ABSTRACT The Mount Diablo region has been located within a hypothesized persistent corridor for clastic sediment delivery to the central California continental margin over the past ~100 m.y. In this paper, we present new detrital zircon U-Pb geochronology and integrate it with previously established geologic and sedimentologic relationships to document how Late Cretaceous through Cenozoic trends in sandstone composition varied through time in response to changing tectonic environments and paleogeography. Petrographic composition and detrital zircon age distributions of Great Valley forearc stratigraphy demonstrate a transition from axial drainage of the Klamath Mountains to a dominantly transverse Sierra Nevada plutonic source throughout Late Cretaceous–early Paleogene time. The abrupt presence of significant pre-Permian and Late Cretaceous–early Paleogene zircon age components suggests an addition of extraregional sediment derived from the Idaho batholith region and Challis volcanic field into the northern forearc basin by early–middle Eocene time as a result of continental extension and unroofing. New data from the Upper Cenozoic strata in the East Bay region show a punctuated voluminous influx (>30%) of middle Eocene–Miocene detrital zircon age populations that corresponds with westward migration and cessation of silicic ignimbrite eruptions in the Nevada caldera belt (ca. 43–40, 26–23 Ma). Delivery of extraregional sediment to central California diminished by early Miocene time as renewed erosion of the Sierra Nevada batholith and recycling of forearc strata were increasingly replaced by middle–late Miocene andesitic arc–derived sediment that was sourced from Ancestral Cascade volcanism (ca. 15–10 Ma) in the northern Sierra Nevada. Conversely, Cenozoic detrital zircon age distributions representative of the Mesozoic Sierra Nevada batholith and radiolarian chert and blueschist-facies lithics reflect sediment eroded from locally exhumed Mesozoic subduction complex and forearc basin strata. Intermingling of eastern- and western-derived provenance sources is consistent with uplift of the Coast Ranges and reversal of sediment transport associated with the late Miocene transpressive deformation along the Hayward and Calaveras faults. These provenance trends demonstrate a reorganization and expansion of the western continental drainage catchment in the California forearc during the late transition to flat-slab subduction of the Farallon plate, subsequent volcanism, and southwestward migration of the paleodrainage divide during slab roll-back, and ultimately the cessation of convergent margin tectonics and initiation of the continental transform margin in north-central California.

ABSTRACT Forearc topography and inferred paleotopography are key constraints on the processes acting at plate interfaces along subduction margins. We used along-strike variations in modern topography, trench sediment thickness, and instrumental seismic data sets over >2000 km of the Chilean margin to test previously proposed feedbacks among subducted sediments, plate interface rheology, megathrust seismicity, and forearc elevation. Observed correlations are consistent with subducted sediments playing a prominent role in controlling plate interface rheology, which, in turn, controls the downdip distribution of megath-rust seismicity and long-term forearc elevation. High (low) rates of trench sedimentation promote long-term interseismic coupled offshore forearc uplift (subsidence) and onshore forearc platform subsidence (uplift). Low trench sedimentation rates also promote deeper megathrust seismic slip, enhancing short-wavelength coastal zone uplift. Shallowing of subducting slabs contributes to a reduction in coastal zone–onshore forearc relief, in turn preventing formation of onshore forearc basins. The extremely low denudation rates of hyperarid northern Chile have allowed better reconstructions of the histories of paleoel-evations and paleoclimate compared to other sections of the forearc. Even if these histories are not sufficiently resolved to unequivocally assign causality among climate variability, changes in plate interface frictional properties, and forearc elevation, they are consistent with the onset of hyperaridity in the coastal zone at 25–20 Ma (1) triggering long-term, long-wavelength offshore forearc subsidence and onshore forearc uplift, and (2) accelerating short-wavelength coastal zone uplift.

ABSTRACT Cretaceous forearc strata of the Ochoco basin in central Oregon may preserve a record of regional transpression, magmatism, and mountain building within the Late Cretaceous Cordillera. Given the volume of material that must have been eroded from the Sierra Nevada and Idaho batholith to result in modern exposures of mid-and deep-crustal rocks, Cretaceous forearc basins have the potential to preserve a record of arc magmatism no longer preserved within the arc, if forearc sediment can be confidently linked to sources. Paleogeographic models for mid-Cretaceous time indicate that the Blue Mountains and the Ochoco sedimentary overlap succession experienced postdepositional, coast-parallel, dextral translation of less than 400 km or as much as 1700 km. Our detailed provenance study of the Ochoco basin and comparison of Ochoco basin provenance with that of the Hornbrook Formation, Great Valley Group, and Methow basin test paleogeographic models and the potential extent of Cretaceous forearc deposition. Deposition of Ochoco strata was largely Late Cretaceous, from Albian through at least Santonian time (ca. 113–86 Ma and younger), rather than Albian–Cenomanian (ca. 113–94 Ma). Provenance characteristics of the Ochoco basin are consistent with northern U.S. Cordilleran sources, and Ochoco strata may represent the destination of much of the mid- to Late Cretaceous Idaho arc that was intruded and eroded during and following rapid transpression along the western Idaho shear zone. Our provenance results suggest that the Hornbrook Formation and Ochoco basin formed two sides of the same depositional system, which may have been linked to the Great Valley Group to the south by Coniacian time, but was not connected to the Methow basin. These results limit northward displacement of the Ochoco basin to less than 400 km relative to the North American craton, and suggest that the anomalously shallow paleomagnetic inclinations may result from significant inclination error, rather than deposition at low latitudes. Our results demonstrate that detailed provenance analysis of forearc strata complements the incomplete record of arc magmatism and tectonics preserved in bedrock exposures, and permits improved understanding of Late Cretaceous Cordilleran paleogeography.

ABSTRACT The Great Valley forearc basin records Jurassic(?)–Eocene sedimentation along the western margin of North America during eastward subduction of the Farallon plate and development of the Sierra Nevada magmatic arc. The four-dimensional (4-D) basin model of the northern Great Valley forearc presented here was designed to reconstruct its depositional history from Tithonian through Maastrichtian time. Based on >1200 boreholes, the tops of 13 formations produce isopach maps and cross sections that highlight the spatial and temporal variability of sediment accumulation along and across the basin. The model shows the southward migration of depocenters within the basin during the Cretaceous and eastward lapping of basin strata onto Sierra Nevada basement. In addition, the model presents the first basement map of the entire Sacramento subbasin, highlighting its topography at the onset of deposition of the Great Valley Group. Minimum volume estimates for sedimentary basin fill reveal variable periods of flux, with peak sedimentation corresponding to deposition of the Sites Sandstone during Turonian to Coniacian time. Comparison of these results with flux estimates from magmatic source regions shows a slight lag in the timing of peak sedimentation, likely reflecting the residence time from pluton emplacement to erosion. This model provides the foundation for the first three-dimensional subsidence analysis on an ancient forearc basin, which will yield insight into the mechanisms driving development of accommodation along convergent margins.

ABSTRACT We investigated the relationship between tectonism and sedimentation in the Karoo Basin by integrating U-Pb single-grain detrital zircon analyses from seven sandstones with U-Pb zircon analyses from 30 volcanic tuffs. U-Pb detrital zircon data from the Karoo Supergroup strata indicate that the source of the turbiditic, deltaic, and fluvial sediments included an active volcanic province, with increasing contribution from the nearby Cape fold belt through time. The depositional ages obtained from the turbiditic strata of the Karoo Basin, based on U-Pb zircon tuff ages, and the published ages for Cape fold belt deformation suggest that the influx of coarse clastic sediment was synchronous with active deformation of the fold belt during the Gondwanan orogeny. Our tuff ages indicate that peak magmatism began prior to a major deformation event and predated turbidite deposition; initial sedimentation in Karoo turbidite systems coincided with a major deformational phase in the Cape fold belt. U-Pb detrital zircon ages reveal that mid-Permian Karoo turbidites are largely composed of Permian volcaniclastic sediment, whereas the Late Permian and Early Triassic sediment was increasingly sourced from the Cape Supergroup, now exposed in the Cape fold belt. While structural development of the Cape fold belt likely controlled the entry points of sediment into the basin, orogenic uplift may have partitioned the sediment routing systems, severing the connectivity between the active magmatic arc and the basin. We present a model in which a combination of volcanic ejecta, transported via atmospheric suspension, and the formation of entrenched drainages in the catchment areas allowed partial bypassing of continental drainage divides and deposition onto the leeward side of the Cape fold belt.

ABSTRACT Orogenic belts, the main factories of continental crust and the most efficient agents of continental deformation, are commonly extremely complex structures. Every orogenic belt is unique in detail, but they are generally similar to each other, having mainly been products of subduction and continental collision. Because of that common origin, they all share common functional organs, such as magmatic arcs, various back-arc and retro-arc features, and multifarious fore-arc environments, collisional sutures, etc. The modern orogenic belts usually display adequate detail about these organs, enabling us to identify them even when they are deformed or otherwise dislocated. In reconstructing now-disrupted orogenic belts, we are after one or more Ariadne’s threads to follow the original structure from one package of rock to another. The most prominent, laterally persistent, and easy-to-follow structures among the major orogenic features are the magmatic arcs. As they are the common expression of their subduction zones, they form linear or arcuate lines along the strike, and they usually move episodically inwards or outwards, being located behind sharply defined magmatic fronts. Present-day dating techniques provide high-resolution dates from magmatic rocks, and the migration of the magmatic front is easily detectable. They form the main Ariadne’s thread in orogenic studies. Where they are absent, the most helpful structures possessing lateral persistence are the now-deformed Atlantic-type continental margins and suture zones. We chose two major fossil orogenic belts, namely, the Tethysides, and the Altaids, to emphasize the methodology of comparative anatomy of orogenic belts. There have been many theories regarding the evolution of these orogenic belts. However, they are either local, only dealing with a small portion of orogen, or they are in conflict with presently active processes. We underline the importance of magmatic fronts as reliable witnesses of the geodynamic evolution of major orogenic collages. This paper aims to disperse the mist upon the reconstructions of complexly deformed orogenic belts with the simplest possible interpretations that help us to form testable hypotheses that can be checked with a variety of geological databases.

ABSTRACT In 1935, Krynine postulated that first-cycle arkose in the humid tropical setting of southern Mexico can be rapidly eroded with minimal chemical weathering and redeposited as second-cycle arkose. Modern quantitative data confirm this hypothesis and highlight exceptions where first-cycle arkosic sediments have been diagenetically altered by intense weathering to yield second-cycle quartz arenites. In this study, extensive sampling of upland source rocks and their derived sediments provided a robust data set with which to quantitatively evaluate the composition and provenance of Holocene sediments. Three upland source terrains were identified: Paleozoic crystalline basement of the Chiapas Massif; Mesozoic to Cenozoic siliciclastic and carbonate rocks of the Chiapas fold belt; and Cenozoic sedimentary rocks in the foothills of the fold belt. Holocene sediments from these source terrains are grouped into seven facies (A–G) based on their provenance and geographic location. Facies A consists of feldspathic sediments from the Mezcalapa-Grijalva River that are sourced from the Chiapas Massif. Facies B consists of lithic-rich sediments from the same area that are derived from the Chiapas fold belt. Facies A and B consist predominantly of first-cycle sand capable of yielding arkosic deposits. Facies C represents a mixture of Facies A and B sands deposited along the course of the Mezcalapa-Grijalva River. Facies D (from Rio Sierra) and Facies E (from Rio Pedregal) represent second-cycle feldspathic sands of the coastal-plain delta and were derived from Cenozoic sedimentary rocks of the foothills. Mild chemical weathering due to rapid mechanical erosion enabled the creation of these arkosic deposits. They are less feldspathic than their parents and have limited occurrence due to mixing with less feldspathic first-cycle sands downstream from their sources. Facies F (from Rio Zanapa) and Facies G (from Lagunas Rosario and Enmedio) represent second-cycle quartzose sands of the low-lying savanna that were also derived from Cenozoic sedimentary rocks in the foothills of the fold belt. Intense, long-term (>10,000 yr) chemical weathering of these sands has precluded the formation of arkoses, instead yielding quartz arenites. They are more weathered than the delta sands (Facies D, E) with a greater loss of feldspar and carbonate detritus. They are enriched in silica and depleted in alumina, CaO, Na 2 O, and K 2 O relative to Facies A arkoses due to loss of feldspars and mafic minerals. Second-cycle sediments eroded from Tertiary sedimentary rocks in the foothills (Facies D–G) contain detrital serpentine and chromite with high abundances of Cr and Ni, suggesting an ultramafic component in their provenance. Cr and Ni are effective tracers for second-cycle components in sands of mixed provenance.

ABSTRACT The Sierra Nevada batholith of California represents the intrusive footprint of composite Mesozoic Cordilleran arcs built through pre-Mesozoic strata exposed in isolated pendants. Neoproterozoic to Permian strata, which formed the prebatholithic framework of the Sierran arc, were emplaced against the tectonically reorganized SW Laurentian continental margin in the late Paleozoic, culminating with final collapse of the fringing McCloud arc against SW Laurentia. Synthesis of 22 new and 135 existing detrital zircon U/Pb geochronology sample analyses clarifies the provenance, affinity, and history of Sierra Nevada framework rocks. Framework strata comprise terranes with distinct postdepositional histories and detrital zircon provenance that form three broad groups: allochthonous Neoproterozoic to lower Paleozoic strata with interpreted sediment sources from Idaho to northern British Columbia; Neoproterozoic to Permian strata parautochthonous to SW Laurentia; and middle to upper Paleozoic deposits related to the fringing McCloud arc. Only three sedimentary packages potentially contain detritus from rocks exotic to western Laurentia: the Sierra City mélange, chert-argillite unit, and Twin Lakes assemblage. We reject previous correlations of eastern Sierra Nevada strata with the Roberts Mountains and Golconda allochthons and find no evidence that these allochthons ever extended westward across Owens Valley. Snow Lake terrane detrital ages are consistent with interpreted provenance over a wide range from the Mojave Desert to central Idaho. The composite detrital zircon population of all analyses from pre-Mesozoic Sierran framework rocks is indistinguishable from that of the Neoproterozoic to Permian SW Laurentian margin, providing a strong link, in aggregate, between these strata and western Laurentia. These findings support interpretations that the Sierran arc was built into thick sediments underpinned by transitional to continental western Laurentian lithosphere. Thus, the Mesozoic Sierra Nevada arc is native to the SW Cordilleran margin, with the Sierran framework emplaced along SW Laurentia prior to Permian–Triassic initiation of Cordilleran arc activity.

ABSTRACT Spatial distributions of widespread igneous arc rocks and high-pressure–low-temperature (HP/LT) metamafic rocks, combined with U-Pb maximum ages of deposition from detrital zircon and petrofacies of Jurassic–Miocene clastic sedimentary rocks, constrain the geologic development of the northern and central Californian accretionary margin: (1) Before ca. 175 Ma, transpressive plate subduction initiated construction of a magmatic arc astride the Klamath-Sierran crustal margin. (2) Paleo-Pacific oceanic-plate rocks were recrystallized under HP/LT conditions in an east-dipping subduction zone beneath the arc at ca. 170–155 Ma. Stored at depth, these HP/LT metamafic blocks returned surfaceward mainly during mid- and Late Cretaceous time as olistoliths and tectonic fragments entrained in circulating, buoyant Franciscan mud-matrix mélange. (3) By ca. 165 Ma and continuing to at least ca. 150 Ma, erosion of the volcanic arc supplied upper-crustal debris to the Mariposa-Galice and Myrtle arc-margin strata. (4) By ca. 140 Ma, the Klamath salient had moved ~80–100 km westward relative to the Sierran arc, initiating a new, outboard convergent plate junction, and trapping old oceanic crust on the south as the Great Valley Ophiolite. (5) Following end-of-Jurassic development of a new Farallon–North American east-dipping plate junction, terrigenous debris began to accumulate as the seaward Franciscan trench complex and landward Great Valley Group plus Hornbrook forearc clastic rocks. (6) Voluminous deposition and accretion of Franciscan Eastern and Central belt and Great Valley Group detritus occurred during vigorous Sierran igneous activity attending rapid, nearly orthogonal plate subduction starting at ca. 125 Ma. (7) Although minor traces of Grenville-age detrital zircon occur in other sandstones studied in this report, they are absent from post–120 Ma Franciscan strata. (8) Sierra Nevada magmatism ceased by ca. 85 Ma, signaling transition to subhorizontal eastward underflow attending Laramide orogeny farther inland. (9) Exposed Paleogene Franciscan Coastal belt sandstone accreted in a tectonic realm unaffected by HP/LT recrystallization. (10) Judging by petrofacies and zircon U-Pb ages, Franciscan Eastern belt rocks contain clasts derived chiefly from the Sierran and Klamath ranges. Detritus from the Sierra Nevada ± Idaho batholiths is present in some Central belt strata, whereas clasts from the Idaho batholith, Challis volcanics, and Cascade igneous arc appear in progressively younger Paleogene Coastal belt sandstone.

ABSTRACT The Coast Ranges Province of central California provides an important record for the timing of convergence of the North American–Pacific plate boundary along the San Andreas fault system. The sedimentary record, in conjunction with seismic interpretation and backstripping methods, constrains the age of onset of Diablo and Temblor Range uplift and concurrent subsidence of the San Joaquin Basin along the San Andreas fault to 6.2–5.4 Ma. This age of convergence and uplift of the Coast Ranges is compatible with plate-tectonic circuit models, where clockwise rotation of Pacific–North American plate motion produced plate convergence at this latitude. However, changes in plate motion do not explain a widespread structural and sedimentary event at ca. 3.5 Ma that is evident in the western San Joaquin Basin and other basins in California. Possible drivers for the 3.5 Ma event include eustatic sea-level change, geomorphic forcing, epeirogeny related to mantle lithosphere removal, and flexural tilt of the Sierra Nevada–Great Valley microplate.

ABSTRACT The upper Middle Eocene to Lower Miocene Sespe Formation is the youngest part of an ~7-km-thick Cretaceous–Paleogene forearc stratigraphic sequence in coastal southern California. Whereas Upper Cretaceous through Middle Eocene strata of southern California record a transition from local (i.e., continental-margin batholith) to extraregional (i.e., cratonal) provenance resulting from Laramide deformation (75–35 Ma), the Sespe Formation records the reversal of this process and the re-establishment of local sediment sources by Middle Miocene time. In contrast to underlying dominantly marine strata, the Sespe Formation primarily consists of alluvial/fluvial and deltaic sandstone and conglomerate, which represent terminal filling of the forearc basin. Prior to Middle Miocene dissection and clockwise rotation, the Sespe basin trended north-south adjacent to the west side of the Peninsular Ranges. The integration of paleocurrent, accessory-mineral, conglomerate, sandstone, and detrital zircon data tightly constrains provenance. Sespe sandstone deposited in the Late Eocene was supplied by two major rivers (one eroding the Sonoran Desert, to the east, and one eroding the Mojave Desert, Colorado River trough area, and Transition Zone, to the north), as well as smaller local drainages. As the Farallon slab rolled back toward the coast during the Oligocene, the drainage divide also migrated southwestward. During deposition of the upper Sespe Formation, extraregional sources diminished, while the Peninsular Ranges provided increasing detritus from the east and the Franciscan Complex provided increasing detritus from the west (prerotation). As the overall flux of detritus to the Sespe basin decreased and deposition slowed, nonmarine environments were replaced by marine environments, in which the Miocene Vaqueros Formation was deposited. The provenance and paleogeographic information presented herein provides new insights regarding the unique paleotectonic setting of the Sespe forearc from the Late Eocene through earliest Miocene. Nonmarine sedimentation of the Sespe Formation initiated soon after cessation of coastal flat-slab subduction of the Laramide orogeny and terminated as the drainage divide migrated coastward. Competing models for movement along the Nacimiento fault system during the Laramide orogeny (sinistral slip versus reverse slip to emplace the Salinian terrane against the Nacimiento terrane) share the fact that the Peninsular Ranges forearc basin was not disrupted, as it lay south and southwest of the Nacimiento fault system. The northern edge of the Peninsular Ranges batholith formed a natural conduit for the fluvial system that deposited the Sespe Formation.

ABSTRACT Conglomerate-clast analysis, sandstone petrology, and detrital zircon age data determine the provenance and correlation of the Middle Miocene Mint Canyon and Caliente Formations. Detrital zircon age assemblages and sandstone compositions confirm that the Mint Canyon Formation is the upstream equivalent of the southern Caliente Formation and that both are dissimilar to the Punchbowl Formation. The Mint Canyon and Caliente Formations are remnants of an axial drainage system that was likely confined by the ancestral Sierra Pelona/Blue Ridge to the north and ancestral San Gabriel Mountains to the south; sediments derived from these local highlands dominate in the Mint Canyon Formation. This study highlights the importance of integrated-methods analysis. Sandstone detrital modes capture the variability in sandstone composition and the degree of overlap between formations; conglomerate-clast compositional data show differences within the drainage system; distinct detrital zircon age assemblages implicate particular source terranes. Analysis of all these data sets provides a robust and complete characterization of the provenance of this confined drainage system.

ABSTRACT Blocks of variably serpentinized oceanic mantle peridotite (harzburgite, olivine orthopyroxenite, and dunite) are entrained within a latest Cretaceous (“Laramide”) low-angle subduction channel, the Orocopia Schist, exposed at Cemetery Ridge, southwest Arizona. Oceanic peridotite, serpentinized by seawater, is strikingly out of place in this region of Paleoproterozoic to Jurassic continental crust. Correspondingly, the Cemetery Ridge peridotite contains four serpentinization-related sulfide or intermetallic minerals quite unusual for an inland, continental tectonic setting: pentlandite, cobalt pentlandite, heazlewoodite, and awaruite. The peridotite also contains two Ni-arsenide minerals, orcélite and maucherite; and, less commonly, the sulfides pyrrhotite, bismuthinite, bornite, and parkerite(?). These minerals form tiny to small (~3–100 μm) grains sparsely scattered amongst the profusion of serpentine and magnetite produced by serpentinization of olivine; many grains are enclosed in magnetite. The three principal sulfide minerals at Cemetery Ridge are pentlandite [(Ni,Fe) 8.93 S 8 ] in all samples studied, and cobalt pentlandite [(Co,Ni,Fe) 9.01 S 8 ] and heazlewoodite (Ni 2.98 S 2) in many or most. Pentlandite and cobalt pentlandite form a discontinuous series to 74 molar percent of the Co end member. High-Co pentlandite has systematically elevated Ni/Fe. Pyrrhotite (Fe 7.88 S 8) occurs only in one high-S sample. Although orcélite (Ni 4.71 As 2) and maucherite (Ni 11.06 As 8) are volumetrically rare, one or both are found in most samples. Awaruite (Ni 5 Fe) is extremely rare, a few minute blebs in two samples. Sulfide assemblages at Cemetery Ridge indicate the highly reduced pore fluid typical of serpentinization. Sulfur was introduced into Cemetery Ridge peridotite during the early stages of serpentinization, as one aspect of a general enrichment in fluid-mobile elements in a suprasubduction (mantle-wedge) environment. Further serpentinization was accompanied by progressive desulfurization, with concomitant transformation from minerals of higher to lower sulfidation, and partial transfer of first Fe then Ni from sulfide to oxide phases. Five Ni or Ni-Co sulfide or arsenide minerals at Cemetery Ridge are new and unexpected for Arizona, where serpentinite is quite rare and Ni or Co deposits unknown. Sulfide minerals and assemblages at Cemetery Ridge are comparable to those of serpentinites in such places as the California and Oregon Coast Ranges and Mid-Atlantic Ridge, emphasizing the uniqueness of the tectonic setting of the subducted oceanic peridotite at Cemetery Ridge, and enhancing the status of the Orocopia and related schists as the world’s type (first-recognized, best-known) low-angle paleosubduction complex.

ABSTRACT Different depositional environments (fluvial, deltaic, lacustrine, tidal-marine) have been proposed for the Upper Miocene to Pliocene Madre de Dios Formation exposed in the upper reaches of the Amazon River catchment in the Andean retroforeland region. This study constrains the stratigraphy, depositional environment, and drainage evolution in southwestern Amazonia through petrographic and provenance analysis of the sand and mud fractions of the three recognized members (A–C) of the Madre de Dios Formation at three stratigraphic sections measured previously along riverbank outcrops: Cerro Colorado, Piedras River, and Candelaria. Petrographic analyses of thin sections of sand separates from 32 sandy samples showed them to be litho-quartzose to quartzo-lithic in composition, with variable feldspar content and a recycled-orogen provenance. Sand components were predominantly monomineralic to polymineralic quartz, and sedimentary and metamorphic lithic fragments. Muscovite, potassium feldspar, plagioclase, and volcanic lithics were less abundant. These sand components are consistent with derivation from the Andean range to the west. Quartz and feldspar content generally increased up section from member A to member B. Sand composition in member C is similar to the modern river sand composition, consistent with recycling of Madre de Dios Formation sand into the modern river system. Petrographic analyses of 144 smear slides of the mud fractions showed no significant changes in silt composition, i.e., mainly quartz, feldspar, and mica, among the three members. X-ray diffraction of eight mud samples showed their clay mineralogy to be dominated by kaolinite and illite, with some smectite and chlorite in member B. None of the 144 smear slides or the 32 thin sections contained marine or marginal-marine biogenic debris. Mud-rich samples from the Madre de Dios Formation exhibit six main colors that characterize distinct intervals (designated I to VI) that occur in the same stratigraphic order in each measured section, from I at the base to VI at the top. The boundaries of these intervals do not correspond directly to member (A, B, C) boundaries; therefore, the colors are at least partly secondary. If primary, the red to orange to brown mudstone (which is dominant in Madre de Dios Formation members A and C) would suggest development in oxidizing environments, consistent with fluvial systems. Based on its light olive-gray color and smectite content, interval IV in member B may have been deposited in, or subjected to, a more reducing environment, such as a lacustrine-deltaic setting, with low-lying topography and poor drainage. In sum, the Madre de Dios Formation exhibits up-section compositional and thickness trends that are consistent with changes in depositional environment from fluvial (member A) to lacustrine/deltaic (member B) to fluvial (member C), as proposed by previous workers.

ABSTRACT High-resolution sand petrography and heavy mineral analyses help to frame U-Pb age and Hf isotope data from zircon grains, integrated in turn with geochemical data from detrital apatite, rutile, garnet, and monazite, and with Raman spectroscopy data from detrital amphibole, pyroxene, and epidote-group minerals. This multitechnique approach, including stream-profile analysis, was used to characterize components of the sediment flux and define erosion patterns across the Lhasa block, a complex continental arc terrane caught in the Himalayan collision. Litho-feldspatho-quartzose detrital modes and hornblende-dominated heavy mineral assemblages suggest that the majority (four fifths) of the sand bed load in the Lhasa River catchment is derived from erosion of granitoid batholiths. Gravel composition, however, is markedly different and dominated by volcanic pebbles in the trunk river, as in all of its four major tributaries, testifying to an order-of-magnitude difference in apparent erosion rates between granitoid batholiths and arc lavas. This marked contrast, partly explained by wide exposures of granitoid rocks in the rugged Nyainqêntanglha Range characterized by active incision, is notably amplified by the high sand-generation potential of granitoid rocks, which, in contrast to dense joint blocks of andesitic lavas, tend to disintegrate to sandy grus upon weathering. Sedimentary strata, making up a good half of exposed rocks, are also underrepresented in sand bed load, suggesting selective mechanical breakdown of nondurable shale/slate grains. This exposes a serious bias affecting estimates based on sand only, and it highlights the necessity for taking into account the entire size spectrum from mud to gravel in order to improve the accuracy of sediment budgets. Provenance analysis should involve multiple methods applied to multiple minerals, rather than be based solely on a single rare mineral, even if it is exceptionally laden with potential provenance information, such as zircon. We here divide arc-derived suites into those eroded from undissected arcs, in which nearly continuous volcanic cover is present, and those from dissected arcs, in which cogenetic plutons are widely exposed from erosional unroofing. —Dickinson and Suczek (1979, p. 2175)