Tectonic Stages Research Articles

Metallogeny and tectonic development of the Tasman Fold Belt System in Queensland

The northern part of the Tasman Fold Belt System in Queensland comprises three segments, the Thomson, Hodgkinson- Broken River, and New England Fold Belts. The evolution of each fold belt can be traced through pre-cratonic (orogenic), transitional, and cratonic stages. The different timing of these stages within each fold belt indicates differing tectonic histories, although connecting links can be recognised between them from Late Devonian time onward. In general, orogenesis became younger from west to east towards the present continental margin. The most recent folding, confined to the New England Fold Belt, was of Early to mid-Cretaceous age. It is considered that this eastward migration of orogenic activity may reflect progressive continental accretion, although the total amount of accretion since the inception of the Tasman Fold Belt System in Cambrian time is uncertain. The Thomson Fold Belt is largely concealed beneath late Palaeozoic and Mesozoic intracratonic basin sediments. In addition, the age of the more highly deformed and metamorphosed rocks exposed in the northeast is unknown, being either Precambrian or early Palaeozoic. Therefore, the tectonic evolution of this fold belt must remain very speculative. In its early stages (Precambrian or early Palaeozoic), the Thomson Fold Belt was probably a rifted continental margin adjacent to the Early to Middle Proterozoic craton to the west and north. The presence of calc-alkaline volcanics of Late Cambrian Early Ordovician and Early-Middle Devonian age suggests that the fold belt evolved to a convergent Pacific-type continental margin. The tectonic setting of the pre-cratonic (orogenic) stage of the Hodgkinson—Broken River Fold Belt is also uncertain. Most of this fold belt consists of strongly deformed, flysch-type sediments of Silurian-Devonian age. Forearc, back-arc and rifted margin settings have all been proposed for these deposits. The transitional stage of the Hodgkinson—Broken River Fold Belt was characterised by eruption of extensive silicic continental volcanics, mainly ignimbrites, and intrusion of comagmatic granitoids in Late Carboniferous Early Permian time. An Andean-type continental margin model, with calc-alkaline volcanics erupted above a west-dipping subduction zone, has been suggested for this period. The tectonic history of the New England Fold Belt is believed to be relatively well understood. It was the site of extensive and repeated eruption of calc-alkaline volcanics from Late Silurian to Early Cretaceous time. The oldest rocks may have formed in a volcanic island arc. From the Late Devonian, the fold belt was a convergent continental margin above a west-dipping subduction zone. For Late Devonian- Early Carboniferous time, parallel belts representing continental margin volcanic arc, forearc basin, and subduction complex can be recognised. A great variety of mineral deposits, ranging in age from Late Cambrian-Early Ordovician and possibly even Precambrian to Early Cretaceous, is present in the exposed rocks of the Tasman Fold Belt System in Queensland. Volcanogenic massive sulphides and slate belt-type gold-bearing quartz veins are the most important deposits formed in the pre-cratonic (orogenic) stage of all three fold belts. The voicanogenic massive sulphides include classic Kuroko-type orebodies associated with silicic volcanics, such as those at Thalanga (Late Cambrian-Early Ordovician. Thomson Fold Belt) and at Mount Chalmers (Early Permian New England Fold Belt), and Kieslager or Besshi-type deposits related to submarine mafic volcanics, such as Peak Downs (Precambrian or early Palaeozoic, Thomson Fold Belt) and Dianne. OK and Mount Molloy (Silurian—Devonian, Hodgkinson Broken River Fold Belt). The major gold—copper orebody at Mount Morgan (Middle Devonian, New England Fold Belt), is considered to be of volcanic or subvolcanic origin, but is not a typical volcanogenic massive sulphide. The most numerous ore deposits are associated with calc-alkaline volcanics and granitoid intrusives of the transitional tectonic stage of the three fold belts, particularly the Late Carboniferous Early Perman of the Hodgkinson—Broken River Fold Belt and the Late Permian—Middle Triassic of the southeast Queensland part of the New England Fold Belt. In general, these deposits are small but rich. They include tin, tungsten, molybdenum and bismuth in granites and adjacent metasediments, base metals in contact meta somatic skarns, gold in volcanic breccia pipes, gold-bearing quartz veins within granitoid intrusives and in volcanic contact rocks, and low-grade disseminated porphyry-type copper and molybdenum deposits. The porphyry-type deposits occur in distinct belts related to intrusives of different ages: Devonian (Thomson Fold Belt), Late Carboniferous—Early Permian (Hodgkinson—Broken River Fold Belt). Late Permian Middle Triassic (southeast Queensland part of the New England Fold Belt), and Early Cretaceous (northern New England Fold Belt). All are too low grade to be of economic importance at present. Tertiary deep weathering events were responsible for the formation of lateritic nickel deposits on ultramafics and surficial manganese concentrations from disseminated mineralisation in cherts and jaspers.

On the tectonic events correlation in the northwestern Pacific Ocean and Asian continental margin

We consider the correlation of Cretaceous-Cenozoic tectonic events in the oceanic and continental parts of the northwestern Pacific. The first results are based presumably on the data about Sikhote Alin, Sakhalin, Japan, the Japan Sea and Okhotsk Sea, the Kuril-Kamchatka arc and trench, and the northwestern part of the Pacific Ocean. The subjects of discussion are the structural rebuildings at the age boundaries of 100–110 and 25–30 Ma, connected with the fundamental changes of the northwestern Pacific tectonic plan and replacing of the continent-ocean boundary to the east. When correlating the tectonic events we discovered not only the coincidence in time of the major structural rebuildings but also the affinity of the stress fields and counter movements of the ocean and continental margin structures, with some signs of compression on their boundary. These structural peculiarities, the data about the source regions in the sedimentation process, the preliminary comparison of paleolatitude data allow us to question that the relative movements of the Pacific and Asian structures may possibly be limited. Inside the Pacific plate we found some signs of the considerable dislocations and the layering of the lithosphere. For every tectonic stage we discuss the succession of tectonic events on the ocean-continent boundary which result in the continental crust formation. The connection of major moments of the structural rebuildings with the reorganisation of the lithospheric plates boundary is emphasized.

Post‐depositional history of the Willyama Supergroup in the Broken Hill Block, NSW

Abstract The Broken Hill Block, in far western New South Wales, comprises complexly folded and metamorphosed Proterozoic metasediments and metavolcanics of the Willyama Supergroup, and pre‐ and post‐folding intrusives. The history of the Block is described in four tectonic stages. The precratonic stage commenced with separation of crustal material from the mantle at 2100–2300 Ma. Deposition of the Willyama Supergroup occurred at about 1820 Ma. These rocks were metamorphosed to amphibolite‐granulite facies and deformed twice at about 1660 Ma, in the Olarian Orogeny. A third deformation with accompanying retrograde metamorphism occurred soon after. Before 1570 Ma, retrograde schist zones had formed, some with associated kyanite and staurolite bearing metamorphic assemblages. The transitional tectonic stage commenced at 1490 ± 20 Ma, with the emplacement of Mundi Mundi type granites and minor muscovite pegmatites. Between 1490 Ma and about 1100 Ma, the presently exposed rocks were elevated from 13 to 20 km b...

Geologic and geochronologic constraints on the evolution of the Red Sea-Gulf of Aden and Afar depression

New K-Ar age determinations show that the development of Southern Afar since 14 Ma ago involved five stages of tectonism and volcanism at 14-11, 11-10, 9-7, 5-4 and post-1.6 Ma ago; while the major phases of rifting for the Red Sea-Gulf of Aden have been shown by geological and geophysical data to be 25, 14, 10 and 4 Ma ago. It is suggested that since 14 Ma ago, identical periods of volcanism preceded by rifting have affected both areas.Continental breakup in the Red Sea region was initiated along large transcurrent faults followed by extension manifested by normal faulting, block tilting in the brittle crustal region and repeated dyke injection. Two large scale transcurrent faults were identified, the Marda (NW-SE) and the Ambo (ENE-WSW) which, it is suggested, controlled the trends of rifting in the Red Sea and Gulf of Aden, respectively. Structural and geological evidence indicate that these faults have had a relatively long history, since they were active in pre-Jurassic times. Because at least the Ambo fault cuts across the N-S trends of the late Proterozoic basement, its orientation must be primarily related to the Phanerozoic rift tectonics.This model avoids overlap of the Arabian plate with the Danakil and Aisha blocks because it involves only limited true oceanic crust. The new age data indicate that the initiation of the Aden and Red Sea rifts was not accompanied by active volcanism on land until the Oligocene-Miocene (30-5 Ma ago). Following that time, volcano-tectonic rifting continued without substantial hiati.

Linee generali dell’evoluzione paleogeografica dell’Italia meridionale nel quadro delle interazioni tra Africa ed Europa

Linee generali dell’evoluzione paleogeografica dell’Italia meridionale nel quadro delle interazioni tra Africa ed Eur0paq‘ LUIGI DAZZARO, GIOVANNI PALMENTOLA e LUIGI RAPISARDI Dipar!imem‘o dz’ Geologfa e Geofisicap. Uiziuersitri deg11'Sutd1', Bari SUMMARY The broad lines of evolution in Southern Italy during the various stages of African-European interaction, which started in the late Paleozoic, are described. During the Triassic continental rifting stage. some oi platforms, basins and intracontinental scblthas were formed running parallel to the rift axis with syrtsedimentary basic effusive activity. in the oceanic rifting stage, due to the drift in the ESE direction during thejutassic and the Lower Cretaceous. Africa moved relative to Europe by a leftwarcl transcurrent movement. This movement resulted into the formation of an oceanic rift and of the European and African continental margins, Due to the stretching of the African margin, :1 number of structural highs and lows were formed which developed into platforms and basins. During the Upper Cretaceous and the Eocene, when the North Atlantic Ocean opened, relative move- ment of Europe and Africa was reversed, and the oceanic basin was closed. Here the Europc4/ergcnt Aipinc range was built while sedimentation continued in platform and basin areas on the African Continent. Between the Oligocene and the Middle Miocene, Africa moved towards Europe with a mild counterclo- ckwise rotation and the African promontory was deformed by collision. The Alpine range was thrust over the Insubric domain and as these two elements approached the Apennine basins, the arenaceous debris was produced. This was followed by a narrowing of the most inland Apennine domains and by taphrogenesis of the external domains and of the foreland. Between the Upper Miocene and the Quaternary, there was a succesion of late tectogcnetic and neotecto- nic phases. The Tortonian tectonic phase stopped the sedimentation process in the lrpinian basin and a part of the Apennirte range was built. During the Mes.-ainian, at the time of the salinity crisis, evaporitic sediments‘ tion occurred in the outermost section of the lrpinian basin and in the inner part of the Apulian forelancl. The Attic tectonic stage reopened the Metliterrancan basin and this was followed by sedimentation until the latest important translations of the mesoplioccnic age. During the Upper Pliocene and Quaternary, the forecleep was filled up and provided the connection between foreland anrl chain; this in turn was subject to compres- sion and isostatic uplift in the external portion and lo rifting in the internal perithyrrenian belt. Concurrently with the above mentioned events. wich occurred in succession since the Upper Miocene. the Thyrrenittn Sea was formed. Geological. volcanological and geophysical data indicate that this sea results from increased African-European compression in the SSE-NNW direction. INTRDDUZIONE E SCliEl\/IA GEOLOGICO L’Appennino e una clelle catene circummecliterrartec formatcsi per l’intera- zione tra l’Africa e l’Europa. L'cvoluzione, prima distensiva c poi compressiva, del margine occidentale del promontorio africano (intcso nel senso di Channel et al. 19'/'9)(') ha portato alla formazione di piu unitit geograficostrutturali: favampaese, l'avanfossa, la catcna appenninica e l’arco calabropeloritarto. l*) Lavoro eseguito con i contrihuti l\Il.P.l., Foncli (10% e 40%. ‘ (‘l Questi Atttori intendono per Promontorio Africano 0 Adriatico, non solo l':1re:1 uttualmente occupata tiai Mare Adriatico, ma artche quella dclla fascia orngcnica (Appennino, Alpi meridionali, Dinaridi, Ellenidi).

Des rides paléogéographiques aux anticlinaux. Permanence des directions structurales dans la Meseta marocaine occidentale au cours du Paléozoïque / From paleogeographic ridges to anticlines. Permanence of structural lines in Moroccan Meseta throughout Paleozoic times

The regional repartition of chaotic fades and volcanic flows of Late Devonian-Lower Carboniferous age allows to reconstruct several ridges all around a subsident basin. During Hercynian movements, these ridges evolved into anticlines. The parallelism of these structures during sedimentary and tectonic stages suggests a movement, throughout Paleozoic times, of ancien basement fractures.

Rifting of the northern margin of the Australian continent and the origin of some microcontinents in Eastern Indonesia

Continental Australia is bounded on the east, south and west sides by passive margins, and the geological histories of these are well documented (Falvey and Mutter, 1981). The northern margin of the Australian continent is now an active collision margin. Its previous history as a passive margin has rarely been examined. This paper shows how the Late Palaeozoic to Mesozoic sequence which forms the northern margin of the Australian continent, in the island of New Guinea, is readily related to the tectonic stages of a rift-drift sequence. Rifting (start of breakup) began at about 230 m.y. ago at the Permian-Triassic boundary. The onset of seafloor spreading is marked by a post-breakup unconformity and ranges in age along the northern margin of the continent, from 185 m.y. in Papua New Guinea to 170 m.y. in Irian Jaya. From there the age of the post-breakup unconformity continues to young in a southwesterly direction along the western margin of the Australian continent reflecting the opening of the Indian Ocean off Western Australia. The timing of the onset of spreading in central Papua New Guinea is consistent with the timing of the initiation of spreading in the proto-Pacific ocean proposed by Nur and Ben Avraham (1977). By the end of the Jurassic the northern margin of the Australian continent faced a seaway which linked the proto-Indian and proto-Pacific oceans. This newly formed ocean was separated from the pre-existing oceans of the Neo-Tethys and Panthalassa by a screen of continents or microcontinents. The identity of this screen is discussed and it is suggested that part of it is preserved in the microcontinents of Eastern Indonesia.

Tectonic stages in the pre-Mesozoic evolution of the Carpathians

(1983). Tectonic stages in the pre-Mesozoic evolution of the Carpathians. International Geology Review: Vol. 25, No. 6, pp. 641-650.

Uppermost Jurassic unconformity in Hokkaido, Evidence for an early tectonic stage.

Uppermost Jurassic unconformity in Hokkaido, Evidence for an early tectonic stage.

Tectonic Elements of the Southern Part of the Gulf of California

Closely spaced seismic and bathymetric surveys in the southern province of the Gulf of California reveal a tectonically active and structurally complex plate boundary. While heat-flow measurements, seismicity, large sediment accumulations on continental margins, and the presence of oceanic crust in deep basins strongly suggest divergent plate motion, magnetic anomaly patterns are conspicuously absent in the Gulf as a whole. The rate of sediment accumulation (range: 6-1,000 m/m.y.), however, provides an alternative for the differentiation of two types of fault traces and suggests a process by which recently formed plate boundary zones evolve. Active faults manifest themselves as steep, elongate scarps delineating the active plate boundary zone. These fault traces appear to converge and diverge, for example in the Pescadero Basin complex, in a manner similar to the Southern California borderland. The floors of the intervening basins are flat and relatively sediment free. Relict faults, also elongate fracture zones, are distinguished from active traces in that they are completely filled with hemipelagic muds. The relict faults are located along both sides of the active plate boundary zone and subparallel to it. These features infer that plate motion is relatively rapid, thus explaining the paucity of sediment accumulation in actively spreading basins in spite of high sedimentation rates. Substantial plate boundary readjustment in short time periods is suggested by the periodic abandoment and subsequent infilling and attachment of formerly active transform faults to the more stable margins of the plates involved. The Gulf of California, therefore, represents a working model of the evolution of a plate boundary and its sedimentological consequences in the early tectonic developmental stage of a passive margin ocean basin such as the Atlantic.

Wrench related thrusting along a Mesozoic-Cenozoic continental margin: Antalya Complex, SW Turkey

Summary An imbricate thrust belt and a geometrically related wrench-fault terrain are reported from the Antalya Complex, SW Turkey. The deformation occurred between late Cretaceous and Miocene time along a former Mesozoic rifted continental margin. The thrusting appears to have been temporally and mechanically related to at least the early stages of wrench tectonics.

Evolution of lower proterozoic epicontinental deposits: Stromatolite-bearing Aravalli rocks of Udaipur, Rajasthan, India

Abstract The Aravalli rocks (> 2060 Ma old) which crop out around Udaipur, Western India, comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement, known as the Banded Gneissic Complex (ca. 2585 Ma old). The rocks have undergone a very low grade of metamorphism, and display a complex structure resulting from two major and several minor episodes of folding. There are two distinctly different ‘facies sequences’ in the Aravalli rocks, indicating deep-sea and nearshore shelf environments. The stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs (altered to greenschists) and quartizites with arkosic conglomerate. In the next sequence carbonates predominate in association with orthoquartzites, carbonaceous phyllites, phyllites, and stromatolitic rock-phosphate. The carbonate sequence passes upward into greywacke-phyllite-lithic arenite in the distal parts and conglomerate-arkose-orthoquartzite in proximal areas. The youngest sequence comprises orthoquartzite with silty arenite. The distribution of different facies, particularly that of dolomite with stromatolitic rock-phosphate, is controlled by sea-floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals. Sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault-controlled troughs along craton margins. Terrigenous debris was deposited in newly-developed troughs with contemporaneous volcanicity along trough margins. With the erasing of the ephemeral relief in the provenance, carbonate deposition was initiated. The environment encouraged algal growth and formation of stromatolitic rock phosphate. Carbonaceous phyllites developed in areas of restricted circulation. Rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke-phyllite and lithic arenite in the deeper parts of the epicontinental sea, and conglomerate-arkose-orthoquartizite in the marginal areas. The final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling. The nature of the terrigenous clasts indicates a predominantly granitic source of sediments. Supply of sediment was mainly from the continent to the east and partly from a landmass to the west. The cycle of sedimentation noted in the epicontinental Aravalli sea is broadly similar to the model of tectonic stages suggested by Krynine (1942).

The origin and history of the Kapuskasing structural zone, Ontario, Canada

Reexamination of geological, geophysical, and geochronological evidence suggests three principal conclusions. Firstly, the Kapuskasing line came into existence very shortly after the stabilization of the Superior Province. The upfaulting of strips of granulites and associated rocks, and probably the redistribution of densities in the lower crust responsible for the regional positive Bouguer anomaly, took place before the emplacement of the Matachewan dike swarm at ~2690 Ma. Secondly, relative movements along the line during this tectonic stage had an important sinistral transcurrent component and sinistral movements continued during the emplacement of the Matachewan swarm. Thirdly, displacements of various kinds took place on the Kapuskasing line during a number of later events spaced out over a period of more than 2000 Ma; some of their effects can be deduced from the relationship of regional dike swarms and of cratonic cover formations. These findings suggest that the Kapuskasing line did not originate as a rift structure. Analogies are seen with deep transcurrent shear zones of other continents.

Polymetamorphism of ores in Precambrian stratiform massive sulfide deposits at Ambaji-Deri, Western India

Massive stratiform zinc-lead-copper sulfide ores, in association with cordierite-anthophyllite rocks, occur in adjacent localities of Ambaji and Deri, in Western India. The metasedimentary country rocks, interlayered with amphibolites and intruded by acidic to intermediate plutonic rocks, belong to the Precambrian Delhi Supergroup. The ore minerals identified by detailed mineragraphic studies include: sphalerite, galena, chalcopyrite, pyrite, pyrrhotite (both monclinic and hexagonal phases), magnetite, ilmenite, rutile, arsenopyrite, molybdenite, cubanite, mackinawite, boulangerite, gudmundite, meneghinite, lautite, tenantite, native bismuth, native silver, chalcocite and covellite. The common sulfide-silicate schistosity in the ores, flowage of sulfide streaks and tails around rotated poikiloblasts and in their pressure shadow region developed during early folding (F1) and regional metamorphism of the rocks under green schist facies condition. These were superimposed by a pervasive hornfelsic fabric involving sulfides and silicates and including microfabrics due to annealing and grain growth in sulfides, during a subsequent phase of low pressure thermal metamorphism and related tectonism (F2). Finally certain deformation features and some uncommon fabrics like martensitic lamellae in galena and subgrains in sphalerite developed during a mild deformation episode (F3) in the waning stages of tectonism in the area. Compositional change in the ores during thermal metamorphism was minimal.

Stratigraphic Evolution of the Benue Trough and Its Implications for the Upper Cretaceous Paleogeography of West Africa

The main stages of tectonic evolution of the Benue Trough are well documented in the stratigraphic succession, basically comprising three depositional sequences: an Albian-Cenomanian pyroclastic, paralic, shallow marine, and fluviatile sequence corresponding to the graben and transitional tectonic stages; a Turonian-Coniacian paralic, marine and fluviatile sequence that resulted from downwarping and a consequent widespread marine transgression; and a Campanian-Maastrichtian paralic, marine and fluviatile offlap sequence which followed the Santonian compressional deformation episode that displaced the depositional axis westward. Sedimentation in the trough was strongly influenced by the Late Cretaceous eustatic rise of sea level. The paleogeography of the Benue Trough in the Late Albian was similar to that of the Gulf of Tajura of the Afar triple junction. Thus, the equatorial and South Atlantic Oceans were open at that time and an embayment projected into the Benue Trough. Paleobiogeographic evidence and ...

Evolution of Sverdrup Basin, Arctic Canada

Sverdrup basin, a structural depression near the northern margin of the North American craton, contains a succession up to 13,000 m thick of Lower Carboniferous to Tertiary, marine and nonmarine sedimentary rocks, basalt flows, and gabbro dikes and sills. The basin evolved in five phases: (1) late Paleozoic, when evaporites and marine muds were deposited in the axial region and carbonates and mature sands on the margins; (2) early Mesozoic, when great thicknesses of siltstone and shale accumulated in axial parts of the basin; (3) middle Mesozoic, when terrigenous clastic deposits accumulated slowly; (4) late Mesozoic, when clastic deposits widely overstepped former basin margins; and (5) late Mesozoic-Cenozoic, when the basin underwent three stages of tectonism (Eurekan orogeny). Some evaporite diapirs in the basin developed halokinetically, beginning probably in the early Mesozoic; others were generated during Tertiary folding and faulting. Mafic volcanism and intrusion occurred episodically at times of basin foundering. Depending on the availability of imported terrigenous detritus, phases of mafic activity (and foundering) were accompanied by widespread marine transgression or by vigorous basin filling. The thick sedimentary fill masks the fundamental character of the basin. If acutely starved depositional conditions had prevailed through its Paleozoic and Mesozoic history, Sverdrup basin in the latest Cretaceous would have been a small ocean basin with steep sides, a basaltic floor about 3,500 m deep, and a thin succession of deep marine sediments--fundamental attributes of some modern small ocean basins.

Development of main structures of the siberian platform: history and dynamics

The location of the regions with maximum subsidence within the Siberian platform and the age volumes of the structural stages have been determined in relation to the location and periods of development of the adjacent geosynclines. The western and eastern parts of the platform differ in the character of the structures and magmatism, in the volume and age of the structural stages, as well as in the asynchronism of the main tectonic stages, which is connected with the boundary location of the platform between the Atlantic and Pacific Ocean segments of the earth's crust. The Siberian platform sharply differs from other ancient platforms of the northern hemisphere by the intensity of magmatism and style of tectonic deformation in the Meso-Cenozoic, and it is similar in this respect to the southern ancient platforms. The subcrustal processes in the adjacent geosynclines have played a leading role in the development of the platform structure; but the release of the stresses during the epochs of the magmatism and rift formation took place directly on the platform itself.

Tectonique et structures post-paléozoïques en Manche occidentale

Recent seismic reflexion surveys (Géomanche I and II) have made it possible to draw up a new geological map of the western English Channel. The structural evolution of the basin can be better defined from the synthesis of available data. The structural pattern of the Mesozoic and Cenozoic cover results from the reactivating of ancient faults in the underlying basement: NE-SW (Caledonian), E-W (Hercynian), NW-SE (late Hercynian). The various tectonic stages which have shaped the structure of this basin are discussed; a new tectonic event has been observed between the Ypresian and the upper Lutetian.

Tectonic and Stratigraphic Evidence for Miocene Gulf of California: ABSTRACT

The correlation of clasts in Eocene river gravels in Baja California with the sources for the clasts in Sonora has indicated the necessity for the dilation of the northern Gulf of California depression prior to, or concurrent with, the 23-18 m.y. opening of the Basin Range province in Sonora. The appearance of an extensive, basin-filling seaway in Baja California and Sonora during the Miocene is consistent with a regional subsidence resulting from crustal extension. The presence of shelf-type lower Miocene marine sediments on both coasts of Baja California Sur, between Loreto and La Paz, suggests that the marine waters entered the early Gulf across a shallow, nontectonic seaway which opened to the Pacific Ocean. The recognition of the early Miocene seaway extending into the northern Gulf, coupled with a later (4-10 m.y.) opening of the modern Gulf, supports the idea of a Gulf formed in at least 2 tectonic stages. End_of_Article - Last_Page 795------------

Geology and tectonic setting of the Mule Creek caldera, New Mexico, U.S.A.

The 10-km diameter Mule Creek caldera is the youngest felsic eruptive center in the Mogollon-Datil volcanic field of southwestern New Mexico. The caldera forms a topographic basin surrounded by a raised rim. The caldera wall is well displayed on the south and west sides of the structure where it dips 20–30 degrees toward the center of the basin. Mudflow breccia fills the caldera and is banked up against the caldera wall. Post-caldera porphyritic quartz latite domes and flows crop out along the ring-fracture zone. The caldera is superimposed upon an older volcanic complex of flow-banded rhyolite and porphyritic andesite lava. The Mule Creek caldera probably originated by explosive eruption of about 10 km3 of pumice and ash, in part preserved in the matrix of the mudflow breccia. Periods of explosive volcanism during the deposition of mudflow breccia are documented by tuffaceous beds interbedded with the breccia. A thin rhyolite ash-flow sheet originated in the caldera and overlies the mudflow breccia. The youngest felsic rocks around the caldera are (1) domes and flows of crystal-rich porphyritic quartz latite of variable mineralogy, interpreted as a defluidized magma, and (2) widespread crystal-poor, flow-banded rhyolite, dated at 18.6 m.y., which is not directly related to the caldera sequence. The Mule Creek caldera and other volcanic features farther south represent the only documented overlap of felsic volcanism with early stages of Basin-Range tectonism in the Mogollon-Datil field.