Structure of the Continental Margin of Southwestern Panama

Home , Caribbean Plate, Cocos Plate, Nazca Plate, South American Plate

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

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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

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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. 3b). As the and Recchi, 1976) reflects the initiation of north-south-trending transform faults jumped Geology of Western Panama and the ocean-ocean subduction and the development of westward, new slices of the Cocos plate became Gulf of Chiriqui a volcanic island chain. A possible unconformity attached to the Nazca plate. With several succes- separates Eocene shallow-marine clastics and sive transform jumps, the Nazca-Cocos-Carib- Geological studies of western Panama were volcanism from the underlying Mesozoic rocks bean triple junction swept westward across the made by Lloyd (1963), Olsson (1942), and (Terry, 1956). Surficial rocks of Coiba Island southern Panama continental margin (Figs. Terry (1956); Weyl (1980) provided an exten- and southern Sona Peninsula consist mainly of 3c-3e), so that by 2 m.y. ago, the triple junction sive review of the published literature on the Eocene: sediments (tuffs, arenites, carbonates) was situated at northwesternmost Panama and geology of Panama and of the rest of Central and volcanics (diabase and basalts; Metti and the once-convergent (Cocos-Caribbean) margin America. Published geologic information of the Recchi, 1976). of southern Panama became a (Nazca-Carib- Gulf of Chiriqui region is limited to the investi- While volcanic activity east of Azuero Penin- bean) transform margin. Jordan (1975) has in- gation by Metti and Recchi (1976) of Sona Pe- sula reached a maximum phase in Oligocene to ferred from adjacent plate pairs that the ninsula and Coiba Island, to a portion of Terry's early Miocene time, the peak in volcanism in Nazca-Caribbean margin had 6.5 ± 1.3 cm/yr (1956) reconnaissance study of entire Panama, western Panama occurred in mid-Miocene time left-lateral relative motion in an over-all east- and to Weyl's (1980) review. Geologic maps (Woodring, 1957). Early Miocene marine and northeast-west-southwest direction. No other have been released by Del Giuice and Recchi nonmarine clastic and volcaniclastic deposition gave way to widespread mid-Miocene dikes, flows, and tuffs. Sedimentary deposits include tuffaceous sandstones and carbonates. A major Mid-Oligocene Mid - Miocene

present land mass

subducted oceanic crust

hot spot ridge

andesitic volcanism

Figure 2. Development of Panama island arc (from Lonsdale and Klit- gorcl, 1978). a, 30 m.y. (mid-Olig- ocene); b, 15 m.y. (mid-Miocene); c, 9 m.y. (late Miocene); d, 4 m.y. (eai'ly Pliocene); e, 1 m.y. (Pleistocene).

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PRE-11 MY 10 MY 10 to 4.5 MY MULTICHANNEL SEISMIC PROFILE

General Description

An exploratory seismic reflection/refraction study of the Gulf of Chiriqui was conducted by Texaco in 1971. Five multi-channel seismic reflection profiles broadly covering the Gulf of Chiriqui were recorded by Panama Exploration, Inc. The seismic reflection data were obtained using a 48-channel cable with a group spacing of 50 m, a near-offset of 150 m, and a time sam- pling rate of 4 milliseconds. Shot spacing of the array of 16 air guns was set at 50 m, producing 2-0 MY 4.5-3 MY 2400% common depth point coverage. Process- ing by Texaco includes gain modification, statics, deconvolution, time-varying filtering, stacking, and amplitude normalization. Playback of six seconds of two-way traveltime represents a depth penetration of ~12 km. Three seismic refraction profiles along the seismic reflection tra- verses were recorded but were not available. In this paper, we discuss the results of the analysis of line 3 which was made available by Texaco. This seismic profile extends from near the Panama mainland, across the broadest portion of the continental shelf comprising the Gulf of Chiriqui just west of Coiba Island, out to the Figure 3. Termination of subduction under western Panama by ridge jumping (from Lowrie continental slope for a total length of 88 km. and others, 1979). a, pre-11 m.y.; b, 10 m.y.; c, 10 to 4.5 m.y.; d, 4.5 to 3 m.y.; e, 3 to 2 m.y.; Artifacts from the processing sequence are f, 2 to 0 m.y. By 2 m.y. to 0 m.y., subduction under Panama has ceased. found to exist on the seismic profile (Fig. 6). The entire seismic profile has undergone a down- ward static shift of .070 sea Adjustment for this delay allows for the identification of many multiple reflections. An airgun waveform of varying period of uplift, erosion, folding, and faulting of contains exposures of Mesozoic, Oligocene, and duration is associated with the sea-floor reflec- all of the Panamanian isthmus also occurred Miocene rocks along with undifferentiated Ter- tions. In some areas, shallow low-amplitude sed- during mid-Miocene time (Terry, 1956). tiary volcanics; the southern half contains pre- imentary reflections are characterized by an Late Tertiary-Quaternary deposition was con- dominantly Eocene and scattered Cretaceous airgun waveform .25 to .45 sec long with ampli- tinuous in western Panama. The base of the late exposures and is very similar to the geology of tudes strong enough to obscure the geological Miocene-Pliocene sequence is conglomeratic, Coiba Island (Metti and Recchi, 1976). The reflectors. In zones of strong amplitude, shallow grading into marine shales and siltstones. Terry same fault extends through the Azuero Penin- acoustic basement reflections, the airgun wave- suggested that Pliocene sediments which are sula to the southeast; post-Oligocene sediments form is not easily identifiable. found in Burica Peninsula near Costa Rica ex- and flows may cover the extension of the fault to tend in a thin belt under the Gulf of Chiriqui. No the northwest. A second regional fault which Results other outcrops are found in the main isthmus. separates the Nicoya and Osa Peninsulas in Terry also suggested that Pliocene deposition Costa Rica and Burica Peninsula in westernmost Interpretation of the line reveals a period of was due to a transgressive sea moving from the Panama from the mainland (B-B' in Fig. 4) was acoustic basement uplift followed by partial ero- northeast to the southwest over a major fault speculated by Terry to extend southeastward sion and subsequent unconformable deposition. block which was subsiding faster to the north- into the Gulf of Chiriqui. If correct, it may pos- Portions of the shallow, subhorizontal deposits east. Extensive lava flows are associated with sibly define the edge of the Gulf of Chiriqui were later folded due to faulting and truncated Mount Baru, Panama's largest volcano; these continental shelf. It is possible, on the other by erosion. A broad depression near the late Tertiary to Quaternary flows cover much of hand, that this regional fault curves or is offset southwestern end of the profile reveals a present- the previously exposed geology in western within the continental shelf under the Gulf of day subsiding basin. Panama (Weyl, 1980). Volcanism is not active Chiriqui and actually correlates with a fault The seismic profile shows two basement at present. found in the continental shelf or mainland. block uplifts separating two active basins (Fig. Structurally, western Panama is dissected by Coiba Island, in the Gulf of Chiriqui, is cut by 7). The outer basement uplift is located at the numerous straight, throughgoing faults which many faults (Fig. 5). Two possible prominent seaward edge of the continental shelf; the inner trend northwest-southeast (Weyl, 1980). A large faults on Coiba Island are parallel to the regional basement uplift is located in the middle of the regional fault oriented north-northwest-south- fault found on Sona Peninsula. Mapping by shelf. The (outer) basin between the two uplifts southeast (A-A' in Fig. 4) divides Sona Penin- Metti and others (1972) does not show any di- forms a bathymetric depression (Fig. 1, inset; sula into two portions. The northern half rection of movement for these faults. and Fig. 4). The second, inner, basin is located

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Time Geology Event

Recent al luvium Q

Pleistocen J a i a a a a Pliocene a marine 8 non-marine a a a late a clastics, volcanics, a a intrusives

Laaaaaaa ] f a L a ^aaaaaa J unconformity M iocene mid main volcanic pulse uplift, eros on T early marine 8 non-marine volcanic s clastics, tuffs, lavas, Oligocene intrusives Eocene volcanics-- basalts,diabase start of volcanics sediments; tuffs, carbonates unconformity carbonates,arenites, tuffs

early undifferentiate d 1u n late chert K ! J late ii1 basement (ophiolitic) complex I

Figure 4. Generalized geology of southwestern Panama (from Terry, 1956). Heavy dashed line represents location of seismic profile line 3. Bathymetry in metres. The Gulf of Chiriqui lies between Burica and Azuero Peninsulas. The continental shelf of southwestern Panama extends between the Panama coastline and the 1,000-m bathymetric contour.

between the mid-shelf basement uplift and the tions (events m) and an irregular surface charac- energy (events m). There are, however, coherent Panama coastline. terize this interface; portions represent an events which are visible. Intermediate reflections Each structural zone in the seismic profile re- unconformable contact. For much of the seismic (events B) between 2 and 4 sec are subhorizontal veals reflections ¡issociated with the acoustic profile, the top of the acoustic basement is sev- and occur intermittently across the seismic pro- basement, deeper reflectors below the acoustic eral cycles in length. This thickness is due to the file. Below 4 sec, there are several reflections basement, and recent sedimentation above the shot waveform and water-bottom multiples (events C) which exhibit greater dip than the acoustic basement. The acoustic basement (events m) which have not been totally removed intermediate reflections and when viewed as a (horizon A) is defined as the horizon separating by deconvolution. whole saggest a broad domal feature (C3). For the subhorizontal r.ear-sea-floor reflections from The entire seismic profile below 2.0 sec is both the intermediate and deep reflections the underlying, more-disturbed events. Diffrac- composed primarily of diffraction and multiple below the acoustic basement, the horizons can-

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highly discontinuous reflections (CDP 470- 560). The over-all acoustic basement horizon here is not subhorizontal, suggesting that numerous faults are present or that the horizon is an eroded surface. Fault F3 is active and penetrates the sea floor at CDP 560. Direction of movement of this fault is not well indicated by the seismic data; however, local offset of the acoustic basement suggests a normal fault with downdip motion to the southwest. A strike-slip component of motion perpendicular to the plane of the seismic profile is possible but is not detectable within the line. gure 5. Geology of Steeply dipping reflections or diffraction ia Island (from Metti limbs from the discontinuous reflections and others, 1972). Direc- multiples dominate to the bottom of the seismic of movement of faults profile. Deep events, however, can be identified lines) are not (C2). by Metti and Inner Basement Uplift

The zone of discontinuous acoustic basement found in the outer basin gently rises to 80 m beneath the sea floor within CDP 240-470. While the outer basement uplift resembles a not be extended across the seismic profile with a Surrounding sea-floor reflections show a notice- block uplift, the inner basement uplift appears to large degree of certainty. Insufficient two-way able change in dip, suggesting that this protru- be gently dipping to the south. This basement traveltime was recorded to receive possible re- sion is trapping sediments derived from the outer uplift is bounded by a major steeply dipping flections off the previously subducted oceanic basement uplift to the southwest and preventing normal fault to the northeast (F4). Reflections plate. the sediments from reaching the adjacent (outer) from sediments which extend from the north- The acoustic basement is covered with as basin. eastern portion of the outer basin show an onlap much as 1,600 m of recent sediments. Seismic Reflections below the acoustic basement are relationship with the underlying acoustic base- reflections of the recent sediments (events D) obscured by multiples and water-bottom diffrac- ment. The acoustic basement within the uplift is show recent and active deposition and structural tions (event m). Some, however, are present a fairly smooth and gently southwest-dipping deformation. (event Bl). surface which reaches a local high at CDP 265. Beneath the acoustic basement reflection, there Outer Basement Uplift Outer Basin are many generations of primary and water- bottom multiples of the acoustic basement. The outer basement uplift is located at the The outer basin between CDP 470-700 forms These multiples obscure up to 2 sec below the seaward edge of the continental shelf between a bathymétrie depression (Fig. 4) and appears to acoustic basement. CDP 720-870. It comes within 60 m of sea be actively subsiding. This basin is divided into Reflections below 4 sec (C3) suggest a deep, level. The acoustic basement is located near the two parts by an active high-angle fault (F3). To domal structure. Diffractions and multiples ob- sea floor; there is little sediment cover. Near-sea- the southwest of the fault (CDP 560-700), shal- scure the structural details, however. floor reflections are characterized by many sea- low sedimentary reflections are overprinted by floor multiples and an airgun waveform of the airgun wavelet associated with the sea- Inner Basin several cycles (events m). Two protrusions bottom reflection. Sediments at the southwest- above the sea floor exist to the northeast of the ern end of the basin (CDP 700) exhibit updip The inner basin (CDP 012-240) is character- basement block. Known geologic information thinning but are truncated against the acoustic ized by a discontinuous and faulted acoustic shows no evaporitic material in the stratigraphic basement by either Protrusion 2 or a steeply basement with overlying deformed sediments. sequence surrounding the shelf (Terry, 1956); dipping fault (Fl). Sediments northwest of fault The acoustic basement is cut by a large number thus the existence of salt diapirs is ruled out. F3 are relatively more subhorizontal and show of small normal faults across the basin; a moder- These protrusions may have a volcanic origin; onlap to the northeast. ate normal fault is located at CDP 130 (F5). zero-offset ray tracing also suggests that they Acoustic basement for the outer basin (event A series of high-angle normal faults (F6) dis- may be due to irregular basement block or sea- A) is characterized by strong continuous reflec- rupt the acoustic basement in the region of CDP floor topography. Protrusion 1 (event PI at tions which are sharply truncated and by highly 012-50. Reflections from recent sedimentation CDP 750) is off the plane of the seismic profile, discontinuous reflections with associated steep are obscured by sea-floor multiples; however, as sea-floor reflections on either side extend dips or diffractions. Acoustic basement under they can be seen to be tilted and folded above through the structure; the dips of these surround- the southwestern (most active) portion of the the faults. The sediment reflections in the region ing sea-floor reflections project toward each basin (CDP 560-700) is dissected and deformed of CDP 40-70 are truncated by the sea floor, other. Protrusion 2 (event P2 at CDP 720) is by normal faults F2 and F3. Acoustic basement suggesting that (1) movement of the faults and approximately in the plane of the seismic profile. to the northeast of fault F3 is characterized by (2) folding and uplift of the sediments are recent.

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ssw 870 800 700 600 50 0 420 o -J i I i i i- n

Figure 6. Texaco line 3. Sea level is at 0.070 sec due to a bulk static shift introduced by the original seismic processing.

DISCUSSION probably composed of (1) Late Jurassic Nicoya telltale, depression. The inner basement uplift Complex-equivalent opliiolite at the bottom, has been recently raised (fault F4), tilting gently The seismic profile (line 3) presented in this progressing upward to mid-Miocene marine sed- to the southwest so as to slightly increase the paper is the only multichannel profile in iments and volcanic rocks and (2) island- southwestward dip of the acoustic basement and southwestern Panama (Gulf of Chiriqui) in arc-related intrusions of pre-Miocene age. the overlying sediments. The sediments above available literature and is the first geophysical Fore-arc deformation of the basement may have fault F6 are folded and beveled at the sea floor, traverse to depict the structure of the entire con- occurred during the time of subduction. The indicating recent deformation and erosion asso- tinental shelf. The identification of chronologic exact structural relationship of the basement sed- ciated with fault movement. horizons within this seismic profile is not well iments, however, is difficult to identify in the constrained due lo the very limited geological seismic profile because the sediment reflections Faults in the Gulf of Chiriqui and geophysical information available. The are obscured by seismic artifacts. structural development of this area may be bet- The end of the mid-Miocene unconformity Regional faults described by Metti and Rec- ter understood by correlating the profile with the and the return of marine deposition to the chi (1976) on Coiba Island can be projected known surrounding land geology and by com- southwestern Panama continental shelf can be northwestward into the outer basin in line 3. paring it with another multichannel seismic pro- seen best in the inner basement uplift. Here, re- They can be further projected into the bathy- file which crosses the adjacent continental shelf flections from post-mid-Miocene sediments on- metric depression located some 30 km north- to the northwest. lap progressively onto the irregular acoustic west of the island. This suggests that the faults on basement toward the northeast. This onlap indi- Coiba Island are active. It is possible, therefore, Interpretation of the Seismic Profile cates the existence of a bathymetric high during that a large northwest-southeast-trending gra- the time of deposition. Other occurrences of ben or pull-apart basin extends for more than Using published geological studies of the sur- onlap are difficult to identify in other portions of 100 km from the bathymetric depression in the rounding land region, the history of the conti- the seismic profile due to recent deformation of northwest through Coiba Island to the sharp nental shelf may be reconstructed. A major the sediments and the acoustic basement. bend of the continental slope at 81°30'W period of uplift during mid-Miocene time has The faults which have produced the major (Fig. 8). The outer block uplift bounds the left a major erosional unconformity across structural blocks in the continental shelf of graben to the southwest and also extends for Panama. This unconformity surface is possibly southwestern Panama are still active. Faults F1 more than 100 km, exposed at the Mantuosa present in the seismic profile as the acoustic and F3 border the subsiding outer basin; fault Islands and at the small companion island south basement (A in Fig. 7). The thick basement is F3 penetrates the sea floor, leaving a small, but of Coiba Island.

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inner base- ssw outer basement uplift outer basin ment uplift 870 800 700 600 500 420

Figure 7. Line drawing of line 3. Horizontal line at 0.070 sec represents sea-level datum. Symbols: A, acoustic basement; B, intermediate reflections below acoustic basement; C, deep reflections; D, shallow sedimentary reflections; F, faults; m, multiples and/or diffractions; P, basement protrusions. Horizontal scale: 100 CDP = 10 km.

The Parida, Cavado, and Contreras Islands in nental margin. Jordan (1975), using adjacent Nazca-Cocos-Caribbean triple junction is lo- the inner continental shelf lie along a northwest- plate pairs, predicts this (Nazca-Caribbean) cated just to the southeast. The continental shelf southeast-trending line. Based on the interpreta- transform boundary to have left-lateral motion of the Gulf of Osa has developed while under tion of line 3, these islands protrude from the with an over-all east-northeast orientation. the influence of subduction since late Mesozoic inner basin portion of the shelf. Fault F4 sepa- Faults within the continental shelf of the Gulf of time. The continental shelf of the Gulf of Chiri- rates the inner basin from the inner basement Chiriqui are oriented northwest-southeast and qui, on the other hand, has been influenced first uplift at CDP 240; southwest of this fault, the are parallel to the curvature of the Panamanian by subduction from Mesozoic time to ~2 m.y. inner basement uplift comprises a broad, gentle isthmus. Within the continental shelf, these ago and since then by transform motion (Lons- southwesterly dipping region not marred by northwest-southeast faults, if treated as left- dale and Klitgord, 1978; Lowrie and others, faults or protruding islands. The northwest- lateral strike-slip faults, may bound active pull- 1978). southeast trend of the islands found in the inner apart basins found in the shelf; the series of small Geologic studies of Costa Rica have been basin suggests that the inner basin-inner base- normal faults associated with fault F5 in the presented by Dengo (1962), Henningsen (1966), ment uplift boundary (fault F4) is also oriented inner basin may be interpreted as step faults Victor (1976), de Boer (1979), Galli-Oliver northwest-southeast (Fig. 8) for at least 80 km. within one such pull-apart basin. The initiation (1979), Kuijpers (1980), and Lundberg (1982); Recent faults which are parallel to line 3 and and cause of the major northwest-southeast Weyl (1980) provided an excellent review. Sim- are adjacent to Coiba Island or Pavida Island faults can be better understood by comparing ilar to Panama, Costa Rica is underlain by ocean can account for marked lateral changes in the the continental shelf in the Gulf of Chiriqui to material, chiefly the ophiolitic Mesozoic Nicoya continental-shelf bathymetry (that is, southwest that of the Gulf of Osa, southern Costa Rica, Complex (Galli-Oliver, 1979). Early Tertiary of Parida Island). Such faults do not have pro- immediately to the northwest. rocks consist of deep-water marine volcanic tur- jections onto known surface outcrops or the bidites and mudstones; late Tertiary sediments Comparison with the Continental Shelf of the seismic profile. are of shallow-marine origin. Volcanism has Gulf of Osa, Costa Rica The regional northwest-southeast-trending continued throughout the geologic history of faults are probably part of the transform plate The Gulf of Osa is located at the diffused Costa Rica. Fischer (1980) interpreted bioero- boundary now active along the Panama conti- southern end of the Middle America Trench; the sional morphologic evidence along the Pacific

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Figure 8. The Gulf of Chiriqui. Geology denoted by patterns which are the same as those in Figure 4. Shading in Gulf of Chiriqui represents inner and outer basins as identified in seismic profile line 3. Small dotted lines are faults projected rom the downthrown sides Faults FI, F2, and F3 project onto Coiba Island and into the bathymetnc basin found m the continental shetf. Sense ot moTonZuufon Coiba Island is not shown iJ mapping by Metti and others (1972) but can be derived from the seismic profile. Bathymetry m metres. Interval distance of 100 CDP's on the seismic line is 10 km.

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coastline to suggest recent minor uplift along the Nicoya and Osa Peninsulas. Similarly, he has interpreted slight subsidence inland of these peninsulas. This rise and subsidence is interpreted by Fischer to have been caused by compressive movement related to current subduction of the Middle America Trench. Two major multichannel seismic reflection surveys were produced by the University of Texas Institute for Geophysics during 1977— 1978, resulting in 17 seismic profiles represent- ing more than 1,500 km of coverage (Fig. 9a). Crowe and Buffler (1984, 1986) present several profiles crossing the Middle America Trench and the continental margin along Costa Rica. Figure 9b shows another profile which extends across most of the continental shelf of the Gulf of Osa and is nearest to Panama. This seismic profile (NCY-7) and its line interpretation were made available by R. T. Buffler (unpub. data). Line NCY-7 reveals the Middle America Trench, the continental slope, and the Figure 9a. Location of seismic surveys by the University of Texas Institute for Geophysics continental shelf (CDP 450-2300+). When the during 1977-1978 (from Crowe and Buffler, 1984). Seismic line NCY-7 is the southeastern- continental shelves in the Gulf of Osa (line most line. NCY-7) and in the Gulf of Chiriqui (line 3) are compared, similarities and differences are ap- parent. The prominent unconformity in line NCY-7 has seismic character similar to the acoustic basement in line 3. Both horizons occur generally between 1 and 2 sec on the profiles. SW NCY-7 IO km NE Basement in both seismic profiles reveal scattered reflections but are heavily obscured by 0200 0000 seismic artifacts and noise. The subhorizontal reflections above the acoustic basement in line NCY-7 reveal internal deformation similar to the reflections at the far northeastern portion of line 3. A major difference between the two seismic profiles is the presence of the prominent structural blocks in the Gulf of Chiriqui which are not present in the Gulf of Osa. The structural blocks and their associated bounding faults have clearly altered the southwestern Panama continental shelf. SW NCY-7 10 km NE The similarities and differences between the two continental shelves may be explained by the 0900 0800 0400 0000 2300 Nazca-Cocos-Caribbean triple junction which is 0 currently located between the two continental shelves. The features seen in the Gulf of Osa 2 were formed while that continental shelf was adjacent to an active subduction zone (late 4 Mesozoic to present). The continental shelf in the Gulf of Chiriqui (line 3) was also adjacent to the same subduction zone until ~2 m.y. ago. 6 This can account for the similar development of the acoustic basement, basement, and sedimen- e tary reflections above the acoustic basement. Sec With the northwest migration of the triple junction, subduction along the Gulf of Chiriqui was Figure 9b. Seismic line NCY-7 with interpretations, courtesy of Richard Buffler. Note converted into a left-lateral transform motion. similarity of prominent unconformity to the acoustic basement in line 3 (Panama).

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The major structural blocks were probably Faults found in the Gulf of Chiriqui extend 1986, Multichannel seismic records across the Middle America Trench and Costa Rica-Nicaragua convergent margin, NCY-7 and NIC-1, in created by transform-related faulting. Presuma- across the continental shelf (Fig. 8) parallel to Ladd, J. W„ and Buffler. R. T.. eds., Middle America Trench off western Central America; Atlas 7, Ocean Margin Drilling Program, Re- bly, in a few milli on years when the triple junc- the shelf s trend. Major faults extend from the gional Atlas Series: Woods Hole, Massachusetts, Marine Science tion will have migrated farther to the northwest, shelfs bathymétrie depression in the southwest International, Woods Hole, sheet 12. De Boer, J., 1979, The outer arc of the Costa Rican orogen (oceanic basement the continental shelf in the Gulf of Osa will be through the outer basin shown on the seismic complexes of the Nicoya and Santa Elena Peninsulas): Tectonophysics, v. 56, p. 221-259. subjected to similar faulting and may develop profile (line 3), across Coiba Island and into the Del Giuice, D-, and Recchi, G., 1969, Geologica del area del proyecto minero structural deformation as seen today in the Gulf continental slope (Fig. 8). This suggests that a de Muero: Administración de Recursos Minerales, República de Pan- ama, Mapa 5, scale 1:250,000. of Chiriqui. northwest-southeast graben more than 100 km Dengo, G., 1962, Tectonic-igneous sequence in Costa Rica, in Petrologic studies (Buddington volume): New York, Geological Society of America, in length may exist in the outer shelf. Within the p. 133-161. inner continental shelf, the fault which defines Fischer, R., 1980, Recent tectonic movements of the Costa Rican Pacific coast: CONCLUSIONS Tectonophysics, v. 70, p. T25-T33. the inner basement uplift in the seismic profile Galli-Olivier, C., 1979, Ophiolite and island-arc volcanism in Costa Rica: Geological Society of America Bulletin, Part I, v. 90, p. 444-452. The continental shelf of southwestern Pan- (line 3) may extend for more than 80 km in a Henningsen, D., 1966, Notes on stratigraphy and paleontology of Upper Cre- northwest-southeast direction. The faults in the taceous and Tertiary sediments in Southern Costa Rica: American As- ama was adjacent to an active subduction zone sociation of Petroleum Geologists Bulletin, v. 50, p. 562-566. from the late Mesozoic Era to ~2 m.y. ago. Due continental shelf are parallel to the regional Kuijpers, E. P., 1980, The geological history of the Nicoya Ophiolite Complex, Cosía Rica, and its geotectonic significance: Tectonophysics, v. 68, to a series of fracture jumps, the Nazca-Cocos- faults in southwest Panama; it is possible that p. 233-255. these faults are predominantly left-lateral strike- Jordan, T. H., 1975, The present-day motions of the Caribbean plate: Journal Caribbean triple junction moved parallel to of Geophysical Research, v. 80, p. 4433-1439. Panama toward Costa Rica (Lonsdale and slip faults which contain large components of Lloyd, J. J., 1963, Tectonic history of the south Central American orogen: American Association of Petroleum Geologists Memoir 2, p. 88-100. Klitgord, 1978; Lowrie and others, 1979). With normal motion and are part of the transform Lonsdale, P., and Klitgord, K. D., 1978, Structure and tectonic history of the plate boundary in this area. With further migra- eastern Panama Basin: Geological Society of America Bulletin, v. 89, this migration, subduction along Panama was p. 9.31-999. converted to left-lateral transform motion, as tion of the triple junction, structural blocks such Lowrie, A., 1978, Buried trench south of the Gulf of Panama: Geology, v. 6, p. 434-436. suggested by Jordan (1975) from plate-motion as those seen in southwestern Panama may de- Lowrie, A.. Aitken, T., Grim, P., and McRaney, L., 1979, Fossil spreading velop in the continental shelf of southern Costa center and faults within the Panama Fracture Zone: Marine Geophysi- data; however, direct evidence of this direction cal Research, v. 4, p. 153-166. of motion has not been collected. Rica. Lowrie, A., Stewart, J., Stewart, R. H., van Andel, Tj., and McRaney, L., 1982, Location of the eastern boundary of the Cocos plate during the Mio- A seismic profile across the southwestern Pan- cene: Marine Geology, v. 45, p. 261-279. Lundberg, M., 1981, Evolution of the slope landward of the Middle America ama continental margin in the Gulf of Chiriqui ACKNOWLEDGMENTS Trench, Nicoya Peninsula, Costa Rica, in Forearc geology: London, England, Geological Society of London, p. 431-447. results in new information about the continental Metti, A., and Recchi, G., 1976, Geologia de la peninsula de Sona e Isla de shelf. The structures revealed by the seismic pro- Coiba: Segundo Congreso Lationamericano de Geologia, p. 541-553. The writers would like to thank Texaco Metti, A., Recchi, G., and Esquirel, D., 1972, Mapa geologico Sona-Isla de file reflect the change in plate-margin tectonics. LA/WA, which has graciously donated the use Coiba: Ministerio de Comercio e Industrias, República de Panama, scale 1:250,000. The acoustic base ment in the line may correlate of the seismic profile in the Gulf of Chiriqui. We Ministerio de Comercio e Industrias, República de Panama, 1976a, Mapa with a mid-Miocene unconformity which ex- geologico de Panama, scale 1:1,000,000. appreciate the efforts of Barry Katz, Texaco, 1976b, Mapa geologico region occidental Chiriqui Bocas, series HOJA- tends across all of Panama. Basement is com- USA, who steered our correspondences to 2, scale 1:250,000. Minster, J. B., and Jordan, T., 1978, Present day plate motion of the Caribbean posed of late Mesozoic ophiolites and cherts Texaco always in the right direction. We also plat:: Journal of Geophysical Research, v. 83B, p. 5331-5354. progressively up to mid-Miocene marine sedi- wish to thank Richard Buffler, who has allowed Olsson, A. A., 1942, Tertiary deposits of northwest South America and Panama, in American Scientific Conference, Proceedings, v. 8, ments and volcanics, reflecting the initiation of us the use of seismic data collected off Costa p. 187-231. Pennington, W. D., 1981, Subduction of the eastern Panama basin and seismo- Farallon plate subduction and the development Rica by the University of Texas. In-depth re- tectonics of northwestern South America: Journal of Geophysical Re- of the Panama island arc. The subhorizontal re- views were provided by Allan Lowrie and Paul search, v. 86, p. 10753-10770. Ross, D. A., and Shor, G. G., Jr., 1965, Reflection profiles across the Middle flections above tie acoustic basement indicate Mann. This work was supported by National America Trench: Journal of Geophysical Research, v. 70, p. 5551-5572. that after the mid-Miocene unconformity the Science Foundation Grants EAR81-09294 and Terry, R., 1956, A geological reconnaissance of Panama: California Academy continental shelf was subjected to marine depo- EAR83-06406. of Science Occasional Paper 23,99 p. van Andel, Tj., Heath, G. R„ Malfait, B. T„ Heinrichs, D. F„ and Ewing, J. I., sition with deformation associated with trans- 1971, Tectonics of the Panama basin, eastern equatorial Pacific: Geo- logical Society of America Bulletin, v. 82, p. 1489-1508. form faulting. van Andel, Tj., Heath, G., Bennett, R., Bukry, J., Charleston, S., Cronan, D., Dinkelman, M., Kaneps, A., Rodolfo, K„ and Yeats, R„ 1973, Ship- The continental shelf of southwest Panama board site reports 155-158, 1973, in Kaneps, A., ed., Initial reports of REFERENCES CITED the Deep Sea Drilling Project, Volume 16: Washington, D.C., U.S. underwent a history similar to that of the adja- Government Printing Office, p. 19-230. cent continental shelf located in the Gulf of Osa, Bandy, O. L., 1970, Upper Cretaceous-Cenozoic paleobathymetric cycles, Victor, L., 1976, Structures of the continental margin of Central America from eastern Panama and northern Colombia: Gulf Coast Association Geo- northern Nicaragua to northern Panama [M.S. thesis]: Corvallis, southern Costa Rica. A seismic profile recorded logical Society Transactions, v. 20, p. 181-193. Oregon, Oregon State University, 76 p. Bandy, O. L., and Case, R. E., 1973, Reflector horizons and paleobathymetric Weyl, R., 1980, Geology of Central America: Berlin, West Germany, Gebruder across the Gulf of Osa, where subduction still history, eastern Panama: Geological Society of America Bulletin, v. 84, Borntraeger, 371 p. occurs, reveals a profile with an acoustic base- p. 3081-3086. Woodring, W. P., 1957, Geology and paleontology of Canal Zone and adjoin- Bnceno-Guarepe, L. A., 1978, The crustal structure and tectonic framework of ing parts of Panama: U.S. Geological Survey Professional Paper 306-A, ment, basement, and shallow sedimentary reflec- the Gulf of Panama [M.S. thesis]: Corvallis, Oregon, Oregon State 145 p. University, 71 p. tions similar to those in the Gulf of Chiriqui. A Buffler, R. T., 1982, Geologic structure of the forearc region off the west coast major difference, however, exists between the of Costa Rica in the vicinity of the Nicoya Peninsula: Results of a multifold seismic reflection survey: University of Texas Institute for two continental shelves. Recently developed Geophysics Technical Report, v. 37, 56 p. Case, J. E., 1974, Oceanic crust forms basement of eastern Panama: Geological structural block u plifts and bounding faults exist Society of America Bulletin, v. 85, p. 645-652. in the Gulf of Chiriqui, but not in the Gulf of Crowe, J. C., and Buffler, R. T., 1984, Regional seismic reflection profiles across the Middle America Trench and convergent margin of Costa Osa. This suggests that the uplifts and faults are Rica, in Bally, A. W., ed., Seismic expression of structural styles—A MANUSCRIIT RECEIVED BY THE SOCIETY APRIL 4,1985 picture and work atlas: Tulsa, Oklahoma, American Association of REVISED MANUSCRIPT RECEIVED APRIL 20, 1987 related to the transform faulting. Petroleum Geologists, p. 3.4.2-147-3.4.2-162. MANUSCRIIT ACCEPTED APRIL 29, 1987

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