Forearc response to subduction of the Cocos ... - www .gsapubs .org

Forearc response to subduction of the Cocos Ridge, Panama-Costa Rica JEFF CORRIGAN Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78713 PAUL MANN Institute for Geophysics, University of Texas at Austin, 8701 Mopac Boulevard, Austin, Texas 78759 JAMES C. INGLE, JR. Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT Stratigraphic, paleontologie, and structural data from two Panama-Costa Rica forearc peninsulas located landward of the aseismic Cocos Ridge along the southernmost Middle America Trench document rapid Pliocene subsidence and basin infilling followed by Quaternary deformation and uplift. On the Burica Peninsula, an approximately 3,000-mthick, Pliocene-Pleistocene clastic sedimentary sequence, characterized by fine-grained turbidite deposits and volumetrically minor coarser-grained channel-fill, slump, and debris-flow deposits, bears depth-diagnostic foraminifera that document rapid Pliocene subsidence and deposition on a south-dipping paleoslope. New paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2,000+ to 1,200 m during the late Pliocene. Present-day exposure of these rocks indicates an average Quaternary uplift rate of ~ 1 mm/yr. The turbidite section on the Burica Peninsula is interpreted to represent a trench-slope deposit and is correlated to a similar, marine sedimentary section on the adjacent Osa Peninsula of Costa Rica. Structures developed within Pliocene-Pleistocene strata of the outer-forearc Burica and Osa Peninsulas record minor (

7J |Tp(b)| BURICA M E M B E R PLIOCENE

N

|Tp(p)| PENITA

MEMBER

EOCENE |-= 1 PALEOCENE I l p e I LATE , , C R E T A C E O U S Kvs UPPER N I C O Y A C O M P L E X (?)

L O C A T I O N MAP

SSW

ISLA

4 km ( NO VERTICAL EXAGGERATION )

S o> C

KEY Intertidal z o n e / b e a c h .Aligned ridge .Scarp, tick on down thrown side, dots indicate topographic low Stream Hogback, direction of dip Edge of w a v e - c u t marine t e r r a c e Measurement strike and dip y Tepid spring/gas seep 0

1

2

1

I

I

3 km I

Figure 10. (A) Photomosaic of the southern Medial fault zone, central Burica Peninsula (provided by R. H. Stewart); (B) photointerpretation and field observations defining structure of the Medial fault zone (stippled).

FOREARC RESPONSE TO SUBDUCTION, COCOS RIDGE, PANAMA-COSTA RICA

Figure 9. Cross sections of the Burica Peninsula, based on l:25,000-scale geologic mapping.

Interpretation of Charco Azui Formation Depositional Environments The depositional environments of the Charco Azul Formation record rapid subsidence during the early Pliocene, basin infilling from early Pliocene to early Pleistocene time, and rapid uplift during the late Quaternary (Fig. 8). The Pliocene Penita Member at the base of the Charco Azul Formation is approximately 300 m thick and is interpreted on the basis of benthic foraminifera, molluscs, and sedimentary structures to represent shallow-marine sedimentation above a basaltic basement, with uppermost exposed Penita Member strata representing transition from a shelf to slope environment. At the base of the Corotu 1 well in the northeastern part of the peninsula, 760 m of basaltic breccia and conglomerate were drilled above basement without penetrating a basal horizon similar to the massive siltstones of the Penita Member (Terry, 1956) (Fig. 9). This stratigraphic relationship and the upsection decrease of lathworktextured volcanic fragments found in sandstone samples collected from the Burica Member suggest that areas of the basaltic basement remained as topographic highs during deposition of shallow-marine strata of the lower Penita Member and during deposition of the lower part of the deep-marine Burica Member. Rocks of the Pliocene to lower Pleistocene Burica Member are interpreted to represent gravitationally driven deposition on a submarine slope. This interpretation is based on (1) the presence of fine-grained, thinly bedded turbidites that contain minor, slump horizons; (2) the relative abundance of debris-flow deposits; and (3) local conglomeratic deposits that incise a sequence composed predominantly of fine-grained turbidites. A southward-facing trench slope setting for the Burica Member is proposed on the basis of (1) foraminiferal-derived paleobathymetric estimates of 2,000+ m, (2) turbidite facies associations characteristic of convergent-margin settings, and (3) south-directed paleocurrent indicators. Deviations from the southward paleocurrent pattern occur in some sandstones of the slope basin facies association and are interpreted to represent diverted paleoflow of sand into relatively small (=?5 km 2 ) slope basins (Fig. 6). The vertical lithologic sequence and biostratigraphy of the upper Burica Member and lower Armuelles Member indicate that gradual shoaling occurred from early Pliocene through early Pleistocene time, followed by rapid uplift approximately 1 m.y. ago (Fig. 8).

STRUCTURE OF THE BURICA PENINSULA Our interpretation of late Neogene deformation of the Burica Peninsula is based on analysis of macroscopic (kilometer scale) and mesoscopic (outcrop scale) structures mapped in sedimentary rocks of the Charco Azul Formation. Structural mapping was carried out by making systematic traverses up stream valleys and along coastlines. Drilling results from the Corotu 1 well constrain the depth of basement beneath the northeastern Burica Peninsula (Terry, 1956; Fig. 9). Macroscopic Structures A prominent, 15-km-long and 600- to 1,800m-wide, linear valley strikes northward from the southwest coast of the Burica Peninsula to the headwaters of the Rio Corotu (Fig. 10). Near the Rio Corotu, the valley assumes a more northwestward trend and becomes poorly defined topographically. Using aerial photographs, Stewart (1977) suggested that this prominent valley marked the presence of a major fault zone. Our mapping confirms that this valley is occupied by a poorly exposed fault zone, which we name herein the "Medial fault zone" (MFZ). Geologic evidence for the Medial fault zone includes (1) omission of approximately 800 m of the lower Burica Member along the eastern edge of the valley (Fig. 9); (2) the presence of fault-bounded slivers of basaltic basement and vertically dipping strata of the Burica Member, where the fault valley changes from a north to a northwest strike; the fault slivers range from 5 to 7 km in length and are as much as 600 m in width (Fig. 3); (3) large vertical displacement as shown by approximately 2,500 m of up-to-thewest vertical throw between the basaltic fault sliver exposed in the upper Rio Corotu and the depth to basement in the Corotu 1 well (Fig. 9, section C-C'); (4) the juxtaposition of deepmarine strata of the Burica Member against slightly older, shallow-marine deposits of the Penita Member for at least 20 km along the eastern edge of the valley (Fig. 3); (5) the change in orientation of bedding in the Burica Member from northwest-striking, northeast-dipping beds to north-striking, east-dipping beds adjacent to the valley (Fig. 3); (6) parallel, 3- to 6-km-long, north-trending, linear ridges found along the valley floor; these ridges display as much as 120 m of topographic relief, despite being composed of easily erodible, massive siltstones of the Penita Member (Fig. 10); and (7) the presence of tepid springs and gas seeps within the fault valley (Fig. 10). In addition to the MFZ, several local macroscopic fold structures have been mapped in rocks of the Burica and Armuelles Members (Fig. 3).

13

Near the southern coast of the peninsula south of Limones (Fig. 3), two open folds with subhorizontal fold axes define a northwest-trending, asymmetric, anticline-syncline pair (Fig. 9, section A-A'). In the northern Burica Peninsula in the Rio Corotu area, two north-trending open folds with subhorizontal fold axes define an anticline-syncline pair that is offset 1.5 to 2.0 km in an apparent left-lateral sense along an unnamed, northwest-striking fault (Fig. 3). Mesoscopic Faults Excellent exposures of the Burica Member on wave-cut platforms around the peninsula exhibit mostly northeast-striking, high-angle normal faults with horizontal separations typically between 1 and 3 m. The most extensive exposure of faults is found on a wave-cut platform that we name herein the Tallo intertidal zone (TIZ). The TIZ was mapped in detail in the field with the aid of enlargements of low-altitude, aerial photographs provided by R. H. Stewart (Fig. 11). The orientations of approximately 70 faults were measured on the TIZ along with magnitude of horizontal fault separation, or the distance between two traces of a displaced marker bed measured on the virtually horizontal surface of the intertidal zone. Unfortunately, no slickenside striations along the fault planes were observed in the TIZ or in other areas because of weathering. The complex fault pattern exhibited in the TIZ is composed of two fault sets: (1) a fault set Fl, consisting of subparallel, steeply dipping (65°-90°) fault planes striking N50° ± 20°E and spaced tens of meters apart, and (2) a fault set F2, consisting of subparallel, steeply dipping (60°-90°) fault planes striking either N80° ± 10°E or N20° ± 10°E and spaced hundreds of meters apart; fault set F2 defines a rectilinear fault pattern with an acute bisector angle of 58°-68° (Fig. 11). Although far more numerous than F2 faults, the Fl faults are less continuous and form less prominent, and in many cases gently curved, fault traces. The F2 faults generally have linear traces that can be followed as continuous features for as much as 1,200 m. The F2 faults truncate and, therefore, are younger than the Fl faults in almost all cases (Fig. 11). Both Fl and F2 faults show similar amounts of horizontal separation of bedding planes, which generally range between 0.5 and 3.0 m. Displacement along Fl faults is predominantly normal slip, as right-lateral, horizontal separation of bedding planes occurs on southeast-dipping faults, whereas left-lateral, horizontal separation of bedding planes occurs on northwest-dipping faults (Fig. 1 IB). The oppositely dipping faults of the Fl fault set are, therefore, interpreted as conjugate normal faults. On the southern part of the TIZ, bedding consist-

EL TALLO INTERTIDAL ZONE

»Right Lateral Separation o Left Lateral Separation

Disrupted zone of closely s p a c e d brittle faults Chaotic slump deposit with soft sediment deformation Distinctive marker bed or set of beds Edge of w a v e - c u t platform and beach

Q Ul z < Ul a z o o . K u, to > §

o S oS

S< a

ui

tc z tu O z

09

il z

Figure 11. Structural map of the Tallo intertidal zone (TIZ) redrawn from low-altitude aerial photographs made during low tide; location of map shown in Figure 3. Inset A: Lower-hemisphere, equal-angle stereoplot of poles to all faults measured in TIZ. Inset B: Stereoplot of poles to all faults with horizontal separations measured in the TIZ. Inset C: Stereoplot of poles to all faults measured on the Burica Peninsula exclusive of the TIZ faults.

500m

642

CORRIGAN AND OTHERS

ently shows right-lateral separations, which suggests that normal-fault blocks are progressively dropped down to the southeast on a single set of southeast-dipping normal faults (Fig. 11). We interpret F2 faults as a conjugate pair of strike-slip faults, based on (1) a rectilinear, conjugate fault pattern with acute angles of 58°-68° between conjugate fault pairs; (2) N80°Estriking, F2 faults show predominantly rightlateral, horizontal separation of bedding planes, whereas N20°E-striking faults generally show left-lateral, horizontal separation of bedding planes; and (3) horizontal bedding separations of 2-40 m on N80°E-striking faults are consistently greater than 0-2 m of separation on N20°E-striking faults, although neither fault set consistently offsets the other. A comparison of all poles to fault planes from TIZ (Fig. 11 A) with all poles to faults from scattered and less well exposed localities in the Burica Member (Fig. 11C) indicates that the fault pattern of the TIZ is typical of the Burica Peninsula as a whole.

southwest-northeast-directed (N50°E) subhorizontal shortening and northwest-southeastdirected subhorizontal extension. The principal direction of shortening is indicated by the N50°E-striking bisector of the acute angle between N80°E- and N20°E-striking, conjugate strike-slip faults in the TIZ. The N50°E direction of shortening inferred from the orientation of conjugate strike-slip faults is consistent with both the orientation of the N50°E ± 20°E F1 normal fault set and the N48°W-striking anticline-syncline fold pair exposed to the north of the TIZ (Fig. 3). On the basis of crosscutting relationships, the F1 normal-slip mesoscopic faults formed first and were subsequently offset by the F2, conjugate strike-slip faults. The temporal relationship between the F1 and F2 fault sets and macroscopic folds of the southern Burica Peninsula is not clear from field data, although it is reasonable that faulting accompanied folding. The timing of mesoscopic faulting and macroscopic folding relative to movement on the MFZ is completely unconstrained.

Structural Interpretation

THE MIDDLE AMERICA FOREARC LANDWARD OF THE COCOS RIDGE

The predominant macroscopic structure of the Burica Peninsula is the MFZ, which we infer to be a high-angle, right-lateral fault zone displaying between 800 and 2,500 m of up-to-thewest vertical displacement. Our interpretation of right-lateral strike-slip movement is based on (1) the linear topographic expression of the fault zone over a distance of 15 km and (2) higher topographic relief and the presence of uplifted basement slivers where the MFZ changes strike from north to northwest; we interpret the uplifted topography and exposed basement rocks to be, in part, a consequence of localized shortening at a left-stepping segment in a right-lateral fault zone (Fig. 3). The lack of piercing points across the MFZ does not allow direct measurement of its strike-slip component. The easterly tilt and strike change in sedimentary rocks along much of the length of the Burica Peninsula is attributed to as much as 2,500 m of vertical displacement along the MFZ. Macroscopic folds in the southern Burica Peninsula suggest less than 5% of post-Pliocene southwest-northeast-directed, subhorizontal shortening. The north-trending open folds in the northeastern Burica Peninsula are related either to forced folding above subsurface basement fault blocks or to strike-slip movement along the unnamed, northwest-striking faults that bound the northern edge of the peninsula (Fig. 3). We interpret mesoscopic normal-slip and strike-slip faults in the TIZ and in river sections to be the result of a small amount of Quaternary

Tectonic Setting of the Forearc Landward of the Cocos Ridge The forearc region landward of the Cocos Ridge, extending from the Nicoya Peninsula to the Panama fracture zone, represents a morphologically and seismically distinct segment of the southern Middle America arc-trench system. The Middle America Trench shoals from a depth of approximately 5 km immediately west of the Nicoya Peninsula to a depth of less than 2 km off the Osa and Burica Peninsulas (Fig. 12). Burbach and others (1984) found that northwest of the southeastern Nicoya Peninsula, the Benioff Zone is well defined and dips at angles of 30°-45° beneath Costa Rica and Nicaragua. In the vicinity of the Cocos Ridge, they concluded that the Benioff Zone is poorly defined and characterized by a noticeable absence of hypocenters deeper than 70 km. More recently, Adamek and others (1987) estimated that the subducted part of the Cocos Ridge dips northeastward at an angle of 23° ± 3° beneath the Osa and Burica Peninsulas (Fig. 13). The Burica Peninsula provides an excellent study area for investigating the sedimentary, paleobathymetric, and tectonic history of the southern Middle America forearc because of the presence of well-exposed upper Neogene sedimentary rocks. The structures described in the Pliocene-Pleistocene Charco Azul Formation formed within the past 5 m.y. and, therefore,

provide important constraints on the style, magnitude, and rate of deformation along this segment of the Middle America forearc. Morphology and earthquake seismicity observations indicate that the effects of Cocos Ridge subduction extend along the margin to the northwest as far as the Nicoya Peninsula. Therefore, integration of regional data is necessary to establish the forearc response to the subduction of this prominent bathymetric feature. The purpose of this section is to integrate our field observations from the Burica Peninsula with tectonic studies by previous workers in other forearc areas landward of the Cocos Ridge. After reviewing the geology of the forearc region landward of the Cocos Ridge, we discuss (1) deformation of the forearc region related to Pleistocene subduction of the Cocos Ridge, (2) mechanisms to explain the observed forearc and arc deformation, and (3) the geologic history of the forearc area prior to Cocos Ridge subduction. For ease of discussion, we subdivide the Middle America forearc landward of the Cocos Ridge into an inner-forearc region between the late Neogene volcanic arc and the BallenaCelmira fault zone and an outer-forearc region between the Ballena-Celmira fault zone and the Middle America Trench (Fig. 12). The beststudied area of the inner-forearc area is the uplifted Terraba belt in Costa Rica (Henningsen, 1966; Lowery, 1982; Phillips, 1983; Yuan and Lowe, 1987). We use the terms "Terraba belt" to refer to the uplifted and deformed rocks presently exposed within a coastal mountain range (Fila Costena) and "Terraba Trough" to refer to the basin in which these rocks were deposited (Fig. 13). The Ballena-Celmira Fault Zone The Ballena-Celmira fault zone (BCFZ) strikes approximately N55°W and extends 120 km as a prominent lineament on LANDSAT imagery from near the village of Celmira on the Panama-Costa Rica border to the island of Ballena west of the Osa Peninsula of Costa Rica (Corrigan and others, 1987) (Fig. 12). The linear trace of the BCFZ through varied topography suggests a near-vertical fault plane. The BCFZ may represent a westward extension of one of several northwestward-striking, leftlateral strike-slip faults known in southwestern Panama (Mann and others, 1986; Okaya and Ben-Avraham, 1987). North of the BCFZ, approximately 3,500 m of marine volcaniclastic sedimentary rocks of Eocene to Miocene age dips monoclinally arcward and forms the Terraba belt (Henningsen, 1966; Phillips, 1983) (Fig. 13). South of the BCFZ, sub-Eocene basal-

BURICA POLES TO BEDDING n=26

P O L E S TO n = 1 16

FAULTS

POLES TO n=228

BEDDING

P O L E S TO n = 130

FAULTS

• ^ m m 1-2% 2-4% 4-6%

> 6%

Figure 12. Generalized geologic map of the Burica and Osa Peninsulas and surrounding outer-forearc region immediately arcward of the subducting Cocos Ridge. Bathymetric contour interval is 200 m; bathymetry is taken from Case and Holcombe (1980). Kv, Cretaceous to Paleocene basaltic basement (Nicoya Complex); Tem, Eocene to Miocene predominantly clastic, marine sedimentary rocks; Tp, Pliocene to Pleistocene clastic, marine sedimentary rocks (Charco Azul Formation); Qal, Quaternary alluvium. Thick dashed and barbed line is the projected axis of the Middle America Trench. Data sources include Terry (1956) and Sandoval and others (1982), outcrop pattern of northwestern Burica Peninsula; Mora (1979), outcrop pattern and structural data for the Terraba belt; Lew (1983) and Sandoval and others (1982), outcrop pattern and structural data for western and central Osa Penisula; this study, outcrop and structural data from eastern coast of Osa Peninsula and aerial-photograph interpretation of Osa lineaments. Sources for lower-hemisphere, equal-area contour plots of structural data from the Burica Peninsula are from this paper. Sources for the Osa Peninsula stereoplots include our own unpublished field notes (26 bedding and 18 fault measurements) and Lew (1983) (98 fault measurements used with permission of Lew). Cross section along line A-A' shown in Figure 13.

644

CORRIGAN AND OTHERS

SW

NE

Figure 13. Cross section at no vertical exaggeration of the forearc region landward of the subducting Cocos Ridge. Top of subducting ridge inferred from the April 3, 1983, main event and associated aftershocks relocated by Adamek and others (1987). Single-letter designation following date of events denotes thrust (T) or normal (N) type focal mechanisms. Event hypocenters have been projected from 50 km on either side of plane of section, which is shown in Figure 12. Note that the two large events (Ms > 7.0) occurred near the boundary between the outerand inner-forearc regions.

tic basement of the Burica and Osa Peninsulas is unconformably overlain by Pliocene-Pleistocene marine sedimentary rocks (Fig. 12). This contrast in geology across the BCFZ indicates that this structure has played an important role in accommodating deformation within the forearc. Outer-Forearc Region The outer-forearc area landward of the Cocos Ridge includes the Burica and Osa Peninsulas and the offshore shelf and trench slope from the Panama fracture zone to the Nicoya Peninsula (Fig. 14). The following summary of the geology of the Osa Peninsula combines observations made by us during 3 days of reconnaissance field work along the eastern coast of the peninsula with Lew's (1983) more detailed mapping of the southern coast of the peninsula. The source of our information on the offshore region is previously published marine geophysical and geologic data (Crowe and Buffler, 1983,1985; von Huene and others, 1985). Osa Peninsula. The geology of the Osa Peninsula is very similar to that of the Burica Pe-

ninsula (Fig. 12). The basement of the Osa Peninsula consists of complexly faulted basalt, overlain by, and locally intercalated with, Paleocene to lower Eocene pelagic limestone (Azéma and others, 1981; Salsipuedes Formation of Lew, 1983). Marine sedimentary rocks of the Punta La Chancha Formation (Lew, 1983) unconformably overlie this basement and are equivalent in age and similar in lithology to the Charco Azul Formation of the Burica Peninsula. Tidal exposures along the east coast of the peninsula provide near-continuous outcrop of this Neogene sedimentary cover tilted 5°-10° to the northeast (Fig. 13). Assuming no major, covered faults, we estimate a total thickness of 3,000 m for the Punta La Chancha Formation. Along the southern coast of the Osa Peninsula, Lew (1983) estimated a minimum thickness of 850 m near the type locality of the Punta La Chancha Formation, although outcrops here are less continuous than along the eastern coast, owing to block faulting (Fischer, 1980) (Fig. 12). Rocks of the Punta La Chancha Formation consist of a fining-upward sequence of siltstone, sandstone, and conglomerate that unconforma-

bly overlies basaltic basement along the southern coast of the peninsula (Lew, 1983). Along the southern coast of the Osa Peninsula, conglomerates and turbiditic sandstones grade upward into a slope facies association consisting of a monotonous sequence of facies G mudstone with some facies D interbedded sandstone and mudstone. Fossils reported by Lew (1983) from the Punta La Chancha Formation include shallow-water, molluscan fragments in the lower part of the section and a mixed mollusc and benthic foraminifera fauna in the higher parts of the section. On the basis of 5 mudstone samples, the deepest-dwelling benthic foraminifera indicate deposition at 600 to 1,200 m and are assigned ages ranging from early to late Pliocene (J. C. Ingle and H. C. Chou in Lew, 1983). On the basis of lithologic similarities and the occurrence of shallow-water molluscan fragments in both sections, we correlate the lower part of the Punta La Chancha Formation with the coeval Penita Member of the Charco Azul Formation. We correlate the deep-marine, sedimentary rocks of the upper part of the Punta La Chancha Formation with the coeval Burica Member of

\ 0

ft

COCOS RIDGE-

i r i. •» • > A

U

"565

Quaternary calc-alkaline volcanics and active volcanoes (dots)

location of seismic profiles

outcrops of post-Miocene shelf and slope deposits

1 , 0 0 0 - 2 , 0 0 0 m slope apron thickness

pre-Eocene "basement" rocks

thickness

Middle America Trench axis

topographic elevations exceeding 2,000 m

> 2 , 0 0 0 m slope apron

DSDP site 5 6 5

Figure 14. Compilation map of geologic and tectonic features of the southern Middle America forearc area. Onshore features based on Case and Holcombe (1980), and offshore slope sediment thicknesses extrapolated from Crowe and Buffler (198S). Key to abbreviations: SEP, Santa Elena Peninsula; NP, Nicoya Peninsula; OP, Osa Peninsula; BP, Burica Peninsula; PFZ, Panama fracture zone; ENFZ, East Nicoya fault zone. Approximate extent of subducting Cocos Ridge is indicated.

646

CORRIGAN AND OTHERS

uous section (328 m) of uppermost Miocene to Quaternary, homogeneous, dark greenish-gray mudstone was recovered from the drill hole (von Huene and others, 1985). The axis of the offshore slope apron depocenter defined by the seismic profiles extends from west of the Nicoya Peninsula to the limit of the seismic data just west of the Osa Peninsula (Fig. 14). Crowe and Buffler (1983) described a major, unnamed down-to-the-northwest highangle fault southeast of the Nicoya Peninsula that strikes approximately perpendicular to the MAT. We refer to this fault as the "East Nicoya fault zone" (ENFZ). Vertical offset of Cocos plate oceanic crust and overlying pelagic strata along this fault is approximately 300 m. Near the eastern edge of the Nicoya Peninsula, the slope depocenter axis is truncated in a left-lateral sense by the northeastward extension of the ENFZ (Fig. 14). Apparent left-lateral offset of the slope apron isopachs by the ENFZ is approximately 20 km.

Figure 15. Schematic Pleistocene evolution of the southern Middle America Trench. Key: 1, active volcanic chain; 2, inner-forearc basin including the Ten-aba Trough of Costa Rica; 3, outer-forearc region including the present-day Burica, Osa, and Nicoya Peninsulas and the Middle America Trench slope; MFZ, Medial fault zone; ENFZ, East Nicoya fault zone; barbs indicate trace of the Middle America Trench and mapped thrust faults in the Terraba belt. Inset shows inferred orientations of principal stress axes and their interpreted relation to conjugate strike-slip and normal mesofault sets observed in Pliocene-Pleistocene strata exposed on the Burica Peninsula (Fig. 11). In the left inset, subhorizontal margin-perpendicular shortening and subhorizontal margin-parallel extension is accommodated along conjugate strike-slip and normal faults; in the right inset, relief of the subducting Cocos Ridge and isostatic uplift causes arching of the forearc and extension in a direction parallel to the trench.

the Charco Azul Formation, based on lithologic and biostratigraphic similarities. Mesoscopic structures affecting Pliocene rocks of the Punta La Chancha Formation include (1) small, open folds with east-west-trending subhorizontal axes and wavelengths of 3 to 5 m and (2) numerous high-angle normal faults. Fault measurements indicate two sets of conjugate normal faults with one set striking parallel and the other set striking perpendicular to the long axes of the peninsula (Lew, 1983) (Fig. 12). Offshore Region. Widely spaced, marine seismic reflection profiles between the Nicoya and Osa Peninsulas reveal a sedimentary slope apron as much as 2,400 m in thickness overlying an acoustic basement of unknown lithology

(Crowe and Buffler, 1983, 1985) (Fig. 14). An extensive slope depocenter can be defined off Costa Rica by interpolation of slope apron thickness between seismic profiles spaced tens of kilometers apart (Crowe and Buffler, 1983) (Fig. 14). Where relatively undeformed, the sedimentary slope apron appears to thin downslope and downlap the underlying acoustic basement. The younger part of the slope apron onlaps basement along a prominent unconformity near the landward edges of the seismic lines to the west of the Osa Peninsula. The only age determinations for sedimentary rocks from the slope apron offshore Costa Rica are from DSDP site 565, approximately 40 km southwest of the Nicoya Peninsula on the lower slope landward of the Middle America Trench (Fig. 14). A contin-

The ENFZ, located at the approximate western margin of the Cocos Ridge, coincides with three important changes in slope structure and morphology of the southern Middle America forearc. (1) To the northwest of the ENFZ, the trench slope is relatively smooth and undisrupted, whereas in the vicinity and southeast of the ENFZ, the bathymetry of the lower slope is very irregular and is characterized by large, uplifted basement blocks (Crowe and Buffler, 1983); (2) the Middle America trench axis loses its well-defined morphologic expression east of the ENFZ and shoals from a depth of greater than 4 m to depths of less than 1,600 m near the Osa and Burica Peninsulas; and (3) the inferred axis of maximum slope apron thickness narrows and shifts landward east of the ENFZ (Fig. 14). These observations suggest that the ENFZ defines the western edge of Cocos Ridge-related deformation of the overriding forearc area. Inner-Forearc Region The Terraba belt of the inner-forearc region of Costa Rica consists of at least 3,500 m of Eocene to Pliocene, predominantly marine volcaniclastic sedimentary rocks (Henningsen, 1966; Phillips, 1983). Middle to upper Eocene shallow-water carbonate rocks of the Brito Formation are believed to have been deposited on basement blocks surrounded by deeper marine basins that accumulated volcaniclastic and carbonate debris (Yuan and Lowe, 1987). A 1,000-m-thick mud-dominated, coarsening-upward turbidite sequence, the Oligocene to lower middle Miocene Terraba Formation, conforma-

FOREARC RESPONSE TO SUBDUCTION, COCOS RIDGE, PANAMA-COSTA RICA

bly overlies the Brito Formation (Phillips, 1983). The Terraba Formation was deposited at maximum estimated water depths of 2,000 m (Phillips, 1983). Overlying the Terraba Formation is approximately 800 m of middle to upper Miocene strata (Cunt Formation), which grades upward from marine-deposited gravity-flow breccia and conglomerate to fluvial and lacustrine units (Henningsen, 1966; Lowry, 1982). This shallowing-upward trend suggests that depositional rates exceeded subsidence rates during this period. The contact between the Curré Formation and the overlying Pliocene Paso Real Formation is not clear (Henningsen, 1966). The Paso Real Formation consists of approximately 900 m of predominantly subaerial volcanic flows and pyroclastic deposits (Henningsen, 1966). An angular unconformity separates Pleistocene to Holocene alluvial fan and lacustrine deposits (Brujo Formation) from older folded and tilted deposits of the Terraba belt (Henningsen, 1966; Yuan, 1984). DISCUSSION Deformation Related to Cocos Ridge Subduction Outer-Forearc Region. Foraminifera-derived, paleobathymetric estimates of subaerial exposures of Pliocene-Pleistocene marine sedimentary rocks found on the Burica, Osa, and Nicoya Peninsulas constrain the timing and rates of uplift in these areas. Rapid uplift of the Burica Peninsula occurred approximately 1 m.y. ago, based on benthic foraminifera identified in the upper Burica Member and on benthic foraminifera and molluscs identified in the Armuelles Member. These foraminifera indicate deposition of the upper Burica Member at water depths of approximately 1,500 m. Molluscs in the uppermost exposed Armuelles Member indicate that the Burica Peninsula was characterized by shallow-marine deposition by late Pleistocene time. These observations indicate that the Burica Peninsula has undergone approximately 1,500 m of uplift since middle Pleistocene time. If an earliest Pleistocene age (1.6 m.y.) is used, a minimum, average Quaternary uplift rate of 0.94 mm/yr is calculated for the Burica Peninsula. Uplift of the Osa Peninsula occurred sometime during the Pleistocene, based on middle to late Pliocene benthic foraminifera identified in the Punta La Chancha Formation (Lew, 1983). Benthic foraminiferal assemblages indicate that deposition took place at minimum water depths of 200 m and probable water depths between 600 and 1,200 m. If an average water depth of

600 m during the earliest late Pliocene (3.4 Ma) is used, a poorly constrained minimum average uplift rate of about 0.2 mm/yr is calculated for the Osa Peninsula. A third area of Pliocene marine sedimentary rocks (Montezuma Formation) occurs on the southeastern corner of the Nicoya Peninsula (Lundberg, 1982) (Fig. 14). Studies of benthic foraminifera indicate that these rocks were deposited during early Pliocene times at water depths of about 300 m (McKee, 1985). Thus, a minimum, average uplift rate of about 0.05 mm/yr since 5 m.y. ago can be inferred for the southeastern Nicoya Peninsula. The much lower rate of uplift for the Nicoya Peninsula as compared to the Burica and Osa areas is consistent with the location of the Nicoya Peninsula to the west of the down-to-the-west East Nicoya fault zone, which we propose as the northwestern extent of deformation related to Cocos Ridge subduction (Fig. 14B). Structures in the Pliocene-Pleistocene sedimentary rocks of the Burica Peninsula and Pliocene rocks of the Osa Peninsula record marginperpendicular, normal faulting as well as minor, margin-parallel folding. The orientation of macroscopic folds and mesoscopic conjugate strikeslip faults on the southern Burica Peninsula is consistent with an inferred N50°E direction of maximum shortening, which is approximately parallel to the N30°E direction of predicted convergence between the Cocos and Caribbean plates in this area (Minster and Jordan, 1978). This inferred shortening direction in the southern Burica Peninsula is consistent with the orientation of other faults and folds observed on the northern Burica Peninsula and on the Osa Peninsula (Lew, 1983) (Fig. 12). The predominance of Pleistocene margin-perpendicular normal faults over strike-slip faults and marginparallel folds on the Burica Peninsula, the Osa Peninsula, and the Costa Rican coastline west of the Osa Peninsula (Fischer, 1980) may reflect arching and margin-parallel extension of the outer-forearc area above the subducting Cocos Ridge. In this interpretation, the direction of near-surface, maximum stress is vertical, whereas the direction of minimum stress is arc parallel (Fig. 15B, inset). We favor this latter interpretation as being most indicative of time-averaged Quaternary stress state because we find no direct evidence for large (>5%) margin-perpendicular, subhorizontal shortening within the outer-forearc blocks. Shallowing and marginward deflection of the Middle America Trench coincide approximately with the proposed extent of the subducting Cocos Ridge from the East Nicoya fault zone to the Panama fracture zone (Fig. 15B). Margin-

647

ward deflection of the trench axis of approximately 15 km in the vicinity of the East Nicoya fault zone coincides with approximately 20 km of left-lateral apparent offset of an extensive slope depocenter defined by seismic profiling (Fig. 14). The right-lateral(?) Medial fault zone of the Burica Peninsula may play a similar role in accommodating deflection of the MAT along the eastern edge of the subducted Cocos Ridge. Both faults have a significant component (as much as 2 km) of downward vertical displacement away from the margins of the subducting Cocos Ridge. Inner-Forearc Region. Arcward tilting and uplift of the Terraba belt also occurred in Pleistocene time; however, deformation of the innerforearc region differs from that observed on the outer-forearc blocks, in that clear evidence for subhorizontal shortening exists. Numerous northwest-striking reverse faults have been mapped in the arcward-tilted Eocene-Pleistocene sedimentary rocks of the Terraba belt (Mora, 1979; Heywood and Silver, 1983; see Fig. 12). Uplift of the southwestern edge of the Terraba Trough is recorded by an angular unconformity between thrusted and arcwarddipping, Eocene to Pliocene marine sedimentary rocks of the Terraba belt and nearly horizontal sedimentary rocks of the upper Pleistocene to Recent Brujo Formation (Henningsen, 1966; Yuan, 1984). These lines of evidence suggest the initiation of subhorizontal shortening of the inner-forearc region during Pleistocene time. Volcanic Arc. Farther to the northwest, the volcanic arc (Cordillera de Talamanca and Cordillera Central) reaches elevations of more than 2 km (Fig. 14). These high elevations correspond approximately with a 200-km-wide gap in active arc volcanism from approximately the Panama-Costa Rica border to central Costa Rica along the projected trend of the Cocos Ridge (McGeary and others, 1985; Fig. 14). On the basis of a geodetic releveling survey, Miyamura (1975) calculated a present uplift rate of 1-2 mm/yr for the Cordillera Central of Costa Rica. Mechanisms for Margin Deformation We argue that isostatic compensation related to initial subduction of thick (~14 km) oceanic crust of the Cocos Ridge is primarily responsible for uplift and deformation of the outer forearc landward of the Cocos Ridge. Increased subhorizontal compression due to basal shear stresses resulting from impingement of the Cocos Ridge against the base of the inner-forearc crust may play a subsidiary role in producing deformation

648

CORRIGAN AND OTHERS

observed along the inner-forearc region. These two types of responses are possible because a major crustal discontinuity, the surface expression of which is the Ballena-Celmira fault zone, separates the inner- and outer-forearc regions (Fig. 13). This interpretation is consistent with earthquake seismicity data (Adamek and others, 1987) that show that the two largest (Ms > 7.0) recorded earthquakes in this region are located near the boundary between the inner and outer forearc along what probably is the contact between the base of the inner-forearc crust and the subducting Cocos Ridge (Fig. 13). On the basis of the size of the aftershock expansion area and the duration of the aftershock period associated

PRE-COCOS RIDGE SUBDUCTION

with the April 3, 1983, main thrust event, Adamek and others (1987) inferred that passage of the Cocos Ridge beneath the arc is marked by relatively weak, inhomogeneous coupling (small aftershocks, large aftershock expansion area) beneath the Osa-Burica outer-forearc region and stronger, more homogeneous coupling (main rupture, large-moment release) beneath the area arcward of these peninsulas. We suggest that relatively strong coupling between the overriding and underriding plate beneath the forearc region arcward of the Osa and Burica Peninsulas represents interaction between the subducted part of the Cocos Ridge and the crystalline basement of the arc. Weaker, more heterogene-

POST-COCOS RIDGE SUBDUCTION

Ah

OC +

I w = water depth l s = sediment thickness ' a c = a r c c r u s t thickness l o c = oceanic crust thickness

p w = water density p s = sediment density Pac= arc crust density Poc= oceanic crust density

l0c

P m =

+AI0C

= Cocos Ridge thickness x = A l o c

y= [ A l

Ah = { x

o c

A l o e

mantle density

( P m " P o c ) / ( P r t r P w )

( P m ' P o c ) - l w

( P m " P w ) J / P m

:xl w } Figure 16. Amount of isostatic uplift predicted by replacing normal-thickness oceanic crust (loc) with crustal thickness of the Cocos Ridge (!„. + Al^), assuming that both columns are compensated at depth. Average outer-forearc crustal thickness (l ac ) beneath the Burica Peninsula is constrained by depth to the interplate coupling zone (Adamek and others, 1987). The crustal thickness and density of the Cocos Ridge is constrained by marine seismic refraction studies of this feature (Bentley, 1974). Late Pliocene paleodepth of the Burica Peninsula prior to subduction of the Cocos Ridge (l w = 1 . 5 km) is based on foraminifera-derived paleodepth determinations. If it is assumed that the two columns are compensated at depth, the magnitude of isostatic uplift expected is given by Ah = l w + y for uplift above sea level (x > l w ). On the basis of these assumptions and values discussed in the text, the amount of isostatic uplift expected due to Cocos Ridge subduction is on the order of 2 to 3 km.

ous coupling beneath the Burica and Osa Peninsulas occurs because the outer-forearc crust is effectively "detached" from the arc crust along a major high-angle discontinuity that is manifest today at the surface by the Ballena-Celmira fault zone (Fig. 13). The isostatic component of forearc uplift related to Cocos Ridge subduction can be estimated if variations in crustal thickness and density are assumed to be compensated at depth (Fig. 16). An uplift amount on the order of 2 to 3 km can be estimated by assuming a 14-km thickness for the Cocos Ridge (Bentley, 1974), an average mantle density of 3.4 g/cm 3 , and an average density difference between the oceanic upper mantle and crust of 0.6 ± 0 . 1 g/cm 3 . This estimate of isostatic uplift is compatible with the minimum 2,000 m of uplift estimated for the Burica Peninsula from benthic foraminifera and suggests that gravitational body forces associated with subduction of thickened oceanic crust of the Cocos Ridge exert primary influence on margin deformation. In addition to rapid, post-Pliocene uplift of the Osa and Burica Peninsulas, other deformational features observed in the area of Cocos Ridge subduction include (1) arcward-tilted (10°-30°) Pliocene-Pleistocene strata on the Osa and Burica Peninsulas; (2) minor, marginparallel folds and margin-perpendicular conjugate strike-slip and normal faults in PliocenePleistocene sedimentary rocks of the Burica and Osa Peninsulas; and (3) the north-striking, highangle, right-lateral (?) strike-slip fault along the center of the Burica Peninsula (Medial fault zone) and the northeast-striking, high-angle, leftlateral (?) strike-slip fault along the southeastern Nicoya Peninsula (East Nicoya fault zone). These observations indicate both marginparallel, subhorizontal extension and minor margin-perpendicular, subhorizontal shortening. The relative contributions and the interplay of subhorizontal compression and isostatic gravitational forces to the overall deformation pattern observed in this and other areas of aseismicridge subduction are unclear. Along the central New Hebrides arc, Collot and others (1985) have emphasized the possible role of subhorizontal compression in forearc deformation landward of the subducting aseismic D'Entrecasteaux Ridge. In this same area, Fisher (1986) has used seismic reflection data to illustrate a lack of folding and thrust faulting in the forearc area landward of the D'Entrecasteaux Ridge. The lack of observed shortening is consistent with the hypothesis that deformation is primarily the result of isostatic uplift due to vertical gravitational forces associated with subduction

FOREARC RESPONSE TO SUBDUCTION, COCOS RIDGE, PANAMA-COSTA RICA

Figure 17. Inferred depositional paleodepths through the Neogene for the innerforearc and outer-forearc regions. Age and paleodepth estimates of Terraba belt strata from Henningsen (1966), Lowery (1982), Phillips (1983), and Yuan (1984). Age and paleodepth for the outer-forearc region are from this paper. Vertical bars indicate qualitative error.

§

649

500

•C Q. 1000 (1) O 1500 'S c o 2000 ü o 2500 a. a> a

— < >

3000

O

Inner forearc (Terraba Trough)

•

Outer forearc (Burica-Osa area)

3500

of the D'Entrecasteaux Ridge (Chung and Kanamori, 1978; Moretti and Ngokwey, 1985). Similarly, Dupont and Herzer (1985) have pointed out the lack of evidence for subhorizontal shortening of the Tonga forearc landward of the subducted Louisville Ridge. Geologic History Prior to Cocos Ridge Subduction Subduction of the Cocos Ridge has affected this part of the southern Middle America arctrench system for approximately the past 1 m.y. It is interesting to compare this Pleistocene phase of deformation to the longer-term, late Cenozoic history of this part of the arc-trench system. Stratigraphic and paleodepth observations by previous workers from the inner-forearc area of the Terraba belt demonstrate that when the inner-forearc block is uplifted and eroded, the outer forearc subsides and receives sediment (Fig. 17). Conversely, when the outer-forearc block is uplifted and eroded, the inner forearc subsides and receives sediment. A major hiatus between Eocene and lower Pliocene sedimentary rocks on the Burica and Osa Peninsulas suggests that the outer-forearc region was a topographic high during the time of greatest subsidence of the Terraba Trough (Fig. 17). This is supported by paleocurrent measurements indicating that the Terraba Trough was an elongate basin parallel to the trend of the volcanic arc throughout the Tertiary (Lowery, 1982; Phillips, 1983; Yuan, 1984). The outerforearc high probably acted as a barrier for trenchward transport of sediment along the southwestern edge of the Terraba Trough. Therefore, arc-derived volcaniclastic sediments ponded in the elongate, forearc trough.

A reversal in the subsidence pattern of the inner-forearc and outer-forearc regions occurred near the Pliocene-Miocene boundary. Shallowmarine, fluvial, and lacustrine depositional environments in the Terraba Trough indicate complete filling by the late Miocene (Lowery, 1982). Since late Miocene time, sediment deposition in the Terraba Trough took place near sea level (Fig. 17). Rapid, outer-forearc subsidence of the Burica-Osa area began approximately 5 Ma, based on the presence of Pliocene molluscs in the shallow-marine section (Penitas Member) and early Pliocene foraminifera at the base of the deep-marine section (Burica Member). A satisfactory mechanism to explain this rapid subsidence is lacking, although we speculate that this subsidence may be related to the proximity of an unstable Cocos-Nazca-Caribbean triple junction presently located at the northern end of the Panama fracture zone (Lonsdale and Klitgord, 1978; Okaya and Ben-Avraham, 1987). The contrast in geology between the outerand inner-forearc regions indicates that the outer forearc was essentially decoupled from the inner forearc throughout the Tertiary, probably along the arc-parallel Ballena-Celmira fault zone (Fig. 12). As argued above, vertical movements along this major structural discontinuity may explain the present-day difference in seismic coupling inferred beneath the outer- and inner-forearc regions (Adamek and others, 1987). Over longer periods, this poorly studied margin-parallel fault may allow the outer-forearc region to act as a "bumper zone," which accommodates variations in the rate, direction, or in the case of the Cocos Ridge, thickness of the subducting plate. Similar longitudinal fault systems have been described along the South America convergent margin (Dalziel and Forsythe, 1985).

CONCLUSIONS 1. Sedimentary facies, paleocurrent indicators, and biostratigraphic and paleobathymetric evidence from predominantly fine-grained turbidite strata of the Burica and Osa Peninsulas suggest that these areas formed a rapidly subsiding outer-forearc block in the early Pliocene. Sedimentation occurred on a southward-facing slope in a near-trench environment. Studies of benthic foraminifera from the Burica Peninsula indicate that water depths shoaled from 2,000+ m in the early Pliocene to less than 1,500 m in the early Pleistocene. 2. Estimates of middle Pleistocene uplift and timing of deformation for both the inner- and outer-forearc regions based on biostratigraphic evidence are in agreement with an independent estimate of the initiation of Cocos Ridge subduction approximately 1 m.y. ago based on marine magnetic anomalies (Lonsdale and Klitgord, 1978). Paleodepth determinations of benthic foraminifera from Neogene strata on the Burica and Osa Peninsulas indicate minimum, Quaternary averaged uplift rates for these outer-forearc blocks of 0.9 to 0.2 mm/yr, respectively. 3. Faults and folds in Pliocene-Pleistocene sedimentary rocks of outer-forearc Osa and Burica Peninsulas indicate minor (