Journal of Petroleum Geology, 3,4, pp. 413-425,1981 113

THE STRUCTURE OF A MONOCLINE IN THE SYRIAN ARC SYSTEM, MIDDLE EAST-SURFACE AND SUBSURFACE ANALYSIS

Ze'ev Reches", David F. Hoexter" and Francis Hirsch""

The long Syrian Arc belt in the Middle East consists oftensof'folds arid monoclines with their ussociutedfuults. The structure of one nmnocline of this belt, the Hebr-on nionoclirie in Israel, is analyzed b-v construction of' accurate structural traverses, nzeasurenient ofthe internal strain ofthe rocks, and geological mapping. The sut-face structure indicates that three modes of monocline development, draping, buckling and kinking, operated iii the Hehron monocline. The subsurface structure, which includes a steep reverse fault. is deduced through mechanical and tectonic models, and structural similarity with other nzonoclines in Israel.

INTRODUCTION The analysis of the exposed structure of the mapping as well as a good understanding of the monocline of Hebron, Israel, and an interpre- mechanism of monocline development. tation of its subsurface structure are presented The Hebron monocline is one of tens of in this study. folds and monoclines comprising the Syrian Monoclines usually develop in sedimentary Arc system (Fig. 1). This belt of folds, about sequences of intermediate thickness, com- 1,000km long, is developed on the margins of monly with involvement of the crystalline the Arabian Shield, within a sequence of basement (Harding and Lowell, 1979). sedimentary rocks of platform type. The system Typically, monoclines contain one main fault consists mostly of open flexures and folds, zone, with throw comparable to the uplift of frequently associated with reverse and wrench the monocline, and many small faults; faults faults (e.g. Krenkel, 1929; Hensen, 1951; with intermediate throw are relatively rare. Freund, 1965). The tectonic deformation of Large detachment surfaces have been sug- this belt was initiated by differential subsidence gested to explain the gross shape of a few in Triassic-Jurassic times, but the main phase monoclines, and therefore, to determine the that shaped its present structure is the tectonic compression of Senonian to at least Neogene subsurface structure of a monocline, one has time. The breaking up of the Levant along the to evaluate the throw, attitude and the location Dead Sea rift, during the Neogene, slightly of the major fault of the monocline, and the modified the already-developed folds. Only a level and amount of possible detachment. The few, small, oilfields have been found in the quality of the proposed subsurface geometry Syrian Arc system, in spite of the promising depends on the accuracy of the surface combination of simple closed structures with 41.1 Z. Reches. D. E Hoen-ter.and E Hirsch

0

10

?O

10

30

140 I50 160 I 70 Fig. 1. Location map of the study area. The regional tectonic map is shown in upper left: a-study area; b-northern Negev. Included in the main map: locationsof structural traverses, A-K and Z; location of stations of small-scale structures with the derived strain axes and generalized geology. Grid numbers refer to Israel Grid of co-ordinates. thick shallow marine and continental sedi- both on the mechanism of the development of mentary rocks. monoclines and on structural similarity with In the first part of this paper. the gross monoclines in Israel and elsewhere. Finally, structure of the Hebron monocline is described, implications for structural traps in the Hebron together with the internal strain of its rocks. monocline are briefly discussed. Based on these data, the mechanism which formed the monocline is derived, followed by a THE HEBRON MONOCLINE discussion of the subsurface geology based The two NNE-trending large monoclines of The structure of a monocline in the Svrian Arc svstem 41.5 AGE NOMENCLATURE QUATERNARY Alluvium

Beit Nir Conglomerate - > JEOGENE U 5qlaq - Formation - U- c EOCENE Zor'a - Formation 50 - 200 W c 'ALEOCENE I Toqi ye - Fm- AASTRICHTIAK 'atrur'm , chmm / ARIPANIAN - Mishosh Fm 1 - - ;ANTON IAK Menuha Fm. 25 :ONIACIAN - -90 TURONIAN Bin'a - Fm 80-I60 ~- JPPER Weradim - Fm v) 45-180 2 E NOMANIAh -

-Kfor -ShoT%& ~ 3 0-75 Amminadav Fm ___ - 50 - 95 - 0 -0WER _-Motro - Fm 15- - XNOMANIAI -0- W Beit Me'ir - Fm 40-95 - .- Kesalon - Fm 20 - 45 V -- _.~ - Soreq - Fm

a ~ . Givat Yearim - Fm 30 -70 c - .-__ Kefira - Fm 90 - I80 W ALBIAN Qatana - Fm 40 - 80 LT En Qinia - Fm V ~ Tamun - Fm __-- APTIAN "Blanche " 50 -~ - - -- 615- ARREMIAN- Hatira - Fm 200 ~. ERRIASE ~~ - - - 815- Beersheba - Fm. 85 3XFORDIAN - 900- Kidod- Fm 300 __ __ - -. __ 1200- :ALLOW AN Zohar Haifa -Fm V - - 1350 v) v) 3AJOCIAN a - - - ._~ -2550- a 4ALENIAN Rosh Pinna Fm. 50 ______- -.. ~ 3 2600 7 'OARCIAN Jn Nirim Fm. (1200') LIENSBACHII 1 - ----1R03- LEGEND : EJ EJ ppj 1.1 ~1 I 2 3 4 5 6 7 8 9;- 10; - I1 12 ------13 \ 14 Fig. 2. Generalized stratigraphic section of the Hebron monocline. Legend: 1 alluvium, soils; 2 fluviatile gravels; 3 polymict conglomerate; 4 limestone; 5 chalk; 6 marl; 7 dolomite: 8 sandstone; 9a chert, massive; 9b chert, concretion; 1Oa oolite; lob limestone and marl; 11 fauna abundant; 12 metamorphic; 13 unconformity; 14 lateral change. 4lh Z. Reches, D. E Hoexter and F. Hirsch

Hebron and Ramallah form a prominent Earlier Cretaceous (Kurnub Group) and backbone between the Mediterranean Sea and Jurassic (Arad Group) strata have been the Dead Sea Rift valley (Fig. 1 !. The cities of penetrated by deep drillings (Fig. 2). The Jerusalem. Bethlehem and Hebron are located limestones and dolomites which are exposed close to the crestal zone of the Hebron on the upper region of the Hebron monocline monocline. compose part of the intake area for the The study area is approximately 25 X 40 km Cretaceous aquifers of Israel. in size; the work included mapping the surficial Sub.sLir;fucestrutigraphy. Several deep wells geology of much of this area. at a scale of have reached the Arad Group of Jurassic 1 :50.000 (Hirsch. in /IW/J. 1% and conducting strata in the Ramallah, Jerusalem and Hebron detailed surface traverses at intervals of areas. Some 4,000 m composed mainly of between three and six km. By mapping tectonic carbonates of Oxfordian to Liassic age were stylolites. extension veins. small-scale faults found in Ramallah-1 16 km north of Jerusalem. and similar features. the internal strain in the In Halhul-1 (Fig. 1) almost 3,000 m were monocline was determined and finally. the penetrated, and a thickness close to that found subsurface structure was constructed. using a in Ramallah-1 might have been expected. The modified Busking method at a scale of 1 :20.OOO. top of the Jurassic sequence is truncated by the Detailed cross-sections were then fashioned. regional Early Cretaceous unconformity. The Previous work in the Hebron area includes thickness of the Kurnub Group of Early studies by Rofe and Rafferty (1963). Cook Cretaceous age reaches some 350 m in ( 1971 I. Kolton (1973).and Arkin (10761: Gilat Halhul-I. ( 19771 recently mapped the southern part of the Hebron structure. A large segment of the The .surfuce str-ut;gruphv of the Hebron region is currently being mapped by Hirsch as monocline includes the Judea Group of Albian- part of the 1:.50.000 geological map of Israel. Cenomanian-Turonian age, and the Mount One wildcat borehole. the Halhul- I. was drilled Scopus Group of Senonian-Paleocene age. The predominant carbonate sequence of the to ii depth of 3.860 ni in 1966. and was found dn. Judea Group can be divided according to its exposures and lithology into three main sub- divisions: mainly limey to shaley lower part Stratigraphy IKleq, Klq, KI) of Albian age in the deep The stratigraph! exposed in the studied area gorges cutting the central domes of the Hebron i5 dominated by limestone. dolomite and chalk and Ramallah uplifts; dolomitic, shaley and of Albian (Early Cretaceous) to Middle Eocene minor limey middle part (Kugy, Kus, Kuke, age. Neogene limestones and conglomerates K ubm, K umo) that builds the terrace landscape cocer plateaux between 400 and 550 m above of the higher zones of the Hebron and Ramallah m.s.1.. cutting Cretaceous to Eocene strata. uplifts ( Albian-Cenomanian); upper part of

I I I I -0 10 0.2 1 64) 220 ~ -0 10 0.3 200 260 660 3.5 I" 1.0 370 370 610 3.6 90 1.6 620 620 445 3.1 90 1.6 560 560 520 6.8 310 1.5 I040 1040 20 9.0 330 I..? 1ooO 1000 5 9.3 400 2.1 1 160 1160 - 100 10.0 2x0 1.3 Xi0 1220 - 80 I 1.5 300 1.5 880 I 170 - 160 5.8 160 1.1 680 8(X1 40 I I I I Table 1. Structural data of the 11 sections across the Hebron monocline; locations of sections are shown in Fig. 1. The shortening is the difference between the length along the deformed Moza Formation and the horizontal distance that it occupies. Section A parallels the axis of monocline. The structure qf a monocline it1 the Syrian Arc .system 417 mainly dolomites and lithographic limestones about 1.2', to the north and south. The on top of the steep flank of the monoclines and structural plunge steepens to about 3O at the Iower zones of the Ramallah and Hebron northern and southern ends (Sections A, B and uplifts (Cenomanian-Turonian) (Kua, Kuks, J, K respectively). The structural uplift Kuw, Kub). increases continuously from the north (Section B), towards the south (Section I), with an Structure abrupt increase between Sections D-E and F The gross shape. The Hebron structure is a (Table 1). This increase is due to a marked doubly-plunging "E-SSW trending monocline, decline in the elevation of the trough of the about 35 km long, with some branches to the synclinal bend (Table l), rather than an south (Figs. 1,3).The steep monoclinal flexure, increase in the elevation of the crest, and the which dips up to 50°, separates a down-thrown increase of uplift correlates to the increase of western block and an uplifted eastern block. the thickness of the sequence between the The topographic units correlate in general Moza (Kumo) and Bina (Kub) Formations with these structural divisions and the details (Hirsch, in prep.). These changes occur within of the gross structure are revealed by the overlap zone between the Hebron and construction of ten accurate cross sections, Ramallah monoclines. presented in Figs. 1 and 3, and Table 1. The The ten profiles of the monocline show both construction method is a modified Busking a synclinal bend and an anticlinal bend; at the technique, and is described in the Appendix. Moza Formation level, the former is usually The gross shape of the Hebron monocline is the tighter of the two. Open, domal anticlines presented in a block-diagram of the structure are associated with the anticlinal bend in at the Moza Formation (Kumo)level, based on several section (Fig. 4, Sections B, C, D, E. F eight traverses (Fig. 3). Structural contours and G),while the other sections have a straight and the crestal line of the base of the Moza tilted segment above the anticlinal bend Formation were drawn between the individual (Sections I, J and K). traverses (Fig. 3). Structural details between The profiles of the Hebron monocline display the traverses are inferred. both continuous flexures (Sections B and K; The Hebron monocline has a relatively flat, Fig. 4), and kinked flexures (Sections C, D, E. undulatory crest line (Fig. 3).Highest structural F, G, H, I and J). The former type is elevation of the base of the Moza Formation is characterized by a smooth transition between 1,140 m in section I, and it plunges gently, concentric sections, whereas the latter type

Fig. 3. The surface structure of the Hebron monocline at the base of the Moza Formation. Eight accurate cross sections were located in their relative positions, displaying the gross shape of the monocline (Fig. 1). Structural contours and the crestal line are traced on the block diagrams. Contours between sections are inferred. Every section has a base line which is horizontal at sea level. All elevations are in m above m.s.1. 118 Z, Reches. D. i? Hoexter and E Hirsch

ESE WNW

Fig. 4. The profile of the Hebron monocline shown in ten accurate sections of the base of Moza Formation. The presented profiles are segmen6 of the complete sections (Figs. 1, 3). The inclined lies in the flexure zone show the binge zone, of "mis-match" of concentric arcs.

1 I' \ \\

0 = -2930

13m0-3500 14000

Fig. 5. The measured surface and interpretative subsurface structure of the Hebron monocline along Section H (location in Fig. 1). Stratigraphy after Fig. 2. Elevation in meters. Vertical and horizontal scales are equal. The structure ofa monocline in the Svr-ian Arc system 1IY contains zones of “mismatch”. For example, in shape of the structure, as described above, and Section H (Fig. 5) the section with center R, its internal deformation, which will now be does not continue smoothly to the section with discussed (Reches and Johnson, 1978). The center R, (see Appendix); the zone between internal deformation of the Hebron monocline the two concentric sections, Hz in Fig. 5, is a has been evaluated by measuring small-scale zone of fast change of dip, and high curvature, structures with apparent displacement: mostly and is a possible locus for rock brecciation and tectonic stylolites, small faults, filled veins and minor faulting (e.g. Stearns, 1971). The hinge slickolites were measured, all indicating the zones of the kinked flexures are marked also sense and the nature of the internal deforma- on the other profiles (Fig. 4). tion (Fig. 6). Many road cuts, quarries and The shortening and structural uplift of the natural cliffs were surveyed in search of stations monoclinal flexure, as well as the overall with ample data, although in many sites no structure, were measured along the profiles mesoscopic structures were found. Eighteen (Table 1). The shortening is the difference stations in the Ramallah and Hebron between the length along the flexed Moza monoclines (Fig. 1) were located, and at each Formation and the horizontal distance it station several tens of individual structures, occupies; the structural uplift is the difference commonly of several types (Table 2) were in the altitudes between trough and crest of the measured; the pattern of the small structures base of the Moza Formation. The results of was remarkably consistent at each site and was these measurements (Table 1), indicate that furthermore consistent for most stations in the the maximum overall shortening of the overall study area (after retilting bedding to the structure is 400 m, or 2.1%, in Section H. The horizontal) (Figs. 1, 7; Table 2). maximum shortening of the monoclinal The predominant strain pattern in the flexure is 11.5% in Section J, where maximum Hebron monocline consists of a horizontal dip is 50’. compression axis with mean trend of N64’W- Small-scale structures. The mechanisms S64’E (Fig. 7; Table 2); this axis is associated which govern the development of monoclines in several stations with a horizontal extension can be revealed by knowing both the gross with mean trend of N26’E-S26’W. This strain

~~~ ~ ~ Fig. 6. Schematic presentation of the small-scale structures measured in this study. 32( 1 Z. Reches. D. E Hoester arid E Hirwh

I 7\,/"' of ,Ytrn1bcr. Loctrl \t?lllll of dartr clip iriicii/w$ pa'ni\ Qicirli/~~

19.4 Kub FL S 30 E 20- A Kuh FL S. F. V -17 E co-2Y7° EX-025' 22-27-4 Ku~ c S. F 5 2 E CO-2YO' - I02 Kuh FL V. S. F 41 E EX-020' CO-20'/ 130' 103 Kugl c V 52 G EX-293' EX-008' 104 Kuh FL s. v 62 E CO-287' EX-286' - .- - - - - 177 Kus FL-U V 22 P 200 Kum FL F 12 P 203 Kuh FL F 13 G 211 K ub D F. V .i7 G 267 K ua FL F I0 G - 2hk Kus U-FL F 15 G EX-276' - 2W Kuhni UFL F 30 G EX-024' co-292' 170 Kubni u F 13 P 28 1 Kuh FL-D S 7 G 297 8 Kuh u F. S 4) E

Table 2. Summary of the stations of small-scale structures; locations of stations are shown in Fig. 1. Rock units after Fig. 2. FL-located in the monoclinal flexure; U-located in the uplifted block; D-located in the downthrown block. Type of small structures measured in a given station: S- tectonic stylolites; F- small faults; V- extension vein. Quality of results: E-excellent; G- good; P- poor. Strain axes are sub-horizontal unless otherwise marked. CO-compressive strain; EX-extensive strain. Stations 19-A, 2@A, 22-A and 23-A from Eyal and Reches (it1 ptxJp.1.

Furthermore, this strain field is consistent with the regional strain field that prevailed in Israel A from the Senonian to the Eocene, as t determined through a regional study of small structures (Reches and Eyal, 1979). In twostations predominant extensions, with mean trends of N7S0W-S7S0E were found subparallel to the dominant compression noted above. These two stations were located in the crestal region of Section I, which has the largest overall uplift (Fig. 4; Table 1). The B significance of these two stations is discussed in the next section.

Mechanism of development of the Hebron monocline Monoclines can be generated through three E main modes: buckling, draping and kinking 21 (Reches and Johnson, 1978). In the first mode (buckling), the sedimentary sequence is Fig. 7. The orientation of the horizontal strain axes subjected to layer-parallel shortening, and it measured in 18 stations in the Hehron monocline (Table 2). All data after retilt of bedding to horizontal. buckles into the asymmetric monocline due to Primary and secondary strain axes {Table2) have the an initial dip or an underlying fault. In the same weight. A-compression axes; 6-extension secoird mode, the sedimentary sequence is axes. draped by differential vertical displacement above a fault or an igneous intrusion. The third field has a simple relationship to the trend of mode (kinking), is dominant in sedimentary the Hebron monocline: its compressive axis is sequences with high frictional resistance subperpendicular to the trend of the flexure. between the layers and here, as with the and the extension axis is subparallel to it. buckling mode, layer-parallel shortening is The sti-ucture of n monocline in the Syrian Arc system 121 essential. The dominant modes in the for the monoclinal flexure. In later stages. generation of a given monocline can be when extension dominates (Freund, 1979), determined by using the criteria suggested by small normal faults developed in the anticlinal Reches and Johnson (1978, p. 301), summarised bend of the area with the largest uplift. below. Drape Folding 1 monoclinal flexure is simple, with open DISCUSSION anticlinal and synclinal bends; The study of the surface structure of the 2 curvature increases downwards; Hebron monocline reveals an elongated 3 vertical displacement is constant or structure, with a smooth or kinked monoclinal decreases upwards; flexure, which is most probably underlain by a 4 there are zones of layer-parallel extension reverse fault-the Hebron fault. The deter- and zones of layer-parallel shortening. mination of the inclination, throw and location Buckling of the fault which underlies the monocline, is 1 monoclinal flexure is associated with an difficult (e.g. Reches, 1978).Here, however, an anticline and a syncline; attempt at deducing the subsurface structure 2 curvature is constant, increasing or of the Hebron monocline is made by decreasing upwards: comparison with similar monoclines in 3 vertical displacement is constant, or southern Israel together with theoretical increases upwards; models. 4 layer-parallel shortening prevails at all Several reverse faults which are not exposed levels. at the surface have been penetrated by wells in Kinking Israel's Negev region, south of the Hebron area 1 straight limbs and distinct hinge zones are (Fig. 1). These reverse faults were detected in present; five boreholes penetrating four monoclines 2 internal strain indicates layer-parallel (e.g. Coates etal., 1963;Mimran, 1976; Aharoni, shortening at all levels: 1976). These faults have considerable throw, 3 there is yielding or faulting in tight hinge from one third to the complete uplift of the zones. host monoclines at depths of up to 2.5 km The application of these criteria to the (Table 3). They are steeper than 45' and Hebron monocline indicates that all three usually fall within the 60'-80' range. In two models were involved in its development. For boreholes normal faults were found close to example, the Hebron monocline has an open the reverse ones. simple flexure in Section K, in agreement with These reverse faults could be generated by Criterion 1 for draping. Also, other Sections several mechanisms, of which one is reverse (B, C, D, E, F and H), have anticlinal domes, as faulting in the core of a concentric flexure, with Criterion 1 for buckling, while straight during the development of the monocline. segments, typical of kinking, dominate Sections Such faulting occurs through the yielding of I, J and K. Another criterion is the internal rocks in the loci of high curvature; in this case, strain distribution. The evidence for prevailing the fault initiates at the center of a concentric layer-parallel shortening in the Hebron mono- section of a fold (points R2,Rlin Fig. S), and its cline is characteristic for the buckling and displacement increases downward. However, kining modes (criteria 4 and 2 respectively); as the center of concentricity of the monoclinal however, the layer-parallel extension found in flexures of the northern Negev is 1.5-4.0 km the anticlinal bend of Section I is typical of the deep, and the fully-developed reverse faults draping mode (Criterion 4). are found at shallower depths, one must reject Thus, criteria of all three modes (draping, this mechanism for this area. buckling and kinking) were found to occur in Another mechanism of fault generation the Hebron monocline. An appealing model suggests that the fault preceeded the mono- that fits all these features is that the Hebron cline, and that it was reactivated by the monocline overlays a steep reverse fault, which tectonic stresses which generated the was activated by horizontal tectonic compres- monocline, the fault thereby determining the sion. The displacement along the reverse fault location, shape and strain of the monocline. during the early stages of deformation provided The pre-existing fault may occur within a both the subhorizontal compression observed crystalline basement (e.g. Steams, 1971). or in the rocks, and the localization mechanism within a sedimentary pseudo-basement (e.g. .Llavitiiiii?1 Pcrlc~llurccl Vci-~icu I llplitl ot dq.l”h 10 si;liumriori riioiiociiiie 1rii1 fililll illii ot,/u1rIi (111)

510 1096 I48 Barbur 420 2427 1x0 + Rechnir 440 227.5 27s + Shrrif 1x0 2.790 2.7 I

Reches. 19%). Both true and pseudo-base- relatively small effect on the monoclines. nients apply di~phcerneriron the base of the compared to the shallow faults in the pseudo- overlying sequence. and thus are equivalent basement. from a mechanical point of view. This mechanical model is in good agreement It has been observed in simple monoclines with a tectonic model for Israel, suggested by that the major fault propagates only a short Freund e[ a/.. (1975). They proposed that, distance into the overlying sediments. For during Triassic-Jurassic time, the region of example. in the Palisades monocline. Colorado Israel was subjected to regional extension in a Plateau. the major fault propagated about WNW-ESE direction, and long basins (probably 60 m into the sediments above it, even though bounded by normal faults) developed during the total throw along the fault at depth is about this stage (Fig. 8a). This old extensional field 250 m (Reches, 1978). In the Palisades mono- was replaced by a new regional tectonic cline. the pre-existing fault in the Precambrian compression, in WNW-ESE direction, in the rocks is replaced by a tight, continuous flexure Senonian. This new tectonic stress has been in the Lower Paleozoic rocks. This short recorded (Fig. 7, Table 2). According to Freund distance of propagation is due to the tendency et d..the new tectonic stress reactivated the of the layered sedimentary rocks to fold, steep normal faults as reverse faults, and rather than to fault, above a steep reverse generated the large monoclines (Fig. 8b). fault. Therefore. the “reversal of structure”mode1of It seems that the pre-existing fault concept Freund ef a/.. (1975), is the most likely fits the reverse faults of the northern Negev. However. it is not clear if these faults propagated from a crystalline basement, or if they existed within the sedimentary sequence (see also Mimran, 1976).If the faults existed in the crystalline basement. at depths of 4-5 km. they should have propagated 2.5- 4.0 km in order to be detected in the oil boreholes (Table 3). Such a long distance of propagation of reverse faults through the clastic sedimentary rocks of Paleozoic age is very C unlikely. according to the discussion above. 7i-J 1 However, it is possible that the steep faults B. =S P -== existed within the Triassic-Jurassic sequence. which behaved as a pseudo-basement at depth . I. . ‘ . I .t-_------.P(.‘’. ‘ I.LI.II. x ,..,.,.. ...I.... of 1-3 km. before the development of the ,..../...... I... monoclines. During reactivation in Senonian age. the faults propagated only short distances Fig. 8. Schematic presentation of the “reversal of structure” model for a monocline in the northern into the Lower Cretaceous rocks, and thus are Negev (following Freund el al., 1975). PC-Pre- not exposed today. This suggestion. that the camhrian crystalline basement, P - Paleozoic, reverse faults in the northern Negev propagated T-1- Triassic-Jurassic sequence, C - Cretaceous. from a sedimentary pseudo-basement, does A - extension phase during Triassic-lurassic age; B - compressive phase of Senonian-Eocene age. Note not exclude the possibility of large reverse reversal of separation along the main fault, and faults in the deep crystalline basement. negligible propagation of the fault into the Cretaceous However. such deep faults would have a rocks. The structure 0.f a monocline in the Svr-ian Arc system 423 mechanism to explain the main features of the 2 The total throw of the Hebron fault at reverse faults in the Negev: large throw at great depth should be approximately equal shallow depth, no surface penetration, steep to the uplift of the host monocline. planes of the faults and the close occurrence of provided that no major dis-harmonic zones normal and reverse faults. exist (Reches and Johnson, 1978). Thus, it The Hebron monocline is similar structurally is estimated that under Sections H-I, the to the monoclines of the Negev, both in gross lower layers of the Jurassic, at a depth of shape and internal strain. Thus, a reasonable 4.0 km below sea level (md.),are offset assumption is that the reverse fault under the by 1.2 km. If the model is correct, the Hebron monocline has similar characteristics throw should decrease gradually, and is to the reverse faults under the Negev expected to be replaced by a closed monoclines. The main parameters of the intense flexure. The fault is inferred to Hebron fault are described below. terminate at the base of the Lower 1 The inclination can be estimated as Cretaceous beds. follows: the horizontal shortening pro- vided by the Hebron fault in the “pseudo- 3 The lateral location of the postulated basement” of the Triassic-Jurassic rocks is reverse fault is less restrictive than the accommodated in the upper levels by vertical variation. In the northern Negev, both monoclinal flexing and internal for example, the reverse faults can be strain. During early stages of monocline located laterally between the anticlinal development, the horizontal shortening bend and the kinked zone for four of five provided by the reverse fault exceeds the faults found in the monoclines (Aharoni, shortening accommodated by monoclinal 1976). Direct observations in monoclines flexing (Freund, 1979), and the additional of the western US locate the major faults horizontal shortening is probably absorbed under the anticlinal bend (e.g. Reches, by compressional internal strain of the 1978; Stearns, 1971). The consistency in rocks. However, during later stages of location and altitude of the hinge zones in development, the horizontal shortening the Hebron monocline (Fig. 4), suggests associated with the monoclinal flexing that they are related to a single source, exceeds the shortening provided by the most likely the Hebron fault. Thus, it steep reverse fault (Freund, 1979). At this is suggested that the Hebron fault lies stage, extensional internal strain develops along the continuation of the kinked in the anticlinal bend (Reches and zone, Hz, in the flexure (Figs. 4, 5). Johnson, 1978). Evidence of extensional However, similarly to the Negev mono- internal strain was only found in the clines, this fault may be closer to the proximity of Sections H-I (Fig. 1). As anticlinal bend. these sections have the largest uplift in the Hebron monocline (Table l), it is Based on the study of the Hebron mono- concluded that these sections reached the cline, a subsurface structure for Section H. later stages of monocline development. which runs through the Halhul-1 borehole Following Freund (1979, Eq. 6), one can (Fig. 5) has been delineated, and its main calculate the inclination, 0,of a reverse features are as follows: the Hebron fault is fault for the stage that the shortening due located within a zone of 1 km width, inclined at to faulting equals the shortening due to about 73’, under the “mismatch” zone of the flexing: monocline (Fig. 5). The layers above the fault are slightly folded and dip gently, and those 1 -cos 6 west of the fault are intensely flexed in the arc tan ___ p= proximity of the fault, with an open synclinal 6 -sin 6 bend further to the west. The throw along the where 6 is the maximum dip in the main fault decreases from about 1.2 km at monoclinal flexure. Thus, for the Hebron 4.0 km below m.s.1. to zero at about 0.2 km monocline, with 6 = SOo, the maximum below m.s.1. Normal faults with displacement inclination of Hebron fault is 73’. It of a few tens of meters may be found in the should be noted, however, that the proximity of the reverse fault. inclination of the fault may change with In the present study the subsurface structure depth (e.g. Mimran, 1976). has been determined by construction of 324 Z. Reches, D.F. Hoexter and F. Hitsch accurate surface traverses across the elongated projected on to the cross section (Fig. 5). This monocline. The modified Busking method is done by applying an overlay traced with (Appendix). seems to be a reliable, simple many concentric arcs over the section, and technique of lineation of both surface as well searching for the arcs that fit as many data as wbsurface structure. For example. the points as possible. The centers of the best fits crestal zone in Section H (Fig. 5) is practically are marked (R,, Rz and RJin Fig. 5), and con- vertical; thus. drilling for crestal traps here centric arcs are constructed with a compass. should be on the surface crest, rather than In the common Busking method, a center down-dip on the uplifted block. The Halhul-1 for the concentric arc is taken between every borehole is located at the crest of Section H two neighbouring data points, thus implicitly (Fig. 5):however. the highest point along the assuming that there are no measurement errors crest is at Section I, about 2 km south of and no natural deviation between the points. Section H (Figs. 1. 3). On the other hand. by the present method a Another structural trap could exist in the search is made for best-fit centers, and large, downthrown block at the proximity of the consistent zones of concentricity are obtained. reverse fault (e.g. Levorsen. 1967. Ch. 6). However. due to the great difficulty in locating this fault I the choice of well location for such a trap requires more information on the subsurface structure. REFERENCES AHARONI. E., 1976. The surface and subsurface structure of the southern Judean desert, Israel. ACKNOWLEDGEMENTS Library of the Israel Geol. Surv., Jerusalem, 100 p. (Hebrew with English summary, unpublished Discussions with numerous colleagues have report). contributed to this study. Our thanks go to ARKIN. Y.. 1976. Geological Map, Jerusalem and I. Shachnai, U. Bieda, Y. Arkin. Y. Eyal. E. Vicinity. Geol. Surv. Israel, Scale 1:50.000. Diamant, H. Fligelman and particularly to the COATES. J.. GOTTESMAN. E.. JACOB, M. and late R. Freund. Y. Eyal kindly let us use some ROSENBERG, E., 1963. Gas discoveries in the western Dead Sea region. Proc. hih World Peirol. of his unpublished data. and the technical Coq..Frankfurt. I, Paper 26. assistance of S. Fliegehann and Y. Barbut is COOK. P.. 1971. A columnar section of the Judeli greatly appreciated. Group (Cretaceous) west of Hebron. Geol. Surv. This study was supported by a grant from Israel, Rept. No. M.M. 100/7/0. FREUND. R., 1965. A niodel of structural develop- the Ministry of Energy and Infrastructure, ment of Israel and adjacent areas since Upper Israel. Cretaceous times. Geol. Max.. 102, 189-205. FREUND, R.. GOLDBERG, M., WEISSBROD, T., DRUCKMAN, Y. and DERIN, B., 1975. The APPENDIX Triassic-Jurassic structure of Israel and its relation The modified Busking method to the origin of the Eastern Mediterranean. Geol. The Hebron monocline is a long structure; Sirw. Israel Bd1. 65. due to its high length/width ratio, the structure FREUND. R., 1979. Progressive strain in beds of is assumed to be essentially two dimensional, monoclinal flexures. Geology. 7,269-271. GILAT. A,, 1977. Geological Map, Eshtemoa. Geol. and thus itsgrossshape can be studied through Surv. Israel, Scale 1:50.000, Sheet 15-1. accurate cross sections. The sections have HARDING. T. P. and LOWELL. J. D., 1979. been constructed using the following method Structural styles, their plate-tectonic habitats. and I Fig. 51: field measurements of stratigraphic hydrocarbon traps in petroleum provinces. Anzer. Anoc. Peirol. Geol. Bull.. 63, 1016-1058. contacts and dips along traverses sub-normal HENSEN. F. R. S., 1951. Observations on the to the main monocline were plotted at a scale Geology and Petroleum Occurrences of the Middle of 1 :2O.O00. The author‘s own measurements East. Pwc. 3rd World Perrol. Cong., 1, 118-140. together with the results of Arkin (1978). KOLTON. Y., 1972. Geological Map, Hebron Kolton (1972)and Gilat (1977) have been used. Region. 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RECHES, 2.. 1978. Development of monoclines: RECHES, Z. and EYAL, Y., 1979. Regionaldistribu- Part I. Structure of the Palisades Creek branch of tion of paleostress in the vicinity of the Dead Sea the East Kaibab monocline, Grand Canyon, Rift. Israel labs.) Amer. Geophj~.Union. Trans.. Arizona. In; MATTHEWS. V., 111, 1978. Laramide 60.948. Folding Associated with Basement Block Faulting in the Western United States, Geol. SOC.Amer. ROFE and RAFFERTY, Consulting Engineers Ltd.. Mem.. 151. 1963. Jerusalem and District Water Supply. RECHES, Z. and JOHNSON, A. M., 1978. Develop- Geologic and Hydrologic Rept.. London ment of monoclines: Part 11. Theoretical analysis (unpublished). of monoclines. In: MATTHEWS, V., 111, 1978. Laramide Folding Associated with Basement Block STEARNS, D. W., 1971. Mechanism of drop folding Faulting in the Western United States, Geol. Soc. in the Wyoming Province. Wyoming Geol. hsoc., Amer. Mem.. 151. 23 Ann. Field Conf., Guidebook, 125-143. 426