The Chixoy-Polochic Fault and Its Associated Fractures in Western Guatemala

Home , Cayman Trough, Chiantla, Cuilco, Huehuetenango, Motagua Fault

The Chixoy-Polochic fault and its associated fractures in western Guatemala

RICHARD J. ERDLAC, JR. Gulf Oil Exploration & Production Co., P.O. Box 1150, Midland, Texas 79702 THOMAS H. ANDERSON Department of Geology and Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

ABSTRACT aerial photographs and field mapping rep- Polochic fault in this area is probably no resent the accommodations of displace- more than a few kilometres, since the In western Guatemala, post-Cretaceous ments along the underlying fault. accumulation of the overlying beds of prob- volcanic and sedimentary rocks locally Comparison of fracture trends in the able post mid-Tertiary age. Earlier lateral cover the trace of the Chixoy-Polochic Chixoy-Polochic fault zone with models displacements of large magnitude are not fault. No single throughgoing fault cuts from experimental studies and field ex- precluded, although none have been docu- these units. However, a complicated series amples of strike-slip faults suggest that mented by field study. of en echelon lineations identified from left-lateral displacement along the Chixoy-

Figure 1. Outcrop distribution of probable pre-late Paleozoic (Chuacus) metamorphic and igneous rocks in part of Central America and southern Mexico showing their relation to major tectonic and geologic features. Also shown are the physiographic-tectonic provinces of Guatemala and the quadrangles along the Chixoy-Polochic fault zone discussed in the text. 1 = Motozintla; 2 = Canibal, 3 = Cuilco, 4 = San Sebastian Huehuetenango; NB = Northern Boundary Fault; B = Bladen Fault; CP = Choxoy-Polochic Fault; M = Motagua Fault; JC = Jocotan-Chamelecon Fault; LC = La Ceiba Fault; A = Aguan Fault. Adapted from Home and others (1976), Muehlburger and Ritchie (1975), and Kesler (1971).

Geological Society of America Bulletin, v. 93, p. 57-67, 7 figs., January 1982.

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INTRODUCTION Figure 2. Location map of rivers, towns, roads, and Colotenango beds (stipple pattern). The regional tectonic grain of Guatemala Map also shows the location of major lineatious which comprise the Chixoy-Polochic fault is east-west. This trend is shown not only zone. by faults and outcrop patterns but also by parallel orientation of physiographic axes MOTOZINTLA QUADRANGLE of ridges and valleys of folded mountains of CUILCO QUADRANGLE the Central America Mountain System (Fig. 1). Two extensive fault zones, Chixoy- Polochic and Motagua, which underlie major river valleys, traverse Guatemala as arcs gently convex to the south. These two fault zones, and possibly the Jocotan- Chamelecon (Fig. 1), appear to be the land- ward extension of the fault system that occupies the Bartlett or Cayman trough and extends from the Gulf of Honduras 1,600 km eastward to the Windward Pas- sage between Cuba and Hispaniola (Meyer- hoff, 1966; Holcombe and others, 1973;

Perfit and Heezen, 1978). SCALE 1:25 0,000 The Chixoy-Polochic and Motagua faults 10 15 20 25 30 •1.. divide the Central America Mountain Sys- 10 15 20 tem into three structural provinces. At most places the Chixoy-Polochic fault, which extends about 400 km through the Central fracture pattern using a method employed Perfit and Heezen (1978) provided a America Mountain System to the Mexican by Tchalenko (1970). somewhat different interpretation as to the border, juxtaposes sedimentary rocks of amount of left-lateral offset along the Cay- late Paleozoic and Mesozoic age to the PREVIOUS WORK man trench and implied a similar amount of north against pre-Permian igneous and motion along each individual fault or dis- metamorphic rocks to the south (Fig. 1). On Muehlberger and Ritchie (1975) state that tributed across the structures that may the other hand, the Motagua fault breaks the prominent fault zones that they ob- comprise the plate boundary in Guatemala across igneous and metamorphic rocks for served on Skylab photographs of Guatemala (that is, Chixoy-Polochic, Motagua, and much of its length, locally separating rocks have acted as the Caribbean-North Ameri- Jocotan faults). Their evidence points to- of different metamorphic grade. At its west- can plate boundary at various times. They ward an average east-west full spreading ern extension, the fault cannot be traced in maintain that the present boundary is the rate of 0.4 cm/yr for the mid-Cayman Cenozoic volcanic rocks. Chixoy-Polochic fault zone. Although they spreading center for the past 5 m.y. By gave no opinion as to the amount of motion assuming that this average rate has pre- PURPOSE OF INVESTIGATION that has occurred along the fault, they cited vailed from Eocene (~ 50 m.y. B.P.) to the Kupfer and Godoy (1967), who suggested present, then a total of about 200 km of The area of study consisted of a section that Holocene left-lateral strike-slip offsets left-lateral displacement has occurred across 86 km long along the Chixoy-Polochic fault total 122 m, and Kesler (1971), who pro- the plate boundary. in western Guatemala, extending west from posed that the Chixoy-Polochic fault is a Schwartz and others (1979) have reported the border of Chiantla quadrangle across Cenozoic plate boundary with less than 150 Quaternary faulting along the Motagua and the quadrangles of San Sebastian Huehue- km of left-lateral offset. the eastern portion of the Chixoy-Polochic. tanango, Cuilco, and Canibal in Guate- Major displacement was also suggested Evidence for this faulting included sag mala, and continuing past the town of by Burkart (1978), who argued that if the ponds, shutter ridges, scarps, springs, and Motozintla into the surrounding regions of block north of the Chixoy-Polochic fault offset streams and river terraces. These Mexico (Fig. 1). Emphasis was placed on trace is moved eastward about 132 ± 5 km, a authors were able to determine a slip rate of characterizing the distinctive nature of the unique match of Cenozoic and older struc- between 0.45 and 1.8 cm/yr for Quaternary Chixoy-Polochic fault in Cuilco quadran- tures and pre-Cenozoic stratigraphy results. motion along the Motagua fault. They gle, where its trace had been inferred on the Burkart (1978) suggested that if a 2 cm/yr believed that the Chixoy-Polochic and pos- basis of reconnaissance mapping. The pur- average strain rate exists along the Polochic sibly the Jocotan-Chamelecon faults, along pose of investigation was three-fold: (1) to fault, and the fault is still active, then its with the Motagua, share portions of the identify the previously unmapped Chixoy- first movement would have occurred 6.5 total strain produced by the Cayman trough Polochic fault by analyzing observed frac- m.y. B.P., or middle Pliocene. If this strain spreading center. According to Schwartz ture patterns, (2) to determine the age of were shared along the Jocotan or Motagua and others (1979) the cumulative slip for the displacement by observing the youngest faults or both, then an age of two or three Cayman trench would be between 170 and rocks cut by faulting, and (3) to estimate the times would apply, placing age of inception 685 km. The consistent up-to-the-north ver- amount of displacement by interpreting the in middle Miocene. tical movement found across all three faults

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SAN SEBASTIAN HUEHUETENANGO are probably of Paleozoic age. The plutonic CUILCO QUADRANGLE rocks include mainly granodiorite, quartz QUADRANGLE monzonite, and granite with less common diorite. Quartzo-feldspathic gneiss, mylo- nitic gneiss, and amphibolite compose the country rocks for these intrusive bodies.

Post-Cretaceous Volcanic and Sedimentary Rocks in Cuilco

Within Rio Cuilco Valley and along the trend of the Chixoy-Polochic fault zone several kilometres to the east lies a sequence of rocks called the Colotenango beds (And- erson, 1969; Anderson and others, 1973), which is composed of both sedimentary and volcanic units. These strata crop out for about 35 km, from the village of Cuilco at the western edge of Cuilco quadrangle to a point 0.75 km west of the town of San Se- bastian Huehuetenango in San Sebastian as described by Schwartz and others could and Tactic. An incomplete 788-m section of Huehuetenango quadrangle (Fig. 2). They possibly represent as much as 5% of the lat- Chochal was measured by Siesser (1967), lie within a 1- to 4-km-wide band and are eral component of slip along the plate although the estimated minimum thickness confined to the valleys of Rios Selegua and boundary. is about 1,000 m. Cuilco except near Colotenango where they Orogenic movements affecting the Paleo- occupy an uplifted abandoned stream trace GEOLOGIC FRAMEWORK zoic units probably created the basins into between Rio Selegua and Rio Cuilco. The which the terrestrial sediments of the low- greatest known thickness of the Colote- Pre-Tertiary Stratigraphy of ermost Mesozoic unit, the Todos Santos nango beds is about 460 m, south of Ixta- Cuilco Quadrangle Formation, accumulated. It unconformably huacan in eastern Cuilco. overlies the folded and faulted Chochal Colotenango beds consist of a heterogen- In western Guatemala, the Chixoy-Po- Limestone. Todos Santos consists chiefly of eous sequence of conglomerate, sandstone, lochic fault generally separates sedimenta- interbedded conglomerate, sandstone, and and mudstone that grade into tuffaceous ry rocks to the north from crystalline shale. Anderson and others (1973) reported sandstone, lahar and volcanigenic rocks rocks of predominantly pre-Permian age to 1,120 m of Todos Santos near the type such as water-laid tuff, rhyodacitic crystal, the south. Four formations of Paleozoic age locality above the village of the same name. vitric and lithic tuffs, and rare basalt (And- (Chicol, Tactic, Esperanza, and Chochal) Lack of exposures prevented measurement erson, 1969; Anderson and others, 1973). comprise the Santa Rosa Group (Anderson of a section of Todos Santos in Cuilco, but The conglomerate contains clasts of meta- and others, 1973). In Cuilco quadrangle, the a minimum thickness of 350 m has been morphic and igneous rocks, volcanic rocks, oldest exposed formation of this group is estimated. limestone, and serpentinite, in almost any the Tactic Formation, which is composed Marine conditions during Cretaceous combination. mainly of shale beds intercalated with time resulted in accumulation of the Ixcoy In Cuilco the Colotenango beds can be minor carbonate and siltstone units and is Formation, which is composed of lime- divided into distinctive units. Siesser (1967) at least 400 m thick. stone, dolomite, and carbonate breccia described the lower unit as consisting of Above the Tactic is the Esperanza For- with less abundant calcareous wackestone, about 263 m of gently dipping "green beds" mation, although at places within Cuilco packstone, and a few beds of calcareous composed of interbedded volcanic ash, quadrangle it may be obscured because of mudstone. Within Cuilco quadrangle, the mudrock, sandstone, and conglomerate that extensive faulting (Sandstrom, 1978). The contact between Todos Santos and Ixcoy crop out mainly in a belt south of Ixtahua- term "Esperanza" is used for a sequence has not been accurately defined. Poor expo- can stretching from Tunales to 1 km west of consisting of less than 90% but more than sures hampered Siesser (1967) from measur- Estancia. They rest unconformably on a 10% limestone. Within Chiantla quadran- ing the Ixcoy, although it undoubtedly thin residual soil developed upon gneiss. gle, Blount (1967) measured a tilted section attains a minimum thickness of 650 m, and The uppermost unit of the sedimentary 474 m thick. Siesser (1967) described a possibly 1,000 m or more, as suggested by sequence is defined as the uppermost cobble locality along the Interamerican Highway Blount (1967). conglomerate underlying tuff and ash. within Cuilco quadrangle where the Cho- Crystalline rocks in the southern portion The volcanic units, which comprise the chal overlies about 80 m of interbedded of Cuilco quadrangle are composed of both upper half of the Colotenango beds, crop shale and limestone that appear to be litho- plutonic and metamorphic sequences. Al- out mainly north of Rio Cuilco along the logically equivalent to Esperanza. though geologic relations suggest that intru- wall above the valley floor. The units con- Limestone and dolomite of the Permian sions as old as Precambrian and as young as sist of rhyodacitic vitric, vitric-crystal, and Chochal Limestone overlie the Esperanza Tertiary exist, most of the crystalline rocks crystal tuff and other pyroclastic units.

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SYMBOLS 9200 05 CANIBAL 55 -Fault '---Inferred or 9215 10 probable fault Jíf Figure 38 Linear, mapped from aerial T V. photograph. 25 25 •••<• -Concealed fault R . . trace as inter- preted from a linear and . p , - " .P p.. - fracture pattern y R " analysis. D R - r' ' ' '

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Figure 3. Maps of lineations in part of Canibal quadrangle and the region near Motozintla: Area a (upper map) is the base map showing all lineations that were observed; Area b (middle map) shows an interpretation of the lineations; Area c (lower map) is of those lineations possibly associated with the Chixoy-Polochic fault zone.

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They extend from just north of the village of composed of pumice. The ash is more than about 100 m wide, composed of granule and Cuilco eastward into San Sebastian Hue- 100 m thick west of the village of Cuilco, larger sized breccias, cut by planar shear huetenango quadrangle. The thickness of where it overlies an 80-m-thick deposit of zones which locally dip north. Farther east, the volcanic units varies from a minimum of alluvium. This deposit, and higher, younger a more or less continuous band of shattered, about 100 m near Cuilco to at least 200 m alluvium along Rios Cuilco and Selegua, commonly silicified Chochal(?) Limestone, above the green beds in the mountains consist of sand- to boulder-sized debris of tens of metres wide, marks the northern south of Ixtahuacan. These deposits may volcanic, crystalline, and sedimentary ori- edge of the breccia zone. This zone coin- once have been thicker and distributed over gin. cides with offsets of seven streams, all of a somewhat larger area, but the greatest which record left-lateral displacement (An- present thickness and width of the units are EXPRESSION OF THE derson and others, 1973). The maximum near Ixtahuacan, which may have been near CHIXOY-POLOCHIC FAULT displacement is about 1 km. the source area. IN WESTERN GUATEMALA Rock relationships along the fault west of The age of the Colotenango beds is uncer- AND MEXICO the village of San Sebastian Huehuetenan- tain. Anderson and others (1973) described go are much less consistent and involve a them as being directly related to uplift and Canibal Quadrangle wide variety of rocks, including plutonic volcanism along the Chixoy-Polochic fault and metamorphic rocks, Tertiary(?) tuff and zone. They considered the upper beds of The expression of the Chixoy-Polochic conglomerate, and shale beds. Easily de- this sequence to be post-Miocene(?) because fault zone as en echelon fractures in Cuilco formed shale beds in the Esperanza and the beds contain boulders of Miocene(?) quadrangle is distinct from its manifestation Tactic Formations obscure the fault, volcanic rocks derived from the south. as a continous trace in adjacent areas to the although it can still be recognized by McBirney (1963) described the volcanic east (San Sebastian Huehuetenango quad- impressive breccia zones in limestone units rocks of southern Guatemala as falling into rangle) and to the west (Canibal quadrangle exposed in stream cuts and landslides, two principal groups. One group is com- and the Motozintla region). ubiquitous fracturing in outcrops of tuff in posed of Pleistocene and Holocene erup- In the Canibal quadrangle (Fig. 3), the Rio Selegua canyon, and sheared gravel and tions restricted to a narrow line of central fault zone, which separates igneous and titled gravel beds at various localities along vents, with dominant rock types consisting metamorphic rocks from distinctive pink- the highway. of olivine basalt and pyroxene andesite. ish-orange quartz porphyry and volcanic- Near the western end of this quadrangle, Large volumes of dacitic pumice have been lastic rocks, crops out commonly as a single the Interamerican Highway turns north- erupted from some of the older vent com- trace. The zone is marked by breccia, westward and crosses the fault zone (Fig. plexes. The second older group (which triangular facets, and photo linears. Near 2). Here the fault zone is poorly exposed, al- McBirney, 1963, distinguishes) is mainly the Mexican border where the fault zone though intensely sheared nondescript black rhyolitic or dacitic tuffs and glowing ava- crosses Rio Salitre (Fig. 2), granitic quartz shale may indicate its proximity. The fault lanche deposits with intercalated lavas of porphyry is juxtaposed against gneiss. From appears to parallel the Interamerican High- basic or intermediate composition. These this point the fault appears to follow the way for a short distance before it continues volcanic rocks compose a 50- to 70-km-wide channel of Arroyo El Aguacate westward west toward the village of Llano Grande belt, parallel to the Pacific coast, which into Mexico. (Fig. 2) on the western border of the formed during fissure-type eruptions, dur- quadrangle. There, several well-exposed ing late Miocene and Pliocene time. Motozintla, Mexico east-west subsidiary faults record displace- Volcanic units found within the Colote- ment of overlying Tertiary strata (Fig. 4). nango beds appear to relate to McBirney's The Chixoy-Polochic fault can be traced Moderately to steeply dipping beds can be (1963) older group because they are mainly westward from Canibal (Fig. 3) as a photo- found as much as 2 km south of the main rhyolitic and dacitic tuffs and their exist- linear that coincides with a tributary valley zone. ence within the Cuilco valley is quite possi- of Rio Bacanton. The fault appears to cut bly due to a local (fissure-type?) eruption as across Rio Bacanton rather than following Cuilco Quadrangle suggested by their distribution along a 35- the river valley, and passes through material km-long segment of the fault zone. described by Carfantan (1977) as meta- Within the Cuilco quadrangle in the andesite and metarhyolite. It probably en- Ixcoy Formation, and to a lesser degree in Younger Ash and Alluvial Deposits ters Rio Motozintla Valley at the village of Tertiary volcanic units, brecciation extends Mazape and may lie in the valley of Rio from just east of Ixtahuacan, westward to a Above the tuff units lie younger ash Motozintla as far as the town of Motozintla point about 2 km east of the town of Cuilco. deposits, which form a conspicuous terrace where it appears to split into a northwest- The exposures of brecciated rocks vary level along Rios Cuilco and Selegua within trending segment and a west-trending seg- from as little as 2/ 3 m to 1/3 km. In general, Cuilco and San Sebastian Huehuetenango ment. breccia in the volcanics is composed of large quadrangles. These deposits generally man- boulders within a matrix of smaller frag- tle low slopes within river valleys. They San Sebastian Huehuetenango Quadrangle ments. Limestone of Ixcoy Formation (Cre- occur on both sides of the river, overlying taceous) is consistently highly smashed, tuff and crystalline and sedimentary rocks. East of the village of San Sebastian with varying amounts of boulder-sized This younger ash consists of friable, white Huehuetenango, the fault occupies a zone blocks within a matrix of smaller fragments or light gray to light brown lapilli and ash of strongly brecciated rock, commonly and powdered limestone.

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Figure 4. Map of lineations in part of San Sebastian Huehuetenango quadrangle an d the western portion of Chiantla quadrangle: Area a (upper map) is the base map showing all llineations that were observed; Area b (middle map) shows an interpretation of the lineations; Area c (lower map) is of those lineations possibly associated with the Chixoy-Polochic fault zone.

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Figure 5. Map of lineations in part of Cuilco quadrangle. Area a (upper map) is the base map showing all lineations that were observed; Area b (middle map) shows an interpretation of the lineations; Area c (lower map) is of those lineations possibly associated with the Chixoy-Polochic fault zone.

Brecciation in the tuff is quite variable. In measure several metres across. In constrast, area of Rio Agua Caliente, brecciation the valley of Rio Helado, 1 km east of Ixta- tuff that crops out 1 km northwest of Cuilco ranges from boulder-sized blocks that may huacan, highly brecciated volcanic tuff in Arroyo El Zapote is brecciated in a zone measure metres across to sand- or silt-sized 250 m thick crops out along the south wall that is only 1.5 m wide. material. Landslides within this brecciated of the valley. Material in this zone ranges Brecciation in Ixcoy Limestone is much material are common. from sand-sized grains up to blocks that more intense than in the volcanic tuff. In the Scattered slickensided fault surfaces crop

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out within the breccia zone at several of a trend from the upper to lower diagram A second group of northeast-trending localities. These commonly record dip-slip implies the presence of short linears within lineations exists within N34°-50°E. These movement. the group. fraotures are more strongly defined in the The plotted fracture patterns of Figure 6 upper diagram of Figure 6 than in the FRACTURES AND FRACTURE SETS distinguish a series of high-angle northwest- lower, which indicates their abundance but IN POST-CRETACEOUS ROCKS trending lineations located in the central short length. and easternmost portions of Cuilco, whose Finally, a small number of east-west lin- Within Cuilco quadrangle, a distinctive straight traces suggest steep dips, but which ears fall within N86°-94°E occupying an pattern of en echelon fractures occurs do not appear to be related to reverse area of 4° on either side of the east-west mainly in the Colotenango beds. The ex- faults. These linears lie within N58°-36°W trend. Although Figure 6A indicates the pression of the Chixoy-Polochic fault in with a break (accentuated in the lower number of linears occupying this range as these rocks is in constrast to fracture diagram) around N76°W. Thus within this being equivalent to that of any of the patterns to the east and west (Fig. 5). northwesterly trending group, two sets of previous sets already described, Figure 6B Lineations utilized to define the patterns fractures probably exist. The first trends shows that the lengths of these fractures in were mapped on the ground and from aerial N78°-86°W and represents those linears this east-west trend are less than in other photographs, on which they were defined by with the greatest individual lengths. The directions. changes in vegetation cover, saddles be- second set trends N58°-74°W and is com- tween ridges, abrupt changes in the slope of posed of shorter linears. COMPARISON WITH hillsides, and the existence of long, deep, Most lineations trend within N54°-86°E MODEL STUDIES and sometimes parallel systems of stream with several drops in the frequency of the valleys. Most of these lineations occur on linears within this, range. If plotted to Similarities between shear zones of var- the northern slope of the Rio Cuilco Valley, emphasize length, the trends are distin- ious scales have been recognized since the although some lineations lie south of the guished, although not strongly. Not only do early days of structural geology and used in valley. a large number of lineations fall within this model studies of tectonic processes. Tcha- The fracture patterns define four general interval but also the lengths of these linears lenko (1970) performed what is called the trends. The first is composed of westerly to are greater if compared to the remainder. Riedel experiment (Cloos, 1928; Riedel, northwesterly trending fractures that paral- The longest individual linears are within 1929) in which a clay paste (such as kaolin) lel reverse faults that were mapped within this group, with four fractures having is placed over two adjoining horizontal Canibal quadrangle (T. H. Anderson, 1976, lengths of as much as 5 km. These north- boards. As one board is slowly moved past unpub. data). In the Cuilco quadrangle, the east-trending linears comprise a series of en the other, a characteristic en echelon frac- distribution of Ixcoy Formation and Todos echelon fractures that can be traced from ture pattern develops within the overlying Santos Formation along the northern wall the western border of the quadrangle east- clay slab. of the Cuilco Valley suggests the existence ward to the San Sebastian Huehuetenango Tchalenko (1970) observed five stages of of similar reverse faults. These reverse faults border (Fig. 5). development in the en echelon pattern (Fig. are not considered to be directly related to existing strike-slip fracture patterns, and 10 f 10 they are not included in Figure 6. The 253 remaining lineations from area C A of Figure 5 are plotted on two rose diagrams in Figure 6. Figure 6A is constructed by plotting the number of the linears within 4° intervals versus the total number. Figure 6B is a plot of the total length of lineations within each 4° interval against the total length of all lineations (149.5 km). The percentage increase of a particular trend from the upper to lower diagram implies the existence of long lineations within that group. Similarly, a decrease in percentage

Figure 6. Rose diagrams of selected linears. Upper rose diagram is constructed by plotting the number of linears with 4° intervals versus the total number; the lower diagram is a plot of the total length of lineations within each 4° interval against the total length <5 of all lineations (149.5 km).

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lOlrcel Shaarl Fig. 7) is followed by stage a, or what is a called peak structure. During this stage the Riedel shears (R) first appear at an average inclination of 12° ± 1°, but quickly become rotated to a maximum inclination of about 16°. The amount of board displacement accommodated by individual shears rises from 0 to 50%. A second set of shears, the conjugate Riedel shears (R'), form at an inclination of 70° ± 1° and may appear simultaneously or just prior to the Riedel shears. During this stage, tension (T) fractures may also form in addition to, or in place of, Riedel shears. They develop at an inclination of about 45°. Post-peak structure (stages b and c) is associated with the drop in shearing resist- ance from peak to residual strength value. The R and R' shears formed at peak strength become unfavorably oriented to maintain large relative motions. The R' 0 = 19.50 mm I 1 100X shears respond passively to post-peak de- 10 mm formation by rotation and distortion because they are nearly perpendicular to the -10 0 4 8 12 16 i direction of movement. The R shears extend into undeformed material on either side of the shear zone where their relative displacements are reduced toward zero from the maximum in the central part of the shear zone. Some R shears extend into a more horizontal direction, with a few new D= 27.20 mm shears appearing at about 8°. The amount 100% of total displacement accommodated by these shears attains about 75%.

-10 0 4 6 12 16 I With the passive response of R' shears and the decrease in motion along R shears, Tchalenko (1970) observed the formation of new shears called P, or thrust shears, because oblique motion can at times occur

0=38.00 mm along this shear. These are symmetrical to 100« the R shears about the direction of motion, forming characteristic "bull nose" structures (Skempton, J966). More than one-half 10 0 4 8 12 16 I of the shears are now inclined at 4°, with the majority of the total movement at Figure 7. Sequence of structures in the Riedel experiment, i = inclination of shear in this stage accommodated by displacements degrees with respect to general direction of movement. R = Riedel shear. R' = conjugate along shears. Riedel shear. T = tension shear. P = "thrust" shear. D = principal displacement shear. Tchalenko (1970) described stage d as the (a,b,c,d,e) = stages in the deformation process. Adapted from Tchalenko (1970). pre-residual structure, in which the first continuous horizontal shears, the "principal displacement shears" (D shears), are 7) when using kaolin with a 56% water con- cumulative displacement (di) that has oc- formed. They isolate elongate lenses of es- tent. During the development of these curred along the shear groups. sentially passive material between them. shears, the proportion of board movement, Tchalenko (1970) observed that initial Most shears are now inclined at 4° or less. or total displacement (D), taken up by movement of the boards causes homogene- When stage e (the residual structure) is movement on the individual shears (d) can ous strain in the region of the future shear reached, the combination of displacement be directly measured. These displacements zone. Circles inscribed on the clay are trans- along the R and P shears leads to the forma- can be plotted against the inclination (i) of formed into ellipses, indicating that defor- tion of the D shears oriented parallel to the the shears to the general movement direc- mation is of the simple shear type. This direction of motion. The active portion of tion in histograms (Fig. 7) and indicate the pre-peak stage deformation (not shown in the shear zone reduces in width until all

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movement is along a very narrow zone or between 91°55' and those in the western time in excess of the actual duration of the on a single slip surface. border of the quadrangle closely resemble underlying bed-rock rupture, as a result of In the Riedel experiment, the over-all stage b of Figure 7, whereas those in the the time required to transfer the energy of strain is controlled and measured, but in the remainder of the quadrangle more closely motion to the surface. Therefore cover case of earthquake faulting, rates of move- resemble stage c. rocks across the fault not only affect the ment are commonly unknown. However, a This interpretation of the fractures in time of surface displacement, but also affect growing body of evidence indicates that sur- Cuilco suggests that the fractures in Figure the amount of surface motion. Thus the face displacements occur over a time period 6 may be correlative to R, R', P, T, and D deeper the bed-rock fault is buried, the well in excess of the actual duration of the shears. Thus the northeast-trending linears greater the rupture that is required before underlying bed-rock rupture, as a result of within N54°-86°E in the lower diagram are any surface expression is observed. the time required to transfer the energy of considered to be comparable to R shears. If it may be assumed that the thicknesses motion to the surface (Scholz and others, These R shears are inclined between 4° and of cover rocks in Cuilco and Nimbluk val- 1969). 36°, equal to about twice the maximum leys are comparable (that is, varying from a With earthquake faulting, it is not usually observed by Tchalenko, which was 16°. few tens to a few hundreds of metres), then possible to follow the structural evolution Two other northeast-trending shear sets are a comparison of the observed shear lengths continuously, as in the Riedel experiment. also present. The first, within N42°-50°E, may provide insight into the magnitude of Commonly, observations pertinent to a would appear to correspond to the T shears fault displacement. In the Nimbluk Valley, structure have been made sometime after if they are defined as being inclined at Tchalenko (1970) and Tchalenko and Am- the main shock, when most of the move- 45° ± 5°. The second set, less than N38°E braseys (1970) measured shear lengths from ment has been completed. However, even (or the equivalent of greater than 52° incli- metres to hundreds of metres, and actual then, detailed mapping may reveal segments nation), are probably R'shears. The inclina- fault motion of 4.5 m. Within Cuilco quad- that have undergone different amounts of tion of R' shears is defined to be 90°-/2, rangle, shears were observed with lengths relative displacements, and, if conditions where i>/2 is the inclination of the R shear. varying from hundreds of metres to almost are favorable, a time sequence and stages of Because the range of inclination for the R 5 km. To produce the en echelon shear pat- structural evolution may be distinguished. shear in Cuilco is between 4° and 36°, the tern observed in the Colotenango beds, the Such is the case for the Dasht-e-Bayaz minimum angle for the R' shear is 90°-36°, authors suggest that total displacement earthquake of 1968 in Iran as described by or 54°. This compares well with the angle along the buried Chixoy-Polochic fault has Tchalenko (1970) and Tchalenko and Am- of 52° observed. probably amounted to at most a few thou- braseys (1970). The northwest-trending linears lie within sands of metres. The segment of the fault studied by Tcha- N58°-86° W, equal to an inclination of-32° lenko (1970) and Tchalenko and Ambraseys to —4°. The P shear is symmetric with the R CONCLUSIONS (1970) was a zone 2 to 3 km wide and 25 km shear about the direction of motion (Fig. 7). long, located in Quaternary deposits in the Because the R shears are inclined between Movement along the Chixoy-Polochic Nimbluk Valley. Maximum relative dis- 4° and 36° in Cuilco, the symmetry of these fault since the accumulation of the volcanic placements in the valley were 4.5 m left- to the northwest-trending P shears is readily beds is recorded not as a single principal lateral and 2.5 m vertical. seen. displacement shear as in most of the region Tchalenko (1970) showed that this seg- The remaining set of linears lies within near the towns of San Sebastian Huehuete- ment of the fault could be described at three N86°-94°E. These east-west-trending line- nango, Canibal, and Motozintla, but as a distinctive stages of structural evolution., the aments correspond to the D, or incipient series of en echelon fractures extending the peak structure (stage a), the post-peak principal displacement shears. entire length of the Cuilco quadrangle structure (stage c), and residual structure In the region of the Dasht-e-Bayaz earth- within Rio Cuilco Valley. Geomorphic evi- (stage e). Stage a at the fault segment quake, Tchalenko (1970) and Tchalenko dence for movement within the Colote- showed over-all displacements of about 150 and Ambraseys (1970) were able to measure nango beds includes a complicated system cm along the earthquake fault. For stage c, horizontal displacement along the fault of linear drainage valleys, saddles, subtle about 250 cm of displacement was observed (4.5 m) and displacement along individual breaks in slope, changes in vegetation and and, finally, stage e showed displacement of shears (1.5 to 3 m) observed in Quaternary soil color that probably reflect disturbance 300 cm along the earthquake fault. Tcha- cover. In the Chixoy-Polochic fault in of ground-water regime, and zones of lenko (1970) also described fossil shear Cuilco quadrangle, the shears observed in coarse breccia. No values for the amount of zones, which formed in the past but are now the volcanic cover record the structural evo- motion along these linear features could be inactive. These displayed comparable struc- lution of the fault. The shears indicate that measured. However, if lineations observed tures. In the Riedel experiments, Tchalenko motion along the buried Chixoy-Polochic on the ground and on aerial photographs (1970) observed the length of the shears as fault has occurred since emplacement of the are the reflection of motion along the buried being less than or equal to 5 cm, whereas Colotenango beds. However, it was not Chixoy-Polochic fault, these probable frac- those in the earthquake fault were measured possible to directly measure any displace- tures can be likened to those of the Riedel in metres to hundreds of metres. ment across the shears, so the amount of experiment, and the amount of movement The similarity between the fractures des- motion can only be estimated. In the esti- can be estimated. The fracture pattern cribed by Tchalenko (1970) and those mates, thickness of the volcanic cover is represented in the Cuilco quadrangle re- observed in Cuilco (compare Figs. 7 and important. Scholz and others (1969) have sembles that observed in stages b and c of 5C) is strong. The fractures in the region stated that surface motion occurs over a Figure 7. The fracturing in the volcanic

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sequence in Cuilco Valley acts similarly to Austin, Texas, University of Texas, 219 p. Perfit, M. R., and Heezen, B. C., 1978, The geol- that in the Riedel experiment in that the Anderson, T. H., and others, 1973, Geology of ogy and evolution of the Cayman Trench: the western Altos Cuchumatanes, north- Geological Society of America Bulletin, fracture pattern is the result of transferral of western Guatemala: Geological Society of v. 89, p. 1155-1174. a percentage of the total displacement that Amerita Bulletin, v. 84, p. 805-826. Riedel, W., 1929, Zur Mechanik geologischer has occurred along the fault. Because there Blount, D. N., 1967, Geology of the Chiantla Bruckerscheinungen: Centralblatt fur Min- appears to be no coalescense of the fractures quadrangle, Guatemala [Ph.D. thesis]: Baton eralogie und Paleontologie, v. 1929 B, into a single principal displacement shear, Rouge, Louisiana, Louisiana State Univer- p. 354-368. sity, 135 p. Sandstrom, M. A., 1978, Stratigraphy and struc- the amount of motion which has occurred Burkart, B., 1978, Offset across the Polochic fault ture of Chochal Limestone, El Tapon canyon along the fault since the emplacement of the of Guatemala and Chiapas, Mexico: Geol- area, Cuilco quadrangle, Guatemala [M.S. volcanic rocks (Miocene to Pliocene?) is ogy, v. 6, p. 328-332. thesis]: Pittsburgh, Pennsylvania, University likely small, probably at most a few kilome- Carfantan, J. C., 1977, La Cobijadura de of Pittsburgh, 137 p. tres, which is close to the 1-km displacement Motozintla-Un paleoarco en Chiapas: Uni- Scholz, C. H„ Wyss M„ and Smith, S. W„ 1969, versidad Nacional Autonoma de Mexico, Seismic and aseismic slip on the San Andreas observed along offset streams by Anderson Instituto de Geología, v. 1, p. 133-137. fault: Journal of Geophysical Research, and others (1973) in San Sebastian Hue- Cloos, H., 1928, Experimente zur inneren Tek- v. 74, p. 2049-2069. huetenango. Therefore, proposed displace- tonik: Centralblatt für Mineralogie und Schwartz, D. P., Cluff, L. S., and Donnelly, ment of 100 or more kilometres within this Paleontologie, v. 1928 B, p. 609-621. T. W., 1979, Quaternary faulting along the Caribbean North-American plate boundary fault zone must be older than the volcanic Holcombe, T. L„ Vogt, P. R., Matthews, J. E„ and Murchison, R. R., 1973, Evidence for in Central America: Tectonophysics, v. 52, rocks in Cuilco. Earlier displacements may sea-floor spreading in the Cayman Trough: p. 431-445. be responsible for some of the intense brec- Earth and Planetary Science Letters, v. 20, Siesser, W. G., 1967, Geology of the Cuilco ciation recorded in the Ixcoy Formation but p. 357-371. quadrangle, Guatemala [M.S. thesis]: Baton not found within the volcanic strata. Home, G. S., Clark, G. S„ and Pushkar, P., Rouge, Louisiana, Louisiana State Univer- 1976, Pre-Cretaceous rocks of northwestern sity, 87 p. Honduras: Basement terrane in Sierra de Skempton, A. W., 1966, Some observations on ACKNOWLEDGMENTS Omoa: American Association of Petroleum Tectonic shear zones: First Congress of Geologists Bulletin, v. 60, p. 566-583. International Society of Rock Mechanics We are grateful for the maps, photos, Kesler, S. E., 1971, Nature of ancestral orogenic Proceedings, Lisbon, v. I, p. 329-335. zone in nuclear Central America: American Tchalenko, J. S., 1970, Similarities between shear and other logistical support provided by Association of Petroleum Geologists Bul- zones of different magnitudes: Geological Ing. Oscar Salazar of the Geological Divi- letin, v. 55, p. 2116-2129. Society of America Bulletin, v. 81, p. sion of the Instituto Geografico Nacional Kupfer, D. H., and Godoy, J., 1967, Strike- 1625-1639. of Guatemala as well as the privilege of slip faulting in Guatemala [abs.]: American Tchalenko, J. S., and Ambraseys, N. N., 1970, pursuing long-term geological research in Geophysical Union Transactions, v. 48, Structural analysis of the Dasht-e-Bayaz p. 215. (Iran) earthquake fractures: Geological So- western Guatemala. We wish to thank McBirney, A. R., 1963, Geology of a part of the ciety of America Bulletin, v. 81, p. 41-60. James Kilburg and Melissa Sandstrom for central Guatemalan cordillera: California Williams, H., 1960, Volcanic history of the Gua- their support and companionship during University Publications in Geological Sci- temalan Highlands: California University the occasionally arduous field studies in ences, v. 38, p. 177-242. Publications in Geological Sciences, v. 38, Meyerhoff, A. A., 1966, Bartlett fault systems: p. 1-86. Guatemala. Age and offset: 3rd Caribbean Conference Transactions, p. 1-9. REFERENCES CITED Muehlberger, W. R., and Ritchie, A. W„ 1975, MANUSCRIPT RECEIVED BY THE SOCIETY Caribbean-Americas plate boundary in Gua- FEBRUARY 18, 1980 Anderson, T. H., 1969, Geology of the San Se- temala and southern Mexico as seen on REVISED MANUSCRIPT RECEIVED bastian Huehuetenango quadrangle, Gua- Skylab IV orbital photography: Geology, FEBRUARY 23, 1981 temala, Central America [Ph.D. thesis]: v. 3, p. 232-235. MANUSCRIPT ACCEPTED MAY 4, 1981

Primed in U.S.A.

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