Victor E. Camp

Pan-African microplate accretion of the Arabian Shield

The late Proterozoic Arabian Shield is composed of at least five geologically distinct terranes (microplates) separated by four ophiolite-bearing suture zones. Three ensimatic island-arc terranes occur in the western shield, whereas the two terranes in the eastern shield have continental affinities. The western two sutures are island-arc-island-arc joins, whereas the eastern two sutures collectively form a major coilisional orogenic belt. Accretion of the five terranes to form an Arabian neocraton occurred from 715 to 630 Ma. After accretion, intracratonic deformation and magmatism related to collision continued and resulted in the formation of molasse, intermediate to silicic volcanic rocks, peralka-line to peraluminous granites (640-570 Ma), and a major left-lateral wrench fault system (∼630-550 Ma) which displaced the northern part of the Arabian neocraton ∼250 km to the northwest. These tectonic events represent the accretion of the Arabian portion of Gondwanaland during the Pan-African event.

The Sistan suture zone of eastern Iran

A deformed accretionary prism and a flanking forearc basin extending from Birjand southeast to Zahedan, Iran, record the destruction of an arm of the Neo-Tethys during Senonian-Paleocene time and consequent collision of the Afghan and Lut eratonic blocks. The accretionary prism at 32 °N is subdivided into two northwest-trending en echelon belts termed the “Ratuk” and “Neh” complexes, respectively. On the east, the Ratuk complex is characterized by ophiolitic block-against-block or serpentinite-matrix melange and large fault slivers of epidote blueschist tectonite. The Ratuk complex was built prior to Maastrichtian time. The Neh complex to the southwest is Senonian to Eocene in age and includes, in addition to ophiolitic melange, weakly metamorphosed marine sedimentary rock exposed in extensive belts bounded by steeply dipping faults. The Sefidabeh forearc basin deposits onlap both the Neh and Ratuk complexes and the southwest margin of the Afghan block. They make up as much as 8 km of Cenomanian to Eocene terrigenous elastics and carbonates that display a complex but coherent stratigraphy. Facies relations demonstrate the uplift and subaerial exposure of the Ratuk structural high, followed by its subsidence contemporaneous with construction of the Neh complex and calc-alkalic volcanism on the northeast (inner) side of the basin. The accretionary prism-forearc basin polarity, the structural vergence and general younging of the accretionary prism to the southwest, as well as the position of the (relatively) high P T metamorphic rock on the inner side of the prism are consistent with northeast-dipping subduction. Widespread emergence of the entire belt and the initiation of folding of the Sefidabeh basin deposits during middle Eocene are interpreted to be consequences of the entry of the Lut block into the subduction zone. Continued convergence of the continental blocks is expressed by a regional system of folds and transcurrent faults corresponding to east-northeast compression. These structures are buried by mildly deformed Miocene volcanic rocks. Extensive post-Miocene right-slip faulting is inferred to be an effect of Miocene “terminal” collision of Arabia and Eurasia.

Genesis of flood basalts and Basin and Range volcanic rocks from Steens Mountain to the Malheur River Gorge, Oregon

The middle and south forks of the Malheur River provide a unique mapping corridor connecting two flood-basalt successions—Steens basalt to the south and the basalt of Malheur Gorge to the north. Each contains chemically defined subtypes, which merge stratigraphically across the north-south length of the map area. The lowermost flows of Steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of Malheur Gorge. The uppermost flows of Steens basalt pinch out in the map area, but are partly interbedded with the middle and uppermost flows of the basalt of Malheur Gorge, which continue to thicken northward. The upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas— the Venator Ranch basalt flows. Tholeiitic volcanism ceased at ca. 15.3 Ma; the last tholeiitic unit erupted is the Hunter Creek basalt, which also thickens northward. Subsequent (younger than 15.3 Ma), more localized eruptions were dominated by calc-alkaline to mildly alkaline lavas associated with Basin and Range extension. Local uplift generated deep canyons, which were filled by andesitic lavas of the Keeney sequence (ca. 13‐10 Ma). The final eruptive products include the Devine Canyon tuff (ca. 9.7 Ma), the Drinkwater basalt (ca. 6.9 Ma), and the Voltage flow (older than 32,000 yr B.P.). Major and trace element analyses demonstrate that (1) crystal fractionation was a universal process in the derivation of the

Character, genesis and tectonic setting of igneous rocks in the Sistan suture zone, eastern Iran

Abstract Igneous rocks in the Sistan suture zone have characteristics that can be correlated with important tectonic events. A Late Cretaceous ocean basin is recorded by ophiolites now exposed in numerous melange zones. Subduction beneath the Afghan block is indicated by Late Cretaceous-Paleocene calc-alkaline volcanics. Collision of the Lut block with the subduction complex in the middle Eocene produced widespread deformation and was followed by the emplacement of late Eocene-early Oligocene calc-alkaline granitic batholiths that probably formed by widespread anatexis of marine sediments. A dominantly Oligocene magmatic event is represented by widespread alkaline volcanics and minor intrusions that appear to be related to major transcurrent faults. Miocene calc-alkaline activity was limited to sporadic volcanism in the north and minor intermediate intrusions farther south. These units are largely underformed and not related to any major faults. The youngest magmatic event is recorded by late Miocene-Pliocene mafic flows that are weakly alkaline, clearly related to right-lateral faults and probably were derived from a deep crustal or upper mantle source.

A plume-triggered delamination origin for the Columbia River Basalt Group

The Columbia River Basalt Group reveals a complete and detailed stratigraphic succession to assess the interplay of lithospheric and asthenospheric processes. This record of chemical change through time is used to evaluate genetic models for Columbia River Basalt volcanism. We recognize four primary constraints on source melting: (1) a plume component appears to be the dominant source of Imnaha Basalt; (2) Grande Ronde Basalt is best interpreted as being derived from a mafic pyroxenite or eclogite source; (3) the sequence of source melting must correspond with the stratigraphic record; and (4) working models must explain a step-function chemical change at the Imnaha–Grande Ronde stratigraphic boundary. We can envision only three potential models to satisfy these primary constraints: (1) melting of a mantle plume entrained with eclogite, (2) plume interaction with the Juan de Fuca plate, and (3) delamination triggered by plume emplacement. The first two of these are inconsistent with the time-stratigraphic sequence of melting and cannot satisfy all four primary constraints. In contrast, a model of plume-triggered delamination accurately predicts a progressive sequence of melting that satisfies each of the primary constraints. Such a model is consistent with recent numerical experiments demonstrating that delamination is the expected result of plume emplacement beneath thin Mesozoic lithosphere lying adjacent to a thick cratonic boundary. We test this model by comparing the observed history of uplift and tectonism in eastern Oregon and adjacent Washington to that predicted by the numerical models to reveal consistent stress regimes and strikingly similar topographic and structural profiles.

Mid-Miocene propagation of the Yellowstone mantle plume head beneath the Columbia River basalt source region

Columbia River flood-basalt volcanism is the product of mid-Miocene distortion of the Yellowstone mantle plume head against the Precambrian margin of North America. The overall migration pattern of Columbia River basalt vents reflects northward propagation of the plume head beneath the west-southwest–migrating North American plate. Thermal uplift mimics the vent migration. Over the 1.5 m.y. of Grande Ronde basalt extrusion, the projected crest of the plume head migrated 105 km (7 cm/yr), from the Chief Joseph dike swarm east-oortheast into the Clearwater embayment, where uplift occurred over the same period at a rate of 0.67 mm/yr. Chemical variations reflect progressive melting of the plume and mantle lithosphere, followed by the mixing of these components with an enriched source from the overriding craton. Rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes.

Regional waveform propagation in the Arabian Peninsula

Regional waveform propagation is characterized in the Arabian Peninsula using data from a temporary network of broadband seismometers. Between November 1995 and March 1997, 332 regional (delta<15°) events were recorded from nine stations deployed across the Arabian Shield. Regional phase propagation was analyzed in two ways: by individual inspection of the waveforms and by stacking of waveforms. Inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin. Waveforms from events in the Gulf of Aqaba, northwest of the network, possess weak Pn, Pg, and Sn but show a prominent Lg phase. In contrast, clear Pn, Sn, and Lg are observed for events located in the Zagros, a region northeast of the network. Events near the Straits of Hormuz also display Pn and Sn but lack a strong high-frequency Lg. Southern Red Sea and African earthquakes have moderate-amplitude body phases with some Lg. For the stacks the data were high-pass filtered at 1 Hz, rectified, binned, and then stacked by time/distance or by time/slowness. The time/distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms. The time/slowness stacks allow comparison of relative phase velocities and amplitudes. Pn velocity under the network was estimated to be 8.0±0.2 km/s, consistent with data from prior refraction profiles. The area of inefficient Pn and Sn propagation coincides with an area of Holocene volcanism and suggests that anomalous upper mantle underlies much of the Arabian Shield.

Deformation of the southeast part of the Columbia Plateau

Four structural elements north of the Olympic-Wallowa lineament in the southeast part of the Columbia Plateau (Washington, Idaho, and Oregon) are (1) the offlap of progressively younger basalt units from pre-basalt topographic highs; (2) east-west open folds associated with reverse faulting; (3) northwest-southeast, northeast-southwest, and north-south faults with predominantly vertical displacement; and (4) vertical north-northwest–south-southeast feeder dikes. These may be explained by (1) a regional east to west tilting of the plateau caused by the isostatic rise of older rocks on the eastern margin; (2) a stress regime with a horizontal maximum principal stress in a north-northwest–south-southeast direction, and a horizontal minimum principal stress in a west-southwest–east-northeast direction; and (3) reactivation of an older northwest-southeast, northeast-southwest, and north-south structural grain in the pre-Miocene basement. The stress regime is similar to that envisaged for the area southwest of the Olympic-Wallowa lineament, and the difference in the type of deformation on either side of that feature may be attributed to differences in the thickness of the crust across the ancient boundary.

Yellowstone plume trigger for Basin and Range extension, and coeval emplacement of the Nevada–Columbia Basin magmatic belt

Widespread extension began across the northern and central Basin and Range Province at 17–16 Ma, contemporaneous with magmatism along the Nevada–Columbia Basin magmatic belt, a linear zone of dikes and volcanic centers that extends for >1000 km, from southern Nevada to the Columbia Basin of eastern Washington. This belt was generated above an elongated sublithospheric melt zone associated with arrival of the Yellowstone mantle plume, with a north-south tabular shape attributed to plume ascent through a propagating fracture in the Juan de Fuca slab. Dike orientation along the magmatic belt suggests an extension direction of 245°–250°, but this trend lies oblique to the regional extension direction of 280°–300° during coeval and younger Basin and Range faulting, an ∼45° difference. Field relationships suggest that this magmatic trend was not controlled by regional stress in the upper crust, but rather by magma overpressure from below and forceful dike injection with an orientation inherited from a deeper process in the sublithospheric mantle. The southern half of the elongated zone of mantle upwelling was emplaced beneath a cratonic lithosphere with an elevated surface derived from Late Cretaceous to mid-Tertiary crustal thickening. This high Nevadaplano was primed for collapse with high gravitational potential energy under the influence of regional stress, partly derived from boundary forces due to Pacific–North American plate interaction. Plume arrival at 17–16 Ma resulted in advective thermal weakening of the lithosphere, mantle traction, delamination, and added buoyancy to the northern and central Basin and Range. It was not the sole cause of Basin and Range extension, but rather the catalyst for extension of the Nevadaplano, which was already on the verge of regional collapse.

Uplift, rupture, and rollback of the Farallon slab reflected in volcanic perturbations along the Yellowstone adakite hot spot track

Field, geochemical, and geochronological data show that the southern segment of the ancestral Cascades arc advanced into the Oregon back-arc region from 30 to 20 Ma. We attribute this event to thermal uplift of the Farallon slab by the Yellowstone mantle plume, with heat diffusion, decompression, and the release of volatiles promoting high-K calc-alkaline volcanism throughout the back-arc region. The greatest degree of heating is expressed at the surface by a broad ENE-trending zone of adakites and related rocks generated by melting of oceanic crust from the Farallon slab. A hiatus in eruptive activity began at ca. 22-20 Ma, but ended abruptly at 16.7 Ma with renewed volcanism from slab rupture occurring in two separate regions. The eastern rupture resulted in the extrusion of Steens Basalt during the ascent and melting of a dry mantle (plume) source contaminated with depleted mantle. The contemporaneous western rupture resulted in renewed subduction, melting of a wet mantle source, and the rejuvenation of high-K calc-alkaline volcanism near the Nevada-California border at 16.7 Ma. Here, the initiation of slab rollback is evident in the westward migration of arc volcanism at 7.8 km/Ma. Today, the uplifted slab is largely missing beneath the Oregon back-arc region, replaced instead by a seismic hole which is bound on the south by the adakite hotspot track. We attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17-15 Ma, and possible foundering and sinking of slab segments from 16-10 Ma.