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

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. 57 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/1/57/3434362/i0016-7606-93-1-57.pdf by guest on 02 October 2021 58 ERDLAC AND ANDERSON 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/1/57/3434362/i0016-7606-93-1-57.pdf by guest on 02 October 2021 CHIXOY-POLOCHIC FAULT, GUATEMALA 59 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.

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