Darrel S. Cowan

Structural styles in Mesozoic and Cenozoic melanges in the Western Cordillera of North America.

The term “melange” is currently used to describe several different kinds of mudstone-rich rocks that are broadly characterized by an obscure stratigraphy, stratal disruption, or a chaotic, “block-in-matrix” fabric. Four types of melange, which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition, are particularly widespread and distinctive. Type I includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer-parallel extension. Type II consists of similarly deformed, thin layers of green tuff, radiolarian ribbon chert, and minor sandstone originally interbedded with black mudstone. Disruption in both types I and II, which probably occurred while the sediments were incompletely consolidated, has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation. Type III comprises inclusions of diverse shapes, sizes, and compositions enveloped in a locally scaly, pelitic matrix. The ultimate source of fragments is obscure, because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks. Although some type III melanges may have originated deep within accretionary prisms, final emplacement as olistostromes (muddy debris-flow deposits) or mud diapirs seems likely. Type III melanges are mechanically analogous to scaly, “sheared” serpentinites; many probably have been tectonically remobilized or even intruded into shallow-level fault zones. Type IV consists of lenticular inclusions bounded by an anastomosing network of subparallel faults. Their fabric records progressive slicing in brittle fault zones. Each of the four types of melange described here could, in theory, have formed in a variety of settings on or within an accretionary wedge at an active convergent margin; none can yet be singled out as a uniquely diagnostic “subduction melange.”

Regionally extensive mid-Cretaceous west-vergent thrust system in the northwestern Cordillera: Implications for continent-margin tectonism

The Intermontane-Insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the North American Cordillera. Structural, stratigraphic, and geochronologic relations along this boundary indicate that substantial west-vergent compression and concomitant crustal thickening occurred there in mid-Cretaceous time. This orogenic zone extends for more than 1200 km along strike length, between southern southeast Alaska and northern Washington. In southern southeast Alaska and northwest British Columbia, rocks of the Insular superterrane were imbricated along a series of west- to southwest-vergent thrust faults. In northern Washington and southwestern British Columbia, a wide zone encompassing the margins of the two superterranes, as well as numerous intervening smaller fragments, was shortened principally along west-vergent thrusts. Known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid-Cretaceous thrust system.

Early Mesozoic tectonic evolution of the western Sierra Nevada, California

Prebatholithic rocks of Mesozoic age in the Sierra Nevada can be interpreted as remnants of ancient volcanic arcs, subduction complexes, and sequences of oceanic lithosphere. Two partly coeval subparallel volcanic arcs, one in the western foothills and the other in the northern and eastern Sierra Nevada, are juxtaposed. The western arc was an east-facing island-arc complex that evolved through a series of steps including formation of a remnant arc and interarc basin. The eastern arc was a west-facing marginal arc that was constructed on the edge of North America. Both arc-subduction complexes consumed intervening oceanic lithosphere and collided during the Late Jurassic Nevadan orogeny. Generation of magmas in both arcs apparently ceased at about this time, and renewed subduction was initiated west of the island arc in latest Jurassic time, giving birth to the Franciscan-Sierran arc-trench complex. Fault zones and melanges in the western Sierra Nevada reflect the complex suturing at the collision boundary. Pre-Tithonian ophiolite at the base of the Great Valley sequence in the Coast Ranges originated in a back-arc or marginal basin setting with respect to the coeval Sierran foothills arc.

Offscraping and underthrusting of sediment at the deformation front of the Barbados Ridge: Deep Sea Drilling Project Leg 78A

On Leg 78A we drilled Sites 541 and 542 into the seaward edge of the Barbados Ridge complex, and Site 543 into the adjacent oceanic crust. The calcareous ooze, marls, and muds at Sites 541 and 542 are lithologically and paleontologically similar to the upper strata at Site 543 and are apparently offscraped from the down-going plate. A repetition of Miocene over Pliocene sediments at Site 541 documents major thrust or reverse faulting during offscraping. The hemipelagic to pelagic deposits offscraped in the Leg 78A area include no terrigenous sand beds, but they contain numerous Neogene ash layers derived from the Lesser Antilles Arc. Hence, this sequence is quite unlike the siliciclastic-dominated terranes on land that are inferred to be accretionary complexes. The structural fabric of the offscraped deposits at Sites 541 and 542 is disharmonic, probably along a decollement, with an underlying acoustically layered sequence, suggesting selective underthrusting of the latter. The acoustically layered sequence correlates seismically with pelagic strata cored at Site 543 on the incoming oceanic plate. Cores recovered from the possible decollement surface at both Sites 541 and 542 show scaly foliation and stratal disruption. Approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits. In the incoming section, a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata. In the Leg 78A area, the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the Barbados Ridge complex.

Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides

We modeled global positioning system measurements of crustal velocity along a N13°E profile across the southern Adria microplate and south-central Dinarides mountain belt using a one-dimensional elastic dislocation model. We assumed a N77°W fault strike orthogonal to the average azimuth of the measured velocities, but we used a constrained random search algorithm minimizing misfit to the velocities to determine all other parameters of the model. The model fault plane reaches the surface seaward of mapped SW-verging thrusts of Eocene and perhaps Neogene age along the coastal areas of southern Dalmatia, consistent with SW-migrating deformation in an active fold-and-thrust belt. P-wave tomography shows a NE-dipping high-velocity slab to ∼160 km depth, which reaches the surface as Adria, dips gently beneath the foreland, and becomes steep beneath the Dinarides topographic high. The thrust plane is located directly above the shallowly dipping part of the slab. The pattern of precisely located seismicity is broadly consistent with both the tomography and geodesy; deeper earthquakes (down to ∼70 km) correlate spatially with the slab, and shallower earthquakes are broadly clustered around the geodetically inferred thrust plane. The model fault geometry and loading rate, ages of subaerially exposed thrusts in the fold-and-thrust belt, and the length of subducted slab are all consistent with Adria-Eurasia collision involving uninterrupted subduction of southern Adria mantle lithosphere beneath Eurasia since Eocene time.

Provenance of Franciscan graywackes in coastal California

A systematic comparison of available detrital modes for graywacke sandstones of the Franciscan subduction complex and for coeval sandstones of the Great Valley sequence in the California Coast Ranges indicates that both were apparently derived from the same general sources. The inferred provenance terrane was the ancestral Sierran-Klamath magmatic arc, from which mixed volcanic and plutonic detritus readily entered the adjacent Great Valley forearc basin. At intervals along the trend of the arc-trench system, arc-derived detritus also bypassed the forearc region through submarine canyons that fed the Franciscan trench. Longitudinal flow along both the Great Valley trough and the Franciscan trench achieved wide dispersal of the turbidite sediment. Suites of both Franciscan and Great Valley samples include an array of subquartzose compositions ranging from feldspatholithic to lithofeldspathic. Mean framework modes of 17 Franciscan suites comprising 203 individual samples, and of 23 Great Valley suites comprising 410 individual samples, range from 14% to 44% quartz grains, 15% to 54% feldspar grains, and 7% to 71% total lithic fragments. The ratio of quartz to feldspar remains relatively constant as the proportion of lithic fragments changes. The compositional variations reflect differences mainly in the admixture of lithic fragments derived principally from volcanic cover with quartz, and feldspar derived principally from erosion of underlying plutons. Despite major overlap in the compositions of the two sets of samples, some Franciscan sandstones are somewhat more feldspathic and less lithic than any known Great Valley counterparts and were probably derived from segments of the arc terrane where exposures of plutons were more extensive than within typical Great Valley sources. Higher proportions of non-volcanic to volcanic lithic fragments in some Franciscan sandstones probably reflect complex recycling processes on the trench slope. Diagenetic effects in many Franciscan suites include apparent wholesale replacement of K-feldspar by albite. Present age control is inadequate to test fully for time-dependent trends in the compositions of Franciscan sandstones analogous to the known stratigraphic variations in the composition of Great Valley sandstones. This question ought to be investigated in future studies.

Leg 67: The Deep Sea Drilling Project Mid-America Trench transect off Guatemala

Drill cores from a transect of the Mid-America Trench off Guatemala were obtained at three sites on the oceanic Cocos plate, and at four sites on the continental Caribbean plate. An ocean sub-bottom seismometer was successfully emplaced in the deepest hole in the trench landward slope where it was left to record data after departure of the drill ship. Drilling on the Cocos plate recovered a basal chalk sequence deposited during early and mid-Miocene time, a short interval of abyssal red clay, and an upper sequence of late Miocene and younger sediment deposited within an area influenced by a terrigenous source. In the trench, a mud and sand fill less than 400,000 yr old overlies the oceanic sequence. The entire section shows no evidence of compressive deformation even at holes drilled against the trench9s landward slope. In contrast, the section cored on the trench9s landward slope 3 km from the trench axis is affected by tectonism. The section contains a Cretaceous to Pliocene claystone sequence, broken by hiatuses but in a normal stratigraphic succession that is capped by Pliocene to Quaternary hemipelagic slope deposits. Seismic records show that the section overlies probable igneous oceanic crust from which it is separated by a few hundred metres. That thickness of undrilled section is insufficient to accommodate the potential offscraped volume of oceanic sediment carried into the trench during Neogene plate convergence. At the estimated 10 cm/yr rate of convergence, much of the oceanic sediment must have been subducted rather than tectonically accreted to the Guatemalan margin. Current models for convergent margin tectonics do not satisfactorily explain the surprising occurrence of Cretaceous to Miocene mudstone at the base of this trench slope. The recovery of gas hydrates prevented drilling to some landward-dipping reflections presumed to be imbricate thrust slices at two sites near the middle of the trench landward slope.

Origin of blueschist-bearing chaotic rocks in the Franciscan Complex, San Simeon, California

Chaotic rocks exposed in sea cliffs south of San Simeon, California, consist of sub-rounded to lens-shaped fragments of graywacke, greenstone, and less abundant blueschist and chert dispersed in a matrix of argillite. This nonbedded melange has been deformed twice. An earlier deformation, D 1 , produced a strong northwest-striking, northeast-dipping foliation defined by both tectonically flattened inclusions and a parallel penetrative cleavage in argillite. Most inclusions, even of blueschist, are imperfect oblate ellipsoids or display pinch-and-swell structure and, locally, extreme necking and boudinage. Overall ductile behavior during D 1 was succeeded by the development of subparallel shear fractures that record a brittle deformation, D 2 . Displacements on these fractures were generally small or negligible. It is clear that neither D 1 nor D 2 was responsible for the original lithologic heterogeneity and chaotic fabric of the melange. Mixing of foliated, glaucophane-lawsonite blueschist with lower-grade graywacke and greenstone to yield a nonbedded diamictite composed of variously sized clasts in a mudstone matrix must have occurred prior to D 1 , probably by sedimentary processes involving submarine sliding and downslope transport of debris flows. There is no evidence that the melange was bedded or that the blueschist inclusions were tectonically introduced among lower-grade rocks, prior to D 1 . The sequence of sedimentary and tectonic events suggests that as the subduction ultimately responsible for the Franciscan Complex as a whole proceeded, blueschists in elevated parts of previously accreted material were eroded, mixed with lower-grade rock, and deposited as olistostromes that were subsequently accreted and deformed.

Quaternary low-angle slip on detachment faults in Death Valley, California

Detachment faults on the west flank of the Black Mountains (Nevada and California) dip 29°-36° and cut subhorizontal layers of the 0.77 Ma Bishop ash. Steeply dipping normal faults confined to the hanging walls of the detachments offset layers of the 0.64 Ma Lava Creek B tephra and the base of 0.12-0.18 Ma Lake Manly gravel. These faults sole into and do not cut the low-angle detachments. Therefore the detachments accrued any measurable slip across the kinematically linked hanging-wall faults. An analysis of the orientations of hundreds of the hanging-wall faults shows that extension occurred at modest slip rates (<1 mm/yr) under a steep to vertically oriented maximum principal stress. The Black Mountain detachments are appropriately described as the basal detachments of near-critical Coulomb wedges. We infer that the formation of late Pleistocene and Holocene range-front fault scarps accompanied seismogenic slip on the detachments.

Structural geology and kinematic history of rocks formed along low-angle normal faults, Death Valley, California

Several late Cenozoic low-angle normal, or detachment, faults on the western flank of the Black Mountains are each characterized by the following, in descending structural order: (1) a hanging wall of upper Tertiary to Quaternary sedimentary and volcanic strata that displays little evidence for fault-related damage other than widely spaced planar or listric normal faults; (2) a sharp, planar, and locally striated principal slip plane forming the lower boundary of the hanging wall; (3) an upper zone—zone I—of very fine grained, fault-generated rocks composed dominantly of gouge; (4) a lower zone—zone II—of coarser-grained fault rocks consisting chiefly of foliated breccia; and (5) a variably damaged footwall, consisting of partly mylonitic Precambrian units or Tertiary plutonic rocks, that has been exhumed from depths of 10–12 km since late Miocene time. The fault rocks, which were mostly derived from the footwalls, preserve evidence for cataclastic and particulate flow. Fault rocks contain authigenic minerals but lack the cyclically deformed, mineral-filled syntectonic veins that are abundant in some other late Cenozoic high-angle and strike-slip faults. Meso- and microscale fabrics in zones I and II indicate that finite shear strains increase progressively upward, toward the principal slip plane. In a conceptual kinematic model of a shear box, displacements of the hanging wall produced a shearing flow in the fault rocks below. Some of the slip on the principal slip plane was also partitioned into localized slip on discrete sliding surfaces in zones I and II. Since 770 ka, during the latest stages of incremental deformation in the brittle shear zones, distributed flow in the fault rocks alternated with slip that was chiefly localized on the principal slip planes. In this respect, the detachment faults differ from inactive segments of the San Andreas system, along which displacements were progressively and irreversibly localized onto a single principal slip surface. The strain gradients and corresponding changes in grain size in the shear zones resemble those in mylonitic shear zones except in their symmetry. Strain gradients in the Black Mountains are notably asymmetric: they are present only below the principal slip planes, not above.