Mesozoic Tectonic Events Research Articles

The Mesozoic Tectonic Dynamics and Chronology in the Eastern North China Block

Mesozoic tectonic events in different areas of the eastern North China Block (NCB) show consistency in tectonic time and genesis. The Triassic collision between NCB and Yangtze results in the nearly S-N strong compression in the Dabie, Jiaodong, and west Shandong areas in Middle Triassic-Middle Jurassic. Compression in the Yanshan area in the north part of NCB was mainly affected by the collision between Mongolia Block and NCB, as well as Siberia Block and North China-Mongolia Block in Late Triassic-Late Jurassic. However, in the eastern NCB, compressive tectonic system in Early Mesozoic was inversed into extensional tectonic system in Late Mesozoic. The extension in Late Mesozoic at upper crust mainly exhibits as extensional detachment faults and metamorphic core complex (MCC). The deformation age of extensional detachment faults is peaking at 120–110 Ma in Yanshan area and at 130–110 Ma in the Dabie area. In the Jiaodong area eastern to the Tan-Lu faults, the compression thrust had been continuing to Late Mesozoic at least in upper crust related to the sinistral strike slipping of the Tan-Lu fault zone.The extensional detachments in the eastern NCB would be caused by strong crust-mantle action with upwelling mantle in Late Mesozoic.

U‐Pb Dating of Volcanic Rocks and Granites along the Wuyishan Belt: Constraints on Tuning of Late Mesozoic Tectonic Events in Southeast China

Abstract:Five volcanic rock samples and two granite samples taken from the volcanic basins in western Fujian and southern Jiangxi were dated by using the zircon laser albation‐inductively coupled plasma mass spectrometry U‐Pb method. Together with previously dated ages, the dates obtained provide important constraints on the timing of late Mesozoic tectonic events in SE China. The volcanic rock samples yield ages of 183.1±3.5 Ma, ca. 141 Ma to 135.8±1.1 Ma, 100.4±1.5 to 97.6±1.1 Ma, confirming three episodes of late Mesozoic volcanic activities, which peaked at 180±5 Ma, 140±5 Ma and 100±5 Ma, respectively, along the Wuyishan belt. Moreover, based on field investigations of these volcano‐sedimentary basins, we have recognized two compressional tectonic events along this belt The early one was characterized by Upper Triassic to Middle Jurassic NNE‐trending folds that were intruded by late Jurassic granites; and the late one caused the Lower Cretaceous volcano‐sedimentary layer to be tilted. The dated age 152.9±1.4 Ma of the granitic samples from the Hetian granitic pluton in the Changting Basin and that from the Baishiding granitic pluton, 100.2±1.8 Ma, in the Jianning Basin, give the upper boundaries of these two tectonic events respectively. Hence, the late Mesozoic tectonic evolution of SE China was alternated between extension and compression.

Prelithified deformation of the Jurassic Bansong Group along the Gongsuwon Thrust, South Korea

The Gongsuwon Thrust in the Taebaeksan Basin is one of the important regional structures for the Phanerozoic tectonic evolution of the Korean Peninsula. The late Early Jurassic Bansong Group (Daedong Supergroup) is structurally overlain by the early Paleozoic Joseon Supergroup along the thrust in the Yeongwol area. We present four lines of evidence indicating that the siliciclastics of the Bansong Group suffered a prelithified deformation within the fault zone of the Gongsuwon Thrust; (1) strainfree quartz clasts both in the hinge and limb areas of isoclinal folds of the sandstone layer, indicating granular flow, (2) intact gravels in the conglomerate with pressure shadows filled by sandstone or shale matrix, (3) injection of mud into sandy layers, and (4) lensoidal marble blocks (Joseon Supergroup) with plastically deformed calcite grains within siliciclastics of the Bansong Group. We suggest that the deposition of the Bansong Group and the thrusting were coeval during the middle Mesozoic Daebo tectonic event.

Aptian basaltic andesites in the Amur-Zeya depression: First geochemical and 40Ar/39Ar geochronological data

The eastern margin of Asia hosts numerous Mesozoic‐Cenozoic depressions or sedimentary basins: Amur‐Zeya, Bureya, Torom, Uda, Middle Amur, Alchan, and others (Fig. 1). Their formation is closely related to general geological development of the continental margin. Therefore, concepts on their origin were transformed in accordance with the development of concepts on the regional geodynamic evolution. The great body of geological, geochemical, geochronological, paleontological, and paleomagnetic data accumulated in recent years has provided insight into the Mesozoic and Cenozoic tectonic events that produced the present-day structure of East Asia. At the same time, geochronological and geochemical data on volcanic rocks in continental sedimentary basins are scarce, although precisely such information is crucial for understanding the origin of these structures. In the present communication, we discuss geochemical and 40 Ar/ 39 Ar geochronological data obtained for volcanics of the Amur‐Zeya Depression (AZD), which is among the largest structures of this kind in the Russian Far East.

Age of oceanic basalts of Bol’shoi Lyakhov Island (New Siberian Islands): Implication for the western boundary of the South Anyui ocean in the Jurassic

Serpentinites and pillow basalts exposed in the southeastern part of Bol’shoi Lyakhov Island are considered the northwestern continuation of the South Anyui suture of Chukotka [1, 2]. However, some researchers doubt that these rocks represent the lithosphere of the Mesozoic South Anyui oceanic basin. According to the rotation hypothesis of the opening of the Amerasian basin, it should be confined to Chukotka structures and not extend to the west [3]. Previous data on the inferred Paleozoic age of the oceanic pillow basalts exposed on the island [4] suggest that the northwestern continuation of the suture is the Paleozoic collisional zone, which lacks a direct relation to the Mesozoic tectonic events in the East Arctic. We present new data on the Middle‐Late Jurassic age of the oceanic basalts of Bol’shoi Lyakhov Island. Consequently, they formed precisely in the South Anyui oceanic basin. These data are of great importance for reconstructing the Mesozoic tectonic evolution of the East Arctic region and for discrediting the validity of the popular rotation hypothesis of the opening of the Amerasian oceanic basin. The southeastern part of Bol’shoi Lyakhov Island comprises diverse complexes of oceanic and island-arc (basic and ultrabasic) rocks [4, 5]. These complexes make up a series of allochthonous slices thrust onto the southeastern margin of the New Siberian continental block. The outcrops of the aforementioned rocks are considered to mark the northwestern continuation of the South Anyui suture—one of the most important tectonic sutures in the East Arctic region (Fig. 1) [1, 6, 7]. The South Anyui suture is a trace of the eponymous oceanic basin, which closed in the Neocomian simultaneously with opening of the Amerasian oceanic basin [1, 6, 8]. A composite collage of island-arc and continental terranes is located south of the suture [6], while the East Arctic continental massif is located north of the suture. Some geologists (for instance, [3]) believe that the eastern part of this massif (Chukotka microcontinent according to [6]) was a fragment of the North American Platform, while its western part was a continuation of the Siberian structures. These two blocks were separated by a hypothetical transform fault, which was formed during opening of the Amerasian basin

Termination of the Permo-Triassic Songrim (Indosinian) orogeny in the Ogcheon belt, South Korea: Occurrence of ca. 220 Ma post-orogenic alkali granites and their tectonic implications

The ‘Indosinian Orogeny’ is defined in a broad sense as encompassing all the Late Paleozoic to Early Mesozoic tectonic events associated with the closure of the eastern parts of the Paleo-Tethys Ocean. It includes the Permo-Triassic continent–continent collision between the North China and South China blocks that generated the Qinling–Dabie–Sulu orogen containing the largest ultrahigh-pressure (UHP) terrane in the world. The Korean Peninsula, situated at the easternmost end of the Tethyan tectonic regime, was also tectonically influenced by the Indosinian Orogeny, often known as the ‘Songrim Orogeny’ in Korea. The Songrim Orogeny, characterized by intense regional medium- to high-pressure metamorphism, extends from the Imjingang belt through the Gyeonggi massif to the Ogcheon belt. It was initiated as early as the Early Permian Period (ca. 290 Ma) and peaked in the Triassic Period (ca. 255 to 225 Ma). This paper presents new geochemical and SHRIMP U–Pb zircon ages for two alkali granites that postdated the Songrim Orogeny in the Ogcheon belt. Both granites are contemporaneous (219.3 ± 3.3 Ma and 219.6 ± 1.9 Ma) and are geochemically post-orogenic A-type granites intruded in an extensional setting subsequent to a major collision. The data provide a critical time constraint on the termination of the Indosinian Orogeny in the Korean Peninsula. Despite the absence of an UHP terrane in the Korean Peninsula, similarities in the duration, metamorphic characteristics, and post-orogenic magmatism between the Qinling–Dabie–Sulu and Songrim orogens, as parts of the broad Indosinian orogenic regime, are clearly alike, suggesting a genetic linkage in their time–space characteristics.

Fission-track-age records of the Mesozoic tectonic-events in the southwest margin of the Ordos Basin, China

Based on the analysis of apatite and zircon fission track (FT), the FT age distribution and the peck-ages of the Mesozoic tectonic events in the southwest margin of the Ordos Basin (OB) were discussed. (1) The early event mainly occurred at 213–194 Ma with a peck-age of 205 Ma, and corresponded to the tectonic uplift and the mega-clastic deposit in the southwest OB during the Late Triassic. (2) The middle event included at least two episodes. One was at 165–141 Ma with a peck-age of 150 Ma, and the other was at 115–113 Ma with a peck-age of 114 Ma, corresponding to the over-thrusting and the mega-clastic deposits in the southwest OB during the Late Jurassic to the Early Cretaceous. (3) The late event mainly demonstrated the regional uplifting and included at least two episodes. One was at 100–81 Ma with a peck-age of 90 Ma, and the other was at 66–59 Ma with a peck-age of 63 Ma. Additionally, the relationship analysis of the tectonic event and the mineralizing chronology revealed that the extreme environment of the peck-age event and the subsequent moderate activity could be the key factors of the multiple resources coupling and coexistence in the OB.

Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin

The northeastern South China Sea continental margin holds the key to understanding Late Mesozoic tectonics and evaluating hydrocarbon potentials in Mesozoic tectonic and stratigraphic structures offshore southeast China. With newly obtained and processed seismic data, and new drilling and logging data, we correlate regional Mesozoic stratigraphy and analyze major Mesozoic tectonic events and structures. In particular, we focus our study on the three major tectonic units in the area, the Chaoshan Depression, the Tainan Basin, and the Dongsha–Penghu Uplift, which are separated by basement high, thrust fold, and (or) faults. Stratigraphic correlations suggest a major phase of southeastward regression, spanning in time from the late Early Jurassic (∼180 Ma) to the Early Cretaceous (∼120 Ma). Seismic data reveal two major tectonic events, with the first one in the Late Jurassic to the Early Cretaceous, contemporary with the regression, and the second one in the Late Cretaceous. Regional magnetic anomaly map after the reduction to the pole clearly reveals the boundary between the Dongsha–Penghu Uplift and the Chaoshan–Tainan depositional system. The seismic and magnetic data also suggest that, while the Dongsha–Penghu Uplift has highly magnetized sources buried mostly in the upper crust at depths from about 2 km to about 20 km, the Chaoshan–Tainan depositional system has thick Mesozoic sediments of low magnetization.

Geologic and geophysical evidence for the influence of deep crustal structures on Paleozoic tectonics and the alignment of world-class gold deposits, north-central Nevada, USA

Geologic data concur with geophysical and isotopic data that suggest the presence of deep crustal fault zones along the Battle Mountain-Eureka (BME) trend and elsewhere in Nevada. The fault zones may have originated during Proterozoic rifting of the continent and were likely substantially reactivated and modified during Paleozoic tectonism. Five distinct Paleozoic structural and stratigraphic domains are defined that demonstrate the complexity of Paleozoic tectonic events and also lead to hypotheses about ways in which the margin could have been modified. The current locations of these domains adjacent to the geophysically and isotopically defined indicators of the buried continent edge corroborate their interactions with the continental margin. During the Tertiary, preexisting crustal fault zones were intersected and reopened during episodes of extension and served as the conduits for deep-sourced, gold-rich fluids, which were disseminated into Paleozoic slope facies sedimentary rocks, forming sediment-hosted Carlin-type and other deposits. Multiple factors including the locations of these deep-seated structures, the original configuration of the lower Paleozoic continental margin of Nevada, and its subsequent reactivation during the Paleozoic all were fundamental controls on the location of younger mineral deposits. A clearer understanding of the original configuration of the margin and of the effects of subsequent Paleozoic and Mesozoic tectonic events on the margin would provide insight into the locations of these and other prospective mineral belts.

A Large-Scale Cluster of Gold Deposits and Metallogenesis in the Eastern North China Craton

The eastern North China craton contains the largest association of gold deposits in China. Hundreds of gold deposits of different scales, commonly in groups or belts, constitute seven subclusters. In contrast, there are no industrial gold deposits in the neighboring areas of the UHP Sulu region and the northern Yangtze craton. The host rocks of the gold deposits are chiefly Precambrian high-grade metamorphic rocks and granites (anatectic melts derived from Precambrian metamorphic rocks). Traditionally the gold deposits were considered to be of the Archean greenstone type. However, recent geochronology of granitoid bodies and veins related to gold mineralization and gold-bearing minerals in ore lodes reveals that the main metallogenic episode was 110-130 Ma in all seven subclusters. Geochemical data also indicate that the metallogenic materials mainly came from the Precambrian basement of the North China craton and its underlying mantle. Sr-Nb-Pb and S-O-H isotopic data of the ore lodes show crust-mantle mixing characteristics, corresponding to the metamorphic host rocks and basic-alkaline dikes. The main structures controlling the mineralization are Mesozoic faults that trend roughly NE-ENE and NW. Therefore, Precambrian rocks and Mesozoic magma-tectonic events provide fundamental controls on the mineralization. Problematic relationships between the Mesozoic tectonic events (Yanshanian movements) and large-scale metallogenic activity have attracted substantial attention from Chinese and foreign geologists. Mesozoic tectonic inversion in eastern China mainly involved a thick lithosphere that thinned at depth and a reconstructed basin-and-range structure at the surface, which resulted from a catastrophic lithosphere-asthenosphere event. This tectonic event led to a large-scale mantle upwelling, which induced disturbance and adjustment of thermal, density, and chemical phenomena, resulting in large-scale crustal remelting (especially the lower crust), material exchange and mixing between mantle and crust, movement and circulation of fluids, and finally a new magma-fluid-mineralization system.

40Ar-39Ar thermochronology of two ductile shear zones from Yiwul�shan, West Liaoning Region: Age constraints on the Mesozoic tectonic events

Two ductile shear zones trending EW and NNE respectively not only controlled the tectonic framework of the northern North China, but also constrained the geodynamic background for gold mineralization in this region. Field observations and microstructural analyses reveal that the EW trending ductile shear zones are mainly contributed to dextral compressional deformation resulting from top-to-the-southeast oblique thrust shearing, whereas the NNE trending ones are genetically related to sinistral strike-slip and extensional faulting. One sample from the former yielded an 40Ar–39Ar plateau age of (219± 4) Ma (Bi) and two samples from the latter gave 40Ar–39Ar plateau ages of (116± 2) Ma (Bi) and (127±3) Ma (Bi). These ages provide constraints on the top-to-the-southeast oblique thrusting event occurring in Late Triassic and the sinistral extensional and strike-slip faulting event which occurred in Early Cretaceous.

Paleozoic and Mesozoic Evolution of East-Central California

East-central California, which encompasses an area located on the westernmost part of sialic North America, contains a well-preserved record of Paleozoic and Mesozoic tectonic events that reflect the evolving nature of the Cordilleran plate margin to the west. After the plate margin was formed by continental rifting in the Neoproterozoic, sediments comprising the Cordilleran miogeocline began to accumulate on the subsiding passive margin. In east-central California, sedimentation did not keep pace with subsidence, resulting in backstepping of a series of successive carbonate platforms throughout the early and middle Paleozoic. This phase of miogeoclinal development was brought to a close by the Late Devonian-Early Mississippian Antler orogeny, during the final phase of which oceanic rocks were emplaced onto the continental margin. Subsequent Late Mississippian-Pennsylvanian faulting and apparent reorientation of the carbonate platform margin are interpreted to have been associated with truncation of the continental plate on a sinistral transform fault zone. In the Early Permian, contractional deformation in east-central California led to the development of a narrow, uplifted thrust belt flanked by marine basins in which thick sequences of deep-water strata accumulated. A second episode of contractional deformation in late Early Permian to earliest Triassic time widened and further uplifted the thrust belt and produced the recently identified Inyo Crest thrust, which here is correlated with the regionally significant Last Chance thrust. In the Late Permian, about the time of the second contractional episode, extensional faulting created shallow sedimentary basins in the southern Inyo Mountains. In the El Paso Mountains to the south, deformation and plutonism record the onset of subduction and arc magmatism in late Early Permian to earliest Triassic time along this part of the margin. Tectonism had ceased in most of east-central California by middle to late Early Triassic time, and marine sediment deposited on the subsiding continental shelf overlapped the previously deformed Permian rocks. Renewed contractional deformation, probably in the Middle Triassic, is interpreted to be associated with emplacement of the Golconda allochthon onto the margin of the continent. This event, which is identified with certainty in the Sierra Nevada, also may have significantly affected rocks in the White and Inyo Mountains to the east. Subduction and arc magmatism that created most of the Sierra Nevada batholith began in the Late Triassic and lasted through the remainder of the Mesozoic. During this time, the East Sierran thrust system (ESTS) developed as a narrow zone of intense, predominantly E-vergent contractional deformation along the eastern margin of the growing batholith. Activity on the ESTS took place over an extended part of Mesozoic time, both before and after voluminous Middle Jurassic plutons, and is interpreted to have been mechanically linked to emplacement of the batholith. Deformation on the ESTS and magmatism in the Sierra Nevada both ended prior to the close of the Cretaceous.

Timing, style and sedimentary evolution of Late Palaeozoic-Mesozoic extensional basins of East Greenland

Abstract The late Palaeozoic-Mesozoic sedimentary basins of East Greenland record a complex series of events which eventually led to the successful opening of the Norwegian-Greenland Sea in the late Palaeocene. Major tectonic events and regional sea-level changes caused by plate movements and reorganizations, overprinted by the effects of local tectonics and associated sea-level changes are reflected in thick, unconformity bounded sequences. Caledonian crustal shortening and thickening culminated in the Silurian, and was succeeded by extensional collapse, vulcanism and intrusion of post-tectonic granites in the Devonian. Large, transtensional pull-apart basins were formed and became filled with continental red beds. Basin margins underwent repeated episodes of thrusting. A change to rifting took place in the latest Devonian. Rapid subsidence and continental deposition punctuated by episodes of igneous activity and contractional deformation continued from the Devonian until early Permian times for about 120 Ma leaving a record of alluvial fan and flood plain siliciclastics alternating with rhyolitic and alkaline basaltic intrusions and extrusions. Later basin subsidence reflected thermal cooling and contraction following the prolonged late Palaeozoic period of crustal thinning. Important phases of block faulting occurred at several intervals during Mesozoic times climaxing in the Volgian-Valanginian interval. Cenozoic basin inversions resulted in dramatic decoupled uplift and tilting of large blocks. The degree of Devonian extensional collapse, the position and orientation of Caledonian thrust planes, and the localization of mid- and late Palaeozoic transtensional strike-slip basins exerted a profound control on localization and style of the compartmentalized Mesozoic rift. The importance of pre-Mesozoic history, timing of Mesozoic tectonic events and the interplay of tectonism, and rates of subsidence, sea-level change and sediment influx have close parallels in the North Sea area. Examples from the exposed Mesozoic succession of East Greenland may thus serve as excellent predictive guides for subsurface geology around the British Isles.

Sequential Mesozoic and Cenozoic Tectonic Events in Grant Range and Railroad Valley, Nye County, Nevada: ABSTRACT

A series of palinspastic geologic cross sections were constructed for the Grant Range-Railroad Valley (GR-RRV) area. This palinspastic reconstruction differs from previously published interpretations in two major ways. First, new granite age dates and a chronological framework deduced by Fryxell in 1984 in the central and southern Grant Range were applied, where possible, to the northern Grant Range. Second, recent drilling was correlated to outcrop interpretations. Results of this reconstruction suggest that significant tectonism affected the GR-RRV area earlier than previously believed. Mesozoic compression produced east-vergent folding in the GR-RRV area, followed by Late Cretaceous intrusion. Local, possibly blind thrusting may have been associated with the intrusion. Above the granite dome, shallow, brittle, listric faulting and deeper low-angle attenuation faulting accommodated intrusion. After intrusion, relative quiescence prevailed during the deposition of the Sheep Pass Formation and the lower Garrett Ranch Group ignimbrite. Uplift in the middle and late Oligocene produced pervasive listric, normal, east-dipping faulting that may in part be represented by apparent growth faults within volcanics. Subsequent west-dipping listric faulting was in part gravity driven. High-angle Miocene normal faulting initiated the present structure. All commercial oil fields in Railroad Valley are producing from Miocene and younger structures. However, anmore » understanding of older structures will allow more accurate prediction of reservoirs within a particular, prospective, younger structure.« less

Structure of the lower crust beneath the Gulf of Maine

Summary. The variability of deep crustal reflections in USGS line 1A across the offshore New England Appalachians shows the differing influence of Paleozoic and Mesozoic tectonic events. Mesozoic extension has not significantly modified Paleozoic thrust faults penetrating the lower crust in the northern Gulf of Maine. Mesozoic extension or Paleozoic crustal melting could explain the lower crustal character in the central Gulf.

Mesozoic and Cenozoic regional tectonics and metallogenesis in Mainland SE Asia

The major Mesozoic tectonic event in mainland SE Asia was the late Triassic Jndosin­ ian orogeny. Deformation and uplift in the Main Range Malaysia and northern Thailand, emplacement of syn- to late-tectonic two-mica granites of the Central Tin Belt, and the recently reported imbricate thrusting in Bangka Island, can best be explained by early Triassic collision of the Shan-Thai block foreland with the Indochina block to the east. Syncollision east-verging back-thrusts can explain the narrow gap between the Shan-Thai block and magmatic arc. The Mogok Belt of Central Burma was metamorphosed and elevated above a major collision-related west-verging foreland thrust. Regional stress related to foreland thrusting initiated eastward subduction of oceanic lithosphere at a 1urassic spreading centre in a small ocean basin west of the Shan-Thai block. Subduction of the spreading system and ophiolite detachment were followed by eastward subduction of the remnant ocean basin. A continental fragment, exposed in the Jndoburman Ranges, collided with Burma in the latest Jurassic, and-was underthrust beneath the ophiolite. East-verging thrusts of the Shan Scarps and Thai­ Burma border were related to the collision, and caused partial melting and emplacement of anatectic granites of the Western Tin Belt. Generation of mineralized granites in both the Central and Western Tin Belts may have been related to expulsion of fluids from a common source tectonically buried in the late Triassic and Cretaceous. The Cretaceous heating event well-documented in the Main Range granites was possibly related to their position above converging Benioff zones at a depth of a few hundred kilometres. Cenozoic relaxation of compressive stress probably permitted isostatic rise of the Shan Plateau. Dextral movement on the Sagaing transform fault was accompanied to the east, in northern Thailand, by crustal thinning, emplacement of basaltic magma at depth, and uplift of the Khorat Plateau. Hydrothermal activity related to the sub-surface intrusions and crustal extension caused widespread antimony and fluorite mineralization. Similarities in the regional tectonic setting of northern Thailand ~o that of the Great Basin in western USA suggest the possibility that epithermal gold-silver mineralization could occur at depth. Subduction of Indian Ocean floor beneath the Jndoburman Ranges collision belt began in the Aptian, with magmatism in the Burma volcanic arc and probably Its continuation through Sumatra. Olistostromes of probable Campanian age in the Indoburman Ranges are interpreted as debris flows formed by diapiric rise and extrusion of mud-matrix melanges. Bodies of jadeite and schist were possibly elevated in serpentinite melange diapirs.

The Owen Basin off the south-east margin of Arabia and the evolution of the Owen Fracture Zone

Summary. The history of the Owen Basin, which lies between Arabia and the Owen Fracture Zone (OFZ) in the north-west Indian Ocean, is relevant to the break-up of Gondwanaland and the subsequent sea-floor spreading history of the region. Bathymetric, magnetic and gravity studies of the basin were made in 1975 in conjunction with seismic reflection and refraction profiles. A slightly thickened oceanic crust exists under the basin and thickens towards the continental margin. There is limited evidence of an ENE grain in both the magnetic anomalies and the acoustic basement but, on the other hand, there are major ridges which parallel the shelf edge and the OFZ. Two-dimensional gravity modelling indicates areas of abnormally thinned crust under some sediment troughs and areas of abnormally thickened crust under ridges not associated with the OFZ. The Owen Basin does not appear to be a region of simple oceanic crust but possesses features whch possibly relate to the formation of the adjacent continental margin. Free-air gravity anomalies suggest that the OFZ is a continuous feature from 15’ 30” to as far north as, and inclusive of, thg Dalrymple Trough. The arcuate shape of the northern OFZ is probably a relict of an earlier phase of sea-floor spreading before the Gulf of Aden opened although there has been some vertical and horizontal readjustment at the OFZ as a consequence of this opening. Thus the northern OFZ is probably no longer an active transform fault. The Owen and Chain Ridges exhibit rotational symmetry about a point midway between them and the latter ridge, being the inactive extension of a transform fault, traces a history of changing spreading direction during their formation. It is not possible at present to relate the history of the Owen Basin to Mesozoic tectonic events in the Somali. Basin because the age and origin of two intervening seamount chains are unknown.

Mesozoic and Cenozoic Stratigraphy and Tectonic Events of Puno-Santa Lucia Area, Department of Puno, Peru

The Puno-Santa Lucia area is in the higher parts of the Andes in southern Peru. The oldest rocks in the area are quartzite and shale of the lower Paleozoic Cabanillas Group, more than 2,000 m thick. The Jurassic Lagunillas Group is divided into three units with an aggregate thickness of 1,505 m. The Cretaceous rocks are included in the Huancane Group and the Moho Group, the combined thickness of which is 410 m. The Ayavacas Limestone, upper member of the Moho Group, contains fossils of Cenomanian age. Trachytes of Cretaceous age are present near Juliaca and on the Capachica Peninsula. Redbeds, Cotacucho, Vilquechico, and Munani Formations, mapped by Newell within the Tertiary Puno Group, are considered here to be Late Cretaceous on the basis of their stratigraphic positio . Apparently these formations were deposited in a closed basin between the Cordillera Occidental and Cordillera Oriental. The Puno Group of probable Tertiary age consists of two units: the Saracocha Formation below, consisting mainly of conglomerate, is widely distributed in the southwestern part of the area, but is absent in the northeast; and the Tacaza Formation above, a volcanic sequence, also present in the southwestern part, shows the thickest sections along the Cordillera Occidental with thinning toward the east. Olivine basalt flows and tuffs of the Sillapaca Formation unconformably overlie the Puno Group. Three orogenies have been identified.