Moron Fault Zone, North-Central Venezuelan Borderland: Identification, Definition, and Neotectonic Character

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1984-09 MORON FAULT ZONE, NORTH-CENTRAL VENEZUELAN BORDERLAND: IDENTIFICATION, DEFINITION, AND NEOTECTONIC CHARACTER

Schubert, Carlos; Krause, Federico F.

Springer Verlag

Schubert, Carlos and Krause, F. F.. (1984). "MORON FAULT ZONE, NORTH-CENTRAL VENEZUELAN BORDERLAND: IDENTIFICATION, DEFINITION, AND NEOTECTONIC CHARACTER". Marine Geophysical Researches, 6: 257-273. http://hdl.handle.net/1880/44461 journal article

Downloaded from PRISM: https://prism.ucalgary.ca MORON FAULT ZONE, NORTH-CENTRAL VENEZUELAN BORDERLAND: InENTIFICATION, DEFINITION, AND NEOTECTONIC CHARACTER

CARLOS SCHUBERT Centro de Ecologia, I.V.I.C., Apariado 1827 Caracas 1010A, Venezuela and F. F. KRAUSE Petroleum Recovery Institute, 3512 33rd St. N. W., Calgary, Alberta Canada T2L 2A6

(Accepted 11 February, 1984)

Abstract. Location of the southern Caribbean plate boundary has been hindered mainly because it is in large part submerged. Analysis of 28 acoustic reflection pro6les along the north-central Venezuelan borderland, and a review of published data, suggest that this borderland is the site of a complex fault zone, formally defined as the Mor6n fault zone, which encompasses the nodal region of the Bocon6-Oca-El Pilar fault system. The Morh fault zone consists of: (1) an eastward extension of the Oca-Chirinos fault zone at about 10" SUN latitude; (2) a probable eastward continuation of the Bocon6-Mor6n faults along the Venezuelan coast, which splits into the Ada and Macuto faults, north and east of Caracas; (3) the Tacagua fault, which is a southeastward trending splinter fault of the Oca-Chiriios fault zone; (4)and the westward extension of the Cariaco pull-apart basin and the El Pilar fault zone. All of these faults and fault zones are active, as shown by offset sea bottom, offset Pleistocene-Holocene features, and seismicity. It is suggested that the Oca-Chirinos fault zone represents a formerly more active part of the plate boundary. Since the Late Tertiary (?) or Quaternary, the Boconn6 fault zone was incorporated into the plate boundary, and the northwestern block (Bonaire block) was thrust northeastward over the Caribbean crust.

1. Introduction Search for the southern Caribbean plate boundary has been hampered by the complexity of structural and sedimentary features which are found north of the Venezuelan coast to the Venezuelan basin. In a highly significant report, Silver et al. (1975) showed that the Venezuelan borderland consists in large part of a block of deformed sedimentary and volcanic rocks (Bonaire block), which is thrust northeastward over the Venezuelan basin, and which includes the Curacao ridge and the Bonaire basin (Figure 1). The southern boundary of this block was thought to be the eastern continuation of the Oca and Bocon6 fault zones, which join the El Pilar fault zone in the east. In this report, we reinterpret the submarine geophysical data, together with recent land geological data, in order to establish the existence of a major fault zone along the north-central Venezuelan borderland, and its relation to southern Caribbean tectonics. Existence of a fault zone along the north-central'venezuelan coast, between Morbn and Cabo Codera (Figure 2), has been hypothesized since the end of last century by numerous authors (Sievers, 1888a, b; Dengo, 1953; Alberding, 1957; Marine Geophysical Researches 6 (1984) 257-273.0025-323518410063-0257 $02.55. @ 1984 by D. Reidel Publbhing Company. 258 C. SCHUBERT AND F. F. KRAUSE

1 I I U -6.~ CARIBBEAN PLATE Q

Fig. 1. Southern Caribbean tectonic relationship, slightly modified after Silver et al. (1975, Figure 12).

Smith, 1962; Hess, 1966; Stainforth, 1969; Bellizzia, 1972, 1976; Wehrmann, 1972, Schubert, 1981). This hypothesis was based on the following criteria: (1) the linearity of the coast (Figure 2); (2) the great elevation contrast between the Coastal Range and the continental platform; (3) sporadic evidence of faulting parallel to the coast; and (4) the seismic activity of the region. A preliminary report, emphasizing the land evidence for the existence of this fault (Schubert and Krause, 1981) reviewed the varying tectonic schemes proposed by the above authors for this region. That the Venezuelan coast, between the Yaracuy river basin and Trinidad, was a consequence of faulting was proposed in the late eighteen hundreds by Sievers (1888a, pp. 72-73, 78), who considered that the crustal block to the north had subsided. At the same time, he postulated faulting along Lake Valencia and the Tuy river, and related the frequent earthquakes to movements along these faults. Sievers (1888b,p. 57) also postulated the existence of a graben (Einbruch) in the Yaracuy valley (Figure 2), the earliest suggestion of extensional valleys in Venezuela. Later, Alberding (1957) identified faulting between Morh and Cabo Codera, and suggested that it was part of the El Pilar fault, a strike-slip fault mapped between the Cariaco basin and the eastern end of the Northern Range of Trinidad. Subsequently, the name Sebastihn was informally applied to this fault by Smith (1962; Feo-Codecido, pets. comm., 1980). The nearest geographic locality with a similar name is the town of San Sebastih, on the opposite side of the northern Coast Range and removed by at least 50 km from the nearest trace of the fault. Bellizzia (1972) recognized this inconsistency and renamed the fault as the Caribbean fault or Caribbean fault zone. Unfortunately, this name has disad- vantages similar to the previous one, since the Caribbean is a region of continental dimensions. However, Smith's (1962) map shows the name Morbn fault for the Fig. 2. Index and location maps displaying the generalized tectonic setting of northern Venezuela (modified after Schubert, 1981).The Mor6n fault zone is defined as the segment of the Bocono-Oca-Chuios-El Pilar fault systemlocated between Mor6n and Cabo Codera. Abbreviations: A: Aruba; Al: Albarico; APR: Araya-Paria range; B: Bonaire; Ba: Barquisimeto; BB: Bonaire basin; BF. Bocon6 fault (active trace); BFZ: Bocond fault zone; C: Curqao; CAF Carache fault; CB: Cariaco basin; CC: Cabo Codera; CF: Caparo fault; Ch: Chichiriviche; Chu: Chuspa; Cu: Cuyagua; EPFZ El Pilar fault zone: FB: Falcdn basin; G: Guajira peninsula; GF: GuCico fault; GGB: Guarenas-Guatire basin; GT: Golfo Triste; GV: Gulf of Venezuela; HF: Humocarofault; IF: Icotea fault; LB: La Blanquilla; LG: La Guaira; LM: Lake Maracaibo; LO: La Orchila; LR: Los Roques; LS: La Sabana; LVFZ: La Victoria fault zone; M: Margarita; MF. Mor6n fault; OC: Ocumare de la Costa; 0-CFZ: Oca-Chirinos fault zone; P: Paraguana peninsula' Pa: Patanemo; PC: Puerto Cabello; PT: Punta Tucacas; SFF: San Sim6n fault; T: Tortuga; TAR Tkata fault; TB: Turiarno basin; TF: Tigre fault; U: Urama; UF: Urica fault; VB: Lake Valencia basin; VFZ: Valera fault zone; Ya: Yaritagua; YB: Yaracuy basin. 260 C. SCHUBERT AND F. F. KRAUSE

connection between the fault that forms the southern limit of the Yaracuy river valley, a northeastern continuation of the Bocon6 fault zone (Schubert, 1982a), and the fault along the north-central coast. We propose here to designate as the Morbn fault zone the whole of this fault segment and to extend it to Cabo Codera. The Mor6n fault was originally defined by its outcrop south of the town of Mor6n and is, therefore, defined formally by accepted geological criteria. In this report and henceforth, we shall use the name Moro'n fault zone to designate the fault segment and associated parallel fractures between Mor6n and Cabo Codera. This use of fault nomenclature follows that of Crowell (1975), who defined a fault system as a set of extensive high-angle faults; a fault zorte as a nearly continuous zone, several kilometers wide, of roughly parallel fractures that branch and interlace; and a fault as the principal, most recent surface rupture. Further, we wish to emphasize that continued usage of the designations Sebastiin, San Sebastign, Caribbean fault, or Caribbean fault zone are likely to promote confusion. As has been indicated above, the evidence presented for the existence of this fault zone is fragmentary and the following discussion is an effort to establish clearly the evidence for its existence. Schubert (1981) compiled recent data on the regional tectonics of northern Venezuela, and concluded that there is evidence of continuity along the Bocon6- Mor6n-El Pilar Fault system (Figure Z), and that the right-lateral offset along this fault system is most probably post-Middle Tertiary. Total left-lateral offset along the northern boundary of the Caribbean plate has been estimated as less than 500 km (Schwartz et al., 1979). Along the southern boundary of this plate, in the coastal areas and continental platform, offset has been estimated as less than 100 km (Schubert, 1981, 1982a, b, in preparation). Therefore, either the rate of offset along the southern Caribbean plate boundary is lower, or the displacement began geologically later (possibly in latest Tertiary or Quaternary time, as compared with Late Eocene along the northern Caribbean; Ladd, 1976), or the difference in rates is taken up elsewhere.

2. Previous Data

Dengo (1953) described an important fault zone along the northern edge of the Coast Range, in the region of Caracas, which he named the Macuto fault zone (Figure 8). This fault zone strikes east-west and dips steeply to the north. The movement along this fault zone is younger than the folding of the Mesozoic metamorphic rocks and may be normal (north side down) or strike-slip. Dengo preferred the latter interpretation, due to the linearity of the fault. Its width reaches 100m and the fault zone is characterized by the presence of fault gouge and a polished fault plane with horizontal slickensides (Wehrmann, 1972). Weisbord (1957) mapped an east-west trending fault zone south of Cabo Blanco and called it the Brucas fault (Figure 8). It consists of a fractured zone, several meters wide, MORON-FAULT ZONE, VENEZUELAN BORDERLAND 26 1

Fig. 3. Side-looking radar image of the~aracu~river pull-apart basin (Y). The Bocond fault (B) forms the northwestern boundary of the basin, and offsets right-laterally the large alluvial cones, and the Mordn fault (M) forms the southeastern boundary (C: Caribbean Sea). The basin is formed within a right-step in the right-lateral Bocon6-Mor6n fault zone. North is towards the top. (Published with authorization of Cartografia Nacional, Caracas, communication 4141-0316 of 17 October, 1983). which separates Pliocene (?)-Pleistocene sedimentary formations. East of Caracas, Asuaje (1972) mapped an east-west trending fault near La Sabana (Figures 2 and 8), approximately 1 km south of the coast, and several en echelon faults parallel to the coastline; he mentioned no evidence of relative offset. The Yaracuy river valley (Figures 2 and 3) was described as a graben by Bellizzia Rodriguez (1976) confirming Sievers' (1888b) earlier proposal. More recently, Schubert (1982a) suggested that the Yaracuy river valley could be accounted for as a pull-apart basin along the Bocon6 fault zone. Near Morh, this basin narrows and ends as a steep coastal escarpment along the north-central coast. The Mor6n fault, which begins as the southern limit of the Yaracuy valley, has been mapped on land between Urarna and Cuyagua (Figure 1; Gonzilez, 1972, 1975); along this stretch, it separates the Mesozoic Las Brisas and Nirgua Formations. Bell (1972, 1974) found no unequivocal evidence of stxike-slip offset of great magnitude during the Tertiary in the Coast Range, except geomorphological evidence of Miocene (?) to Recent movements along the La Victoria fault zone and the Mor6n fault. He could not document strike-slip offset before Eocene time. Oxburgh (1966) and Murany (1972) suggested that the La Victoria fault zone, and other strike-slip faults in the Interior Range (Figures 1 and 2) might end as thrusts and that, therefore, total offset along them would be small and/or geologically recent. 262 C. SCHUBERT AND F. F. KRAUSE

Feo-Codecido (1972) suggested that the Oca fault zone (recently redefined an Oca-Chirinos fault zone by Piimpin, 1978) is intersected and right-laterally offset by the Bocon6 fault zone in Golfo Triste (Figure 2), where both then presumably join to form the Mor6n fault zone. Continuity between the Oca and Mor6n fault zones was suggested by Vkquez and Dickey (1972); an eastern continuation of the Oca fault zone is suggested by seismic reflection profiles up to 40 km east of Punta Tucacas (Godlez de ~uahaet al., 1980, Figure VII-15; Barbot et al., 1979). This eastern continuation consists of a belt of horsts and grabens with a width between 1 and 10 km, and evidence of Pleistocene faulting. On aerial photographs and Landsat and side-looking radar images (Figure 3), two well-defined active fault traces can be observed along the northwest and southeast boundaries of the Yaracuy basin. These traces represent the northeastern end of the Bocon6 fault zone, which forms a narrow fault zone southwest of Barquisimeto. The northern trace (Bocon6 fault), between Yaritagua and Al- barico, is characterized by an almost continuous fault trace which offsets rivers, creeks, and alluvial fans, with evidence of shutterridges, fault scarps, and other active fault geomorphic features; this evidence suggests a right-lateral offset of up to one or more kilometers of Pleistocene features, and a normal offset of a similar magnitude (south side down). To the northeast of Albarico, the fault trace cuts the alluvial plain of the Yaracuy river and reaches the Caribbean Sea north of Mor6n. The southern trace (Mor6n fault) is characterized by an abrupt and straight topographic change, with numerous triangular facets. The two traces converge towards the Caribbean Sea and, presumably, join east of Mor6n. This tectonic setting of the Yaracuy valley has been interpreted as a pull-apart basin within an en echelon stepover along the Bocon6-Mor6n fault system (Schubert, in press). East of Mordn, in the Puerto Cabello-Patanemo area, there is no unequivocal evidence of faulting on land. Several east-west aligned outcrops of serpentinite have been mapped a few kilometers south of the coast (Gonziilez, 1972). On the other hand, the rocks of the Las Brisas Formation, in the vicinity of Puerto Cabello, show evidence of important shearing. In the northern end of the western part of Patenemo bay, there is probable faulting in rocks of the Las Brisas and Nirgua Formations. In Landsat and side-looking radar images, a fault trace is observed between Mor6n and Patanemo.

Maloney (1965) described the morphology of the central Venezuelan coast and nientiorled the presence of several 'fossil' sediment cones in the region between Mor6n and Cabo Codera. These he interpreted as having formed as alluvial outwash fans during the last glacial age, which were later inundated and eroded. Recent uplift has produced a rejuvenation of the rivers and creeks, and the formation of gorges. The western part of the Moron-Cabo Codera coastline shows evidence of submerged valleys. More recently, Picard (1974) described this coast and divided it into two segments: (1) a western segment, between Puerto Cabello MORON-FAULT ZONE, VENEZUELAN BORDERLAND 263 and Chichiriviche, characterized by cliffs submerged by up to 200 m; and (2) an eastern segment, between Chichiriviche and Cabo Codera, characterized by a continental platform with evidence of uplift and erosion. Picard and Goddard (2975) showed that these two geomorphological segments are separated by the Tacagua fault (Figure €9, at least to the south, and possibly also along the Mamo submarine canyon to the north. To the east of this fault, Sellier de Civrieux (1977) described evidence of a 60 to 70 m subsidence of the continental platform and emergence of the coast, between Playa Grande and Maiquetia. Schubert et al. (1977), based on isotopic ages of beach rock, calculated that the Coast Range was uplifted at a rate between 3 x lo-' and 6 x lo4 m yr-' during the last 2000 to 3000 yr. Galavis and Louder - (1970), in a general description of the Venezuelan continental platform, proposed that the Bonaire basin was limited in the south by a normal fault (Morbn fault zone). The Turiamo basin is oriented parallel to the coast and suggests a platform which has subsided by about 100 fathoms along the Mor6n fault zone. Rial (1973), on the basis of an analysis of seismic reflection profiles (Figures 4 and 5), concluded that the Tacagua tectonic depression (aligned with the Tacagua fault) continued on the continental platform to a latitude of about 1lo N and, consequently, denied the continuity of great east-west strike-slip

I '68'~ 1 '67. '66. I CARIBBEAN SEA

Fig. 4. Location map of acoustic reflection profiles presented in Figures 5,6, and 7. Sources of reflection profiles: (a) numbers 3,4,6,8,12, 13,14,17,20,23, and 24 are from Silver et al. (1975, profiles 22.24, 26,27,28,30,31,32,34,37,39,40 and 41); (b) numbers 1 and 2 are from Gonzilez de Juana et al. (1980, p. 755, profiles A and B); (c) numbers 5,7, and 9 are from Rial (1973, profiles A-A', C-C' and E-E'); (d) numbers 10,11, 15, 16,18,19,21,22,and A,B, and C are from Planesa (1975, profiles 7,10, 15, B, C, NaiguatP, Los Caracas, and La Sabana). I:; --" MORON-FAULT ZONE, VENEZUELAN BORDERLAND 265 faults along the north-central coast. Picard (1975) proposed that the Tacagua fault extended for more than 100 km in a northwest direction. Schubert (1982b) reviewed the structural data concerning the Cariaco basin and showed that it was formed as a pull-apart basin between an en echelon stepover in the Mor6n-El Pilar fault system. Recently, a similar origin has been postulated for other tectonic depressions in the Coast Range, such as the Lake Valencia and the Guarenas-Guatire basins (Schubert, in press).

2.3. GEOPHYSICALDATA Rodriguez and Grater01 (1975) published a gravimetric analysis of northwestern Venezuela, in which they suggested that the trace of the Oca fault zone separates a region of positive anomalies to the north from a region of negative anomalies to the south, across the Falcbn basin. Bonini et al. (1977) showed a strong north-south gravity gradient across the Coast Range (+I0 mgal to -60 mgal in 15 km), which they explained as being most probably due to a deep crustal discontinuity, with a vertical displacement of approximately 12 krn (south side down). Although this sense of displacement is opposite to that derived from all other data, this gravity gradient does suggest a major structural boundary near the coast. Published seismic data seems to be inconclusive concerning the existence of great east-west fault zones along the north-central Venezuelan coast (Fiedler, 1970; Shurbet, 1976; Fernindez et al., 1977). Epicenters of earthquakes (mb > 4.5) are shown in Figure 8 (Fiedler, 1981). Rial (1978) analyzed the Caracas earthquake of 1967, and concluded that it was produced by rupture along a left-lateral fault, oriented in a 350" direction (Tacagua fault). He calculated that this offset was of the order of 1.2 m, and that since the 1900 Caracas earthquake (Fiedler, 196I), tectonic strain of approximately 1.8 cm yr-l has accumulated. Rial (1978) also suggested that, if a large fault along the coast exists, it may be locked and could generate further earthquakes.

3. Definition and Neotectonic Character of the Moron Fault Zone

From the previous review, it is evident that the main obstacle to substantiating the existence of the Mor6n fault zone is the fact that, for the most part, it is submerged. Therefore, marine seismic data are indispensable for its detection. In this section, we have compiled all the acoustic reflection profiles which are available to the public. Figure 4 shows the location of 28 acoustic reflection profiles obtained, between east of Punta Tucacas and east of Cabo Codera; 17 profiles are oriented north-south, 6 are oriented north-east, 1 is oriented north-west, and 4 are oriented east-west. The main characteristicsof these acoustic profiles are shown in the line drawings of Figure 56,and 7. Detailed line drawings of profiles 4,6,8,13, 14,17,20,23, and 24 can be found in Silver et al. (1975); geological interpretations of profiles 1 and 2 were published by Godlez de Juana et al. (1980, p. 755). In this report, we C. SCHUBERT AND F. F. KRAUSE MORON-FAULT ZONE, VENEZUELAN BORDERLAND 267

Fig. 7. Simplified linedrawings of east-west acoustic reflection profiles A to D (for location, see Figure 4). Emphasis was made on locating faults. Depth is shown as two-way travel time (seconds). concentrate on the detection of important structural features, mainly faults and, where possible, sense of offset. In Figures 5 and 6, profiles are oriented as in the map, so as to show possible continuous faulting. In Figures 5 and 6, there are at least four rather clear east-west alignments of faults (from south to north): (1) a possible eastward continuation of the Bocon6 fault (at about 10" 40' N latitude, between profiles 1 and 9); (2) a possible eastward extension of the Macuto fault (at about 10" 40' N latitude, between profiles 15 and 22); (3) the eastern continuation of the Oca-Chirinos fault zone (striking in an east-northeast direction from about 10" 40'N to 11" 00'N latitude, across most profiles); (4) another alignment is that of the Tacagua fault, which strikes southeast between profiles 7 and 9, presumably joining the eastward extension of the Bocon6 fault. Figure 7 shows faulting on line drawings of the east-west profiles. The faulting on profile D is most probably associated with the eastward estension of the Oca-Chirinos fault zone. The horst shown in the eastern end of profile C may represent secondary faulting, associated with the Macuto fault and/or the Oca- Chirinos fault zone; similarly, the frequent minor faulting observed in profile B may be associated with the Macuto fault. The faulting near the eastern end of profile A represents the Tacagua fault, and the fault near the western end represents a continuation of the Bocon6 fault towards the Oca-Chirinos fault zone, or part of the eastward continuation of the Bocon6 fault. The Oca-Chirinos fault zone forms a well-defined trench (or graben) east of Tucacas (profiles 1 and 2). From -profiles 3 eastward, it forms a horst with clear evidence of minor uplift between profiles 8 and 24. The profiles suggest very 268 - C. SCHUBERT AND F. F. KRAUSE

recent offset involving the sea bottom. The possible eastern extension of the Bocon6 fault appears as a single break or several faults between profiles 4 and 6 to 12, and forms a small trench. Small normal offset (north side down) is indicated along most of these profiles. The possible eastern extension of the Macuto fault shows a single break, with normal offset of the northern block, and as a trench from profile 22 eastward, into the El Pilar fault zone and Cariaco basin (Schubert, 1982b). The Tacagua fault forms a trench bounded by several faults in profiles 7 to 9. It must be kept in mind that along north-south oriented acoustic reflection profiles, only normal or reverse offset can be detected; the fact that normal offset reverses itself along strike of the faults, and that the faults change in morphology, is evidence for strike-slip offset along these faults. We have compiled these data in the map interpretation displayed in Figure 8. Other structural features derived from the acoustic reflection profiles, such as folds and sedimentary thickness, were compiled by Silver er al. (1975, Figure 5) and are not included here. From Figure 8. it is evident that the Mor6n fault zone consists of

Fig. 8. Regional interpretation of faulting along the north-central Venezuelan coast, based on acoustic reflection profiles shown in Figures 5 to 7. Other structural features along the north-central Venezuelan borderland were published by Silver et al. (1975, Figure 5). Legend: (a) strike-slip fault; (b) direction of dip of fault plane in the profile; (c) inferred connection between faults in the profiles; (d) fault zones; (e) epicenter of earthquake (5.5 5 M, I7.2); (f) epicenter of earthquake (4.5 5 Mb 5 5.4); (9) basin; (h) area of uplift. Location of epicenter after Fiedler (1980). Abbreviations: AF: Avila fault; BB: Bonaire basin; BF: Bocon6 fault; BRF: Bruscas fault; CB: Cariaco basin; EPFZ: El Pilar fault zone; LVFZ: La Victoria fault zone; MAF; Macuto fault; MF Mor6n fault; 0-CFZ: Oca-Chinnos fault zone; TAF: TBcata fault; TFZ: Tacagua fault zone. MORON-FAULT ZONE, VENEZUELAN BORDERLAND 269 the following faults: (1) a northern, through-going, east-west fault zone (the eastern extension of the Oca-Chirinos fault zone), a suggestion also made by Silver et al. (1975, Figure 5); (2) a southern, probably through-going, east-west fault zone, consisting of the Bocon6, Morhn, and Macuto faults; and (3) many splinter faults, such as the Tacagua and Avila faults. In a right-lateral strike-slip regime, such faults bound crustal blocks which will be uplifted and depressed (basins and push-ups), depending on the local stress field. An example of an uplifted block is that between the Macuto and Avila faults (the area with plus signs in Figure 8), and which includes the highest and most abrupt elevations of the Coastal Range (2765 m above sea level). Depressed blocks are shown as minus signs, and consist mainly of pull-apart basins, formed along en echelon stepovers in the fault zones or at zones of fault divergence.

The geologically recent age of right-lateral offset along the BoconbMor6n-El Pilar fault system and other strike-slip faults of northern and western Venezuela, is suggested by: (1) the mainly Quaternary age of the sediments deposited within pull-apart basins developed along these faults (Schubert, 1980,1982b); (2) offset of Early Tertiary structural features, such as the frontal thrust of the Caribbean Mountains (Schubert, 1981): (3) offset of Late Mesozoic metamorphic terrain of the Caribbean Mountains (Schubert, 1981); (4) the fact that most faults (such as the Bocon6, La Victoria, and other strike-slip faults of the Caribbean Mountains) end as thrusts (Oxburgh, 1966; Murany, 1972). Therefore, it seems probable that right-lateral offset along the major fault zones of northern and western Venezuela is at most post-Middle Tertiary. It is possible that the right-lateral regime was initiated as the South American plate converged diagonally with the Caribbean plate in the north and the Santa Marta block in the west (Silver et al., 1975). This convergence in the west brought about compression and uplift (Venezuelan Andes) and right-lateral offset along the Bocond fault zone and the Caribbean Mountains. In this sense, all of these fault systems (Bocon6-Mor6-El Pilar and Oca Chirinos) are neotectonic systems (i.e. they are of Late Tertiary-Quaternary age). All of the faults within the Mordn fault zone show evidence of neotectonic activity in the acoustic reflection profiles. They offset the sea bottom and are characterized by prominent fault morphological features. Together with the Boconb and El Pilar fault zones, the Mor8n fault zone forms a long tectonic feature, which extends for over 1300 km between the Colombian border in southwestern Venezuela, to the east of Trinidad (Silver et al., 1975; Schubert, 1981, 1982a, 1982b). This fault system has been active in Late Pleistocene and Holocene time, and is responsible for most of the seismic activity in Venezuela (Molnar and Sykes, 1969; Fiedler, 1970). The Caracas earthquake of 29th July, 1967 (mb= 6.5, M, = 6.7) is the only one within the Mor6n fault zone which has been analyzed in detail (Rial, 1978). This 270 C. SCHUBERT AND F. F. KRAUSE author determined that there was 1.2 m of displacement along the left-lateral Tacagua fault (although the epicenter was located about 25 lun northeast of the fault trace, as located on the acoustic reflection profiles; see Figure 8), and that accumulated strain is approximately 1.8 cm yr-' since the 29 October, 1900, magnitude 7.4 Caracas earthquake (Fiedler, 1961). Wallace (1970) determined an empirical relationship to calculate recurrence intervals, which incorporates displacement, rupture length, long-term strain rate, aseismic slip, and total fault length. For the Caracas earthquake of 1967, one has to assume that rupture length is equal to total fault length, as it was a submarine earthquake; displacement was 1.2 m, and aseismic slip is 1.8 cm yr.-l To arrive at a value for the long-term strain rate, one can use the available apparent off set of Early Cretaceous formations by the Bocon6 fault across the western Caribbean Mountains (Schubert and Krause, 1981), which suggests a value of 1.6 x m yr.-l. Using these values in Wallace's (1970) relationship results in a recurrence interval of 67 yr for earthquakes of magnitude (M,)of 6.7. Of course, using a total length of the Mor6n fault zone of at least 200 km, would result in a shorter interval. This result is only indicative of the sort of value to be expected. Accurate values for rupture lengths and displacements associated with specific earthquakes within the Morh fault zone are needed in order to calculate reasonable intervals and maximum expectable earthquake magnitudes.

5. Conclusions Analysis of 28 acoustic reflection profiles along the north-central Venezuelan borderland, suggests that the Mor6n fault zone as outlined in Figure 8, consists of: (1) an eastward extension of the Oca-Chirinos fault zone (approximately parallel to latitude 10" 50'N) which ends as the northern master fault of the Cariaco pull-apart basin; (2) a probable eastward continuation of the BoconBMor6n faults along the north-central Venezuelan coast, which, north of Caracas, splits into the Avila and Macuto faults, and in turn, to the east, forms a series of east-west faults; (3) the Tacagua fault, which is a splinter fault of the Oca-Chirinos fault zone, and which strikes southeastward from about longitude 67' 30' W to join the junction of the Bocon6-Mor6n-Avila-Macuto faults. Thus, the Bocon6-Oca-Mor6n-El Pilar fault system forms a complex structural boundary along northern and western Venezuela. mset and fault activity varies, and has varied in the geologic past. All of these faults and fault zones are active, as shown by offset sea bottom, offset Pleistocene and Holocene features, and seismicity. Following Silver et al. (1975), the western Venezuelan crust (Bonaire block) has been thrust northeastward over the Caribbean crust, to form Curaqao ridge. The southern Caribbean plate boundary seems to be mainly represented by the Oca-Chirinos fault zone, the Moron fault zone, and the El Pilar fault zone, from west to east. North of these faults, according to Silver et al. (1975), the main tectonic evidence suggests northeast thrusting at least to 66OW longitude. MORON-FAULT ZONE, VENEZUELAN BORDERLAND 27 1

These conclusions suggest, in turn, that the Oca-Chirinos fault zone probably was part of the southern Caribbean plate boundary until the Bocon6 fault zone became part of this boundary (Late Tertiaty? -Quaternary), and the Bonaire block moved northeastward over the old plate boundary. In order to test these conciusions, a detailed seismic survey of the western end of the Mor6n fault zone has to be undertaken to unravel the details of fault chronology. Also, a close seismic network is needed along the whole of the north-central Venezuelan coast and the Netherlands and Venezuelan Antilles, to establish the relative fault activity and sense of motion. Neotectonic studies along other, accessible faults which belong to the system, such as the La Victoria and TBcata faults (Schubert, in prep) are needed to evaluate the seismicity of north-central Venezuela.

Acknowledgments We thank Planesa (Caracas) for providing copies of acoustic reflection profiles. The Direcci6n de Cartografia Nacional kindly allowed us to use side-looking radar images. J. Sebastian Bell reviewed and significantly improved the manuscript.

References

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