Eldridge M. Moores

Southwest U.S.-East Antarctic (SWEAT) connection: A hypothesis

A hypothesis for a late Precambrian fit of western North America with the Australia-Antarctic shield region permits the extension of many features through Antarctica and into other parts of Gondwana. Specifically, the Grenville orogen may extend around the coast of East Antarctica into India and Australia. The Wopmay orogen of northwest Canada may extend through eastern Australia into Antarctica and thence beneath the ice to connect with the Yavapai-Mazatzal orogens of the southwestern United States. The ophiolitic belt of the latter may extend into East Antarctica. Counterparts of the Precambrian-Paleozoic sedimentary rocks along the U.S. Cordilleran miogeocline may be present in the Transantarctic Mountains. Orogenic belt boundaries provide useful piercing points for Precambrian continental reconstructions. The model implies that Gondwana and Laurentia rifted away from each other on one margin and collided some 300 m.y. later on their opposite margins to form the Appalachians.

Model for the origin of the Troodos massif, Cyprus, and other mideast ophiolites

Any comprehensive model for the origin of the Troodos complex and other Mideast ophiolite complexes must explain the geochemical evidence for subduction-zone involvement, the thin inferred oceanic crust, the extensional environment indicated by sheeted dikes, the existence of fault zones perpendicular to the inferred spreading axes, and the discontinuous nature of ophiolite exposures around the Arabian block. The Andaman Sea region of the Indian Ocean may provide an actualistic model for the origin of these ophiolites. There, spreading takes place in short segments above a subduction zone in the region of active spreading. The Andaman Sea model substantially accounts for the known geologic and geophysical data from the circum-Arabian ophiolite belt, and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt. The model also predicts that Andaman Sea spreading-center magmas may be similar in composition to those found in the Mideast ophiolites.

Spreading structure of the Troodos ophiolite, Cyprus

Orientations of dikes within the sheeted dike complex of the Troodos ophiolite reveal primary spreading structure produced at a complex ridge/transform intersection. Three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes. Faults flatten at depth into a detachment within the upper parts of the plutonic complex. Large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids. We suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north-trending (present coordinates), slow-spreading ridge crest. A simple model for ridge migration indicates eastward jumps of 8–13 km; changes in ridge orientation are suggested by changes in trends of dikes and graben axes. Comparison of the pattern of dikes near the Arakapas fault zone with the structure of active ridge/transform intersections suggests that the fault is a right-offset (sinistral) transform, in contrast to earlier models in which a ridge to the west of the exposed Troodos complex was proposed.

Pre–1 Ga (pre-Rodinian) ophiolites: Their tectonic and environmental implications

A global survey of sutures predating formation of the supercontinent Rodinia, i.e., older than ca. 0.9–1.0 Ga, has yielded evidence for at least 35 described and possible ophiolite complexes. Results indicate that ophiolitic complexes older than 1.0 Ga mostly lack mantle tectonites below the magmatic rocks. Geochemically, these pre-Rodinian ophiolites display a mix of supra-subduction zone (SSZ), mid-oceanic-ridge, and oceanic- island compositions, with higher proportions of komatiite in complexes older than 1.6 Ga. Magmatic ages of identified and inferred complexes tend to cluster at times of 1.0–1.5 Ga, 1.8–2.3 Ga, ca. 2.5–2.7 Ga, and ca. 3.4 Ga. The data mostly support the hypothesis that magmatic oceanic crust was thicker prior to 1.0 Ga and thinned abruptly at 1.0 Ga. Mesozoic oceanic plateaus, e.g., the Caribbean, may provide an analogue to pre–1 Ga oceanic crust. A modified ophiolite columnar section to accommodate pre–1 Ga complexes includes a “1972 Penrose Conference–type” section in which the igneous rocks are overlain by a thick accumulation of interlayered sedimentary and volcanic rocks, as well as high-level intrusive complexes and plutons. Thinning of oceanic crust, the end of the anorthosite-anorogenic granite “event’ at 1000 Ma, and discontinuous hotspot activity through time suggest discontinuous magmatic evolutionary processes in Earth history, similar to other planets. Differences in thickness and “subductability” of oceanic crust may provide clues to the Archean-Proterozoic tectonic transition. The stratigraphic and sea-level effects of the inferred 1 Ga thinning of oceanic crust include widespread continental emergence, development of seasons, and increased atmospheric oxygen leading to the Phanerozoic.

The Vourinos Ophiolite, Greece: Cyclic Units of Lineated Cumulates Overlying Harzburgite Tectonite

Re-examination of the Vourinos ophiolite shows it to be composed of metamorphic tectonites, cumulates, plagiogranites, dikes, and lava. The contact between the tectonites and the cumulates is exposed and sharp. Beneath the cumulate contact, the rocks have been highly deformed and complexly folded; above that contact, they simply have been tilted vertically to expose a stratiform complex 1,500 m (4,800 ft) thick. The stratiform intrusion is characterized by cyclic units, rich in olivine at the base, and rich in feldspar at the top. Some cumulus diorites are present at the top of the section which grade into quartz diorites (plagiogranites) with hypautomorphic textures. Lineate lamination characterizes the cumulates and may indicate the direction or orientation of the Mesozoic mid-oceanic ridge crest with respect to the present position of the complex. A siliceous dike swarm cuts the upper part of the stratiform complex. The section suggests that in the case of Vourinos, a large magmatic chamber formed at a mid-oceanic ridge crest and that intrusion was a much more important process than extrusion in the formation of oceanic crust in that area. The reported presence of cumulates in many other ophiolite complexes suggests that these relations may obtain generally at most or all spreading ridges. The contact between the tectonites and the cumulates of the complex would not have corresponded with seismic M.

Structure and tectonics of the northern Sierra Nevada

Recent detailed mapping suggests a new working hypothesis for the structure of the northern Sierra Nevada. We propose that pre-Cretaceous rocks are deformed by a series of eastward-directed over-thrusts modified by west-directed folds and faults. The highest and westernmost tectonic unit is the Jurassic Smartville complex. Structurally below it, there are imbricate thrust slices of Jurassic and older ophiolitic and oceanic sedimentary rocks in the Central belt and of Paleozoic to Jurassic sedimentary and volcanic rocks in the Eastern belt. The Paleozoic Feather River peridotite separates the Eastern and Central belts, and our hypothesis suggests that both of its margins may be important thrust faults. The east-directed faults and folds are deformed by northwest-trending, upright, and west-vergent folds and reverse faults that control the present outcrop pattern. These later structures have so modified the earlier east-directed structures that the latter have been recognized only recently. An important key to understanding the structure has been the recognition of ophiolitic complexes that can be correlated across major faults and that contain a pseudostratigraphy useful in determining local structures. The events recorded by both the east- and west-directed deformations occurred during Callovian through Kimmeridgian time and represent the Nevadan Orogeny. Our hypothesis of early east-directed overthrusts followed by west-directed back folding and faulting implies shortening and thickening of the crust during the Nevadan Orogeny and is consistent with the idea that the northern Sierra Nevada is the result of a crustal collisional process.

Neoproterozoic oceanic crustal thinning, emergence of continents, and origin of the Phanerozoic ecosystem: A model

Ophiolites emplaced in collisional orogens prior to 1 Ga generally have a thicker magmatic crust than younger complexes. This fact suggests a model in which Mesoproterozoic oceanic crust was two or three times as thick as at present, possibly owing to partial melting of a more fertile mantle. Earth had less continental freeboard and 90%-95% marine inundation, producing a much more uniform worldwide temperature distribution. Neoproterozoic oceanic crust thinning increased ocean basin volume and continental freeboard, and caused a drop of average sea level relative to the level of the continents. The resultant increased erosion, sediment deposition, upwelling, and consequent biological productivity and burial led to increased atmospheric oxygen, which permitted the evolutionary development of animals.

Quaternary blind thrusting in the southwestern Sacramento Valley, California

Patterns of microearthquakes and Quaternary surface deformation suggest that the tectonic setting of the SW Sacramento Valley is similar to areas of the western San Joaquin Valley known to be underlain by seismogenic blind thrust faults. On the basis of previous work and analysis of geologic and seismic reflection data, the following late Cenozoic tectonic features and processes are identified: (1) uplift of the northern Coast Ranges beginning approximately 3.4 Ma, and eastward propagation of uplift into the southwestern Sacramento Valley by 1.0 Ma; (2) uplift and homoclinal flexure of Plio-Pleistocene strata at the eastern Coast Ranges mountain front; (3) uplift and folding above blind thrusts approximately 15 km east of the mountain front in the southwestern Sacramento Valley. Similar associations of structures and processes have been observed in thrust belts in Pakistan, the Peruvian Andes, and the Canadian Cordillera and are commonly attributed to thrusting within an intercutaneous wedge or triangle zone. By using other thrust belts as analogs, the propagation of an eastward tapering triangle zone is interpreted to be the principal mechanism for uplift and homoclinal flexure at the eastern Coast Ranges mountain front. Seismic reflection profiles reveal that (1) the triangle zone consists primarily of east-vergent blind thrusts and (2) west-vergent backthrusts exposed in the the eastern Coast Ranges and southwestern Sacramento Valley are rooted in the east-vergent thrusts. Transfer of slip from the east-vergent blind thrusts to the west-vergent backthrusts occurs locally beneath the southwestern Sacramento Valley. Fault-bend folding in the hanging walls of the backthrusts has created a north-northwest striking chain of low hills approximately 15 km east of the mountain front. The folds deform 3.4–1.0 Ma fluvial sediments and thus are middle Pleistocene in age or younger. Local variations in strike suggest that the fold chain is segmented, like the New Idria-Coalinga-Kettleman Hills segmented fold chain in the southwestern San Joaquin Valley (Stein and Ekstrom, 1989). These data have implications for seismic hazard assessment. Anecdotal accounts indicate that two M = 6.0+ events of the 1892 Winters-Vacaville earthquake sequence probably occurred beneath the eastern Coast Ranges (Dale, 1977; Toppozada et al., 1981). Ground cracking was observed following the main shocks along the mountain front in the southwestern Sacramento Valley. We propose that the earthquakes were generated by slip on a blind thrust beneath the Coast Ranges, and that the ground cracking in the valley represents propagation of the eastward tapering triangle zone. The 1892 earthquake sequence suggests that blind thrusts beneath the southwestern Sacramento Valley are active and capable of generating moderate to large magnitude earthquakes.

Tertiary Tectonics of the White Pine–Grant Range Region, East-Central Nevada, and Some Regional Implications

The White Pine-Grant Range area is a 400 sq mile region located in east-central Nevada, east of the Antler erogenic front, and west of the Sevier-Laramide thrust belt. About 20,000 feet of Paleozoic rock are overlain by 3000 to 5000 feet of Eocene-Oligocene sedimentary and volcanic rocks, and 0 to 10,000 feet of Mio-Pliocene sedimentary rocks. The near concordance of Paleozoic and early Tertiary strata indicates that the major deformation of the area is post-Oligocene in age. Mesozoic tectonic movements were limited to the formation of gentle folds and high-angle faults of limited displacement. The structure of the area is dominated by north-trending folds, some overturned, which involve both Tertiary volcanic and Paleozoic rocks. These folds are cut by a series of low-angle faults emplacing younger rocks over older; six faults are present in the White Pine Range and two in the northern Grant Range. A series of complex discrete slide masses 1 to 3 miles long characterizes the southwestern Horse Range; a relatively unfaulted overturned Paleozoic section characterizes the southeastern Horse Range. Displacement along these faults increases from the center toward the ends of the ranges; the faults are not continuous throughout the length of the range, and probably do not represent features of a regional d ecollement. Relations suggest that deformation or movement on these faults was probably nearly surficial, partly a product of uplift of the ranges and partly the result of basement extension. Much observable movement took place during deposition of Mio-Pliocene sediments, accompanied by emplacement of gravity slide masses and monolithic breccia of Paleozoic and volcanic rock in the Mio-Pliocene sedimentary basin. Differences in intensity of folding and degree of low-angle faulting in the White Pine, Horse Range, and northern Grant Ranges suggest that these areas were deformed separately. The north-trending folds may be the result of gravity effects on the flanks of major uplifted blocks, but probably reflect a period of postvolcanic crustal shortening in this region.

Structural and geophysical expression of the Solea graben, Troodos Ophiolite, Cyprus

Field studies show that extensional structures in the north-central part of the Troodos Ophiolite, including steep normal faults, grabens, dikes, and low-angle detachment faults are related to E-W (present coordinates) spreading. The Solea graben, directly north of the mafic-ultramafic rock outcrops around Mount Olympus, consists of a 10- to 15-km-wide western portion that is underlain by a low-angle detachment fault between the sheeted dike complex and the plutonic section. Structural and paleomagnetic evidence indicates that the sheeted dikes above this detachment have been rotated 40° or more about subhorizontal axes. The eastern portion of the graben contains sheeted dike blocks with varying axes of rotation and significantly different style of intrusive activity. Most structural features in the graben formed simultaneously with or shortly after cessation of spreading at the graben axis based on cross-cutting ophiolitic dikes and hydrothermal mineralization. Timing of the uplift of the central part of the Troodos Ophiolite is uncertain but is probably related to the tectonic thinning of the upper crust by low-angle normal faulting. The Solea graben spreading center provides a model for the structure of modern mid-ocean ridges that are spreading amagmatically.