CONICAL FOLDING AND FRACTURE PATTERNS IN THE PRE-BETIC OF SOUTHEAST SPAM

F. MOSELEY

The Pre-Betic Alpine ranges of southeast Spain consist of a series of ENE trending folds developed mostly in Mesozoic and Tertiary limestones. One of these folds, the Mongo Syncline is described here and is of interest because of firstly its anomalous ESE trend, secondly the clearly demonstrable conical form and thirdly the en-echefan arrangement of the minor fold axes which comprise the axial zone of the main syncline. To facilitate interpretation of this structure a con- siderable number of bedding and fracture orientations were taken (the latter mostly joints) over an area of 3 sq. km. The fractures related to the structure of the syncline. reveal particularly important sets of ENE “sinistral” joints and northerly “dextral” joints. The origin of the fold is debatable but the interpretation favoured here is that of a second order drag fold to a major wrench fault in the pre-Triassic basement. In both cases diapiric squeezing of the incompetent Trias, which outcrops as an elongated (NW-SE) inlier adjacent to the Mongo structure is envisaged.

1. INTRODUCTION

The Costa Blanca region of southeastern Spain is characterised by the series of prominent folds which comprise the Pre-Betic ranges. The usual tectonic trend is east-north-east, parallel to the direction of the Betic Cordilleras, but there are several rather interesting exceptions to this general rule, of which the ESE Mongo syncline, where the Pre-Betic zone is finally truncated by the Mediterranean, and the ESE structures of the Sierra de Bernia 25 km to the southwest are two examples (Fig. 1).

The whole of the region between Denia in the northeast and Alicante 80 km to the southwest is reasonably well known geologically, the principal works being those of Darder 1945, and more recently the maps and reports of Rios et al. 1958 and 1961. These publications have proved of great value as a basis for the more specialised investigation described here. Other relevant geological work is listed in the extensive bibliographies to these publications.

The geological succession starts with gypsiferous marls, dark limestones and basic igneous rocks of Triassic age, mostly to be found on the low ground. Stratigraphically above these rocks there are limited Jurassic outcrops, and extensive outcrops of Cretaceous and Eocene limestones which give rise to the mountains (up to 1,500 m) and steep precipices characteristic of the area. Oligocene and Miocene sediments (also mainly limestones) rest unconformably on the earlier rocks but the major tectonic events were post-Miocene. Pliocene to Quaternary gravel fans, mostly limestone gravels, and calcrete deposits form the post-tectonic element.

The structure is complex as shown on the published sections to the Benisa, Altea and Javea sheets (Nos et ul. 1958, 1961 ; Martinez et ul. 1954). The incompetent Trias is largely in the form of diapirs and has tectonic contacts with younger rocks. Therc are numerous northward directed thrusts and folds resulting from the post-Miocene movements whilst the intra Eo-Oligocene movements resulted in an angular dis- cordance between Oligocene and earlier strata.

That part of the Mongo Syncline referred to in this account (Fig. 1) extends about 2 km eastwards from the summit of Mongo (75i m) and includes the steep sides of the mountain, the level plateau of the Plana de Justa, a 200 m erosion surface extending from the foot of Mongo to the cliffs of Cab0 San Antonio, the dissected regions Geol. J. Vol. 6, Pt. 1,1968 97

98 F. MOSELEY immediately to the south ending in the Javea plain (the southern margin of Fig. l), and similar dissected regions to the north towards Denia. The outcropping strata are bedded Cretaceous limestones with subsidiary marl bands and a thin capping of Eocene limestone on Mongo summit (Rios et al. 1961). The account which follows is divided into three parts; Viz a description of the geometry of the fold, the orientation of fractures and a general synthesis.

2. FORM OF THE MONGO SYNCLINE

The general trend of this fold is ESE and it therefore makes an acute angle with the regional ENE trend. It is a conical fold with an en-echelon arrangement of minor folds in the axial region (Fig. l), and is of a form which does not altogether lend itself to description by standard fold terminology. Much of the structure is basin like (Fig. 1) converging to a sharp apical zone in the west (apex is used here with reference to a cone and not to cross-sections of cylindrical folds (Fleuty 1964 463)). In the open basin structure precise orientation of hinge (axis) and axial surface cannot be stated although this is better seen in the apical zone, where axial traces can be plotted for short distances.

If one considers a hypothetical case of a right circular cone (the simplest case), a determination can easily be made of the apical angle, direction of convergence and orientation of the cone axis (Fig. 2). The orientation of the cone in space is thus dehed and orientation of any part of it can be deduced, whereas such terms as fold axis (or hinge) and axial surface are ambiguous. Admittedly it is unlikely that conical folds of such simple pattern will be common, but nevertheless in the cases examined here bedding poles plotted on the stereographic projection are best fitted to small circles and can therefore be interpreted in terms of right circular cones. The “tectonic trends” can thus be illustrated by the azimuths of the cone axes (Haman 1961), constant for the different parts of one cone, whereas “fold trends” may depend on which part of the cone the fold is situated and will be of variable trend.

Details of the Mongo Syncline are shown on Fig. 1, with the conical form brought out strongly by the strike and dip. The open basin like structure in the east narrows down further west to become a series of sharp en-echelon folds, with occasional vertical faults along fold limbs, and small thrusts (Fig. 1, sections A and B). Bedding poles for the whole area are best fitted to a small circle which also indicates the conical nature of the fold (Fig. lX, note that contours were transferred from an equal area to a Wulf€ net, and that the maximum of westerly dipping beds is caused by a predomi- nance of westerly dip values in the extreme east of the map area). Similarly bedding poles for the sharp apical zone (Fig. lZ) fall on a small circle and reveal that the conical nature of the folding persists here also, a fact that is not immediately obvious on the ground. The orientations of the best fitting right circular cones for 1X and 1Z are given on Fig. 2, and whilst the number and spread of dip readings do not permit great accuracy, these two diagrams do show cone axes plunging WNW at between 10” and 30”, and apical angles of between 30” and 60”.

Fig. 1. Strike and dip for the Mongo Syncline (east). Mongo Summit (751 metres) and the San Antonio road junction (SA) shown on main map. Cape San Antonio (A), Cape Nao (N) and Sierra Bernia (B), shown on inset map (top right). X and Z, bedding poles with best fitting small circles and axial trends of cones. Contours (8 and 4ya transferred to Wulff Net from Schmidt Net.

SOUTH NORTH

Metres v C 0 5 0

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N

Fig. 2. Stereographic projections (Wulff Net, lower hemisphere) showing small circles to bedding poles for -1, a hypothetical cone of apical angle 80" (with reference to the bedding planes) with the cone axis rotated from the vertical (A) through 30,60 and 90" (B, C and D). -2 and 3 com- pleted cones for stereograms of Figs. 1X and 12.

3. FRACTURE ORIENTATION The principal object of this part of the study was to determine whether the fracture

pattern was related to regional structures, to the local structure or even to more local structure within the syncline. The great majority of fractures measured were joints, of which by far the greater proportion were high angle joints (greater than 60" dip). The procedure adopted 'was to take approximately 50 measurements at each of the 78 localities, the localities being situated in all parts of the syncline. For purposes of

. analysis the syncline has been divided into four areas, within each of which the dip is --

PLATE 11 Rose diagrams for joints at localities 1-78. (M-Mongo summit, SA-San Antonioroad junction). Equal area projections 1-4 (lower hemisphere) for joint totals. Contours at 8 ,6 ,4 and 2 per cent.

Inset 5. Equal area projection for total low angle joints, dotted circle at 50°, contours a t 15, Bedding poles (B) and mean bedding plane (dotted great circle) are indicated.

10, 5 and 2 per cent.

CONICAL FOLDING AND FRACTURE PATTERNS, SOUTHEAST SPAIN 101

consistently in one direction. This has been done on the assumption that the bedding is likely to exert some influence on joint orientation, and therefore within each area the bedding effect will be more or less the same for all localities, and there will be more chance of uniformity of fracture pattern. The areas are respectively the northeast, south, west and northwest dipping parts of the syncline as shown on the stereograms and by the strike lines on P1. 11. The high angle joint patterns in each of these 4 areas are described below.

(a) High angle joints on northeasterly dipping beds The stereograms of total joints PI. 11.2 reveals three important trends. The most

important is the ENE trend, with the N and WNW trends of second importance. There is some tendency for the joints to be at right angles to the bedding but there are many exceptions to this. The individual localities, shown as rose diagrams for clarity, mostly develop the three sets (above) but with up to 15" deviation from the mean. However, at certain localities it will be observed that either one or two sets only are dominant (e.g. localities 45 and 53), whilst at others minor sets appear which do not show on the stereographic projection totals of P1. 11.2 (e.g. the NE set of locality 51). This is because PI. 11.2 is a combination of many localities with resultant overlap in azimuth between sets and the submergence of minor sets in the total. These minor sets may nevertheless be significant to interpretation.

Referring to the main trends (rose diagrams), the ENE set is normally represented by one strong maximum, but in a few cases there is a double maximum which does not always show up even on the rose diagram of an individual locality because of overlap in the manner mentioned above. This can be effective even within the limits of one small locality.

The tendency towards a double maximum is more common with the northerly joints (e.g. locality 49), but note that with both ENE and northerly trends the double maxima are submerged in the totals of stereogram PI. 11.2. The WNW joints have a single maximum but a variable trend from one locality to another.

(b) High angle joints on south dipping beds The stereogram (Pl. 11.1) shows mainly vertical joints with a strong north trending

maximum and less important WNW, NW and NE trends. It will be noted that the ' strong ENE sets of all other parts of the syncline are practically absent. Individual

localities represented by the rose diagrams nearly all develop the northerly joints, in some cases forming a single strong maximum at localities 1,2,5,6 and 7, but in others giving a double maximum as at localities 4,9, 10 11 and 13 where the two peaks are about 20" apart. The few localities where the northerly joints are not developed are towards the axial zone of the syncline, where patterns tend to be similar to those of the northeast dipping beds (localities 21,22 and 23). Indeed these are the only localities on south dipping beds where the ENE set is developed. The WNW and NE sets are extremely variable, strong at some localities, weak at others, and for this reason do not give a large maximum on PI. 11.1. Others less important sets not showing on P1.11.1 but visible on some of the rose diagrams include the ENE set referred to above and the NW set.

(c) High angle joints on west dipping beds P1. 11.3 suggests a more complex pattern than those already referred to, but

inspection of the rose diagrams shows that the complexity is largely a result of the combination on one diagram of different localities with different patterns The ENE set is the most important one with the joints dipping south at about 80". Second in importance is the NNW set, which with dips of 60" east is nearly at right angles to the

1 02 F. MOSELEY

bedding. Other sets showing on P1. 11.3. are near vertical and trend to the N, NW and WNW.

Individual localities (rose diagrams) vary between simple and complex patterns. For example localities 76, 77 and 78 show only the two sets of ENE dip joints and WNW strike joints both normal to bedding. These are widely spaced master joints which give rise to large cuboidal blocks. Other adjacent localities in contrast have sets intersecting at acute angles and no longer normal to the bedding (e.g. localities 73 and 74).

(d) High angle joints on NW dipping beds P1. 1 1.4 reveals a strong double maximum of easterly and ENE trends. There is also

an important northerly set and less important WNW and NNE sets. Referring to the rose diagrams it will be seen that the easterly double maximum is to some extent caused by the combination of localities with ENE trends (60 and 65) and others with easterly trends (61, 68, 69) and does not entirely represent a true double maximum.

(e) Low angle joints for the whole area Low angle joints form a small proportion of the total but they are nevertheless

important when considering the tectonic history and have been plotted separately ("1. 11.5). Two well defined sets dip at 40 to 45" to the NE (the strongest set) and 45" to the SW and are clearly of different origin to the high angle joints. These joints, and especially the NE dipping ones, are common on south dipping beds and fairly common on west dipping beds. The SW dipping joints are only locally developed and most of the measurements were made on south dipping beds. The other sets on P1. 11.5 are merely local developments of high angles joints with the same trend, as indicated in the caption to the figure.

( f ) Faults and Veins A few minor and narrow calcite veins have been recorded, all with orientations

parallel to important joint sets. Most of the faults are high angle with an ENE trend, but there are also some high angle NW faults and some small low angle thrusts, the latter with north to northeast dips.

4. ORIGIN OF THE MONGO STRUCTURES Any postulated origin for the Mongo Syncline must take account of the anomalous

ESE trend when compared with the Betic ranges as a whole, the conical nature arid en- echelon arrangement of the fold, and the geological succession in which incompetent Triassic rocks outcrop adjacent to Mongo to the west and must underly the massive Cretaceous limestones at depth.

The regional tectonic trend between Cape Nao and Alicante 80 km to the southeast is ENE (Fig. I), and it is interesting to reflect upon theoretical trends which other structures could be expected to have in this tectonic setting, and to compare them with the Mongo trends. In particular dextral and sinistral wrench faults could be expected to have southeasterly and N-S trends whilst drag folds related to such faults would have ESE and NNE trends as indicated on Fig. 3. Drag folds of this kind normally have an en-echelon arrangement (Moody and Hill 1956 1217) and thus a tendency to a conical form and it is suggested that the Mongo Syncline is best interpreted as such a structure associated with a major dextral wrench in the manner shown on Fig. 3.

Fig. 3. Summary diagram illustrating the supposed origin of the joints and other structures of the Mongo Syncline. Sin. and dex. 1-theoretical trends of 6rst order sinistral and dextral wrench aults . Sin. and dex. 2-trends of supposed second order wrench type fractures. lo and c- ongitudin a1 and cross joints.

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Although positive evidence is lacking one probable line for such a fault is along the low lying southeasterly trending strip of Trias which, partly covered by alluvium and other sediment, extends inland from Javea immediately southwest of the Mongo Syncline., (1 :50000 geological map, 822, Benisa). The highly incompetent Trias has in fact been interpreted by Nos et al. (1961) as diapiric, and could well have squeezed up along the line of a southeasterly wrench fault, conceivably a clean fracture in pre- Triassic basement, but with dishannonic separation from post-Triassic rocks.

The final question to be considered is the relation between the fractures and the other structures. Detailed interpretation of the various high angle joint sets leaves some room for manoeuvre, but there seems no doubt that the overall pattern is related to the local structure of the Mongo Syncline rather than to the regional tectonic trend. The ENE joints and faults, making an angle of about 40” to 50” with the syn- clinal trend, are most likely to be sinistral “wrench type” structures (Fig. 3), whilst the N-S joints, intersecting the syncline at about 60” to 70” are best interpreted as dextral “wrench type” structures. The double maxima, particularly common with the north trending joints (p. 101) and P1. 11, localities 4, 9, 10, 11, 13, 20, 44, 49, 56, may well represent the development of Reidel structures and if so is a further indication of a wrench type origin for these joints (cf. Firman 1960 331). Of the remaining joints NW longitudinal and NNE cross joints are the most important and are probably tensional structures. There are other minor high angle joint sets (Pl. 11) which for the most part are best regarded as products of local stress fields set up largely by local structures such as irregularities of bedding and minor flexures. The low angle joints (PI. 11.5) must clearly be interpreted as “thrust type” structures, with complementary sets (SE and NW dips) developed.

To conclude it is suggested that the fractures were formed in relation to second order stresses which gave rise to the Mongo Syncline. The dominant stress alignment, approximately normal to the synclinal trend, would have been responsible for the orientation of the major joint sets, but other factors such as position within the syncline, local flexures and bedding plane control clearly resulted in differing stress fields and therefore in differing fracture patterns at many of the localities. This is illustrated by PI. 11 in which the four different parts of the syncline (in terms of dip of bedding) each have a characteristic fracture pattern when total joints are plotted, (Stereograms 1 to 4), and in addition individual localities within these four regions, whilst displaying similar patterns to the area mean (1 to 4), nevertheless commonly develop considerable local deviations from that mean.

REFERENCES DARDER, B. 1945. Estudio geol6gico del sur de la provincia de Valencia y norte de la de Alicante.

EVANS, A. M. 1963. Conical folding of oblique structures in Charnwood Forest, Leicestershire. Bol. Znst. Geol. y Min. de Espaiia. 17.

Proc. Yorks. geol. SOC. 34,67. FIRMAN. R. J. 1960. The relationshiD between joints and fault Dattems in the Eskdale Granite

(Cumberland) and the adjacent BorrowdalgVolcanic Series.-Q. J. geof. SOC. Lond. 116, 317. FLEUTY, M. J. 1964. The description of folds. Proc. Geol. Ass. Lond. 75, 461. HAMAN, P. J. 1961. Manual of the stereographic projection for a geometric and Kinematic

analysis of folds and faults. West Canadian Research Publications. Series 1, No. 1 , 67.. , MARTINEZ, D. T. and PARDO, J. M. 1954. Mapa geologico de Espaila (1 :5oooO). Expficacion de lu

hoju No. 823, Javea, 56. MOODY, J. D. and HILL, M. J. 1956. Wrench fault tectonics. Geol. SOC. Amer. Bull. 67, 1207. Rios, J. M., NAVARRO, A., TRIGUEROS, E. and VIFLAL~N, C. 1961. Mapa geologico de Espaiia

(1 :50000). Explicacion de la hoju No. 822. Benisa 106. - , VILLAL~N, C., TRIGUEROS, E. and NAVARRO, A. 1958. Mapa geologico de Espaiia

(1 :50000). Explicacidn de la hoja No. 848, Altea 98. DEPARTMENT OF GEOLOGY, THE UMVERSITY, BIRMINGHAM.