S t r u c t u r e o f t h e S i n u - S a n J a c i n t o fold belt - - A n a c t i v e
a c c r e t i o n a r y p r i s m in n o r t h e r n C o l o m b i a
EL ARBI TOTO and J. N. KELLOGG*
Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, U S A
(Received November 1991; Revision Accepted January 1992)
A b s t r a c t - - - T h e Sinu and San Jacinto fold belts in northern Colombia form a wedge of sediments up to 12 k m thick which has been accreted to the South American margin throughout the Cenozoic. The wedge is characterized by a very low topographic slope ( -<- 2 °) and abundant mud volcanism. Many folds have a core of mobile overpressured mud. Active shortening at a rate of about 1 to 2 cm yr -1 is the result of conver- gence between the Caribbean plate and the northwest margin of South America. Aeromagnetic and gravity anomalies suggest a buttress of rigid basement rock dipping toward the toe of the wedge. A seismic reflector dipping gently (2 °) toward the back of the wedge is interpreted as the basal d~collement. According to the theory of critical taper for a noncohesive wedge of Coulomb material, fluid pressure ratios should approach lithostatic values (0.97) within the wedge and on the basal d6collement. The predicted high fluid pressures within the wedge are consistent with the mud diapirism. The low slip rate and subduction of large quan- tities of young, high-porosity sediments explain the absence of large interplate thrust earthquakes. R e s u m e n m L o s cintures de plegamiento Sinu y San Jacinto, localizados en la parte norte de Colombia, forman una curia sedimentaria de hasta 12 km de espesor, la cual ha sido acrecionada a la margen Sur Americana durante el Cenozoico. La curia se caracteriza por una baja pendiente topogr~fica ( ~ 2 °) y por abundanto vulcanismo de lodo. Los pliegues son frecuentemente atravesados por lodos m6viles sobrepre- sionados. E1 activo acortamiento en el cintur6n plegado de Sinu-San Jacinto es el resultado de la conver- gencia entre la placa del Caribe y la margen noroeste de Sur America a una tasa aproximada de 1-2 cm pot afio. Basados en anomalias aeromag~nticas y gravim~tricas, nosotros hemos predicho un estribo de basa- mento rigido buzando hacia la parte m~s baja de la curia. Un reflector sismico buzando suavemente (2 °) hacia la parte de atrds de la curia es interpretado como el despegue basal (d6collement). Aplicando la teoria de decrecimiento critico para una curia no cohesiva de material Coulomb, la relaci6n de la presi6n de fluidos se a c e r c a a l valor litost~itico (0.97) dentro de la curia y sobre el despegue basal (d~collement). Las altas
p r e s i o n e s de fluido predichos dentro de la curia son consistentes con el diapirismo de lodo. La falta de terremotos de alta intensidad en el limite entre las placas es el resultado de la baja tasa de desplazamiento y de subducci6n de grandes cantidades de sedimentos recientes de alta porosidad.
I N T R O D U C T I O N
SEVERAL RHEOLOGICAL MODELS have been developed for accretionary prisms associated with convergent plate boundaries. Chapple (1978) proposed a perfect plastic model characterized by significant horizontal compressive deformation within the wedge and a weak basal d6collement dipping toward the thick back end of the wedge, below which there is almost no deformation. He showed that shortening of the wedge and sliding over its basal d~collement can occur, given topographic slopes, basal d6collement dips, wedge thicknesses, and yield stresses that are within the range of commonly observed values.
Davis et al. (1983) proposed that wedges consist of Coulomb material, since for depths shallower than 12-15 kin, silicate rocks are expected to behave elastic-frictionally rather than plastically under nor- mal geothermal gradients. The mechanics of the wedge is analogous to that of a wedge that forms in front of a moving bulldozer or snowplow.
The Sinu and San Jacinto fold belts in north- western Colombia (Fig. 1) form a wedge of sediments
*Address all correspondence and reprint requests to Dr. James N. Kellogg.
up to 12 k m thick which has been accreted to the South American margin throughout the Cenozoic (Duque-Caro, 1979). The wedge is characterized by a very low topographic slope and abundant m u d vol- canism. In this paper we examine the Sinu-San Jacinto cross section geometry and m u d diapirism and interpret them in the context of Coulomb wedge theory.
GEOLOGY O F T H E SINU AND SAN J A C I N T O F O L D B E L T S
The Sinu-San Jacinto wedge is bounded to the northwest by the South Caribbean marginal fault and to the southeast by the Romeral fault zone (Fig. 1). The Romeral fault zone is a paleo-suture that separates Paleozoic continental basement rocks to the east from Mesozoic oceanic basement rocks to the west. The Sinu fault zone separates the older sub- aerially exposed San Jacinto fold belt to the east from the younger partially submerged Sinu fold belt to the west.
212 EL ARBI TOTO and J. N. KELLOGG
~ o / ~
J~=., Santa Marta
/ ' I
0 '~ "
t / "/ i
~/ / ~
/ < , '
0 100 200 300 km
I I I I
Fig, 1. Tectonic m a p showing the location of the Sinu and San Jacinto fold belts. Present-day plate motions (bold arrows) relative to the North A n d e a n microplate showing average slip rates (cm yr-1) during the last 5 to 10 million years, after Minster and Jordan (1978) and Kellogg et al. (1985).
ments (Duque-Caro, 1979; Case and Holcombe, 1980). Northwest-verging folds and thrust faults trend approximately N20°E.
The Sinu fold belt includes Oligocene-Miocene shales and extensive fine-grained upper Miocene and Pliocene turbidites overlain by shallow-water Quaternary carbonate facies made up of shales, reef limestones, sandstones, and conglomerates (Duque- Care, 1979). Narrow mud-cored anticlines with steeply dipping limbs are separated by broad, gentle synclines. The belt has a remarkable number of m u d volcanoes, domes, and diapirs (Fig. 2) produced by mobilization of Oligocene-Miocene high-pressure shales (Shepard et al., 1968; Raminez, 1969; Shep-
ard, 1973; Duque-Caro, 1979, 1984; Vernette, 1986). Shortening in the Sinu fold belt is accommodated on NW-verging imbricated fault-bend folds (Fig. 3).
I I I
• • Monteria
0 50 100 km
77 = 76 ° 75°W
Fig. 2. Map showing the distribution of mud domes and volcanoes and reefs over domes (all shown as solid circles) in the Sinu-San Jacinto wedge (modified after Vernette, 1989) and the location of the topographic profiles (see Fig. 6). Bathymetry after Case and Holeombe (1980); contour interval = 400 meters.
bean-South American plate boundary zone (Jordan, 1975; Minster and Jordan, 1978; DeMets et al.,
1990). Seismic reflection profiles across the toe of the South Caribbean deformed belt reveal active folding of the youngest sediments, with undeformed reflectors beneath the fold belt dipping gently to the southeast (Silver et al., 1975; Lu and McMillan, 1982; Ladd and Truchan, 1983; Lehner et al., 1983; Kolla et al., 1984; L a d d e t al., 1984). A weakly defined southeast-dipping Wadati-Benioff zone (Dewey, 1972; Pennington, 1981) has been inter- preted as the product of slow subduction of Carib- bean crust beneath the northern Andes (Kellogg and Bonini, 1982). Preliminary results from CASA
(Central And South America) G P S (Global Posi- tioning System) satellite geodetic measurements over a 3-year period (1988-1991) show on the order of 1 to 2 cm yr-1 convergence between the Caribbean plate and North Andean microplate (Vega et al.,
214 EI, ARBI TOI'O and ,]. N. KELLOGG
7Oc 800 900 I000
S h o t p o i n t
700 800 900 1000
~ ~ < : -
~ / I
k'X', "" "~" ' \ " "1
~ . . . " \ . " 0 1 2 kin ~
~-~... -. ~ t t t I
Fig. 3. W e s t e r n end of m u l t i c h a n n e l seismic profile 156. The s t r u c t u r e is i n t e r p r e t e d as a growth fault-bend fold (bottom). A m u d d i a p i r i n t r u d e s t h e axis of t h e anticline. Average vertical exaggeration ~ 1.8 X. See Figs. 2 and 4 for location of profile.
horizon as the basal d6collement. We estimate its regional dip from the seismic data to be about 2.0°+0.5°SE. The wedge has a thickness of about 5 km at the toe, increasing to over 10 k m under zone 2. Under the folded Cenozoic sedimentary sequence in zone 3, basement rocks of the North Andean plate (Mesozoic oceanic rocks to the west and Paleozoic
~ 1 0 3 2 0
~ 6 0
~ 2 0
0 i i i i ~ i
- 2 0 3 0 8 0 1 3 0 1 8 0
D i s t a n c e in k m
Romeral fault zone
Sea L e v e l Nt' ~I¢ , =_ - _
0 - - v .
0 | I 0 | 20 km |
Fig. 4. Cross-sectional geometry of the Sinu-San Jacinto wedge along traverse 3, with Bouguer gravity (mgal) and aeromagnetic anomalies (gamma). See Fig. 2 for location of profile.
C R I T I C A L T A P E R F R O M C O U L O M B W E D G E T H E O R Y
In 1959, Hubbert and Rubey determined the m a x i m u m length for which a rectangular block can slide over a fiat horizontal d6collement without undergoing internal deformation. For lengths grea- ter than the Hubbert-Rubey m a x i m u m , the rocks will deform as the block is pushed from the back end until they reach a critical taper and then will slide along the basal d~coUement without internal de- formation. The critical taper can be defined as the geometrical shape for which the wedge is at Coulomb failure everywhere and the compressive force in every segment of the wedge is balanced by the fric- tional resistance of the part of the wedge lying in front of that segment.
B y assuming that the wedge is a Coulomb material, Davis et al. (1983) defined an expression for the critical taper. The angle a (Fig. 5) represents the topographic slope, the angle p is the dip of the basal d6collement, and a ÷ p is the critical taper. The stress within the wedge at any point is given by Coulomb fracture strength since the wedge is on the verge of failure everywhere.
For small angles (a + p ~ 1) the critical taper for a cohesionless wedge was determined to be:
c c + ~ = (I - P J P ) ~ + ~tb(l - ~) (I - Pw/P) + 4(I - )~)
where A, the fluid-pressure ratio or Hubbert-Rubey coefficient, is assumed to be the same for the wedge and the d6collement; P w and p are the densities of water (or air for the subaerial case) and the rocks in
216 El, ARBI TOTO and d. N. KELLOGG
the wedge, respectively;/z b is the coefficient of fric- tion along the basal d6collement, lc was defined ap- proximately by:
sine . sin2d~b + COSCb(sin2~ - sin2¢b) In - 1 --S-~¢ +cos2~b. cosCb - (sin2¢ - sin2¢b) In
Cbis the angle of friction along the d~colle- ment and q5 is the angle of internal friction in the wedge.
The taper ( a + p ) approximates a ratio of fric- tional terms to wedge strength terms. An increase in effective basal friction, either by a drop in fluid- pressure ratio ~, or an increase in the basal friction term Pb, will lead to an increase in the wedge taper. Conversely, an increase in wedge strength or effec- tive internal friction term p qu = tan q5) will result in an increase in 7c and a decrease in the taper.
A P P L I C A T I O N O F T H E T H E O R Y T O T H E
S I N U - S A N J A C I N T O F O L D B E L T
Five topographic profiles (Fig. 6) were prepared across the Sinu-San Jacinto wedge, based on unpub- lished seismic reflection surveys (Aquitaine Colom- bie, S.A., 1974; Empresa Colombiana de Petroleos,
1983; Exxon, 1978; Gulf Oil Exploration and Production Co., 1982, 1984; Texas Petroleum Co.~ 1975) and published topographic maps 4Instituto Geografico Agustin Codazzi, 1982a,b, 1987) Mean slopes were computed by fitting linear regressions to the topography across the wedge. The topography could not be well fit by a constant-slope regression. Three zones of different topographic slopes were distinguished: 1) a zone of negligible slope basin- ward (NW) of the toe of the wedge, the mean slope of the set of profiles a = 0.4°_+_0.2°; 2) a zone of gentle slope from the toe of the wedge to the shelf break, the mean slope of the set of profiles a ~- 2 0°_+0.4°; and 3) a zone of relatively flat topography southeast of the shelf break , a = 0.1°_+0.1 °.
In the northernmost topographic profile (no. 5, Fig. 6), zone 2 is much wider, with a slope of only 1.3 ° , and merges gradually with zone 1. This topo- graphy can be attributed to the rapid ongoing deposition of the Magdalena fan (Kolla et al,, 1984; Breen, 1989) that is obscuring the deformation in the fold belt. According to Breen, this deposition pro- duced the indentation of the accretionary wedge and increased deformation inboard of the Magdalena fan. Low topographic slope within an active accre- tionary wedge can be explained within the theory of critical taper in a number of ways: isostasy, high d~- collement dip, subaerial erosion, the thickness of the
S e a level 0.08
Y - -
t8 0 k m I
wedge exceeds the depth of the brittle-plastic transi- tion, rigid rock (backstop) within the overriding plate, and high fluid pressure ratios.
If dynamic horizontal compressive forces were not acting on the wedge, the isostatic load on the lithosphere might reduce the surface slope a and increase the d6collement dipp. A southeastward in- crease in the d6collement dip produced by the weight of the downgoing Caribbean slab would also reduce the surface slope. However, on seismic profiles a reflector, interpreted as the basal d6collement, can be traced with a constant 2 ° dip for a distance of 90 km from the toe of the wedge to a point 20 km southeast of the shelf break (Lehner et al., 1983; Toto, 1991).
Erosion also acts to reduce the topographic slope but leaves the d~collement dip unchanged. Erosion is not likely to be important in the submerged por- tion of the accretionary wedge (zone 2). However, southeast of the shelf break in zone 3, unconfor- mities record uplift and erosion in late Miocene and Pliocene times (Duque-Caro, 1984). In an active fold-and-thrust belt, the processes of erosion and internal deformation should be constantly occurring at rates sufficient to counteract each other. Seismi- city (Daniels, 1991; see Fig. 8) and tectonic tilting and warping of Quaternary erosion surfaces (Page, 1983) indicate that the Sinu-San Jacinto belt is actively deforming.
If the thickness of the wedge exceeds the depth to the brittle-plastic transition, the base will no longer display Coulomb friction behavior because of high temperature. The brittle-plastic transition for quartz (Tullis and Yund, 1977, 1980, 1987) is ex- pected to occur at a depth of roughly 12 to 16 km in regions of moderate geothermal gradients (20-25°C
kin-l). At the base of an accretionary prism, how- ever, with high pore pressures and low geothermal gradient (Shi and Wang, 1988), the brittle-plastic transition may be as deep as 20 kin. The fiat tope- graphy in the Sinu-San Jacinto wedge starts where the basal d~collement is at a depth of about 12 kin, probably well above the brittle-plastic transition.
Byrne et al. (1988) have suggested that many trench-slope breaks or outer-arc highs may be cau- sed by large horizontal gradients in shear strength within the overriding plate. An accretionary wedge in which cohesion increases with distance from the toe will have a convex-upward profile (Dahlen et al.,
1984). Laboratory sandbox experiments show that a trenchward-dipping backstop of rigid material can produce a slope break and a forearc basin (Byrne et al., 1988; in press). Based on aeromagnetic and gravity anomalies (Kellogg et al., 1991), we have proposed a NW-dipping rigid buttress beneath zone 3 of the Sinu-San Jacinto accretionary wedge (Fig. 4). In the San Jacinto fold belt, none of the folds and thrusts verge toward the backstop as predicted by the laboratory experiments, but an east-verging wedge may be hidden beneath west-verging roof thrusts, as in models proposed for the Lesser Antilles
(Torrini and Speed, 1989) and the Great Valley of California (Unruh et al., 1991).
OVERPRESSURED MUD-CORED FOLDS
While no direct measurements of fluid pressures within the Sinu-San Jacinto wedge have been published, the wedge is characterized by numerous active m u d volcanoes, clay domes, and reefs over domes (Fig. 2) (Duque Caro, 1984; Vernette, 1989), demonstrating excess fluid pressure at depth within the sedimentary section. The summits of the diapirs are frequently more than 100 meters above the surrounding shelf and form elongate ridge-like fea- tures, 1-2 k m wide and up to 10-15 k m long, oriented N30°E. The m u d extruded from the land volcanoes manifests high water content and low density (less than 2.0 g cm-S). The age assigned to this m u d is late Oligocene-early Miocene based on foraminiferal de- terminations.
The local rise of argillaceous sediment occurs w h e n undercompacted clays are buried under denser sediments (generally sands), resulting in a density- driven diapiric clay-flow (Bishop, 1976). In areas of active tectonism, undercompacted clays can also be subjected to tectonic stresses; m u d flow will then occur preferentially along anticlinal axes or fault lineations (Jackson and Galloway, 1984). In the north P a n a m a thrust belt, m u d volcanoes occur almost exclusively on the flanks of long anticlinal ridges (Breen et al., 1988). Breen et al. (1988) pro- posed that mobile, overpressured m u d s m a y form the cores of the anticlines in a manner analogous to salt- cored folding.
Seismic line 156 (Fig. 3) depicts a submarine m u d volcano extruding up to a height of 90 meters above the sea floor. The structure beneath the vol- cano is interpreted as a growth fault-bed fold (Fig. 3) (Medwedeff, 1989; Suppe et al., 1991). The m u d diapir beneath the volcano intrudes the axis of the hanging wall anticline. The seismic profile indicates a source depth at least 1650 meters below the surrounding sea floor. The total weight of the 1650 meter sedimentary section plus 73 meters of water balances the total weight of a 1740 meter column of mud:
1650/~ + 7 3 p w = (1655 + 90)pro
where p is the average density of the 1650 meter sedimentary section and P m the average density of the mud. But the rock density p = 1030~b + 2650 (1-~b) kg m -a (Henry et al., 1990) where ~b = 0.7 exp (-0.67 X 10 -S z) represents the variation of poro- sity with depth z in meters within accretionary wedges (Bray and Karig, 1985; Fowler et al., 1985; Le Pichon et al., 1990). Using these equations we estimated the average sedimentary section porosity and density to be 0.43 and 1970 kg m -8, respectively, and the porosity and density of the m u d to be 0.46
218 EL ARBI TOTO and J. N. KELLOGG
and 1910 kg m -~. A value of 4.69 MPa is computed for the effective pressure Pe using the empirical equation proposed by Shi and Wang /1988) for the Barbados accretionary prism:
r,b = 0.7 expl --bPe)
where b = 9X 10-" MPa -1. IfA represents the ratio of the pore pressure to lithostatic pressure, with the sea floor t a k e n as pressure reference:
Plim - Pc - Psea floor
~" = P~th - P ~ . noo~ = 0 . 8 7
C O M P A R I S O N WITH O T H E R ACTIVE W E D G E S
Figure 7 shows the theoretical relationship be- tween the surface slope a and the d6collement dip fl for various values of A, Rock strength parameters are assumed to be ~b ---- 0.85 (Byerlee's law) for friction on the basal d6collement a n d / t = 1.03 with- in the wedge. Byerlee's law is used because of its widespread applicability in describing the results of laboratory m e a s u r e m e n t s for a variety of silicate rock types (Dahlen et al., 1984). The observed geo- metries of the Sinu-San Jacinto wedge and other active wedges (Davis et al., 1983) are superimposed.
W e predict very high fluid pressure ratios A = Ab = 0.97 in the Colombian submarine wedge (zone 2) and even higher fluid pressure ratios in the subaerial part of the Colombian wedge (zone 3). Our estimate ofA = 0.87 from the submarine m u d volcano in Fig. 3 is in adequate agreement with this prediction.
The high fluid pressure coefficient ~ estimated for the Sinu-San Jacinto fold belt is very similar to
predicted and observed values in other active accre- tionary wedges. For example, a profile of the Mak- ran wedge in the Gulf of Oman (White and Ross, 1979) just offshore from mud volcanoes and near a well showing extreme overpressures (J. Harms, pers. commun., 1982) predicts A = 0.98 (Davis et aL,
1983k Platt (1990) proposed that the Makran im- bricate thrust geometry could be explained by fluid pressures in excess of the load pressure, sufficient to cause dilatant fractures to form. The wedge of the Lesser Antilles extending east from Barbados has an overall taper (Westbrook, 1975; Westbrook et al.,
1982; Westbrook and Smith, 1983) corresponding to a predicted fluid pressure ratio A = 0.92 t Davis et al.,
1983). Measured A from mud weight, in a deep well in Barbados reached 0.8 (Moore and yon Heune, 1980). Near the toe the taper narrows and direct measurements indicated .~ ~ 1 at the bottom of DSDP hole 542 (leg 78A) located 1.5 km inward from the deformation front (Moore et al., 1982). Like the Sinu-San Jacinto wedge, the Makran and Barbados wedges also contain abundant mud diapirs and mud volcanoes (Henry ct al.,
1990;White, 1981; White and Louden, 1982).
The Oregon section of' Snavely et al. t1980) predicts A = 0.9 (Davis et al., 1983). Even higher values, A = 0.95, are predicted for the lower slope of the Cascadia accretionary wedge (Davis and Hynd- man, 1989). Sonic-log and mud-weight estimates give pore-pressure gradients below the Cascadia continental slope of about 19 kPa m-" (Shouldice, 1971; Yorath, 1980); this value is equivalent to a value of A = 0.9 for sediments having 30% porosity (Davis and Hyndman, 1989). This value is similar to the pore pressure estimated for the shelf in a well off Oregon, ,l = 0.9 (Moore and von Huene, 1980; Davis
et al., 1983). Tiffin et al. (1972) reported mudstone diapirism associated with disharmonic folding near
~ . ~
0 2 4 6 8 10
03 O .
._o u) 4
~___~4..~ ° Submarine wedges
- ~ ' ~ Guatemala
~.9 Colombi&"-~L.____ ~ J Cascadia upper slope
m zon~ 2 ~
/ Barbados ~ ~ ~rr~'~llk
0 2 4 6 8 10 12
Fig. 7. Theoretical relationships b e t w e e n the surface slope a and the decoUement dip ~ for various values of A. The observed
geometries of the Sinu-San Jacinto wedge, Cascadia (Davis and H y n d m a n , 1989L and other active wedges (Davis et al., 19831 are
80 ° W
M <5.0 5 . 0 < M < 7 . 0 7.0_< M <8.0
10 ° N
S 70 °
Fig. 8. Instrumentally recorded earthquakes in the northern Andes from the National E a r t h q u a k e Information Center.
Vancouver Island, and there are numerous recent studies of fault-related active fluid venting in the Oregon accretionary prism
(e.g.,K u l m and Suess, 1990). The Cascadian accretionary wedge has a flat continental shelf similar to that observed in zone 3 of the Sinu-San Jacinto wedge. Davis and H y n d m a n (1989) attribute the flat topography to over- steepening of the d6collement. The dip of the Cascadia d6collement increases beyond 11 °, too steep to allow upward growth of an accretionary wedge. However, steepening of the d6collement beneath zone 3 in the Sinu-San Jacinto belt is not indicated by the seismic reflection or gravity data (Toto, 1991). W e attribute the flat topography in the Colombian wedge to lithostatic fluid pressures and horizontal gradients in shear strength. Based on aeromagnetic anomalies, we have predicted a NW-dipping backstop of rigid basement rock beneath zone 3.
A S E I S M I C S U B D U C T I O N
The Sinu-San Jacinto wedge between the South Caribbean marginal fault and the Romeral fault is a zone of relatively low seismic activity, as a plot of earthquake epicenters shows (Fig. 8). In Fig. 9, earthquake hypocenters are projected onto a N W / SE-trending profile from a zone extending 1 ° on either side of the profile (Daniels, 1991). T (tension) axes are shown for the focal mechanisms of events in the downgoing Caribbean plate from the Harvard Centroid M o m e n t Tensor catalog, P (pressure) axes are shown for the focal mechanisms of events in the overriding North Andean and South American plates, and the lengths of the axes are proportional to the cosines of the angles between the axes and the profile
(i.e.,the longest axes lie in the plane of the profile). The tight cluster of intermediate-depth (161 kin) earthquakes is the "Bucaramanga nest," with consistently high activity and small source volume (Dewey, 1972; Pennington, 1981; Schneider
North Colombia fold belt
NW I (Fig. 4) I
0 , r , r t ,
I I-- 0_
C A R m B E A N
" PL4 T~-
Santa Marta f. ~ SE
'.,%",'!~'~,~: " - . - ' ~ . ..'Y.'.'.'.'.'.'.'.~-. ",4 : •
" "SOUTH AMERICAN '
• , a o ~ a •
a • ° ,
• I D • ~ . .
l I i l i I
0 200 400 600 800
Fig. 9. E a r t h q u a k e hypocenters projected onto a NW/SE-trending profile from a zone extending 1 ° on either side of the profile
220 EL ARB1TOTO and J. N. KELLOGG
1987). Pennington (1981) and Daniels /1991) esti- mated the dip of the Bucaramanga Wadati-Benioff zone as 20 ° (Fig. 9).
In northern Colombia, as in the southern Lesser Antilles, the Hikurangi subduction zone of New Zea- land, the Makran margin of southern Iran and Pakistan, and the Cascadia subduction zone of North America, few or no interplate thrust events are observed (Byrne et al., 1988). These margins also all have large accretionary wedges with an abundance of young sediments and elevated pore fluid pres- sures. As pore pressure approaches the lithostatic value, effective normal stress approaches zero (Hub- bert and Rubey, 1959), shear strength within the rock is reduced, and stable aseismic slip occurs (Die- terich, 1978). Low slip rates (less than 2 cm yr -1) and subduction of sufficient quantities of young, high-porosity sediments, such as in the Sinu-San Jacinto wedge, may reduce the width of the seismo- genic zone and result in aseismic subduction (Byrne
et al., 1988; Zhang et al., in press).
C O N C L U S I O N S
The critical taper for Coulomb materials is very sensitive to the pore-fluid pressure ratio A, especially for values of A near 1, the lithostatic state. The Sinu- San Jacinto accretionary wedge has a very low angle surface topographic slope (a -< 2 °) that can be ex- plained by a lithostatic value of A. This explanation is consistent with the diapirism and mud volcanic activity that requires high fluid pressure. Fluid overpressures are reported worldwide within accre- tionary wedges where well data is available. How- ever, pore pressure variation with depth and depar- ture from the assumed Coulomb behavior are not accounted for in this model. The active accretionary wedges of Makran, Colombia, Barbados, and Casca- dia are characterized by low critical tapers, mud diapirism or fluid venting, and low levels of seismi- city. We believe that these features are directly at- tributable to fluid overpressures within the wedges and within the basal d6collements beneath the wedges.
Acknowledgments--We wish to thank Angela Daniels for the unpublished earthquake map and profile. We thank Don Secor, Pradeep Talwani, and Richardson Allen for helpful discussions and Daniel Davis and John Ladd for careful reviews of the manu- script. Unpublished seismic reflection profiles and aeromagnetic profiles were kindly provided by Empresa Colombiana de Petro- leos (Ecopetrol). Diana Diaz assisted in the preparation of the text and figures. This research was supported by the Petroleum Research Fund of the American Chemical Society (Grant 21740- G2).
R E F E R E N C E S
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Breen, N. A., 1989. The structural effect of Magdalena fan depo- sition on the northern Colombia convergent margin. Geology 17 (1), 34-37.
Breen, N. A., Tagudin, J. E., Reed, D. L., and Silver, E. A., 1988. Mud-cored folds and possible melange development in the north Panama thrust belt. Geology 16 (3), 207-210.
Byrne, D. E., Davis, D. M., and Sykes, L. R., 1988. Loci and maximum size of thrust earthquakes and the mechanics of the shallow region ofsubduction zones. Tectonics 7 (4), 833-857.
Byrne, D. E., Davis, D. M., and Wang, W., in press. Mechanical role of backstops in the growth of forearcs. Tectonics, in press.
Case, J. E., and Holcombe, T. L., 1980. Geologic.Tectonic Map of the Caribbean Region. U.S. Geological Survey, Miscellaneous In- vestigation Series, No. 1-1100, Scale 1: 2,500,000, 3 sheets.
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