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Structure of the Continental Margin of Southwestern Panama

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Structure of the Continental Margin of Southwestern Panama Structure of the continental margin of southwestern Panama DAVID A OK AY A* 1 ZVI BEN AVRAHAMt J department of Geophysics, Stanford University, Stanford, California 94305 ABSTRACT tinental shelf with the Panama seismic profile (1974); Briceno-Guerape (1978); Lowrie reveals an acoustic basement with similar (1978), who presented seismic data across the In southern Panama, a change from con- characteristics, but it is not cut by the high- now inactive trench and continental slope; Low- vergent to transform plate motion at the con- angle faults which are present in the Gulf of rie and others (1982); and Woodring (1957). tinental margin influenced the history of the Chiriqui. These faults may have been intro- Little published literature is available regarding associated continental shelf. Normal subduc- duced in Panama by the conversion from a the structure or geology of the Gulf of Chiriqui tion of the Faralllon and Co cos plates near convergent to a transform plate boundary. located on the southwestern continental shelf. Coiba Island (82° W, 7.5°N) off southern Pan- Several of these faults may be projected Ross and Shor (1965) presented a single-channel ama ceased —3—£1 m.y. ago when left-lateral across the continental shelf under the Gulf of seismic profile with an accompanying simplified transform motion, was introduced due to the Chiriqui for more than 100 km. line drawing across the outer slope and shelf passage of the Nazca-Cocos-Caribbean triple break under the Gulf of Chiriqui. No gravity or junction. Subduction of the Farallon and INTRODUCTION magnetic data in the Gulf of Chiriqui have been Cocos plates during the Tertiary Period was published. The nearest pertinent surface geologic accompanied by the landward development The development of the continental shelf off study is a report by Metti and Recchi (1976) on of a volcanic basement with a thick overlying southern Panama has been related to subduction Sona Peninsula and Coiba Island; Terry (1956) sedimentary sequence. Mid-Tertiary uplift of of the Farallon, Nazca, and Cocos plates. and Weyl (1980) presented the regional geology the basement resulted in an unconformable Panama is part of the southern Central Ameri- of Panama. A geologic map of Panama is avail- surface which since subsided, was covered can volcanic island arc superimposed over able from the Ministry of Commerce and Indus- with shelf deposits, and was structurally Mesozoic oceanic crust (Case, 1974). Subduc- try (1976a). faulted and folded. tion of the Nazca and Cocos plates during the Here, we present multichannel seismic data An industry 24-fold multichannel seismic Tertiary Period was accompanied by the devel- across the width of the Gulf of Chiriqui, provid- profile which crosses the continental shelf on opment of a volcanic basement with a thick ing new information on the structure and sedi- the Gulf of Chiriqui, near Coiba Island, is overlying sedimentary sequence. The structural ments of this segment of the shelf. We then presented. The outer continental shelf here is and sedimentary development of the southern compare this seismic data with seismic data in topographically downbowed by a sedimen- Panama continental shelf has been influenced by Costa Rica (Buffler, 1982; Crowe and Buffler, tary basin. The mid-Tertiary acoustic base- this subduction. The subduction which had been 1986), where subduction is now taking place ment is about 1 km beneath the basin and occurring since the end of the Mesozoic Era northwest of the Nazca-Cocos-Caribbean triple rises to the ocean floor between the basin and ceased ~2 m.y. ago when left-lateral transform junction, in order to study the differences in the the shelf break. It probably represents a mid- motion was developed due to the northwest- structure and sedimentary pattern of a continen- Miocene erosion:! surface of the volcanic ward migration of the Nazca-Cocos-Caribbean tal shelf in response to subduction and transform basement. Disruption of the acoustic base- triple junction (Lonsdale and Klitgord, 1978). plate motion. ment and the overlying sedimentary reflectors This northwestward migration was performed indicates Quaternary episodes of normal through a series of ridge/transform jumps (Van PANAMA CONTINENTAL MARGIN faulting associated with the transform Andel and others, 1971; Lowrie and others, boundary. 1979). As a result, the continental shelf has been The over-all plate configuration for southern subjected to various deformation processes, The Gulf of Osa, Costa Rica, is situated Central America is shown in Figure 1. Five primarily block faulting, associated with the northwest of the triple junction and is pres- plates are now interacting in this area: the North transform motion. ently undergoing subduction. Comparison of American, Caribbean, Cocos, South American, a seismic profile across the Gulf of Osa con- The continental shelf off southern Panama and Nazca plates. The rate of subduction of the can be divided into two halves separated by the Cocos plate under the North American and Azuero Peninsula (Fig. 1)—the Gulf of Panama Caribbean plates is ~9 cm/yr (Minster and * Present address: CALCRUST, Earth Science Divi- on the southeast and the Gulf of Chiriqui on the Jordan, 1978); subduction of the Nazca plate sion, Lawrence Berkeley Laboratory, Berkeley, Cali- southwest. Geological and geophysical studies of under the South American plate is at a compar- fornia 94720. the southeastern continental shelf in the Gulf of able rate (Van Andel and others, 1971). Al- tOn leave from the Department of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv, Panama and of the surrounding area include though a spreading ridge separates the Cocos Israel. Bandy (1970); Bandy and Case (1973); Case and Nazca plates, their over-all movement is Geological Society of America Bulletin, v. 99, p. 792-802,9 figs., December 1987. 792 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/6/792/3998438/i0016-7606-99-6-792.pdf by guest on 04 October 2021 STRUCTURE OF CONTINENTAL MARGIN, SOUTHWEST PANAMA 793 90° 85e Figure 1. Over-all plate configuration for southern Central America. NOAM, North American plate; CARIB, Caribbean plate; COCOS, Cocos plate; SOAM, South American plate; NAZCA, Nazca plate; go, Gulf of Osa; gc, Gulf of Chiriqui; gp, Gulf of Panama; ap, Azuero Peninsula. Arrows indicate direction of relative plate motion. Bathymetry in metres. Solid circles represent active volcanoes. Inset: western Panama. Symbols: gc, Gulf of Chiriqui; ci, Coiba Island; ap, Azuero Peninsula; gp, Gulf of Panama. Solid line west of Coiba Island is the approximate location of seismic profile line 3. Bathymetry in metres. Bathymetric depression west of Coiba Island reaches below 200 m. eastward, relative to the Pacific plate. A series of speculated (Jordan, 1975). The Cocos and Car- developed as an island arc due to intraplate sub- north-south transform faults offsets the Cocos- negie ridges are thought to have been produced duction (Case, 1974). Ocean-ocean subduction Nazca spreading ridge; the Panama Fracture by volcanic activity associated with the Galapa- within the ancestral Farallon plate began during Zone (PFZ) is the easternmost of these faults gos hotspot (Lonsdale and Klitgord, 1978). late Mesozoic time. By mid-Oligocene time, the and defines the eastern boundary of the Cocos Several tectonic evolutionary schemes of Panama island arc was well developed (Fig. 2a; plate. Lowrie (1978) has shown a buried, inac- Panama have been published (Van Andel and Lonsdale and Klitgord, 1978). Spreading started tive subduction zone under the southern others, 1971, 1973; Case, 1974; Lonsdale and at the location of a former major transform fault Panama margin, where currently a broad zone Klitgord, 1978; Lowrie and others, 1979; also which originated from the East Pacific Rise and of transform motion with left-lateral motion is see Figs. 2 and 3). The Panamanian isthmus divided the Farallon plate into the Nazca and Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/99/6/792/3998438/i0016-7606-99-6-792.pdf by guest on 04 October 2021 794 OKA YA AND BEN-AVRAHAM Cocos plates. Activation of the Galapagos hot- geologic or geophysical evidence has been pre- (1969) for the Azuero Peninsula, by Metti and spot over which the new spreading center was sented for this boundary. others (1972) for Coiba Island, and by the Min- located during Miocene time (Fig. 2b) created The PFZ, suggested to be the youngest of the istry of Commerce and Industry (1976b) for the hotspot trails (a.ncestral Cocos and Carnegie fracture zones due to its lack of sediment fill Chiriqui Bocas region. The geology of western ridges) on both the Nazca and Cocos plates. By (Lowrie and others, 1979), marks the current Panama and the Gulf of Chiriqui probably re- late Miocene time, the Nazca-Cocos-Caribbean boundary between the Cocos and the annexed flects a volcanic island-arc origin due to ocean- triple junction was located south of eastern Nazca plates. Cessation of active volcanism on ocean subduction (Fig. 4). Panama with subduction of the Cocos plate the Panama mainland began in eastern Panama Oldest rocks consist of Mesozoic pillow ba- under southwest Panama still taking place and swept westward (Terry, 1956), following salts and diabase, equivalent to the ophiolitic (Fig. 2c). A more detailed scheme for the past the movement of the triple junction. Seismicity Nicoya Complex found in Costa Rica (Kuijpers, 10-m.y. period is given by Lowrie and others studies by Molnar and Sykes (1969) and Penn- 1980; Lundberg, 1981). The Cretaceous pro- (1979). At ~ 10 m.y. ago, the eastern edge of the ington (1981) have found evidence for a gression from cherts to andesites and diabases Cocos plate begin to migrate westward as a beginning of a new westward jump of the PFZ. overlain by carbonates, arenites, and tuffs (Metti result of transform jumps (Fig.
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