TJEERD H. VAN ANDEL" G. ROSS HEATH BRUCE T. MALFAIT Department of Oceanography. Oregon State University. Corralhs, Oregon 97331 DONALD F. HEINRICHSj JOHN I. EWING Lamont-Doherty Geological Observatory. Columbia University. Palisades. New York 10964

Tectonics of the Panama Basin, Eastern Equatorial Pacific

ABSTRACT from being fully understood. Similar enigmatic The Panama Basin includes portions of the features are found at complex boundaries be- Nazca, Cocos and South America Hthospheric tween continental and oceanic plates. plates and borders the Caribbean plate. The In this paper we describe and attempt to ex- complex interactions of these units have largely plain the morphological and structural features determined the topography, pattern of faulting, of such a complex region; the area bordered on sediment distribution, and magnetic character the east and north by South and Central of the basin. Only heat flow data fail to corre- America, and on the south and west by the late with major structural features related to Carnegie and Cocos Ridges. This region (Fig. these units. 1) contains the aseismic Cocos and Carnegie The topographic basin appears to have been Ridges, portions of the Peru and Middle created by rifting of an ancestral Carnegie America Trenches, an actively spreading east- Ridge. The occurrence of a distinctive smooth west rift zone, several major fracture zones, a acoustic basement and a characteristic overly- complex continental margin between the ex- ing evenly stratified sedimentary sequence on treme ends of the two trenches, and the large virtually all elevated blocks in the basin suggest volcanic block of the Galapagos Islands. It en- that they all once formed part of this ancestral compasses portions of the Pacific, Nazca, South ridge. The present Carnegie Ridge is the rela- America and Caribbean Hthospheric plates. tively undeformed southern half of this feature, This paper synthesizes a large volume of geo- while the Cocos Ridge is the northern half frag- physical data obtained by the Lamont-Doherty mented by left-lateral north-south transcurrent Geological Observatory and Scripps Institution faulting. As blocks of the Cocos Ridge reach of Oceanography prior to 1969, and on cruise the Middle America Trench, they appear to YALOC-69 of Oregon State University. Al- clog the subduction zone and become welded though we offer a hypothesis for the tectonic to the Nazca plate. Thus, the active transform evolution of the region, the available data are fault at the eastern edge of the Cocos plate has insufficient for a full test. The validity of this episodically shifted west as segments of the hypothesis depends in part on an analysis of the trench were deactivated. Such a shift appears to tectonic evolution and the plate movements in be occurring at the present time. adjacent areas, an aspect we hope to return to in a later paper. INTRODUCTION The plate tectonics concept as proposed by RELIEF Morgan (1968), LePichon (1968), and Isacks The first reasonably detailed bathymetric and others (1968), has linked tectonic chart of the Panama Basin was prepared by phenomena at mid-ocean ridges and trenches, Chase (1968). We have revised and updated and provided a simple tectonic model for large this chart using new data obtained in 1969 by portions of the ocean. In many cases, however, Scripps Institution and Oregon State Univer- the details remain obscure. Where three or sity, with some supplementary information more plates join, or at locations where large from the Lamont-Doherty Geological Observa- changes in direction and rate of motion have tory. The new chart is based on approximately occurred, tectonic features exist that are far twice as much data as the old one. The two are

Geological Society of America Bulletin, v. 82, p. 1489-1508, 13 figs., June 1971

1489

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Figure 1. The Panama Basin. Track lines represent are major fracture zones, double dashed lines are axis of seismic reflection profiles taken by Lamont-Doherty rift zone (

similar in gross aspect but differ significantly in Peru Trench and, as a continuous topographic detail. The line spacing compared to the com- unit, the Middle America Trench, do not ex- plexity of the relief is adequate for a broad tend into the Panama Basin beyond the land- topographic synthesis (Fig. 2), but fails to do ward terminations of the Carnegie and Cocos justice to the complexity of the relief. More Ridges. detailed surveys are in preparation for some The Cocos and Carnegie Ridges are similar portions of the basin (P. J. Grim, 1970, oral in profile. Both have relatively level, undulat- commun.). It has not been feasible to incorpo- ing crests studded with small pinnacles and rate these surveys into the regional chart. ledges. Both are bordered by steep slopes The Panama Basin can be subdivided into (Figs. 3 and 4) which descend in steps toward four physiographic units: the Cocos and Carne- the adjacent deep ocean floor. The Carnegie gie Ridges; the rugged, low-lying basin en- Ridge is simple in outline and has a marked closed by them and by a major east-west trend with a northeastward curve at its north-south-trending fracture zone at 83° W. eastern end. The Galapagos volcanic pedestal long.; and a highly fractured eastern basin con- forms its western termination. A broad saddle sisting of several marginal troughs, high blocks, in the center of the ridge separates shoaler east- and intervening rugged deep-sea floor. The ern and western areas. The Peru Trench shoals

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abruptly northward and loses its identity in the the East Pacific Rise near 102° W., 2° N. The gap between the ridge and the continental mar- rift separates the Cocos and Carnegie Ridges gin. north of the Galapagos pedestal and terminates The Cocos Ridge has a more complex out- against the Coiba fracture zone. Within the line, being bordered by alternating north-south western basin the rift is offset northward by and east-west-trending marginal slope segments several smaller fracture zones (Molnar and which give it a distinctive staggered shape. The Sykes, 1969; Grim, 1970a; Heinrichs, in Cocos and Carnegie Ridges do not meet at the prep.). A large fracture zone also extends Galapagos block but are separated by a broad northward from the western end of the Galapa- low zone 2000 to 2600 m deep studded with gos, but is not well defined by our topographic pinnacles and small seamounts and, near the information. Galapagos pedestal, a few larger volcanoes. At The eastern part of the Panama Basin, en- its northeastern end, the Cocos Ridge joins the closed by the Coiba fracture zone, the continental margin. Bathymetric and seismic northeastern hook of the Carnegie Ridge, and reflection control show that the Middle the continental margin, has a complex relief. It America Trench ends against the western slope contains several steep-sided high blocks that are of the ridge with no evidence of the trench or similar in relief and sediment cover to the even a buried equivalent east of 84° W. long. A Carnegie and Cocos Ridges but are much deep, sediment-filled round depression occurs smaller. The two principal blocks are the Coiba between the Cocos and Coiba Ridges at the end and Malpelo Ridges. These blocks are located of the Coiba fracture zone, but it is structurally in an undulating terrain of variable and some- very different from the Middle America times considerable roughness with a regional Trench. depth ranging from 3000 to 3600 m. Our data The Carnegie and Cocos Ridges merge are not adequate to establish detailed trends gradually into the Pacific basin. Several cross- within the deeper terrain, except in the south- ings south of the Carnegie Ridge show this east where a central down warp of 500 to 2000 transition to be interrupted by broad terraces m defines the narrow and sharply bounded with steep outer slopes. Yaquina graben (Fig. 3, profiles 3 and 4). The area between the two ridges is occupied The continental margin is paralleled by a se- by a broad basin that deepens from 2200 m in ries of rather broad, shallow, elongate troughs. the west to 3400 m in the east, where it is Seismic reflection and gravity data show that bounded at 83° W. long, by a series of narrow these are the surface expressions of much more elongate north-south trending troughs and pronounced subsurface depressions. They be- ridges. From the distribution of earthquakes gin just north of the point where the Carnegie and analysis of focal mechanisms at 83° W. Ridge and the continent approach each other long., Molnar and Sykes (1969) named this most closely, but are separated from the Peru boundary the Panama fracture zone (Grim, Trench by a shallow saddle. Swinging around 1970a). In this paper it is referred to by its in a single or double arc, the trough sequence more appropriate name, the Coiba fracture terminates against the eastern edge of the Coiba zone, after the Coiba Ridge of which it forms Ridge, which merges into the continental mar- the western scarp. Although only one of several gin. fracture zones in the Panama Basin, the Coiba fracture is prominent because it separates the STRUCTURE morphologically rather simple western basin Much of the Panama Basin is covered with from the highly complex topography of the sediment ranging up to 1.5 km in thickness. A eastern basin. large number of seismic reflection profiles pro- In the western basin, the bathymetric data are vide information on the nature of this sediment insufficient to portray the relief in detail or to and the depth to basement. The newer records, establish the existence of morphological trends. particularly those of the YALOC-69 cruise, Although relatively smooth on the scale of Fig- have the high resolution necessary to reveal ure 2, individual crossings show numerous fine structural details. ridges or hills and small valleys with relief of a Even cursory inspection of these records few hundred meters (Fig. 4, profiles 8 and 9). (Fig. 5) suggests the presence of numerous The low western basin, paradoxically, is the faults. Only rarely are faults directly recorded site of the eastern extension of the Galapagos by seismic profiling methods. We have inferred rift zone, which begins at a triple junction with their presence from the following criteria: ver-

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Figure 3- Topographic and structural cross sections basement shown with dotted pattern; overlying sedi- Assumed velocity of sound in sediments 2 km/sec. See of major relief features of the Panama Basin. Acoustic ment blank. Only the most important faults are shown. Figure 1 for location of profiles.

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Figure 4. Topographic and structural cross sections basement shown with dotted pattern; overlying sedi- Assumed velocity of sound in sediments 2 km/sec. See of major relief features of the Panama Basin. Acoustic ment blank. Only the most important faults are shown. Figure 1 for location of profiles.

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tical separation of sediment beds, drag on both The Carnegie Ridge west of the longitude of sides of a presumed fault, scarps in the sediment the Coiba fracture zone is a relatively uncom- surface correlated with basement offsets at plicated block, uplifted along a small number of depth, hanging sediments with horizontal or major east-west faults (Fig. 3, profiles 1, 2; Fig. low-angle bedding located on hill sides without 6). The distribution of tracks is not favorable support by basement ridges, and narrow verti- for the identification of north-south faulting but cal zones of distorted bedding or poor and con- the much smaller fault density on profiles with fused reflectivity. Faults so identified are shown a westerly trend (compareFig. 3, profiles 1 and on the profiles of Figures 3, 4, and 10. This is 2) and the straightforward correlation of the a conservative approach and yields a minimum major boundary faults suggest dominance of number of faults—many basement scarps are the east-west system. The uplift on the northern probably also fault-controlled and have been side is abrupt and accomplished by a few closely used occasionally to trace faults from one tra- spaced faults. On the south side are several ma- verse to another. The method is limited to areas jor fault clusters separated by broad steps. of reasonably thick sediment cover. In the rift Thus, the ridge is asymmetric with a well- zone, where the crust is young and sediments defined crestal block. are thin, the tracing of faults is difficult and must The Cocos Ridge is defined by alternating be based mainly on less satisfactory topographic north-south and east-west boundary scarps. criteria. These can be identified with major boundary The most striking and closely spaced faulting fault groups; the two systems appear to be of occurs on the crests and flanks of the ridges and equal importance (Fig. 6). Comparison of pro- high blocks. Two categories of faults, to some files in different directions (for example, 5 and extent intergradational, can be distinguished: 6 tf/Fig. 4), shows little difference in fault den- major faults or fault clusters resulting in signifi- sity and structure. The boundaries of the in- cant vertical displacements, and minor faults dividual blocks vary from a series of short steps with small displacements not consistent in di- (profile 9) to single, large-displacement bound- rection. The latter generally form dense clus- ary faults (profiles 5 and 6). Commonly, the ters, the observable fault density being limited south-facing slopes are more abrupt than the only by the resolution of the records. All ob- north-facing ones, whereas both the east- and served faults are normal, although it must be west-facing scarps are steep and similar in char- recognized that the geometry of the reflection acter. There is some evidence for a system of method largely conceals the existence of any southwest-northeast-trending faults along the reverse faulting. western side of the ridge and on the crest near With rare exceptions (a small folded basin on the northern end. We consider this system to be the southeastern continental margin of Panama real, but since the identification of fault trends on trace V-21), all observed structures are ex- becomes increasingly difficult as the number of tensional. Apparent folding invariably appears trends increases, such an interpretation must be to be the result of closely spaced normal fault- considered questionable until confirmed by de- ing. In the trenches and marginal troughs, the tailed surveys. records are universally of such quality that in- Most faults on the ridges extend to the sur- ternal structures cannot be observed. Compres- face with upward diminishing vertical separa- sional structures may be present in these tion (Fig. 5) and are clearly growth faults. They features. do not appear to be very active. Only three Many of the larger faults and fault clusters epicenters, all of them coinciding with major can be correlated from traverse to traverse, ei- southern boundary faults, occur on the Carne- ther by direct comparison of the structural and gie Ridge (the large earthquake swarm at the topographic characteristics or with the aid of western end of the Galapagos is related to the the topographic chart. In this manner, and with 1968 eruption; see Simkin and Howard, 1970). the aid of the bathymetry which is based on Epicenters on the Cocos Ridge are concen- much more data (Fig. 2), the fault map of Fig- trated along north-south faults (Fig. 6). ure 6 has been constructed. Although only a In the central basin, several groups of well- sketch (individual correlations are rarely very aligned north-south faults can be recognized. certain and some bias is inevitable), we are They mark the Coiba fracture zone and the confident that the map depicts the main struc- group of fractures near 85° W. which were tural trends and the approximate positions of originally defined on the basis of earthquake the principal faults. epicenters (Molnar and Sykes, 1969) and mag-

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Figure 5. Photographs of seismic reflection records exaggeration approximately 40 x. Records from of portions of the Carnegie (A) and Cocos (B) Ridges. YALOC-69 cruise, obtained with 10 cu in. airgun, 5 to Vertical scale in seconds, two-way travel time; vertical 320 cycle filter. Locations on Figure 9.

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Figure 6. Fault patterns of the Panama Basin. Ac- cate the two tilted blocks that form the boundaries of the tively spreading Galapagos rift zone shaded (rf/te/'Hein- marginal trough off Panama. Earthquake epicenters for richs, in prep.), trenches horizontally hatched. Inner 1961-1969 shown as black dots (data provided by Envi- scarps of Yaquina graben stippled. C (Coiba) and X indi- ronmental Sciences Service Administration). netic anomalies (Grim, 1970a; Heinrichs, in must be transcurrent as well as transform. Mol- prep.). Individual faults in both systems can be nar and Sykes (1969) have determined slip vec- traced with confidence over long distances, tors for Coiba and 85° W. earthquakes. The even though the vertical separations on them north-south right-lateral movements in all cases are not large (Fig. 3, profile 4; Fig. 4, profile 6). indicate that the original fragmentation of the Numerous epicenters are located along the Cocos Ridge has ceased and a new phase of fracture zones and generally coincide with fault opposite sense has begun. We will return to this traces within the errors of positioning. The feature later in the paper. western faults of the Coiba fracture zone con- The 85° fracture zone is broad and consists of tinue as the eastern boundary faults of the numerous parallel faults. The distribution of northern segment of the Cocos Ridge, while epicenters within this zone (Fig. 6) indicates the faults of the 85° W. fracture outline a central that it consists of several rather closely spaced Cocos segment and continue across the north- fractures separating short rift segments. There ern crest. In general, the data strongly suggest is considerable earthquake activity along this that the outline of the Cocos Ridge is produced fracture zone north of the eastern spreading by left-lateral offset along north-south faults center. rather than by right lateral movement along Near 88 W., Heinrichs (in pre.) has iden- east-west faults. Since the amount of offset in- tified a fracture zone on the basis of magnetic creases eastward, the fracture zone fault systems anomalies. This fracture is aligned with bound-

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ary faults of the southern segment of the Cocos where it curves abruptly to the west. The mar- Ridge. Unfortunately, the lack of epicenters ginal troughs enclosed by the faults become in the area and the absence of sediment in the progressively deeper toward the north. The rift zone make the identification of fault traces two areas described above are linked by some impossible. Another fracture zone, well defined of the northeast faults which cross the Yaquina by epicenters and magnetic anomaly data (Her- graben and produce offsets in the continental ron and Heirtzler, 1967) occurs near 90° W. margin comparable to those caused by the par- The offset in this case is right-lateral, corre- allel faults. sponding to a left-lateral transform motion op- The northern part of the eastern basin is diff- posite to that of the fracture zones farther east. erent. The Coiba Ridge is a large eastward- As in the case of the 88° zone, too few bathy- sloping slab with its high western edge uplifted metric and seismic reflection data are available along one of the faults of the Coiba fracture to characterize any associated fault pattern. zone. A similar north-sloping slab (Fig. 11, pro- To the south, all fractures terminate against file I, center) probably lies south of the Panama the foot of the Carnegie Ridge. There is no shelf at X on Figure 6. Its southern edge is evidence that they extend into this ridge for any uplifted along several east-northeast-trending significant distance. curved faults near 5° N. Thus, the Coiba slab, East of the longitude of the Coiba fracture the slab at point X, and the continental margin, zone, the Carnegie Ridge broadens and curves outline a broad depression which contains a northeastward. The change in trend is accom- deep trough bounded by east-west-trending panied by the appearance of a system of normal faults near the continental margin. The northeast-trending major faults and the disap- slab at X is considerably fragmented, and is not pearance of the east-west system. Northeast- well defined by existing bathymetric and seis- trending faults also occur in the southern half of mic profiler traverses. However, it appears to the eastern basin, between the Coiba fracture be very similar to the Coiba Ridge in structure zone and the continental margin. and sediment cover. One of the most striking features of the east- The northern extension of the Peru Trench ern basin is the Yaquina graben (Fig. 1; Fig. 3, between the continental margin and the Carne- profiles 3, 4). Between 2° N. and 4° N. the gie Ridge is bordered on the east by large en graben is developed as a deep, steep-sided echelon faults with major vertical separation. trough, with a level floor bordered by faults. The east slope of the Carnegie Ridge and of the The ridges on both sides slope gently outward sea floor to the south is downfaulted against this via a series of normal faults of moderate dis- boundary by a series of smaller normal faults placement. Both sides are covered with thick (Fig. 4, profile 7). Cross-faulting at 60 degrees sediments. The seven profiler crossings of this to the main trend affects the shape of the trench feature all reveal the graben structure illus- as well as of the continental margin. The fault trated in Figure 3. The graben begins at the pattern suggests a spread ing-V motion of this north end of the gap between the Carnegie area with the apex near the northern end of the Ridge and the continental margin. From there Carnegie Ridge. The Middle America Trench it extends northward away from the continental (Fig. 4, profile 5) also consists of a stepped margin and is offset several times by right-lat- downwarp of the ocean floor against a major eral faults of the northeast-trending system. boundary fault at the continental margin. The identity of the graben is lost in the complex Within the Panama Basin all earthquakes are terrain near 5° N. (Fig. 6). We interpret this shallow (Barazangi and Dorman, 1969) and feature as a major tensional fissure separating tensional or strike-slip. A Benioff zone with nu- two tectonically distinct areas of the southern merous intermediate and deep focus earth- part of the eastern basin. West of the graben, quakes lies landward of the Peru Trench. The the structure is dominated by numerous north- dense cluster of foci of this zone extends north- south faults which are intersected and probably ward to 0°30' S., just opposite the northern end offset by the northeast fault system. To the east of the Peru Trench. North of that latitude, in- lies a series of faults which parallel the conti- termediate focus earthquakes occur much less nental margin and define several shallow mar- frequently, indicating that if the zone of under- ginal troughs (Fig. 3, profile 3). These faults thrusting extends northward across western Co- curve progressively to the north in conformity lumbia and the base of the Isthmus of Panama, with the continental margin until they intersect it is much less active than farther south. The the continental slope of Panama at the point boundary between the southern and northern

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intermediate earthquake zones is quite sharp. gravity minimum of the Peru Trench termi- No intermediate or deep focus earthquakes nates near 1° S. at the northern end of the mor- have been observed in the Isthmus of Panama; phological trench. A less-pronounced negative the principal activity here consists of shallow gravity anomaly continues north from there earthquakes on the set of normal faults along along the continental margin to the eastern base the southern continental margin. Shallow and of the Coiba Ridge. Several minima in this band intermediate activity increase west of the Coiba coincide with the marginal troughs discussed fracture zone under Central America, but earlier in this paper. A small negative anomaly, become moderately intense only north of the which can be explained by the relief, is as- latitude where the Middle America Trench ter- sociated with the Yaquina graben. minates. Slip vectors determined by Molnar The morphological, magnetic, seismic, and and Sykes (1969) show that underthrusting of gravity data of the Panama Basin yield a reason- the ocean floor northeastward beneath the con- ably coherent structural pattern. An almost to- tinent occurs along the length of the trench. tally different picture, on the other hand, is Unpublished gravity data obtained on generated by the heat flow distribution (Fig 7). YALOC-69 cruise indicate that most of the Only the zone of high heat flow along the axis Panama Basin region is in isostatic equilibrium. of the Galapagos rift west of the 85° fracture This includes the Cocos and Carnegie Ridges zone is concordant with the structural pattern and the high blocks in the eastern basin. Hayes discussed so far. The principal thermal feature, (1966, Fig. 4) has shown that the pronounced a broad tongue of high heat flow extending

Figure 7. Heat flow in the Panama Basin. Station val- Sclater, and Richard P. von Herzen include all measure- ues are in heat flow units (microcalories per sq cm per ments in the area to early 1970. sec). Data made available by Marcus Langseth, John

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westward from the continental margin between This blanket thins toward the crests of the 4° and 7° N. crosses nearly all structural ele- ridges and thickens markedly in the marginal ments of the basin, including the major tectonic troughs of the eastern basin, although not in the boundary of the Coiba fracture zone. This Peru Trench. The center of the Galapagos rift tongue is bordered in the west and southwest zone is nearly free of sediment or contains only by a zone of unusually low heat flow which localized patches. Elsewhere, the thickness is extends far into the deep Pacific. To the north, rather uniform, ranging from 400 to 600 m. values of the continental marginal zone are nor- Little is known of the nature of the sediments mal to slightly above normal. Values for the beyond the reach of conventional coring equip- Carnegie Ridge vary greatly, perhaps as a ment. Hole 84 of the Deep Sea Drilling Project consequence of the irregular sediment cover lies between the Cocos and Coiba Ridges (Fig. and extensive erosion on this ridge, but on the 1), but the results have not yet been published. average appear to be only slightly above nor- Consequently, we must rely on the reflection mal. At the present time, we are unable to inte- profiles to map gross sedimentary units in a grate the heat flow data into a tectonic synthesis preliminary fashion. On the basis of bedding of the region. characteristics and the uniformity or otherwise of acoustic impedance within the sediment sec- SEDIMENT COVER tion, it is possible, with due allowance for diff- Much of the Panama Basin is covered with a erences in method and quality, to distinguish relatively thick blanket of sediment (Fig. 8). four main sedimentary sequences (Fig. 9).

Figure 8. Sediment isopach map of the Panama Ba- Horizontal shading indicates areas free of sediment or sin, based on tracks of Figure 1; assumed sound velocity with small local patches of sediment. in sediments 2 km/sec. Contours in hundreds of meters.

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1. Thinly and evenly stratified sediments oc- 11, profile G) and the fragmented blocks near cur on all high blocks. The stratified complex area X as well. (Fig. 5, profile A) is distinctive and different 2. Deposits in the troughs near the continen- from all other deposits in the area. It is best tal margin are also stratified. Unlike the ridge- developed on the Carnegie Ridge where it top unit, however, bedding is much less even forms the entire sediment sequence apart from and is commonly interrupted by small uncom- localized thin transparent deposits that overlie formities or cross-stratification (Fig. 10, pro- it uncomformably (Fig. 5, profile A; Fig. 10, files, b, f, g). These deposits often overlie profile c). It also occurs on the Cocos Ridge semitransparent sediments, especially near the where it is restricted to the lower part of the seaward edge of the troughs. The bedding sedimentary sequence (Fig. 5, profile B), par- characteristics and nearness to the continent of ticularly in small grabens on the crest (Fig. 10, such sequences identify them as hemipelagic profile h). This stratified complex also charac- deposits, perhaps containing turbidites. terizes the deposits of the Pacific basin west of 3. Thick semitransparent deposits character- the Cocos Ridge and south of the Carnegie ized by widely spaced, fuzzy, discontinous re- Ridge. It is well developed in downfaulted flectors (Fig. 11,1) cover the deeper part of the blocks of the Malpelo Ridge (Fig. 11, profile E) Panama Basin, with the exception of the young and probably occurs on the Coiba Ridge (Fig. sea floor of the Galapagos rift zone and as-

Figure 9. Distribution of sediment types in the hemipelagic deposits; (5) limit of erosional unconform- Panama Basin based on the tracks in Figure 1. Legend: ity on Cocos and Carnegie Ridges. Blank areas are free (1) semitransparent sequence; (2) thin transparent layer of sediment or contain only local patches. Labeled track of western Pacific basin; (3) stratified complex; (4) lines are profiles of Figures 10 and 11.

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sociated fractures. This semitransparent se- tral saddle and also on the northeast flank of the quence overlies, often unconformably, the Carnegie Ridge (Figs. 8, 9). The products of stratified complex on the Cocos Ridge (Fig. 5, this episode of erosion, which apparently is still B; Fig. 10, h; Fig. 11, F), and may be identical occurring, are found mainly along the north with the thin transparent layer that unconform- side of the ridge (Fig. 8). Since the exposed ably overlies portions of the stratified complex surfaces on the crest are fairly level, current- on the Carnegie Ridge. erosion rather than slumping or volcanic ac- 4. A thin transparent layer rests conformably tivity must be inferred. Apparently, northerly on the stratified complex of the deep Pacific and northwesterly flowing currents at inter- Basin, west of the Cocos Ridge extending south mediate depth are responsible. to about 7° N. (Fig. 11, C, D, F). South of this Acoustic basement is well defined over virtu- latitude, the stratified complex extends to the ally the entire study area. Only in parts of the sea floor. marginal trough south of the Panamanian conti- Little information is available on the primary nental margin is the total thickness of sediment lithologies or the ages of the various sedimen- unknown. Two types of acoustic basement can tary units. A core taken just west of profile D be distinguished; one is very rugged and ap- (Fig. 11), where the stratified complex crops pears as complexes of hyperbolic echoes, out, is upper Miocene in age (Ewing and Ew- whereas the other is smooth and sometimes ing, 1970). Hole 84 of Leg 9 of the Deep Sea shows several parallel internal echoes. The Drilling Project, located in semitransparent rough acoustic basement is assumed to be vol- sediments east of the Cocos Ridge, bottomed in canic, since its morphology is similar to the ex- late Miocene basalt. It is not known whether posed volcanic terrain of the young Galapagos this basalt represents true basement or an intru- rift. Such basement is widespread in the deeper sion (Hays, 1970). parts of the basin. It also forms the pinnacles, On the ridges, evidence for periods of ero- ridges, and rough subbottom on the ridges. sion is widespread. The occurrence of the stra- The smooth basement is generally fairly level, tified complex in small grabens on the Cocos sometimes shows well developed faults (Fig. Ridge suggests that this sequence was depos- 5B; Fig. 10, profiles a, c, d, e, h) and is found ited prior to faulting and possibly to uplift of only on the Cocos, Carnegie, Malpelo, and the ridge. The subsequent long period of sedi- Coiba Ridges, where it floors most of the more mentation has occurred under radically differ- extensive sediment masses. This basement ap- ent conditions. The unconformity between the pears to crop out on the central saddle of the two units reaches the surface in the center of Carnegie Ridge (Fig. 10, profile a). However, the ridge, where erosion, volcanic activity, and cores in the area recovered only a coarse slumping on steep slopes have removed or cov- residual sediment. The smooth acoustic base- ered most of the sediments. The erosion pro- ment can be interpreted as either a consolidated ducts have been deposited on both sides of the sedimentary layer, perhaps similar to the chert ridge as chaotically bedded sediments overly- horizons associated with Horizon A in the At- ing the stratified complex at the foot of the lantic (Ewing and others, 1970), or as smooth boundary scarps (Fig. 10, profile d). lava flows. If the first interpretation is correct, On the Carnegie Ridge, unconformities the age of this layer is critical to the interpreta- within the stratified complex are seen in many tion of the history and age of the high blocks. profiles approximately one-third of the total A forthcoming Glomar Challenger d rilling cam- thickness above basement. In addition, it ap- paign in the Panama Basin planned for early pears that much of the sea floor is being eroded 1971 should resolve this and other stratigraphic (Fig. 10, profiles a and c). On the saddle in the problems. center of the ridge, erosion extends to the acoustic basement. Locally, the upper erosion DISCUSSION surface is overlain by thin transparent sedi- In the preceding pages, the structural compo- ments (Fig. 5A; Fig. 10, profile c). In contrast nents of the Panama Basin have been described to the Cocos Ridge, the lower unconformity in some detail. Certain elements, such as the separates two depositional phases of apparently Galapagos rift, have a clearly defined tectonic similar conditions; there is no visible difference role, whereas others are not well understood. in the nature of the stratified complex below Obviously, the structure of the basin is a func- and above. tion not only of forces operating at the present Surfaces free of sediment occur on the cen- time, but also, to an unusually large extent, of

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forces which are no longer active. We do not This rate, averaged since anomaly 3 time intend to discuss here the plate tectonics of the (Heirtzler and others, 1968), increases slightly greater region of which the basin forms a part, from approximately 2.5 cm/yr at the western but rather will develop hypotheses which may end of the basin to 2.8 cm/yr near the Coiba explain those structural elements that appear to fracture zone (Heinrichs, in prep.). have their origin wholly or largely within the We have cited evidence which suggests that basin itself. the Cocos Ridge has been displaced northward The most problematic features of the basin along extensions of the fracture zones and par- are the Cocos and Carnegie Ridges. They are allel faults. In order to explain the diagonal largely aseismic, show evidence of uplift along position of the Cocos Ridge with respect to the normal faults, are now in isostatic equilibrium, axis of the rift, we must assume either that the and show both striking similarities and some spreading rate at its eastern end has been twice intriguing differences. The early depositional that at the western end since the initiation of histories of the two appear to have been nearly rifting, or that spreading started in the east and identical. In both cases a stratified sediment advanced westward with time. The latter as- complex of remarkable uniformity was depos- sumption appears more reasonable since the ited on a smooth (sedimentary?) acoustic base- foot of the Cocos Ridge lies at the south edge ment, which otherwise occurs in the basin only of anomaly 5 just west of the Coiba fracture on some eastern high blocks. On both ridges zone (P. J. Grim, 1970, oral commun.), some- the stratified sequence terminates in a wide- what north of anomaly 3 in its central portion, spread erosional unconformity which may be and north of anomaly 2 near the Galapagos related to original uplift and faulting of the (Fig. 12). This implies that spreading started ridges. The age of this period of erosion cannot west of the Coiba fracture zone approximately be established with certainty, but an upper Mio- 10 m. y. ago, and proceeded at an average rate cene age is compatible with the very small of about 3 cm/yr. Progressively later starting amount of evidence presently available. Fol- times for more westerly segments then pro- lowing the development of the unconformities, duced the present outline of the Cocos Ridge. the histories of the two blocks diverge. On the More detailed work on the magnetic anomalies Cocos Ridge, a significant change in deposi- of the rift zone by means of north-south tra- tional regime led to the development of a sedi- verses should readily test this hypothesis. ment section similar to that deposited If we return the segments of the Cocos Ridge elsewhere in the Panama Basin. In contrast, to their postulated initial positions (Fig. 12), depositional conditions on the Carnegie Ridge the steep sides fit the Carnegie Ridge reasona- remained essentially the same as those that con- bly well to produce a broad ridge with step- tinue to prevail in the deep equatorial Pacific. faulted flanks. The smooth outline of this ridge The two ridges are separated by an active is marred only where the northeasternmost spreading zone. There is no evidence that this fragment of the Cocos Ridge is too small. The zone affects the Carnegie Ridge—the fracture southern portion of Costa Rica required to fill zones do not extend onto the ridge, and the this gap contains the Nicoya complex of pillow ridge shows no indication of compression. basalts, cherts, and graywackes of oceanic ori- Consequently, the axis of the rift zone must be gin. migratingnorth ward at the half-rateof spreading. This hypothesis for the development of the Cocos Ridge is compatible with the available evidence but is by no means strongly confirmed Figure H). Tracings of reflection records from by it. It should, however, be amenable to test- YAI.OC-69 cruise. Locations on Figure 9. Vertical ing by examination of the older magnetic broken double lines: inferred faults. Heavy subhori/on- tal lines indicate smooth, possibly sedimentary acoustic anomaly patterns within the reconstructed basement. Vertical scale in seconds, two-way travel time. block, by refraction studies of the ridges, and Profiles a and c are, respectively, east-west and north- by determination through deep drilling of the south sections across the Carnegie Ridge showing ero- sion and n near-surface unconformity. Profile b: ages of the various depositional events on the hemipelagic sediment overlying semitransparem depos- ridges. its in eastern marginal trough. Profiles f and g: stratified East of the Coiba fracture zone, the situation hemipelagic deposits in fault troughs along eastern con- is more complex. The distribution of intermedi- tinental margin. Profile h: stratified complex in grabens on Cocos Ridge overlain by semitransparem series. Pro- ate focus earthquakes south of the equator file d: chaotically bedded erosion products overlying shows that the South America plate and its fron- stratified complex at loot of Cocos Ridge. tal trench are being thrust over the nose of the

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Figure 11. Photographs of reflection records of the hemipelagic sediments of the Panama marginal trough; Panama Basin obtained on cruises of R/V Vema and Con- profile I: semitransparent sediments of the eastern basin; rad. Locations on Figure 11. Profiles C and D: transpar- profile E: stratified complex on the Malpelo Ridge; pro- ent layer on stratified complex in the deep Pacific west file J: stratified complex outcropping at base of Carnegie of the Cocos Ridge; profile F: thick transparent and semi- Ridge; profile H: erosional surface on the central saddle transparent sediment overlying faulted stratified com- of the Carnegie Ridge. Vertical scale in seconds, two- plex at the foot of the Cocos Ridge; profile G: stratified way travel time; horizontal scale bars 10 km. complex on the Coiba Ridge, dipping eastward under Carnegie Ridge. The orientation of slip vectors The east side of the Panama Basin is occupied (Molnar and Sykes, 1969) indicates some dis- by a series of fault-bounded marginal troughs. tortion of the ridge toward the north in accord- Similar troughs exist on land in the coastal zone ance with the change in fault trends. The of Colombia (Case and others, 1971). Farther northern edge of the eastern basin along the east, a broad zone of intermediate depth earth- Panamanian continental margin shows no evi- quake activity, much less intense than opposite dence for a plate boundary; there is no under- the Peru Trench, continues through north-cen- thrusting, the marginal trough is a simple tral Colombia. Together with the subdued downfaulted feature, and the westward branch gravity anomalies and the trough morphology of the gravity low mapped by Hayes (1966) can offshore, this reduced activity indicates substan- be explained by the thick sedimentary fill in this tially milder tectonism than is occurring in the trough. Bowin (1970), on the basis of gravity area of the Peru Trench. The zone of mild and seismic reflection data, has postulated the earthquake activity continues northward across existence of a nascent plate boundary north of the eastern end of the Isthmus into the Carib- the Isthmus of Panama with northward under- bean, where Bowin (1970) has shown that it is thrusting of the southern plate under the Carib- paralleled by a zone of negative gravity anomal- bean. He attributes the uplift of the Isthmus to ies probably indicating underthrusting of the this process. Caribbean floor under South America. Thus,

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Figure 12. Reconstruction of the ancestral Carnegie Heinrichs (in prep.) and Grim (1970b, and 1970, oral Ridge. Positions of numbered magnetic anomalies after commun.); E is position of south edge of anomaly 5.

the boundary of the South America plate appar- ing faults, all suggest local dilation of the ocean ently continues northward from the Peru floor in an easterly or northeasterly direction. Trench through western Colombia into the Such dilation could result from a relative clock- Caribbean. This boundary seems to be charac- wise component in the rotation of South terized by combined transform faulting and un- America or from northward movement of the derthrusting in a zone bounded by the Bocono basin floor east of the Coiba fracture interacting fault and its extensions (Dolores megashear), with the eastward bend in the margin of the by the Yaquina graben, and by the southern South America plate. The absence of earth- Caribbean. The diminished compressional ac- quake activity along the northeast-trending tivity compared to that opposite the Peru faults and the Yaquina graben, and the evi- Trench may result from the lower angle of the dence for moderate underthrusting along the boundary separating the eastern Panama Basin eastern margin of the Panama Basin suggest and the adjacent part of South America with the that, whatever its cause, the dilation came to a motion vector of the two plates, or it may be halt some time ago. associated with the onset of a new phase of Several features of the eastern boundary of underthrusting which is not yet fully deve- the Cocos plate suggest that the tectonic com- loped. plexity of this area is second only to that of the The fault pattern of the eastern Panama Ba- northwestern portion of the South America sin, the presence of the Yaquina graben, and plate. Anomalies of the eastern area include: the transcurrent offsets along northeast-trend- 1. The presence of smooth acoustic basement

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and stratified sediment sections of similar char- sively greater age should be sought south of the acter on the Malpelo and Coiba Ridges as well Coiba Ridge and ridge X, and the easternmost as on smaller blocks nearby. Such sections segment of the Galapagos rift zone should be closely resemble those already described from investigated in detail to see if its spreading rate the Cocos and Carnegie Ridges. has recently decreased markedly as under- 2. The existence of a discontinuity between thrusting of the northern Cocos Ridge segment the eastern end of the Middle America Trench ceased. and the northern end of the Coiba fracture Reconstruction of the ancestral Carnegie zone. In fact, the northern Cocos Ridge appears Ridge (Fig. 12) indicates that ridge fragments to be attached to the continental margin. penetrate as much as 50 to 100 km into the 3. Marked seismic activity along the 85° frac- subduction zone before plugging it. The under- ture zone north of the rift-rift portion of the thrust portions may have contributed signifi- fracture. Such activity would be restricted to cantly to the uplift of the Isthmus of Panama, the region between the two rift segments if this and Bowin's (1970) gravity anomaly north of were a pure transform fault within the Cocos the isthmus may be related to this uplift rather plate. than to an incipient zone of underthrusting at a These features suggest that the eastern new plate boundary in the Caribbean. boundary transform fault of the Cocos plate has Although this model explains many of the moved westward in a series of steps. Displace- structural features of the Panama Basin, it does ments appear to have occurred as successive not explain the intense fragmentation of the blocks of the northern half of the ancestral high blocks and ridges east of the Coiba frac- Carnegie Ridge entered the Middle America ture zone. The magnetic patterns in this eastern Trench and plugged the subduction zone. Such basin are not well known, and probably cannot blocks, together with the transform faults that be resolved with existing survey lines. Thus, in initially formed their eastern boundaries, then the absence of closely spaced geophysical tra- effectively became part of the Nazca plate be- verses of this area, the evolution of the eastern tween the Cocos and South America plates. At basin cannot be resolved. the same time, successive new faults developed The hypotheses presented here fail to explain along the old left-lateral transcurrent faults that the nature and origin of the ancestral Carnegie formed the western boundary of the blocks. Ridge and the reasons for its uplift and for the The original eastern boundary of the Cocos continued high elevation and isostatic compen- plate may have been at 79° W. (east of block X sation of the fragments after rifting. The pre- on Fig. 6), with subsequent shifts to 81° W. and sent Carnegie Ridge itself has very low 83° W., as block X and the Coiba Ridge succes- magnetic anomalies which contrast with the sively entered the trench and broke away from large anomalies of unknown age to the south the Cocos plate to become part of the Nazca (Herron and Heirtzler, 1967). At this low lati- plate (Fig. 13). At present, the northern block tude, large anomalies imply north-south spread- of the Cocos Ridge has just plugged the trench, ing. To the north, large anomalies of unknown so that the plate boundary is in the process of age reappear in the Cocos Ridge. shifting from the Coiba fracture zone to the The available data do not oppose an ancestral fracture zone at 85° W. The portion of Costa Carnegie Ridge composed of continental Rica adjacent to this northern block, including material of unknown origin. Such a model is the city of San Jose, is at present rising rapidly supported by reported occurrences of continen- (A. McBirney, 1970, oral commun.), presuma- tal rocks on Malpelo Island (A. R. McBirney, bly as a result of rebound of the sealed trench. 1970, oral commun.) and would account con- Such a hypothesis eliminates an awkward gap veniently for the elevation and isostatic com- in the boundary of the Cocos plate; it explains pensation of the ridge. It would also explain the right-lateral movement on the 85° fracture why transfer of fragments of the ancestral ridge zone suggested by an earthquake first motion into the Middle America Trench tends to seal solution at 6° 30' N. 84° 30' W. (Molnar and the subduction zone. The presence of large Sykes, 1969), which contrasts with the left-lat- magnetic anomalies on the Cocos Ridge re- eral offset of the Cocos Ridge, and it is compati- quires that the ancestral block be complex and ble with the history of andesitic volcanism in possibly contain oceanic as well as continental Panama and Costa Rica (Terry, 1956). It also material. has additional consequences susceptible to test- An alternate, and perhaps more satisfactory ing. For example, fossil rift zones of succes- explanation is that the ancestral ridge marked a

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cocos PLATE

Figure 1 3. Stages in the idealized hypothetical struc- after block X and the Coiba Ridge has successively tural development of the eastern Cocos lithospheric plugged the subduction zone and become welded to the plate. For simplicity, the shape of the Cocos Ridge is Nazca plate; (d) present situation, with the crust be- assumed constant throughout the sequence. The present tween the Coiba and 85° fracture zones partly decoupled outline of Central and South America is for geographic from both the Cocos and Nazca plates; (e) projected orientation only. The accurate location of the fossil rift configuration of the plate boundary following incorpo- zones south of block X (Fig. 6) and the Coiba Ridge is ration of the Coiba fracture zone and associated spread- unknown, (a) configuration of the plate before the ances- ing center into the Nazca plate. tral Cocos Ridge reached the trench; (b, c) plate outlines

zone of slight convergence of two oceanic tional Science Foundation with the Lamont- plates where one has overridden the other Doherty Geological Observatory and the without formation of a trench. Such a situation Scripps Institution of Oceanography. We thank may have resulted from a low angle of conver- the officers, crews and scientific staffs of R/V gence, a slow rate of overthrusting, or a brief Yaquina and the other research vessels for the episode of convergence. The resultant double cooperation that has made it possible to obtain thickness of the lithosphere would then account the observations on which this paper is based. for the initial uplift of the ridge, its isostatic Heat-flow data were made available by Dr. compensation, and its sealing effect on the Mid- Marcus Langseth, Lamont-Doherty, Dr. John dle America Trench. Sclater, Scripps Institution, and Dr. R. P. Von Present data do not permit us to choose be- Herzen, Woods Hole Oceanographic Institu- tween the hypothesized models or other pos- tion, and bathymetric data by T. E. Chase, sibilities. Planned seismic refraction studies and Scripps Institution. Dr. J. E. Case, University of sampling of the basement should help resolve Missouri, Dr. A. McBirney, University of Ore- the problem. gon, and Dr. C. O. Bowin, Woods Hole Accountability for ideas expressed in this section Oceanographic Institution, provided helpful rests with the first four authors. suggestions. T. C. Moore, Jr., T. R. Baumgart- ner and Peter Buhl assisted by critically review- ACKNOWLEDGMENTS ing the manuscript. This investigation has been supported by contract N00014-67-A-0369-0007 under pro- REFERENCES CITED ject NR 083-1021 of the Office of Naval Re- Barazangi, M.; and Dorman, J. World seismicity search with Oregon State University. Older maps compiled from ESSA, Coast and Geodetic data were obtained under contracts and grants Survey, epicenter data: Seismol. Soc. Amer., of the Office of Naval Research and the Na- Spec. Paper 59, p. 369-380, 1969.

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Bowin, C. O. Gravity anomalies north of Panama magnetic anomalies, geomagnetic field rever- and Colombia: Amer. Geophys. Union, Trans., sals, and motions of the ocean floor and conti- Vol. 51, p. 317, 1970. nents:;. Geophys. Res., Vol. 73, p. 2119-2136, Case, J. E.; Duran, L. G.; Lopez, R., Alfonso; and 1968. Moore, W. R. Tectonic studies in western Co- Herron, E. M.; and Heirtzler, J. R. Sea-floor lombia and eastern Panama: Geol. Soc. Amer., spreading near the Galapagos: Science, Vol. Bull, (in press), 1971. 158, p. 775-780, 1967. Chase, Thomas E. Sea floor topography of the cen- Isacks, B.; Oliver, J.; and Sykes, L. R. Seismology tral eastern Pacific Ocean: U. S. Dep. Int., Bur. and the new global tectonics: J. Geophys. Res., Comm. Fish., Circ. 291, 1968. Vol. 73, p. 5855-5899, 1968. Ewing, J.; and Ewing, M. Seismic reflection: /'«The LePichon, X. Sea-floor spreading and continental sea (A. E. Maxwell, ed.), Interscience, New drift: J. Geophys. Res., Vol. 73, p. 3661-3698, York, Vol. 4 (in press), 1970. 1968. Ewing, J.; Windisch, C.; and Ewing, M. Correla- Molnar, P.; and Sykes, L. R. Tectonics of the Carib- tion of Horizon A with JOIDES bore-hole re- bean and Middle America regions from focal sults: J. Geophys. Res., Vol. 75, p. 5645-5653, mechanisms and seismicity: Geol. Soc. Amer., 1970. Bull, Vol. 80, p. 1639-1684, 1969. Grim, P. J. Connection of the Panama fracture zone Morgan, W. J. Rises, trenches, great faults, and with the Galapagos rift zone, eastern tropical crustal blocks: J. Geophys. Res., Vol. 73, p. Pacific: Mar. Geophys. Res., Vol. 1, p. 85-90, 1959-1982, 1968. 1970a. Ross, D. A.; and Shor, G. G., Jr. Reflection profiles Grim, P. J. Bathymetric and magnetic anomaly across the Middle America Trench: J. Geophys. profiles from a survey south of Panama and Res., Vol. 70, p. 5551-5572, 1965. Costa Rica: ESSA Tech. Memo, ERLTM - Simkin, T.; and Howard, K. A. Caldera collapse in AOML 9, U. S. Dep. Comm., Env. Sci. Serv. the Galapagos Islands, 1968: Science, Vol. 169, Adm., 1970b. p. 429-437, 1970. Hayes, D. A geophysical investigation of the Peru Terry, R. A. A geological reconnaissance of Trench: Mar. Geol., Vol. 4, p. 309-351, 1966. Panama: Calif. Acad. Sci., Occas. Pap. 23, 91 p., Hays, J. D. Deep sea drilling project, leg 9: Geo- 1956. times, Vol. 15, p. 11-13, 1970. Heirtzler, J. R.; Dickson, G. O.; Herron, E. M.; MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 11, Pitman, W. C.; and LePichon, X. Marine 1970

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