Thomas W. Donnelly

Quaternary Faulting Along the Caribbean–North American Plate Boundary in Central America

(Revised version accepted May 22, 1978) ABSTRACT: Schwartz, D.P., Cluff, L.S. and Donnelly, T.W., 1979. Quaternary faulting along the Caribbean—North American plate boundary in Central America. In: C.A. Whitten, R. Green and B.K. Meade (Editors), Recent Crustal Movements, 1977. Tectonophysics, 52: 431-445. Recent detailed mapping along the Motagua fault zone and reconnaissance along the Chixoy—Polochic and Jocotan—Chamelecon fault zones provide new information regarding the nature of Quaternary deformation along the Caribbean—North American plate boundary in Central America. The southern boundary of the Motagua fault zone is defined by a major active left-slip fault that ruptured during the February 4, 1976 Guatemala earthquake. The recurrent nature of slip along the fault is dramatically demonstrated where stream terraces of the Rio El Tambor show progressive left-slip and vertical (up-to-the-north) slip. Left-slip increases from 23.7 m (youngest mappable terrace) to 58.3 m (oldest mappable terrace) and vertical slip increases from 0.6 m to 2.5 m. The oldest mappable terrace crossed by the fault appears to be younger than 40,000 years and older than 10,000 years. Reconnaissance along the Chixoy—Polochic fault zone between Chiantla and Lago de Izabal has located the traces of a previously unmapped major active left-slip fault. Geomorphic features along this fault are similar to those observed along the active trace of the Motagua fault zone. Consistent and significant features suggestive of left-slip have so far not been observed along the Guatemala section of the Jocotan—Chamelecon fault zone. In Central America, the active Caribbean—North American plate boundary is comprised of the Motagua, Chixoy—Polochic, and probably the Jocotan—Chamelecon fault zones, with each accommodating part of the slip produced at the mid-Cayman spreading center. Similarities in geomorphic expression, apparent amount of left-slip, and frequency and magnitude of historical and instrumentally recorded earthquakes between the active traces of the Motagua and Chixoy—Polochic fault zones suggest a comparable degree of activity during Quaternary time; the sense and amount of Quaternary slip on the Jocotan—Chamelecon fault zone remain uncertain, although it appears to be an active earthquake source. Uplift of major mountain ranges on the north side of each fault zone reflects the small but consistent up-to-the-north vertical component (up to 5% of the lateral component) of slip along the plate boundary. Preliminary findings, based on offset stream terraces, indicate a late Quaternary slip rate along the Caribbean—North American plate boundary of between 0.45 and 1.8 cm/yr. Age dating of offset Quaternary terraces in Guatemala will allow refinement of this rate.

The chemical discrimination of clastic sedimentary components

ABSTRACT The detrital composition of siliciclastic rocks from six localities in the Taconic and Acadian sections of New York and Quebec can be described as mixtures of two or more differentiable sedimentary components. A sedimentary component is defined as a mineral or group of minerals that sort independently of other sedimentary constituents. Because of grain morphology, phyllosilicates usually dominate the finer-grained component, and quartz or mixtures of quartz and feldspar dominate the coarser-grained components. The chemical composition of a sedimentary component is distinctive, and the differentiation of sedimentary components usually results in the concomitant separation of associated elements. Typical separations include the enrichment of K2O and Al2O3 in the finer-grained, phyllosilicate-rich fraction and the enrichment of SiO2 and Na2O in the coarser-grained, tectosilicate-rich fraction. Fe2O3 and MgO are also enriched in the fine fraction, if they are abundant constituents in the phyllosilicates. The chemical composition of the graywackes from North America was compared to the chemical composition of unlithified turbidite sediments from the Black Sea. The chemical variation trends found in the deeply buried sedimentary rocks are similar to the trends found in the suite of slightly buried and unlithified deep-sea sediments. This indicates that burial alteration does not necessarily result in large-scale redistribution of the constituent elements. It is possible, therefore, to use the chemical variations observed in a suite of deeply buried and mineralogically altered sedimentary rock to infer the original clastic assemblage and to help clarify the processes that produced the sediment.

Worldwide continental denudation and climatic deterioration during the late Tertiary: Evidence from deep-sea sediments

A six-fold increase in the rate of accumulation of Al 2 O 3 in north and central Atlantic and Pacific Ocean sediments indicates vastly increased denudation of the continents during the past 15 m.y. The increase is more apparent in hemipelagic than pelagic sites, demonstrating widely distributed local controls. Similarities in the rate of increase in the Atlantic and Pacific show that tectonic elevation is not responsible for the difference in sedimentation rate. Also, similarities in the difference at sites of low and high latitude suggest that glaciation is not the most significant source. A lack of correspondence between sedimentation rates and Vail9s sea-level curve similarly rule out that effect. The conclusion drawn here is that worldwide climatic deterioration during the late Tertiary is the explanation for the striking increase in detrital sedimentation in the world ocean.

Mesozoic and Cenozoic Plate Evolution of the Caribbean Region

The reconstruction of Caribbean plate history is an uncertain task, but a task that has intrigued generations of geologists. Each worker has turned to the task of historical interpretation influenced by a particular set of experiences or a special approach, and the results have been accordingly varied. A complete history of interpretations would form the subject of a fascinating chapter in the history of geological philosophy, but such is not the purpose of this chapter. Instead, I will dwell on a set of data that call for what I believe to be a relatively conservative view of Cretaceous and Tertiary plate history. My own interpretation is based heavily on my own or my students’ field experiences in the northeastern West Indies, Guatemala, Belize, and Venezuela, as well as extensive field excursions in Hispaniola, Jamaica, the Lesser Antilles, Central America, and the Dutch Antilles. I am further heavily influenced by the results of the Deep Sea Drilling Project (DSDP), Leg 15, which produced information of fundamental interest in the Venezuelan and Colombian Basins, and by several dissertations of the Princeton University group in northern Venezuela.

Chemistry, origin and tectonic significance of metamorphic rocks from the Puerto Rico Trench☆

Abstract Constraints on the evolution of the northeastern Caribbean plate boundary and the tectonic development of the Puerto Rico Trench have been derived from a study of rocks and sediments dredged from thirty-six localities within and surrounding the trench. Marble, calc-schist, mica-schist, greenschist, amphibolite, magnesian schist and serpentinite crop out below the trench—slope break (∼ 3500 m) along the steep inner-trench wall for over 400 km. At shallower depths, limestone, calcilithite, and minor amounts of chert and sedimentary rock were sampled. Potassium—argon ages of two of the metamorphic rocks are 63 ± 3 and 66 ± 5 m.y.B.P. Rocks dredged from Mona Canyon, directly behind the inner-trench wall, are primarily igneous rocks, volcanic breccia, limestone and calcilithite that have been only slightly metamorphosed. Middle Eocene to Miocene (or younger) shallow-water limestones are interbedded and overlie the volcanogenic rocks. The outer-trench wall (oceanic side) is composed of tholeiitic basalts, serpentinite and deep-sea sediments commonly recovered from the Atlantic oceanic crust. Mineral assemblages and phase relations in the metamorphic rocks are similar to those observed in many subaerially exposed melanges believed to be subduction complexes. The conditions of metamorphism within the Puerto Rico Trench metamorphic complex are estimated to have been between 400 and 550°C and 3 to 7 kbar. Chemical analyses of the dredged samples suggest that: 1. (1) The metamorphic rocks from the inner-trench wall were primarily island-arc igneous rocks or sediments derived from them. 2. (2) Protoliths of the marbles and calc-schists were apparently biogenic carbonates and pelagic sediments with an arc-derived sedimentary component. 3. (3) Basalt and serpentinite from the outer-trench wall are geochemically similar to oceanic tholeiites (MORB) and ultramafics. 4. (4) Only two greenschists from the inner-trench wall have geochemical characteristics similar to MORB. 5. (5) Magnesian schists and serpentinite from deep along the inner wall have either been severely affected by metasomatism or represent metamorphosed ultramafic rocks from beneath the Greater Antilles. Our results indicate that, unlike a number of arc-trench systems in the Pacific, a great deal of arc-derived material has been accreted to the inner-trench wall and that the amount of oceanic crust incorporated into the subduction wedge was relatively minor. Directly west of the inner-trench wall, metamorphic rocks, marble and limestone identical to those dredged, crop out in a “blueschist belt” in northern Hispaniola. Behind the trench, to the south, low-grade metavolcanics in Puerto Rico appear to be continuous across Mona Passage to central Hispaniola. This belt parallels the higher-grade complex within the trench, and as such constitutes a paired metamorphic belt that extends for more than 500 km along the northern Caribbean plate boundary. The development of this belt is related to the inception of the Puerto Rico Trench in the middle to late Cretaceous due to the southerly subduction of the Atlantic plate beneath the Caribbean plate. Eastward movement of the Caribbean plate relative to the North American plate in the late Cretaceous and early Tertiary initiated transcurrent and tensional faulting which enhanced the uplift of the metamorphosed subduction wedge. Local tectonic readjustments which began in late Eocene time created the grabenlike trench observed at present and caused at least 3500 m of subsidence along the innertrench wall north of Puerto Rico, since Miocene time.

The scavenging of magnesium and other chemical species by biogenic opal in deep-sea sediments

Abstract A study of siliceous oozes from the Caribbean Sea and Pacific Ocean shows that considerable quantities of Mg can be adsorbed on to biogenic opal. The process is diffusion limited and requires a hiatus in sedimentation in order that a substantial amount of Mg be absorbed. The adsorption is sensitive to pH, and is virtually inoperative in the absence of carbonate. The Mg apparently comes from seawater, and some well-known pore-water Mg gradients evidently result from this adsorption process. Na is also adsorbed, but the process is not diffusion limited. Fe and Mn may be similarly adsorbed, but, in one case studied, possibly are supplied from the basaltic basement by hydrothermal processes. K apparently does not participate in surface adsorption to any considerable extent. The later conversion of opal-A (amorphous opal) to opal-CT or to quartz evidently creates zones of local high Na and Mg activity which, in turn, is probably responsible for the sepiolite, dolomite, clinoptilolite, and, possibly, much smectite at the level of chert formation in these sediments.

Compositions and Sources of Metasediments in the Upper Dharwar Supergroup, South India

Texturally mature, quartz-rich sandstones, texturally mature, mafic-rich sandstones, and texturally immature graywackes are abundant in the late Archean Chitradurga Group of the Dharwar craton in South India. When compared to graywackes of Paleozoic age from northeastern North America, the Chitradurga graywackes have slightly higher average concentrations of Na and ferromagnesian elements, indicating the existence of a more tonalitic and mafic-rich provenance during the late Archean in Karnataka. The regional tonalitic gneiss (Peninsular Gneiss) is the predominant source of these sediments, although mafic/ultramafic volcanic debris from unidentified sources is also important. Neither intermediate nor felsic volcanics nor an extensive metasedimentary terrane were prominent in the provenance. Although the composition of the craton was dominantly sialic, the presence of abundant ferromagnesian debris in some of the texturally mature sandstones that probably formed in shallow water, intracratonic, or platformal environments suggests that mafic rock was also situated on or near the craton during the late Archean.

Preliminary Survey of the Microseismicity of the Northeastern Caribbean

A survey of microearthquakes of the Virgin Islands and eastern Puerto Rico showed that it is possible to find quiet recording sites for high-gain, high-frequency seismographs and to record as many as 10 to 15 micro-events per day. A site at Lameshur Bay, St. John, Virgin Islands, was as quiet as some of the best sites occupied in western North America. Microearthquake activity approximately coincided geographically with activity for larger earthquakes from 1900 to 1964. These events were located along the Anegada trough, the Puerto Rico trench, and the northern end of the Lesser Antillean arc.

Phylogenetic relationships of North American Gomphidae and their close relatives

Intrafamilial relationships among clubtail dragonflies (Gomphidae) have been the subject of many morphological studies, but have not yet been systematically evaluated using molecular data. Here we present the first molecular phylogeny of Gomphidae. We include six of the eight subfamilies previously suggested to be valid, and evaluate generic relationships within them. We have included examples of all genera reported from the Nearctic except Phyllocycla. This sample includes all North American species of Ophiogomphus, which has allowed us to explore intrageneric relationships in that genus. Our particular focus is on the closest relatives of the genus Gomphus, especially those North American species groups that have been commonly treated as subgenera of Gomphus. The Gomphus complex is split into additional genera, supported by molecular and morphological evidence: Phanogomphus, Stenogomphurus, Gomphurus and Hylogomphus are here considered to be valid genera. The genus Gomphus, in our restricted sense, does not occur in the western hemisphere; in addition, G. flavipes is transferred to Stylurus.

A NEW SUBSPECIES OF GOMPHUS (GOMPHURUS) SEPTIMA FROM THE DELAWARE RIVER OF NEW JERSEY, NEW YORK, AND PENNSYLVANIA (ODONATA, GOMPHIDAE)

Abstract Gomphus (Gomphurus) septima delawarensis is described from the Delaware River, NJ, PA, and NY (Type locality: Flatbrookville, NJ). The new subpecies differs from nominate G septima septima Westfall (new status) mainly by differences in the male cerci and eplproct, and in the female vulvar lamina and lateral occipltal horns. The larvae of the two subspecies are similar and differ widely from other Gomphurus by the rounded dorsal tubercles on abdominal segments 6-8, and by the highly shortened row of teeth on the lateral palp of the labium. Gomphus septima is most closely related to the vastus, group of Gomphurus.