University of Calgary PRISM: University of Calgary's Digital Repository Science Science Research & Publications 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.