James L. Aronson

Geology, geochronology, and rift basin development in the central sector of the Main Ethiopia Rift

Based on stratigraphic relationships and K/Ar dating of volcanic rocks from both of the escarpments, flanking plateaus, and from the rift floor of the central sector of the Main Ethiopian Rift, six major volcanic episodes are recognized in the rift9s development over a time span from the late Oligocene to the Quaternary. Using the K/Ar data, correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time-stratigraphic chronozones for the central sector that are related to volcanism in the Ethiopian Cenozoic volcanic province. The oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to, or older than, those in river canyons (for example, Blue Nile) on the adjacent northwest plateau. A thinned Mesozoic stratigraphic sequence along the Guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the Cenozoic. By late Miocene time, at least by 8.3 Ma and 9.7 Ma, the eastern and western faulted margins, respectively, of the rift had formed at Guraghe and at Agere Selam as indicated by containment of flows of that age within the rift wall during eruption. A paroxysm of calc-alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the Pliocene epoch. The Munesa Crystal Tuff (3.5 Ma), a prominent marker tuff exposed on both rift margins, is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption. Structural and stratigraphic relationships in the central sector indicate a two- stage rift development. This is characterized by an early phase (late Oligocene or early Miocene) of a series of alternating opposed half-grabens along the rift with alternating polarity, such as that in the present Gregory and Western Rifts of East Africa and symmetrical rifts that evolved from these grabens in late Miocene or early Pliocene time. Thus, evolution from alternating half-graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process. The study indicates that there are major petrologic and tectonic differences between the Main Ethiopian Rift and the Gregory (Kenyan) Rift.

Late Eocene–Recent volcanism and faulting in the southern main Ethiopian rift

Few constraints on the timing, amount and distribution of lithospheric extension associated with flood-basalt magmatism were available from the southern Main Ethiopian rift system, where the base of the Cenozoic volcanic succession is exposed by faulting. New structural observations, together with K–Ar and 40Ar/39Ar geochronology data from a transect of the Chamo basin–Amaro horst–Galana basin, show that basins are bounded by faults with steep dips at the surface, and the stratal dips of Eocene–Recent volcanic and sedimentary units are generally less than 20°. Little or no extension accompanied the extrusion of a 0.5 to 1 km thick sequence of transitional tholeiitic flood basalts between 45 and 35 Ma. Stratigraphical correlations with basins to the north and southwest suggest that felsic eruption(s) at c. 37 Ma blanketed much of the southern Ethiopian plateau region with a felsic tuff unit. A second, less widespread, episode of alkali basalt and trachyte volcanism occurred between 18 and 11 Ma, and Recent alkali basalt volcanism occurs within the Chamo basin. The attitude, distribution, and diversity of Neo–gene–Recent volcanic and sedimentary strata within the Chamo and Galana basins indicate that crustal extension, basin subsidence, and rift flank uplift began during or after the second flood-basalt phase. Based on cross-sectional reconstruction to the top of the Oligocene tuff, we estimate a minimum of 12 km crustal extension (β ≈ 1.12), and infer that maximum extension across the southern Ethiopian rift is less than 25 km. Extension is primarily accommodated by slip along the border faults bounding the asymmetric basins, with small amounts of extension occurring within the hanging walls. Crude estimates of original basalt layer thickness prior to erosion in the Amaro region suggest that roughly comparable volumes of basaltic material erupted during the two episodes of flood-basalt magmatism (45–35 Ma and 18–11 Ma). The small amounts of lithospheric extension and the large volumes of magma estimated in this study of the southern Main Ethiopian rift suggest a very hot plume and/or efficient thinning of the mantle lithosphere from below by mantle plume processes during the two discrete episodes of flood-basalt volcanism.

New age constraints on the timing of volcanism and tectonism in the northern Main Ethiopian Rift–southern Afar transition zone (Ethiopia)

Forty new K-Ar and / isotopic ages from the northern Main Ethiopian Rift (MER)–southern Afar transition zone provide insights into the volcano-tectonic evolution of this portion of the East African Rift system. The earliest evidence of volcanic activity in this region is manifest as 24–23 Ma pre-rift flood basalts. Transition zone flood basalt activity renewed at approximately 10 Ma, and preceded the initiation of modern rift margin development. Bimodal basalt–rhyolite volcanism in the southern Afar rift floor began at approximately 7 Ma and continued into Recent times. In contrast, post-subsidence volcanic activity in the northern MER is dominated by Mio-Pliocene silicic products from centers now covered by Quaternary volcanic and sedimentary lithologies. Unlike other parts of the MER, Mio-Pliocene silicic volcanism in the MER–Afar transition zone is closely associated with fissural basaltic products. The presence of Pliocene age ignimbrites on the plateaus bounding the northern MER, whose sources are found in the present rift, indicates that subsidence of this region was gradual, and that it attained its present physiography with steep escarpments only in the Plio-Pleistocene. Large 7–5 Ma silicic centers along the southern Afar and northeastern MER margins apparently formed along an E–W-oriented regional structural feature parallel to the already established southern escarpment of the Afar. The Addis Ababa rift embayment and the growth of 4.5–3 Ma silicic centers in the Addis Ababa area are attributed to the formation of a major cross-rift structure and its intersection with the same regional E–W structural trend. This study illustrates the episodic nature of rift development and volcanic activity in the MER–Afar transition zone, and the link between this activity and regional structural and tectonic features.

Areal distribution and age of low-K, high-alumina olivine tholeiite magmatism in the northwestern Great Basin

Field, petrographic, chemical, and geochronologic information has led to the identification and characterization of a widespread low-K, high-alumina olivine tholeiite (HAOT) magma type in the northwestern Great Basin. This basalt covers at least 22,000 km 2 and is estimated to represent a total volume of at least 650 km 3 . The time period over which HAOT lavas were erupted extends from late Miocene to Holocene (10.5–0 m.y. B.P.). This interval overlaps with the timing of Snake River, Cascade, and northwestern Basin and Range volcanism but distinguishes HAOT from the main pulse of Columbia River volcanism (∼15 m.y. B.P.). Furthermore, three major pulses of HAOT magmatism are suggested from the geochronology of this study: 0 to 2.5 m.y. B.P., 3.5 to 6 m.y. B.P., and 7 to 10 m.y. B.P. The distinctive holocrystalline, nonporphyritic, and diktytaxitic texture, the low incompatible-element concentrations, and the high MgO/FeO* of HAOT serve to distinguish this basalt from other basalts of the northwestern United States. The low incompatible-element signature accentuates the similarities between HAOT, mid-ocean–ridge basalts, and back-arc–basin basalts. These similarities, combined with the HAOT chronology, support the idea that the processes giving rise to extensional tectonism and HAOT magmatism in the northwestern Great Basin are similar to those acting in active back-arc–spreading regions.

Mechanism of burial metamorphism of argillaceous sediment: 2. Radiogenic argon evidence

Variations in the 40* Ar of the whole rock and the 2 O in the inferred burial metamorphic reactions of argillaceous sediment from Texas Gulf Coast well CWRU (Case Western Reserve University) 6. These data strongly support the occurrence of the reactions involving the illite/smectite mixed-layer clay inferred from depth-dependent mineralogical and chemical changes reported in a companion paper by Hower and others (this issue). The 40Ar released from all samples was generally greater than 40 percent radiogenic ( 40* Ar), allowing precise measurement of differences of 40* Ar with depth. The whole-rock K-Ar apparent age shows a decrease from about 150 m.y. to 75 m.y. over just the same depth interval (1,850 to 3,700 m) that the illite/smectite mixed-layer clay progressively changes from 20 percent illite layers to 80 percent. This decrease in apparent age of the total shale is due to loss of 40* Ar from the rock. The 40* Ar loss is not caused by outgassing from increasing temperature, because the finest ( 40* Ar with depth. The coarser fraction, from which the 40* Ar loss is occurring, concentrates the older 40* Ar—rich phases of the rock such as K-feldspar and mica. These results strongly imply that the K 2 O for the new illite layers of the illite/smectite is derived by chemical decomposition of the K-feldspar and mica. The gain in 40* Ar of the finest fraction also just corresponds with depth to the change in mineralogy of the illite/smectite and a large gain of K 2 O in this fraction from 2 to 5 percent K 2 O. The 40* Ar gain indicates that the mean time of K 2 O gain, which is the mean time of burial metamorphism, was about 18 m.y. ago.

Model for K-bentonite formation: evidence from zoned K-bentonites in the disturbed belt, Montana.

Thirty-four K-bentonites from the Marias River Formation (Upper Cretaceous) in the Montana disturbed belt have been studied in detail. A 2.5-m-thick K-bentonite is zoned mineralogically and chemically, the upper and lower contacts of the bed being more illitic and K-rich than the middle of the bentonite. K/Ar dates of illite/smectite separated from the bed are also zoned, yielding older ages at the contacts than at the center. A model for the formation of K-bentonites enclosed by K-rich shale is presented in which K-bentonites form from smectite bentonites during a thermal event ( T = 100–200 °C). Potassium is derived from mineralogic breakdown (probably mica and K-feldspar) in the shale, and migrates by diffusion into the bentonite.

Alleghanian episode of K-bentonite illitization in the southern Appalachian Basin

Mixed-layer illite-smectite (I/S) from Middle Ordovician potassium (K)-bentonites was found to be uniformly illitic throughout the southern Appalachian Basin regardless of variable depths of burial of the K-bentonites. The K/Ar ages of illitization are narrowly confined between 272 and 303 Ma (Late Pennsylvanian to Early Permian); this suggests that illitization was a short-lived episode coincident with, and prompted by, the Alleghanian orogeny. The illitization is explainable by the fluid expulsion hypothesis recently proposed by others (e.g., Oliver). Hot saline fluids were flushed to the basin edges from the deeply buried part of the foreland basin during the orogeny; these fluids are thought to be effective agents of illitization. Some of our K-bentonite samples are stratigraphically and geographically close to Mississippi Valley-type lead-zinc deposits, suggesting a similar mode of origin. Rapid illitization of shales, in a fashion similar to that observed for the bentonites, should have led to high pore-water pressures that enabled them to act as ideal decollements during Alleghanian thin-skinned deformation.

Paleogeothermal and paleohydrologic conditions in silicic tuff from Yucca Mountain, Nevada

The clay mineralogy of tuffs from Yucca Mountain, Nevada, the potential site of the nation’s first high-level radioactive waste repository, has been studied in order to understand the alteration history of the rocks and to predict potential future alterations. Bulk-rock samples and clay-mineral separates from three drill holes at Yucca Mountain (USW G-1, USW G-2, and USW GU-3/G-3) were studied using X-ray powder diffraction, and supporting temperature information was obtained using fluid inclusion data from calcite. Twelve K/Ar dates were obtained on illite/smectite (I/S) separated from the tuffs from the two northernmost drill holes, USW G-1 and G-2. The predominant clay minerals in the Yucca Mountain tuffs are interstratified I/S, with minor amounts of chlorite and interstratified chlorite/smectite. The I/S reactions observed as a function of depth are similar to those observed for pelitic rocks; I/S transforms from R = 0 interstratifications through R = 1 and R ≥ 3 interstratifications to illite in USW G-2 and to R ≥ 3 I/S in USW G-1. The R = 0 I/S clays in USW GU-3/G-3 have not significantly transformed. K/Ar dates for the I/S samples average 10.4 my. These data suggest that the rocks at depth in the northern portion of Yucca Mountain were altered 10.0-11 my ago, soon after creation of the Timber Mountain caldera to the north. Both I/S geothermometry and fluid inclusion data suggest that the rocks at depth in USW G-2 were subjected to postdepositional temperatures of at least 275°C, those in USW G-1 reached 200°C, and rocks from USW GU-3/G-3 probably did not exceed 100°C. These data suggest that no significant hydrothermal alteration has occurred since Timber Mountain time, ~ 10.7 my ago.Estimates of the temperature of formation of illite/smectites yield probable stability limits for several minerals at Yucca Mountain. Clinoptilolite apparently became unstable at about 100°C, mordenite was not a major phase above 130°C, and analcime transformed to albite above 175°-200°C. It appears that cristobalite transformed to quartz at 90°-100°C in USW G-2 but must have reacted at considerably lower temperatures (and for longer times) in USW GU-3/G-3. The reactions with increasing depth appear coupled, and clinoptilolite and cristobalite disappear approximately simultaneously, supporting aqueous silica activity as a controlling variable in the clinoptilolite-to-analcime reaction. The reaction of clinoptilolite to analcime also coincides with the appearance of calcite, chlorite, and interstratified chlorite/smectite. Although the hydrothermal fluids may have been a source for some cations, breakdown of clinoptilolite (and mordenite) probably provided the source of some of the Ca for calcite, Mg for chlorite, K for the I/S found deeper in the section, and Na for analcime and albite.Using the rocks in USW G-1, G-2, and GU/G-3 as natural analogs to repository-induced thermal alteration suggests that the bulk of the clinoptilolite- and mordenite-bearing rocks in Yucca Mountain will not react to less sorptive phases such as analcime over the required lifetime of the potential repository. The zeolites in zeolite interval I, directly underlying the proposed repository horizon, may transform at the predicted repository temperatures. However, the reaction of clinoptilolite to analcime in interval I may require the transformation of all of the abundant opal-CT and glass to quartz, an unlikely scenario considering the unsaturated nature of these rocks and the predicted temperatures of <100°C.

Timing and Conditions of Permian Rotliegende Sandstone Diagenesis, Southern North Sea: K/Ar and Oxygen Isotopic Data

Illitic clay, ranging from pure illite to highly illitic illite/smectite (I/S), is the most abundant diagenetic phase in the eolian and sabkha facies of the Rotliegende Formation of the southern North Sea and northeastern Netherlands. Most K/Ar ages of diagenetic illite and I/S in the eolian sandstones are between 100 and 175 Ma. Illite formation is related to two major phases of tectonic activity, the Jurassic Kimmerian orogenic movements and Late Cretaceous-early Tertiary inversion movements. The burial depth of a sample at the time of diagenetic illite formation can be derived from the K/Ar age and the appropriate burial-history curve. Illite formed in different localities at different times and at different depths. Generally, diagenetic illite is more abundant in samples more deeply buried at the time of illite formation. The correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties. Specifically, we would expect reservoir permeability of the Rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the Late Jurassic through the Late Cretaceous. Calculated ^dgr18O values of most illite-forming fluids lie between 0 and 4^pmil (SMOW), indicative of meteoric or marine waters modified by interaction with rock. However, 18O-depleted fluids (as low as -5^pmil), indicative of a major meteoric component, are inferred to have been involved in the formation of illite in the Groningen gas field at a time when nearby sections of the Rotliegende were eroded and exposed at the surface.

Relatively old basalts from structurally high areas in central Iceland

Abstract This study reports K/Ar ages for basalts from four areas in central Iceland where erosion of structural highs has exposed stratigraphically older levels of the lava pile. The four areas are the Eyjafjordur regional anticline and the Tjornes horst in north Iceland and the Borgarnes and Hreppar regional anticlines in south Iceland. Three of the areas have their older plateau basalts within the range of at least 8.5–9.5 m.y. old. Only the Hreppar area does not have any exposed rocks much older than about 2.5 m.y. The Tjornes data confirm that the exposed Husavik faults have played a major role in the transform displacement of the Tjornes Fracture Zone. The results are further evidence that the spreading axes through Iceland have had a history of shifting their location. Analysis of our results suggests that the regional anticlines of Iceland, a seeming structural anomaly in a spreading regime, have resulted from shifting spreading axes which transitionally coexist and create regional anticlines in between.