Eocene Volcanism Research Articles

Kinematics and timing of three superposed extensional systems, east central Idaho: Evidence for an Eocene tectonic transition

Cenozoic crustal extension in east central Idaho began about 50 Ma and continues at present. Three distinct episodes characterize one of the longest intervals of Cenozoic extension yet documented in the North America Cordillera. Crosscutting relationships between NE striking normal faults and volcanic rocks, regional dike trends, and slickenline data indicate NW‐SE extension during peak Eocene volcanism about 49–48 Ma (episode 1). NE striking normal faults, with at most a few kilometers of offset, formed in an intraarc setting during rapid NE subduction of oceanic plates under the Pacific Northwest. North to NNW striking and west dipping normal faults, with offsets up to 10–15 km, formed during a younger middle Eocene to Oligocene basin‐forming event (episode 2). This newly documented episode was the most important extensional event in east central Idaho and began during the waning phases of Challis volcanism. WSW‐ENE to SW‐NE extension during episode 2 was nearly perpendicular to the extension direction during episode 1 and perpendicular to the grain of the Idaho‐Montana fold and thrust belt. The flip in extension direction between episode 1 and episode 2 is tightly constrained by 40Ar/39Ar age determinations to have taken place at the end of Eocene Challis magmatism about 46–48 Ma. I infer that plate boundary forces controlled the geometry of normal faults and dikes during episode 1, whereas internal stresses within previously thickened crust drove major SW to WSW directed extension during episode 2. A drop in convergence rates between the North American and Farallon plates between 59 Ma and 42 Ma (Stock and Molnar, 1988) may coincide with the onset of gravitational spreading during episode 2 and may also explain the abrupt end of Eocene magmatism in the Pacific Northwest. Miocene and younger SW dipping Basin and Range faults (episode 3) extended the region in a NE‐SW direction. Although faults formed during episode 2 and episode 3 are not parallel, slickenlines indicate only small changes in slip vector trends, suggesting little rotation of the extension direction in east central Idaho since 46 Ma.

Origin of Eocene-covered karst bauxites of the Transdanubian Central Range (Hungary): evidence for early Eocene volcanism

Fission track (FT) ages measured in detrital zircon crystals from Eocene-covered bauxites of the Transdanubian Central Range (TCR) appear to reflect the cooling age of the source rock(s). Thus, by means of FT-dating of zircons new information could be obtained to explain the problem of source rocks debated so far. Since the majority of the detrital zircon crystals proved to be euhedral, their typologic analysis permitted conclusions regarding the petrographic character of the source rocks as well. (1) In the light of the age results it is clear that the pyroclastics of a Pre-Middle Eocene volcanism contributed to the bauxite material which was deposited on the exposed karstified carbonate terrain. Andesitic tuffite layers intercalated in the upper part of the overlying Eocene sequence are traces of younger explosions of the same Paleogene volcanic event. Zircon grains found in the bauxite are morphometrically identical are morphometrically identical to those found in the younger Eocene pyroclastics of andesitic charater. (2) A euhedral zircon population of 210 to 260 million years years was also identified in some of the bauxite deposites studied. These crystals derive most probably from Middle Triassic trachyandesite tuffs of the Central Range, though anatectic granitic sources cannot be excluded. (3) Those zircon grains which are the oldest and fairly well rounded are throught to derive from the parametamorphic of Hercynian age of the surrounding areas. The bauxites of the TCR, though obviously of polygenetic origin, clearly received considerable amounts of pyroclasic contribution related to a Lower Eocene to Lower Lutetian volcanic event and this probably played an important role in the formation of the deposits

Implication of [formula omitted] dating of Early Tertiary volcanic rocks from the north-Chilean Precordillera

Since the Early Jurassic, subduction-induced volcanic activity in the north-Chilean Andes (21–23,5°S) has migrated 200–300 km eastward from the present coast (Jurassic magmatic arc) to the Western Cordillera (Miocene-Recent volcanic rocks). Due to the paucity of chronological data, however, the exact age and extent of various magmatic sub-belts within this area are not well constrained. In the Precordillera, a sequence of 1500–5000-m-thick red bed sediments is partly intercalated with and mostly covered by predominantly intermediate calc-alkaline tuffs and lavas. Presently, the age of this sequence is considered to be Upper Jurassic and/or Cretaceous. From this sequence eleven biotites and seven hornblendes were analyzed and yielded 40Ar 39Ar plateau ages of between 48 and 38 Ma, indicating Upper Eocene volcanic activity in the eastern part of the Precordillera. In contrast, volcanic activity in the western part of the Precordillera has been dated at 92 ± 2.5 Ma, indicating an earlier onset of volcanism and, therefore, sedimentation of the red beds. In the Late Eocene, the intense volcanism ceased in this area contemporaneous with decreasing relative convergence rate of the Nazca Plate and the South American continent. This final phase of Eocene volcanism coincided with a period of important deformation, the “Incaic phase”. Biotites from two ash-fall tuffs, separated by a distinct angular unconformity, yield identical ages of 38.50 ± 0.90 Ma and 38.45 ± 0.60 Ma. These dates provide the best age estimate of the “Incaic phase” in the North Chilean Precordillera.

The volcanic section at Nazas, Durango, Mexico, and the possibility of widespread Eocene volcanism within the Sierra Madre Occidental

Descriptions of volcanic rocks in the Sierra Madre Occidental of western Mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the Oligocene ignimbrite flare‐up. However, much less attention has been given to the pre‐Oligocene volcanic sequence beneath this ash flow blanket. At Nazas, Durango, on the eastern flank of the Sierra Madre Occidental, a well‐exposed section includes voluminous felsic volcanic rocks of both Eocene and Oligocene age. The oldest igneous rocks at Nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie Cretaceous limestones and apparently were deformed with them during Laramide tectonism. More commonly, the limestones are overlain by Tertiary volcanic rocks and continental clastic deposits. The Tertiary volcanic section is about 800 m thick, not including intercalated fanglomerates. The section records three distinct magmatic episodes: from 51 to 40 Ma, comprising felsic ash flow tuffs and intermediate lava flows and domes; a pulse at about 30 Ma, comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m; and from 24 to 20 Ma, comprising alkalic basalts. The Ahuichila Formation, a molasse‐type conglomerate up to 200 m thick that underlies the Tertiary volcanic section, was deposited during or just after the Laramide deformation. The Santa Inés Formation is a widespread fanglomerate, up to 110 m thick, that underlies the basalts and apparently accumulated during normal faulting. Eocene volcanism in the Nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes. A similar sequence of Eocene volcanic rocks has been mapped in central Chihuahua state, about 350 km to the northwest of Nazas. Felsic tuffs interbedded with intermediate volcanic rocks older than 40 Ma are also exposed at Tayoltita, Durango, about 200 km southwest of Nazas. These occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the Eocene than during the Oligocene in the Sierra Madre Occidental. Eocene volcanism was more typical of orogenic magmatic belts developed at continental margins, whereas the Oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension. In addition to these three areas, there are several other localities in western Mexico having volcanic rocks with reported ages between 40 and 53 Ma. It is possible that the volcanic field during the Eocene in Mexico was comparable in extent to that of the Oligocene. The Eocene volcanism in western Mexico was in part contemporaneous with the Challis volcanic episode of northwestern United States and its extension into western Canada. However, Eocene magmatism in the Pacific Northwest apparently developed in an extensional tectonic setting, whereas in Mexico no evidence for Eocene extensional deformation is known.

Bermuda and Appalachian-Labrador rises: Common non-hotspot processes?

Other than the Corner Rise-New England seamounts and associated White Mountains, most postbreakup intraplate igneous activity and topographic uplift in the western North Atlantic and eastern North America do not readily conform to simple hotspot models. For example, the Bermuda Rise trends normal to its predicted hotspot trace. On continental crust, Cretaceous-Eocene igneous activity is scattered along a northeast-trending belt ∼500-1000 km west of and paralleling the continent-ocean boundary. Corresponding activity in the western Atlantic generated seamounts preferentially clustered in a belt ∼1000 km east of the boundary. The Eocene volcanism on Bermuda is paired with coeval magmatism of the Shenandoah igneous province, and both magmatic belts are associated with northeast-trending topographic bulges—the Appalachian-Labrador Rise to the west and the Bermuda Rise (Eocene?) to the east. The above observations suggest the existence of paired asthenosphere upwelling, paralleling and controlled by the deep thermal contrast across the northeast-trending continental margin. Such convection geometry, apparently fixed to the North American plate rather than to hotspots, is consistent with recent convection models by B. Hager. The additional importance of plate-kinematic reorganizations (causing midplate stress enhancement) is suggested by episodic igneous activity ca. 90-100 Ma and 40-45 Ma.

The Sorvestsnaget Basin, Southwest Barents Sea

The Sorvestnaget basin lies to the south of Spitsbergen at the present continental margin of the Barents Sea platform. At present there is little subsurface well data in this area. However, extensive seismic surveys of exceptional quality indicate that a westward thickening wedge of Tertiary sediments is present, with a maximum thickness of some 6 km. The seismic data give a clear picture of the basin's structural and seismic stratigraphic framework, and indicates that strike-slip faulting had a marked influence on Tertiary basin development. Sedimentation possibly follows similar patterns to those observed on the Spitsbergen. The basal part of the Tertiary section is mainly represented by irregular high amplitude seismic facies thought to indicate the presence of late Paleocene to early Eocene volcanics. The Eocene was a period of tectonic quiescence with the development of a westward prograding passive margin wedge. The base of the Eocene sequence comprises a thick transparent seismic package interpreted to be a clay prone unit possibly with source rock potential. Seismic anomalies in this package suggest the presence of hydrocarbons. From Oligocene times through to the beginning of the Pliocene the area was affected by transpression with the resulting development of hanging basins with wedge-shapedmore » prograding fills. In several places at this time inversion occurred between these basins leading to the main structures at top Eocene level. The latest phase of evolution of the west Barents Sea was dominated by passive margin sedimentation in the form of a thick progradational wedge. Sedimentation of this wedge was strongly influenced by glacial processes.« less

Indus tsangpo suture zone in ladakh—its tectonostratigraphy and tectonics

The Indus Tsangpo suture zone in Ladakh lies between the Phanerozoic sequence of the Zanskar Zone of Tethys Himalaya in the south and Karakoram zone in the north. The five palaeotectonic regimes recognized in the suture zone are: The Indus palaeosubduction complex, the Ladakh magmatic arc, the Indus arc-trench gap sedimentation, the Shyok backarc and the Post-collision molasse sedimentation. The Ladakh magmatic arc, comprising intrusives of the Ladakh plutonic complex and extrusives of the Dras, Luzarmu and Khardung formations, owes its origin to the subduction of the Indian oceanic plate underneath the Tibet-Karakoram block. The Indus Formation, lower Cretaceous to middle Eocene in age, was laid down in a basin between the magmatic arc and the subduction complex. The Shergol and Zildat ophiolitic melange belts exhibit green-schist and blue-schist facies metamorphism and show structural geometry and deformation history dissimilar to that of the underlying and overlying formations. The melange belts and the flysch sediments of the Nindam Formation represent a palaeosubduction complex. The Shyok suture zone consists of tectonic slices of metamorphics of the Pangong Tso Crystallines, Cretaceous to lower Eocene volcanics and sedimentaries, together with ultramafic and gabbro bodies and molasse sediments. This petrotectonic assemblage is interpreted as representing a back-are basin. Post-collision molasse sedimentaries are continental deposits of Neogene age, and they occur with depositional contact transgressing the lithological and structural boundaries. Two metamorphic belts, the Tso Morari crystalline complex and the Pangong Tso Crystallines, flank to the south and north respectively of the Indus suture zone in Eastern Ladakh. Three generations of fold structures and associated penetrative (and linear) structures, showing a similar deformation history of both the metamorphic belts, are developed. The shortening structures developed as a result of collision during the postmiddle Eocene time.

Basement Rocks of Halmahera, Eastern Indonesia: Implications for the Early History of the Philippine Sea: ABSTRACT

The oldest rocks known on Halmahera, eastern Indonesia, are petrologically and chemically similar to supra-subduction ophiolites and include boninitic volcanics resembling those dredged from the Marianas forearc. The age of the ophiolitic rocks is unknown; in east Halmahera they are overlain by Late Cretaceous and Eocene volcanics and associated sediments. Similar volcanics form the basement of western Halmahera. Plutonic rocks intruding the ophiolite and associated metamorphic rocks also yield Late Cretaceous to Eocene radiometric ages. The petrology and chemistry of the igneous rocks indicate an island arc origin. These rocks are locally overlain by shallow-water Eocene limestones and all are overlain unconformably by Neogene sediments. The Halmahera basement rocks have many structural, petrological, and stratigraphic similarities to submarine plateaus of the southern and northern Philippine Sea and basement terranes of the eastern Philippines. The authors suggest that these similarities indicate the existence of an extensive region of Late Cretaceous and Eocene volcanism built upon probable Mesozoic ophiolitic basement. The resultant thickened crust was later fragmented by spreading in the West Philippine Sea Central Basin and backarc spreading in the Eastern Philippine Sea. It is difficult to reconcile the present distribution of these crustal fragments with a linear arc, but equally more » difficult to propose a simple alternative. A proto-Philippine archipelago, with short-lived arcs separated by small oceanic basins, may be the closest modern analog. The development of younger subduction zones has been influenced by the distribution of thickened crustal fragments as they have re-amalgamated since the Miocene. « less

Late Miocene Fanglomerates in Lower Member of Starlight Formation, Northern Portneuf Range, Idaho: ABSTRACT

The lower member of the late Miocene Starlight Formation in the northern Portneuf Range, southeastern Idaho, represents sedimentation influenced by both Basin-and Range faulting and eastern Snake River plain (SRP) volcanism. The lower Starlight contains at least 610 m of fanglomerates and thin, interbedded basalt flows. The formations lower half is interpreted as medial alluvial fan facies and consists of coarse tuffaceous heterolithologic conglomerate lenses and sheets, and interbedded immature tuffaceous pebbly lithic arenites. The fine-grained upper half of the member is interpreted as distal-fan facies. The sandy deposits of the distal facies are cut by numerous gravel-filled channels. On the basis of clast assemblages and preservation of fragile rhyolite tuff grains that could not survive long transport distances, these deposits were likely derived from nearby sources. In addition, paleocurrent data indicate a general westerly transport direction. Individual drainage systems apparently controlled sedimentation, as indicated by different clast assemblages at various locations. The dominant clast type in The Cove, southwest of the Blackfoot River, is Eocene andesitic volcanics. In outcrops 20 km south, Paleozoic and lower Mesozoic sedimentary clasts predominate. Chert pebble conglomerate clasts, found only in southern exposures, may have been derived from distant eastern outcrops of Cretaceous conglomerates.more » Alluvial fan deposits in the lower member of the Starlight Formation developed as a response to early Basin-and-Range uplift. However, they were also influenced by eastern SRP volcanism, as indicated by the presence of tuffaceous sediments and basaltic lava flows. The fan deposits likely originated from a nearby eastern source that is now covered by Tertiary volcanics of the Blackfoot laval field.« less

Geology and tectonics of the Ryukyu Islands

The Ryukyu Islands are divided morphologically and geologically into three island groups: north, central and south Ryukyus. North-central Ryukyu represents the geological continuation of the Outer Belt of southwest Japan, composed of Mesozoic-Eocene sedimentary sequences, whereas south Ryukyu is characterized by high-pressure metamorphic rocks, Eocene volcanics and limestone, and lower Miocene sediments. The geological and structural contrasts between north-central and south Ryukyus are conspicuous before the late Miocene transgression covered the whole area. The Ryukyu Islands have been established since then. The successive developments of the Goto-Tunghai-Senkaku basins since the Paleogene, of the Okinawa Trough since the Miocene, and of the grabens near the Island group since the Pliocene, signify a southeastward shift of the basins in relation to the activity in the granite diapir zones accompanied by volcanism.

Extension across the Eocene Okanagan crystal shear in southern British Columbia

Okanagan Valley follows a gently west-dipping crustal shear, across which the upper plate moved west above the lower plate during the middle Eocene. Matching the lower- and upper-plate rocks indicates about 90 km of offset. Mylonitic gneiss at the top of the lower plate records the Eocene extensional strain, registering the zone's evolution through progressively shallower depths and cooler temperatures. Middle Eocene volcanics, normal faults, and slide blocks demonstrate the upper-plate extension.

Seismic Stratigraphy, Sediments, and Basin History of Tonga Forearc Basin, Late Eocene to Pleistocene: ABSTRACT

Four seismic reflectors (A, B, C, V) define primarily unconformity-bounded sedimentary sequences. Basement (V) is a block-faulted surface, apparently of Eocene volcanics. Above this, an upper Eocene sequence (CV) mainly buries the fault topography, pinching out locally on fault-block and volcanic highs along the eastern side of the basin. This sequence includes volcaniclastics and, on paleohighs, shallow-water limestones. Overlying this is a widespread upper Oligocene-lower Miocene sequence (BC), which also thins and pinches out locally against the high eastern side of the basin. Volcaniclastics are common, but limestones may occur locally. Seismic interpretations indicate little faulting during deposition of this sequence; prominent lenticular bodies could be either sills or reefs. Sequence AB, of middle and late Miocene to early Pliocene age, is composed of volcanopelagics deposited when the Lau arc was active, adjacent to the Tonga platform. No volcanic centers are seen in this sequence in the forearc, but shallow intrusions are common. Major tensional faulting developed toward the end of this depositional cycle. The uppermost sequence (SBA), of late Pliocene to Pleistocene age, also comprises volcanopelagic sediments. The volcanics are derived from the nearby Tofua arc, which developed with the opening of the Lau back-arc basin. Doming and tensionalmore » faulting in the late Pliocene-Pleistocene raised parts of the Tonga forearc basin, allowing wide reef platforms to develop.« less

Paleomagnetism of the triassic nicola volcanics and geotectonics of the quesnellia subterrane of terrane I, British Columbia

The Triassic Nicola Volcanics are the oldest strata in the Quesnellia Subterrane of composite Terrane I which is located in the southcentral Cordillera of British Columbia. A paleomagnetic study using AF and thermal demagnetization methods of 563 speciments from 56 sites in three petrologically and tectonically distinct belts of the Nicola isolates two remanence components residing dominantly in hematite. The prefolding A component found after 560°C thermal cleaning gives a pole position of 11.3°W, 52.7°N ( δ p = 4.7°, δ m = 7.2°), and appears to be caused by alteration associated with major Early Jurassic intrusive events. The dominant postfolding B component is best defined by 565°C thermal cleaning and gives a pole of 303.0°W, 82.5°N ( δ p = 6.6°, δ m = 8.6°). It likely reflects alteration associated with Eocene volcanism. Combined with previous paleomagnetic data, these results indicate that Quesnellia was translated northward by ∼ 13° of paleoatitude along with the rest of Terrane I and rotated ∼ 45° clockwise during Late Cretaceous to Oligocene time with ∼ 7° of the translation and ∼ 15° of the rotation occurring during Eocene to Oligocene time.

Reef exploration in the East Sengkang Basin, Sulawesi, Indonesia

Reconnaissance seismic shot in 1971/72 showed a number of well defined seismic anomalies within the East Sengkang Basin which were interpreted as buried reefs. Subsequent fieldwork revealed that Upper Miocene reefs outcropped along the southern margin of the basin. A drilling programme in 1975 and 1976 proved the presence of shallow, gas-bearing, Upper Miocene reefs in the northern part of the basin. Seismic acquisition and drilling during 1981 confirmed the economic significance of these discoveries, with four separate accumulations containing about 750 × 10 9 cubic feet of dry gas in place at an average depth of 700 m. Kampung Baru is the largest field and contains over half the total, both reservoir quality and gas deliverability are excellent. Deposition in the East Sengkang Basin probably started during the Early Miocene. A sequence of Lower Miocene mudstones and limestones unconformably overlies acoustic basement which consists of Eocene volcanics. During the tectonically active Middle Miocene, deposition was interrupted by two periods of deformation and erosion. Carbonate deposition became established in the Late Miocene with widespread development of platform limestones throughout the East Sengkang Basin. Thick pinnacle reef complexes developed in the areas where reef growth could keep pace with the relative rise in sea level. Most reef growth ceased at the end of the Miocene and subsequent renewed clastic sedimentation covered the irregular limestone surface. Late Pliocene regression culminated in the Holocene with erosion. The Walanae fault zone, part of a major regional sinistral strike-slip system, separates the East and West Sengkang Basins. Both normal and reverse faulting are inferred from seismic data and post Late Pliocene reverse faulting is seen in outcrop.

Alkaline character of Eocene volcanism in the middle part of central Iran and its geodynamic situation

Alkaline volcanic rocks, sometimes feldspathoid bearing, appear in variable stratigraphic positions within the Eocene of the middle part of Central Iran. No magmatic zonation and no K2O enrichment in both time and space can be found in this volcanic sequence. These facts together with other geochemical and geological features of the Iranian Eocene provinces do not agree with a volcanism related to a subduction zone model.

The evolution of the Rio Grande Rise in the southwest Atlantic Ocean

The Rio Grande Rise, a major aseismic rise in the western South Atlantic Ocean, is composed of two portions with distinct morphologies and geological histories. The western portion of the Rise (WRGR) is a large elliptical bulge with its crest at a mean depth of about 2000 m. Numerous guyots protrude from its main platform. The crust underlying this portion of the Rise is older than 80 m.y.B.P. and was formed at a spreading center probably situated close to the sea level. A widespread volcanic event affected this region during the Eocene and generated several volcanic islands located at the center of the present WRGR. This Eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots. Much less is known about the eastern portion of the Rio Grande Rise (ERGR). It has a north—south trend, parallel to the present South Atlantic spreading center, is bounded by fracture zones, is in a conjugate position to part of the Walvis Ridge in the eastern South Atlantic and may represent an abandoned spreading center.

Provenance and Depositional Environments of Middle Eocene Canoe Formation, Big Bend National Park, Brewster County, Texas: ABSTRACT

The middle Eocene Canoe Formation contains the first sedimentologic evidence of local volcanism in the Big Bend region. Sediments comprising the formation's lower member, the Big Yellow Sandstone, were deposited by sandy braided streams which were scoured by ancient carbonate highlands and volcanic terranes to the west. The unit represents a continuation of the depositional styles and compositional trends recorded in the Paleocene and early Eocene strata of the region. In contrast, sediments comprising the upper, unnamed member of the Canoe Formation were deposited as a volcanic sediment apron on the fringes of the newly forming Chisos Mountains volcanic center. The sandstones (feldspathic litharenites and lithic arkoses) are dominated by volcanic rock fragments and, as s ch, document an abrupt change in depositional style and sediment composition brought about by the onset of local volcanism. A comparison of Canoe Formation and earlier Tertiary sediment compositions results in the delineation of distinct petrologic trends which record the tectonic evolution of the early Tertiary sediment source area. The Paleocene sediments of the area were derived primarily from ancient magmatic arcs in northeastern Mexico. With the onset of the Laramide orogeny in late Paleocene-early Eocene, a new source of sediment--newly uplifted carbonate highlands--was added. Local volcanism in the middle Eocene produced yet another source of sediment, lava flows, ash flow tuffs, and sand-size pyroclastic materials from the Chisos Mountain volcanic center. Rapid erosion of these materials produced volcanic sediment aprons such as the one described here. As regional volcanic activity increased, typic l Paleocene and early Eocene depositional styles may have been completely abandoned, especially in areas proximal to the volcanic centers. End_of_Article - Last_Page 521------------

Late Cretaceous and Eocene volcanism in the southern Line Islands and implications for hotspot theory

Rocks dredged from a seamount 100 km northwest of Caroline Island, at the southern end of the Line Islands chain, contain Late Cretaceous fossils associated with volcanic debris. This association is evidence for the existence of a reef-bearing volcanic edifice with a minimum age of Late Cretaceous, 70 to 75 m.y., near Caroline Island. With the discovery of this seamount, the known occurrences of Late Cretaceous, reef-capped, volcanic edifices now extend a distance of 2,500 km, from Deep Sea Drilling Project Site 165 to 100 km northwest of Caroline Island. The apparent synchroneity of Late Cretaceous volcanism over this distance argues against the proposition that a single hotspot of the Hawaiian-Emperor type produced the Line Islands chain. Biochronologic data from the Line Islands indicate that the chain is not the temporal equivalent of the Emperor chain. Volcanic edifices of Cretaceous age are now known to extend from the Line Islands through the Mid-Pacific Mountains to the Marshall Islands and the western margin of the Pacific plate from Japan to the Marianas. A volcanic event occurred in the southern Line Islands during middle Eocene time; Eocene sediments were engulfed and altered by a volcanic eruption. The occurrence of both Cretaceous and Eocene volcanism in the southern Line Islands indicates that the history of the Line Islands is similar to that of the Marshall Islands.

Provenance and Depositional Environments of Eocene-Oligocene Baca Formation, Catron County, New Mexico: ABSTRACT

The Baca Formation, which crops out discontinuously in east-west-trending exposures from Socorro, New Mexico, to the New Mexico-Arizona border, is composed of fluvial, alluvial-fan, and lacustrine deposits. It was deposited in a basin which formed during the Laramide orogeny in New Mexico and Arizona. Uplifts bordering the basin include the Defiance, Zuni, Lucero, and Sierra-Sandia, and the Mogollon highland. In the late Eocene, volcanism began in the Datil-Mogollon volcanic field in southwestern New Mexico and adjacent Arizona, south of the study area. Sediments were derived from adjacent uplifts and from the volcanic rocks. The Mogollon highland and the Zuni uplift supplied metamorphic, sedimentary, and plutonic detritus to the Baca Formation. There is a westward increase in volcanic detritus in the Baca Formation, and faunal evidence shows an early Oligocene, Chadronian, age for western Baca outcrops north of Quemado, younger than outcrops further east which are Eocene in age. With the beginning of Eocene volcanism, volcanic detritus was supplied to the Oligocene Baca sediments which were deposited in a coarse-grained meander belt. Other depositional environments identified include fine-grained meander belt, alluvial fan, and distal, braided alluvial plain. Flow direction was dominantly eastward. End_of_Article - Last_Page 568------------

Paleomagnetism, Rock Magnetism, and Aspects of Structural Deformation of the Butte Mining District, Butte, Montana

Previously, field relations in the Butte District, Montana, have been interpreted to indicate a regional, 40-60° or greater westward tilting of the Late Cretaceous Butte quartz monzonite (BQM), the host rock to mineralization. The tilting was inferred to be Eocene or younger in age, based upon structural features in the overlying Eocene Lowland Creek Volcanics. Extensive paleomagnetic data reveal a more complicated picture. For surface exposure BQM, the in situ mean direction of characteristic magnetization of 13 site means is D = 1.5°, I = 73.3° (K = 27.6, $$\alpha_{95} = 2.6^{\circ}$$). Characteristic directions from BQM affected by early mineralization and high-temperature potassic alteration (antipodes) and two sets of quartz porphyry dikes, which pre-and post-date this mineralization, coincide with results from the surface and Berkeley Pit exposures of BQM. Studies of the magnetic oxides, along with the results of alternating field and thermal demagnetization, indicate a primary thermoremanent origin for the characteristic directions from all units except the high-temperature altered BQM, where a secondary thermoremanent magnetization was imparted. District-wide tilting of the magnitude visualized in the past is precluded by the fact that characteristic magnetization directions of Late Cretaceous units in the Butte district correspond closely to those of time-equivalent units elsewhere in North America. Systematic regional or non-systematic local tilting of as much as 15° could have occurred, but movements of 40-60° or more are ruled out for the BQM units analyzed. The Eocene volcanics overlying the BQM present an interesting tectonic enigma. Immediately northwest of Butte, the units dip steeply to the west or northwest in a graben-like structure; farther west or northwestward, the dips become more shallow. When compared with the predicted Early Tertiary field direction, the data for these units suggest that they have indeed been rotated westward by 30-50° or more about an approximately north-south axis. The immediately underlying BQM is unexposed, but inliers (probably horsts) of BQM surrounded by the volcanics within the graben give characteristic directions identical to those of untilted BQM elsewhere in the district. The discrepancy must relate to movements within the Eocene volcanic units after the magnetic carriers had acquired a stable thermoremanent magnetization, possibly during differential compaction and slumping. The study supports long-standing interpretations that the zonal patterns of mineralization and hydrothermal alteration in BQM have not been extensively reoriented by tectonic tilting.