. Uplift of the Internal Zone

of the Cordillera produced an extensional setting in

the upper crust by gravitational processes, although

this may have been synchronous with the develop-

ment of large thrust structures in the lower crust

(Galindo-Zaldıvar et al., 1997).

Stresses determined from solution marks and striae

in Plio-Quaternary rocks from the northern border of

the Granada depression are radically different, and

indicate prolate NW–SE and NE–SW subhorizontal

compression, in the western and eastern parts, respec-

tively, in spite of being located in the vicinity of the

normal fault that bounds this depression. In the central

segment (Trinchera outcrop), both trends have been

found (Fig. 6, Table 1) and there are no evidences

about their relative chronology. Major structures as-

sociated with these compressions have not been

observed, suggesting that these stresses involved were

of low intensity. The paleostress determinations

obtained from these striated pebbles may provide an

indication of the process suggested to explain the

earthquake focal mechanism solutions in the southern

part of the Granada basin (Galindo-Zaldıvar et al.,

1999). The orthogonal relationship between the trends

of r1, regionally subvertical and locally subhorizontal

with NW–SE and NE–SW trends, suggests that stress

axis permutation, consisting of a switch of the main

axes of the stress ellipsoid (mainly r1 and r2 axes) but

keeping the same orientations, may have been active

in the northern border of the Granada depression. This

process may be related to stress release or local block

interaction. Similar stress permutations have been

described for the development of orthogonal tensional

joint sets (Hancock, 1985). Besides these stresses, the

Pinos outcrop also indicates N–S oblate extensional

stresses associated with the activity of normal faults

on the northern border of the depression.

In the Alcala la Real depression, north of the

Granada depression, a NW–SE horizontal prolate

compression ellipsoid is obtained. This probably

formed under deep conditions, producing distributed

solution marks along pebble surfaces, and, therefore,

probably occurred prior to recent exhumation. The

Late Burdigalian–Lower Langhian age of the Alcala

la Real conglomerate is older than the Plio-Quaternary

conglomerates of the Granada depression and, al-

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198196

though both conglomerates have the same compres-

sion trend, the difference in age does not allow them

to be directly related to one another. However, this

suggests that this compression has recurred during

mountain range development since the Miocene.

In front of the Betic Cordillera, the Late Messi-

nian–Pliocene Jaen outcrop, deformed probably under

several hundreds of meters depth, registers a NE–SW

prolate compression that contrasts with the present-day

regional stresses and with the stresses that have acted

from the Miocene in adjacent areas, such as the Sierra

of Cazorla (Galindo-Zaldıvar et al., 1993). Although

these stresses seem to be anomalous with respect to

regional structures, they are compatible with other

minor structures observed in this mountain front as

sinistral E–O faults.

Data presented in this contribution show that the

recent state of stresses is heterogeneous and affected

by switching principal stress axes, but maintains the

same NW–SE orientation, prevailing extension to-

wards the Internal Zone and compression toward the

External Zone. These stresses are compatible with the

development of the Betic Cordillera in the context of

NW–SE convergence between the Eurasia and Africa

plates. In this context of crustal thickening, compres-

sional stresses are related to the development of large

folds in whole cordillera and thrusting that deform the

External Zone at shallow levels and the Internal Zone

at deep levels, while extension is produced at shallow

depths during the uplift in the Internal Zone.

5. Conclusions

Striated and pitted carbonate pebbles, surrounded

by siliciclastic fragments, constitute useful structures

for paleostress studies. These include the determina-

tion of the orientation of the main axes and axial ratio

of stress ellipsoids, taking into account that in a small

rock volume, it is possible to obtain a high number of

striae orientations. The best practical procedure

involves sampling five pebbles per outcrop for rocks

affected by polyphase deformation of minor complex-

ity, and then repositioning them in the laboratory to

make the measurements. The pebble shape is not

significant in paleostress determination and both

spherical pebbles, as in theoretical models, and irreg-

ularly shaped pebbles may be used. Methods based on

Bott’s (1959) equation, that were initially designed for

microfault analysis, yield accurate paleostress results

for each pebble. The consistency of paleostresses in

different pebbles from the same outcrop confirms the

validity of the results and prevents the possibility of

including redeposited striated pebbles in the samples.

An independent check is provided by the consistency

between the results obtained from striae and those

obtained from solution marks.

The distribution of pits on pebbles depends on the

burial and tectonic stresses. For deep levels, where

burial was greater than 400 m, pits may develop over

the whole of the pebble surface, especially in clast-

supported conglomerates. However, for shallow lev-

els, pits and striae only develop during horizontal

compression, when the maximum compressional stress

exceeds the minimum value required for solution.

The study of several striated pebble outcrops of

Miocene–Quaternary age along a central transect of

the Betic Cordillera provides new insight on paleo-

stress distributions. While the Internal Zone shows

evidence of radial extension, in the northern Gran-

ada depression, there is evidence of N–S oblate

extension orthogonal to a major normal fault. In

addition, NW–SE and NE–SW prolate compression

ellipsoids have been found from conglomerates of

different ages along this border of the Depression

and in the External Zone. These two compression

direction affects rocks as young as the Plio-Quater-

nary fluvial terrace deposits of the northern Granada

depression although there is no clear chronological

relationship.

The paleostresses determined are compatible with a

tectonic model in which uplift of the Cordillera was the

consequence of NW–SE compressional structures

developed by the convergence between the Eurasian

and African plate boundary, which dominate deforma-

tion in the External Zone. In the Internal Zone,

compression at depth produced uplift and extensional

stresses, with subvertical compression at shallow lev-

els probably by gravitational effects. In addition,

switching of stress axes between vertical, NW–SE

and NE–SW horizontal orientations has occurred

which is also suggested by earthquake focal mecha-

nisms in the southern Granada depression and revealed

by striated pebbles from the northern border of the

Granada depression. These data indicate that during

the recent evolution of the Betic Cordillera, the stresses

P. Ruano, J. Galindo-Zaldıvar / Tectonophysics 379 (2004) 183–198 197

have been heterogeneous and not allow the establish-

ment of large tectonic phases for whole the Cordillera.

Acknowledgements

The constructive reviews in the last stage of this

research by Dr. T. H. Bell and Dr. F. Gonzalez Lodeiro

have greatly improved this contribution. Also the

comments by Dr. N. Fry and Dr. L. Arlegui have

enhanced the quality of the paper. Thanks are due to

Dr. A. Jabaloy for the helpful contribution on the field

work. This research was supported by project BTE-

2000-1490-C02-01 of the CICYT.

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