(2) 212. EL ARBI TOTO and J. N. KELLOGG. 12.+. /. j. ~.'~. ~o/~. O'. /. NAZCA. ,:7. t/". 4°7-~9°~ /L. ~. ,,,o,o'/. J~=., SantaMarta. /. /. ,. ~. ~ssi,. a. J '~. /. 'ql" ~. /'. : iYdl. I. 0'~ ". ×. ~I~. ~/ / ~. /<,'. J. /. /. '. 7~72°W. 0. 100. 200. I. I. I. 300 km 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 Andean 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 (DuqueCare, 1979). Narrow mud-cored anticlines with steeply dipping limbs are separated by broad, gentle synclines. The belt has a remarkable number of mud 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-bendfolds(Fig.3). Crustal shortening in the Sinu-San Jacinto fold belt is the result of convergence between the Caribbean plate and the northwest margin of South America. Plate tectonicreconstructionssuggest that the Caribbean-NW South America plate margin was transpressive during Paleocene to late Eocene time and that as much as 1000 k m of N W - S E convergence has occurred in the last45 million years (Malfaitand Dinkleman, 1972; Jordan, 1975; Pindell and Dewey, 1982). Present-day global plate reconstructionspredict ongoing N W - S E convergence across the Carib-.
(3) Structure of the Sinu-San Jacinto fold belt, northern Colombia. I. I. 213. I. 12°N. 11 °. 5". 9. 4. 10 °. /. ?/. •. i,. / i. I. •. 9° • •. I 77 =. 76 °. Monteria. 0. 50. I. I. 100 km. I. I 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 interpreted as the product of slow subduction of Caribbean crust beneath the northern Andes (Kellogg and Bonini, 1982). Preliminary results from CASA. (Central And South America) GPS (Global Positioning 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., 1991). The predicted cross-sectional geometry of the wedge is shown in Fig. 4. The basal d~collement has never been intersected by drilling. Multicb~n-el seismic profiles show a strong, deep horizon below which little deformation occurs extending from the toe of the wedge well into zone 3 at a distance 20 km southeast of the shelf break (Lehner et al., 1983; Toto, 1991). Lehner et al. (1983) interpreted this.
(4) 214. EI, ARBI TOI'O and ,]. N. KELLOGG. 7Oc. 800. 700. 800. 900. I000. 900. 1000. 1. c. Shot. "o. ~/I. ~ ~ < : -. c. point. k'X', I. "" "~" '\" "1. o (,,) G). u'}. 3. ~.... "\. ~-~.... -.. " ~. 0. 1. 2 kin. t. t. t. ~ I. Fig. 3. Western end of m u l t i c h a n n e l seismic profile 156. The structure is interpreted as a growth fault-bend fold (bottom). A mud diapir intrudes t h e axis of the 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 km 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. continental rocks to the east) may be providing a rigid buttress (Fig. 4). Toto (1991) estimated depths to basement from the wavelengths of total field magnetic anomalies (Empresa Colombiana de Petroleos, 1979) using the steepest gradient method (Vaquier et al., 1951)..
(5) Structure of the Sinu-San Jacinto fold belt,northern Colombia. 215. 10420 .~. 10370. ~. 10320. '22t ~. 60. 4o ~. 20 0. i. i. i. i. -20. ~. i. 30. 80. 130. 180. in km. Distance. Romeral fault zone. i. Sea Level 0. - -. "--°'°"d'"""" ~I¢ l. Nt' v. ,. =_. - _. .. 2O 0|. I0 |. 20 km |. Fig. 4. Cross-sectionalgeometry of the Sinu-San Jacinto wedge along traverse3, with Bouguer gravity (mgal) and aeromagnetic anomalies (gamma). See Fig. 2 for locationofprofile.. C R I T I C A L T A P E R F R O M COULOMB WEDGE 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 greater than the Hubbert-Rubey maximum, the rocks will deform as the block is pushed from the back end until they reach a criticaltaper and then will slide along the basal d~coUement without internal deformation. The criticaltaper 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 frictional resistance of the part of the wedge lying in front of that segment. By assuming that the wedge is a Coulomb material, Davis et al. (1983) defined an expression for the criticaltaper. 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 criticaltaper for a cohesionless wedge was determined to be:. cc+~=. (I - PJP)~. + ~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. Fig. 5. Schematic diagram of wedge topographic slope, basal d~collementdip,and criticaltaper (a + p)..
(6) 216. El, ARBI TOTO and d. N. KELLOGG. the wedge, respectively;/z b is the coefficient of friction along the basal d6collement, lc was defined approximately by: sine . sin2d~b+ COSCb(sin2~ - sin2¢b) In - 1 --S-~¢ +cos2~b. cosCb - (sin2¢ - sin2¢b)In where Cb is the angle of friction along the d~collement and q5 is the angle of internal friction in the wedge. The taper ( a + p ) approximates a ratio of frictional terms to wedge strength terms. An increase in effective basal friction, either by a drop in fluidpressure 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 effective internal friction term p qu = tan q5) will result in an increase in 7c and a decrease in the taper.. APPLICATION SINU-SAN. OF THE THEORY TO THE JACINTO FOLD BELT. Five topographic profiles (Fig. 6) were prepared across the Sinu-San Jacinto wedge, based on unpublished seismic reflection surveys (Aquitaine Colombie, S.A., 1974; Empresa Colombiana de Petroleos,. Zenel. 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 basinward (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 topography 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 produced the indentation of the accretionary wedge and increased deformation inboard of the Magdalena fan. Low topographic slope within an active accretionary 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. /.m~2. I. Zmm$. Sea level. 0.3. Sea level. 0.0. J 0.08. S e a level. 0.1S. Sea level. 0.25. Y-0.2. Sea level. 0.3. 2. f. 4 km. VE:IO/1. 2 4O. t. 80km. I. Fig. 6. Topographic profiles across the Sinu-San Jacinto wedge. Mean slopes were computed: zone 1, a = 0.4°+_0.2°;zone 2,, -~ 2.0°_+0.4°; zone 3, a = 0.1°+ 0.10. Vertical exaggeration = 18 ×. See Fig. 2 for location ofprofiles..
(7) Structure of the Sinu-San Jacinto fold belt,northern Colombia wedge exceeds the depth of the brittle-plastic transition, 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 increase 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 portion of the accretionary wedge (zone 2). However, southeast of the shelf break in zone 3, unconformities 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. Seismicity (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 expected 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, however, 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 topegraphy 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 caused 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 SAES 5 / 2 ~. 217. (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 features, 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 determinations. The local rise of argillaceous sediment occurs when undercompacted clays are buried under denser sediments (generally sands), resulting in a densitydriven 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 Panama thrust belt, m u d volcanoes occur almost exclusively on the flanks of long anticlinal ridges (Breen et al., 1988). Breen et al. (1988) proposed that mobile, overpressured muds may form the cores of the anticlines in a manner analogous to saltcored 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 volcano 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 profileindicates 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/~ + 73pw = (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 porosity 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.
(8) 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:. predicted and observed values in other active accretionary wedges. For example, a profile of the Makran 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 imbricate 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 Hyndman, 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. 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 taken as pressure reference:. ~" =. Plim - Pc - Psea floor P~th - P~. noo~ = 0.87. C O M P A R I S O N WITH OTHER ACTIVE WEDGES Figure 7 shows the theoretical relationship between 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 within the wedge. Byerlee's law is used because of its widespread applicability in describing the results of laboratory measurements for a variety of silicate rock types (Dahlen et al., 1984). The observed geometries 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. loL 8. 10 ~___~4..~ °. Subaerial wedges. ~.~. - ~ ' ~. Submarine wedges Guatemala. 6 03 O.. ._o u) 4. 8_. O. 60. ~.9 Colombi&"-~L.____ ~ m. / 0. 2. 4 6 8 Decollement. 10. I. 12. 0. 0. zon~ 2. Barbados ~ 2. J Cascadiaupperslope. ~. ~. 4 6 8 Decollement. ~rr~'~llk 10. 12. Fig. 7. Theoretical relationships between 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 Hyndman, 1989L and other active wedges (Davis et al., 19831 are superimposed..
(9) Structure ofthe Sinu-San Jacinto foldbelt,northern Colombia. 10 ° N. 5°. 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 shelfsimilar 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 oversteepening 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 reflectionor gravity data (Toto, 1991). W e attribute the flat topography in the Colombian wedge to lithostaticfluid pressures and horizontal gradients in shear strength. Based on aeromagnetic anomalies, we have predicted a NW-dipping backstop of rigidbasement rock beneath zone 3.. ASEISMIC. 0°. 5°. S 80 ° W. 75 °. 70 °. M <5.0 5.0<M<7.0 7.0_< M <8.0 M_>8.0. Fig. 8. Instrumentally recorded earthquakes in the northern Andes from the National Earthquake Information Center.. 0. ,. r,. 128" Santa Marta f.. rt. ,. CARmBEAN. E. ~ SE. •. I. I. A. SUBDUCTION. 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 Moment 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 clusterof intermediate-depth (161 kin) earthquakes is the "Bucaramanga nest," with consistently high activity and small source volume (Dewey, 1972; Pennington, 1981; Schneider et al.,. North Colombia fold belt I (Fig. 4) I. NW. 219. "-.-'~. '.,%",'!~'~,~:. .. :. •. "SOUTHAMERICAN '. ". ". ..'Y.'.'.'.'.'.'.'.~-. ",4. I. PL4T~•. ,a. a. o •. ~ °,. a. •. v •. I I-0_ w O. ID. •. ~. .. .. i. l. 0. I. 200. i. l. 400. i. I. 600. 800. DISTANCE (kin) Fig. 9. Earthquake hypocenters projected onto a NW/SE-trending profile from a zone extending 1° on either side of the profile (Daniels, 1991). See Fig. 8 for location of the profile..
(10) 220. EL ARB1TOTO and J. N. KELLOGG. 1987). Pennington (1981) and Daniels /1991) estimated 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 Zealand, 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 pressures. As pore pressure approaches the lithostatic value, effective normal stress approaches zero (Hubbert and Rubey, 1959), shear strength within the rock is reduced, and stable aseismic slip occurs (Dieterich, 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 seismogenic zone and result in aseismic subduction (Byrne et al., 1988; Zhang et al., in press).. CONCLUSIONS 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 SinuSan Jacinto accretionary wedge has a very low angle surface topographic slope (a -< 2 °) that can be explained 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 accretionary wedges where well data is available. However, pore pressure variation with depth and departure from the assumed Coulomb behavior are not accounted for in this model. The active accretionary wedges of Makran, Colombia, Barbados, and Cascadia are characterized by low critical tapers, mud diapirism or fluid venting, and low levels of seismicity. We believe that these features are directly attributable 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 manuscript. Unpublished seismic reflection profiles and aeromagnetic profiles were kindly provided by Empresa Colombiana de Petroleos (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 21740G2).. REFERENCES Bishop, R., 1976. Shale Diapirism and Compactmn of Abnormally Pressured Shales in South Texas. 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