Jeffrey M. Trop

Mesozoic and Cenozoic tectonics of the eastern and central Alaska Range: Progressive basin development and deformation in a suture zone

Analysis of late Mesozoic and Cenozoic sedimentary basins, metamorphic rocks, and major faults in the eastern and central Alaska Range documents the progressive development of a suture zone that formed as a result of collision of an island-arc assemblage (the Wrangellia composite terrane) with the former North American continental margin. New basin-analysis, structural, and geochronologic data indicate the following stages in the development of the suture zone: (1) Deposition of 3–5 km of Upper Jurassic–Upper Cretaceous marine strata (the Kahiltna assemblage) recorded the initial collision of the island-arc assemblage with the continental margin. The Kahiltna assemblage exposed in the northern Talkeetna Mountains represents a Kimmeridgian–Valanginian backarc basin that was filled by northwestward-flowing submarine-fan systems that were transporting sediment derived from Mesozoic strata of the island-arc assemblage. The Kahiltna assemblage exposed in the southern Alaska Range represents a Valanginian–Cenomanian remnant ocean basin filled by west-southwestward–flowing submarine-fan systems that were transporting sediment derived from Paleozoic continental-margin strata uplifted in the along-strike suture zone. A belt of retrograde metamorphism and a regional anticlinorium developed along the continental margin from 115 to 106 Ma, roughly coeval with the end of widespread deposition in the Kahiltna sedimentary basins. (2) Metamorphism of submarine-fan deposits of the Kahiltna ba sin, located near the leading edge of the island-arc assemblage, occurred at ca. 74 Ma, as determined from a new U-Pb zircon age for a synkinematic sill. Coeval with metamorphism of deposits of the Kahiltna basin in the southern part of the suture zone was development of a thrust-top basin, the Cantwell basin, in the northern part of the suture zone. Geologic mapping and compositional data suggest that the 4 km of Upper Cretaceous nonmarine and marginal marine sedimentary strata in this basin record regional subaerial uplift of the suture zone. (3) Shortening and exhumation of the suture zone peaked from 65 to 60 Ma on the basis of metamorphic and geochronologic data. In the southern part of the suture zone, submarine-fan deposits of the Kahiltna basin, which had been metamorphosed to kyanite schists at ∼25 km depth and ∼650 °C, were exhumed and cooled through the biotite closure temperature (∼300 °C) by ca. 62 Ma. In the northern part of the suture zone, this time period was marked by shortening, uplift, and erosion of sedimentary strata of the Cantwell basin. (4) From 60 to 54 Ma, ∼3 km of volcanic strata were deposited over deformed sedimentary strata of the Cantwell basin, and several granitic plutons (the McKinley sequence) were emplaced along the suture zone. (5) Following igneous activity, strike-slip displacement occurred from ca. 54 to 24 Ma along the Denali fault system, which had developed in the existing suture zone. Late Eocene–Oligocene strike-slip displacement resulted in the formation of several small sedimentary basins along the Denali fault system. (6) Regional transpressive shortening characterized the suture zone from ca. 24 Ma to the present. Flexural subsidence, related to regional shortening, is represented by late Eocene to Holocene nonmarine deposits of the Tanana foreland basin. Regional subsidence resulted in Miocene coal seams up to 20 m thick and well-developed lacustrine deposits. Overlying the Miocene deposits are ∼1.2 km of Pliocene and Holocene conglomeratic deposits. Compositional and paleocurrent data from these younger deposits record regional Neogene uplift of the suture zone and recycling of detritus from older basins to the south that had become incorporated into the uplifted suture zone. Geologic mapping of major thrust faults along the northern and southern margins of the suture zone documents Paleozoic strata thrust over both Pliocene fluvial deposits and Quaternary glacial deposits of the Tanana basin. These mapping relationships provide evidence that regional shortening continues to the present in the eastern and central Alaska Range.

Mesozoic sedimentary-basin development on the allochthonous Wrangellia composite terrane, Wrangell Mountains basin, Alaska: A long-term record of terrane migration and arc construction

The ∼7000 m of Upper Triassic–Upper Cretaceous strata of the Wrangell Mountains basin depositionally overlie the allochthonous Wrangellia composite terrane in south-central Alaska. New sedimentologic, compositional, and geochronologic data from these strata provide a record of the migration of the terrane from an intraoceanic setting at low paleolatitudes (∼12°N) to its present position along the continental margin of southern Alaska (∼61°N). We recognize several distinct stages of basin development: (1) Upper Triassic–Lower Jurassic carbonate strata represent an intraoceanic carbonate platform built on a remnant volcanic arc at low paleolatitudes (∼12°N). (2) During the Middle to Late Jurassic, a subduction zone formed along the southern margin of the Wrangellia composite terrane, prompting development of an intraoceanic arc and backarc basin on top of the terrane. (3) A narrow thrust belt and retroarc foreland basin formed along the inboard margin of the arc during the latest Jurassic. The foreland basin and arc were subsequently folded, uplifted, and eroded during the latest Jurassic–Early Cretaceous as recorded by an angular unconformity and isotopic ages from clasts in conglomerate. Regional deformation of the foreland-basin strata, shortening and uplift of the Wrangellia composite terrane, and uplift and erosion of the Middle to Late Jurassic arc are interpreted to represent the initial collision between the terrane and the continental margin of western North America. (4) Following regional deformation, a new volcanic arc was constructed inboard (northward) of the Wrangell Mountains basin. Upper Lower to Upper Cretaceous siliciclastic strata were deposited by shallow- to deep-marine deposystems in a continental-margin forearc basin. Distributions of lithofacies types and formation-thickness changes across intrabasinal normal faults document synextensional deposition. (5) The final stage of basin development was characterized by shortening and coarse-grained sedimentation along a fault system that separated the trenchward (southern) margin of the forearc basin from the subduction complex. (6) The Wrangell Mountains basin arrived at its current position by northward translation along orogen-parallel strike-slip fault systems. Comparison of the sedimentary record of the Wrangell Mountains basin, located on the outboard margin of the Wrangellia composite terrane, with the sedimentary record of the Nutzotin basin, located along the inboard margin of the terrane, demonstrates distinct changes in the locations of depocenters, the timing of deformation, and the composition of sediment. Similar stratigraphic and structural variations characterize outboard and inboard segments of the Wrangellia composite terrane in southeastern Alaska and coastal British Columbia.

Sedimentology and provenance of the Upper Jurassic Naknek Formation, Talkeetna Mountains, Alaska: Bearings on the accretionary tectonic history of the Wrangellia composite terrane

Analysis of the Upper Jurassic Naknek Formation in the Talkeetna Mountains, Alaska, documents synorogenic sedimentation in a forearc basin along the outboard (southern) margin of the allochthonous Peninsular terrane during accretion to the western North American continental margin. New geochronologic, sedimentologic, and compositional data defi ne a two-part stratigraphy for the Naknek Formation. Microfossil, megafossil, and U-Pb clast ages document early Oxfordian to early Kimmeridgian deposition of the lower 690 m of the Naknek Formation and early Kimmeridgian to early Tithonian deposition of the upper 225 m of the Naknek Formation. Lithofacies and paleocurrent data from the lower Naknek Formation demonstrate initial deposition on a high-gradient, southward-dipping basin fl oor. Submarine mass fl ows deposited poorly sorted, cobble-boulder conglomerate in proximal fan-delta environments. Gravelly mass fl ows transformed downslope into sandy turbidity currents on a muddy prodelta slope. During early Kimmeridgian to early Tithonian time, fan-delta environments were replaced by lower gradient marine shelf environments characterized by deposition of cross-stratifi ed sandstone and bioturbated mudstone. Source-diagnostic clasts, feldspathic sandstone compositions, southwarddirected paleocurrent indicators, and U-Pb zircon ages of plutonic clasts (167.6 ± 0.3 Ma; 166.5 ± 0.2 Ma, 164‐159 Ma, 156.2 ± 0.4 Ma) indicate that the Naknek Formation was derived primarily from volcanic and plutonic source terranes exposed along the northern basin margin in the southern Talkeetna Mountains. Geologic mapping documents the Little Oshetna fault, a newly identifi ed northward-dipping reverse fault that bounds the northern margin of the Naknek Formation in the Talkeetna Mountains. The concentration of boulder-rich mass-fl ow deposits in the footwall of the fault in combination with geochronologic and compositional data suggest that sedimentation was coeval with Late Jurassic shortening along the fault and exhumation of plutonic source terranes exposed in the hanging wall of the fault. From a regional perspective, coarse-grained forearc sedimentation and pluton exhumation along the outboard (southern) segment of the Peninsular terrane were coeval with crustal-scale shortening and synorogenic sedimentation in retroarc basins along the inboard (northern) margin of the Wrangellia terrane (Kahiltna, Nutzotin, and Wrangell Mountains basins). We interpret the regional and synchronous nature of Late Jurassic crustal-scale deformation and synorogenic sedimentation in south-central Alaska as refl ecting either initial collision of the Wrangellia and Peninsular terranes with the former continental margin of western North America or amalgamation of the two terranes prior to collision.

Thrust-top basin formation along a suture zone, Cantwell basin, Alaska Range: Implications for development of the Denali fault system

The Cantwell Formation consists of a lower sedimentary sequence as much as 4000 m thick and an upper volcanic sequence with a maximum thickness of 3750 m that was deposited in the Cantwell basin, south-central Alaska. Previous to this study, the Cantwell basin was interpreted as a Paleogene, nonmarine (mainly fluvial), pull-apart basin that formed in response to dextral, strike-slip displacement on the Denali fault system. This study proposes that the Cantwell basin formed as part of the Mesozoic accretionary phase of deformation, prior to the development of the Cenozoic postaccretionary Denali fault system. Our reinterpretation is based on several new lines of data. (1) Age . New data based on palynologic analyses of 135 fine-grained samples indicate that the lower Cantwell Formation was deposited during the late Campanian and early Maastrichtian. On the basis of previous regional tectonic studies and this new age constraint, the formation of the Cantwell basin was coeval with regional Late Cretaceous shortening associated with accretionary tectonics in southern Alaska. (2) Depositional systems . Our analysis of the Cantwell Formation demonstrates that sedimentation occurred mainly in stream-dominated alluvial fan, axial braided stream, and lacustrine settings. These depositional systems were strongly influenced by a southward dipping, asymmetric basin floor. The presence of abundant terrestrially derived organic material, together with palynological assemblages that include marine dinoflagellates and the associated presence of oncolites, may be suggestive of a time of marginal marine influence during the deposition of the upper part of the lower Cantwell Formation. The late Campanian to early Maastrichtian timing of this possible marine influence is within the range of the Bearpaw transgressive event of the Cordilleran foreland basin and allows for regional stratigraphic correlation of the Cantwell basin with other sedimentary basins in northwestern North America. (3) Structural controls on basin formation . Mapping of intraformational angular unconformities and progressively tilted strata along the southern margin of the Cantwell basin provides direct evidence that thrust fault deformation and lower Cantwell Formation sedimentation were synchronous. Distinctive Cantwell Formation conglomerate clasts derived from the uplifted hanging walls of nearby thrust sheets adjacent to the southern basin margin also support a syndepositional thrusting interpretation. Provenance data and the concentration of proximal alluvial fan deposits along the northwestern basin margin adjacent to the Hines Creek fault indicate that it, too, was active during deposition of the Cantwell Formation. On the basis of the new data, the Cantwell basin is interpreted to have formed as a thrust-top basin (i.e., piggyback basin) along the Late Cretaceous suture zone between the accreting Wrangellia composite terrane and the North American continental margin. In contrast to previous studies, this reinterpretation of the formation of the Cantwell basin implies that the lower Cantwell Formation is not a synorogenic deposit directly associated with strike-slip displacement along the Denali fault system. Therefore, the Cantwell basin cannot be used to constrain the timing for the early development of the Denali fault system.

Late Cretaceous paleogeography of Wrangellia: Paleomagnetism of the MacColl Ridge Formation, southern Alaska, revisited

Volcanic and sedimentary strata of the Late Cretaceous MacColl Ridge Formation were sampled and demagnetized to reevaluate the paleomagnetically derived paleolatitude of the allochthonous Wrangellia terrane. Characteristic directions from 15 sites representing ∼750 m of the MacColl Ridge Formation (80 Ma) reveal a reversed-polarity primary magnetization yielding a paleomagnetic pole at 126°E, 68°N, A 95 = 9°. Comparison of this pole with the Late Cretaceous reference pole for North America indicates 15° ± 8° of latitudinal displacement (northward) and 33° ± 25° of counterclockwise rotation. In contrast to previously reported low paleolatitudes (32° ± 9°N) for the MacColl Ridge Formation, these new results place the Wrangellia terrane at a moderate paleolatitude (53° ± 8°N) in the Late Cretaceous.

Latest Cretaceous forearc basin development along an accretionary convergent margin: South-central Alaska

Upper Cretaceous sedimentary strata exposed in south-central Alaska provide insight on tectonic processes that shaped the northern Pacific margin following accretion of the Wrangellia composite terrane, the largest addition of crust to North America over the past 100 m.y. Sedimentologic, geochronologic, biostratigraphic, and petrographic data from the Matanuska Formation permit reconstruction of the tectono-sedimentary history of strata in a forearc basin constructed upon accreted oceanic-arc crust. The Matanuska Formation consists of >3 km of sedimentary strata exposed in the northern Chugach Mountains, Matanuska Valley, and southern Talkeetna Mountains of interior south-central Alaska. Measured stratigraphic sections and lithofacies analyses demonstrate that mass slumps and slides, debris flows, and turbidity currents deposited Campanian–Maastrichtian sandstone, conglomerate, and mudstone on a gullied, trenchward-dipping submarine ramp. Benthic foraminifera, inoceramid bivalves, and Nereites ichnogenera indicate deposition mainly at bathyal water depths. Sandstone and conglomerate petrofacies are characterized by monocrystalline quartz, plagioclase feldspar, and volcanic lithic fragments (Q39F40L21, Qm29F40Lt32, Lm25Lv42Ls32, and Qm42P54K4). Jurassic–Cretaceous arc plutons exposed north of the basin were an important sediment source, based on U-Pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone. Coeval arc plutons were unroofed relatively quickly, judging by the presence of 77–71 Ma detrital zircons in sandstone and 79–77 Ma granitic clasts in conglomerate, together with Maastrichtian (71–65 Ma) ammonite and foraminifera fossils. Sparse Paleozoic–Triassic detrital zircons indicate minor sediment contribution from inboard sources, including the Yukon-Tanana composite terrane and recycled Jurassic–Cretaceous sedimentary strata (Kahiltna assemblage). New data from the upper Matanuska Formation, together with recent studies from age-equivalent strata exposed in the Alaska Range and Wrangell Mountains, provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc. Forearc basin development was dominated by subsidence and sediment gravity flow deposits enriched in plutonic and volcanic clasts eroded from both remnant- and coeval-arc plutons. Within the arc, newly recognized conglomerate in the northern Talkeetna Mountains records erosion of coeval- and remnant-arc source terranes to the south and Precambrian–Paleozoic sources to the north. Farther inboard, syndepositional shortening prompted thrust-top basin development and accumulation of alluvial-lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south. Regional subsidence and basin development terminated during late Maastrichtian–early Paleocene time, coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center.

Sedimentary record of transpressional tectonics and ridge subduction in the Tertiary Matanuska Valley-Talkeetna Mountains forearc basin, southern Alaska

The Chickaloon, Arkose Ridge, Wishbone, and Tsadaka Formations consist of more than 2800 m of Paleocene-Oligocene sedimentary and volcanic strata that are the products of sedimentation, volcanism, and faulting in the Matanuska Valley-Talkeetna Mountains forearc basin. These deposits provide a record of early Tertiary tectonic processes that formed the southern Alaska convergent margin. The northern margin of the forearc basin is characterized by nonmarine sandstone, conglomerate, and minor mudstone that interfinger with volcanic strata. On the basis of lithofacies, paleocurrent, and compositional data, the northern basin margin deposits are interpreted to represent southward prograding alluvial-fluvial systems. New 4 0 Ar/ 3 9 Ar ages from detrital feldspars in volcaniclastic sandstone and from igneous clasts in conglomerate suggest that these deposits were derived from Middle Jurassic and Paleocene-Eocene volcanic arc-related rocks. Stratigraphic and structural data from the northern basin margin adjacent to the Castle Mountain fault document syndepositional faulting that produced footwall growth synclines in the forearc basin. Paleocene-Oligocene strata exposed along the southern margin of the forearc basin are characterized by nonmarine sedimentary deposits that lack volcanic strata. Lithofacies, paleocurrent, and compositional data from these deposits are interpreted as recording northward prograding alluvial-fluvial systems that were derived from metavolcanic and metasedimentary source terranes of the accretionary prism. Both northern and southern basin-margin deposits merge into basin-axis deposits characterized by thick sections of carbonaceous mudstone and coal, and minor channelized sandstone. Basin-axis strata are interpreted as products of high-sinuosity fluvial and lacustrine environments that drained southwestward into the ancestral Cook Inlet basin. Unlike most previously studied ancient forearc basins, the Matanuska Valley-Talkeetna Mountains basin contains a fairly complete stratigraphic record of nonmarine sedimentation and volcanism. These deposits record multiple episodes of transpressional deformation that may be related to northward translation of the forearc basin along the continental margin, oroclinal bending of Alaska, and/or subduction of a spreading ridge. To evaluate the record of ridge subduction in Paleocene-Oligocene forearc basin deposits, two reconstructions are presented. In one reconstruction, the forearc basin and accretionary prism were translated northward as a single block with most displacement accommodated along inboard dextral strike-slip faults such as the Castle Mountain and Denali fault systems. In this reconstruction, ridge subduction beneath the forearc basin would have occurred at ca. 54-50 Ma, coeval with basinward progradation of coarse-grained deposystems, and with syndepositional displacement and growth-syncline development along the Castle Mountain fault. In the second reconstruction, in addition to displacement on inboard strike-slip faults, significant northward displacement was accommodated along the Border Ranges and Hanagita faults. These fault systems separated the forearc basin from the accretionary prism. In this reconstruction, the forearc basin and accretionary prism were translated separately and have different displacement histories; ridge subduction beneath the forearc basin would have occurred at ca. 61-58 Ma. The sedimentary record of ridge subduction in this reconstruction is represented by a basinwide unconformity and/or deposition of relatively fine-grained deposits in the forearc basin. We prefer the first reconstruction, but until additional high-resolution geochronological data are available, and the displacement histories of major fault systems are better known, both reconstructions are feasible.

Integration of Field Observations with Laboratory Modeling for Understanding Hydrologic Processes in an Undergraduate Earth-Science Course

Understanding how water is transported and stored in the subsurface is a difficult concept for introductory earth-science students. We have developed a hydrology minicourse that integrates field and laboratory experiences to help undergraduate students gain a better understanding of ground-water flow in aquifers. The centerpiece of the minicourse is an investigative field trip that permits analysis of a local aquifer that provides drinking water for the university community. Students collect qualitative and quantitative field data on grain size, thickness, and geometry of different stratigraphic horizons within the aquifer and then construct a small-scale laboratory model of the aquifer using boundary conditions determined from the field investigation. The aquifer model allows students to test hypotheses of ground-water flow by conducting a series of modeling experiments. The experiments test questions such as: “What is the influence of porosity and permeability on ground-water flow?” and “What is the effect of regional dip on ground-water flow?” Analysis of pre- and post-minicourse examinations demonstrates that students are able to better communicate fundamental hydrologic concepts after completing the minicourse.

Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat–North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene–Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and 40 Ar/ 39 Ar ages of lavas indicate that basin development and volcanism commenced by 12.5–11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5–9.3 Ma) than western sections (9.6–8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

Cenozoic strata from forearc basins in southern Alaska record deposition related to two different types of shallow subduction: Paleocene–Eocene spreading-ridge subduction and Oligocene–Recent oceanic plateau subduction. We use detrital zircon geochronology (n = 1368) and clast composition of conglomerate (n = 1068) to reconstruct the upper plate response to these two subduction events as recorded in forearc basin strata and modern river sediment. Following spreading-ridge subduction, the presence of Precambrian and Paleozoic detrital zircon ages in middle Eocene–lower Miocene arc-margin strata and Early Cretaceous ages in lower Miocene accretionary prism–margin strata indicates that sediment was transported to the basin from older terranes in interior Alaska and from the exhumed eastern part of the Cretaceous forearc system, respectively. By middle-late Miocene time, diminished abundances of these populations reflect shallow subduction of an oceanic plateau and associated exhumation that resulted in an overall contraction of the catchment area for the forearc depositional system. In the southern Alaska forearc basin system, upper plate processes associated with subduction of a spreading ridge resulted in an abrupt increase in the diversity of detrital zircon ages that reflect new sediment sources from far inboard regions. The detrital zircon signatures from strata deposited during oceanic plateau subduction record exhumation of the region above the flat slab, with the youngest detrital zircon population reflecting the last period of major arc activity prior to insertion of the flat slab. This study provides a foundation for new tectonic and provenance models of forearc basins that have been modified by shallow subduction processes, and may help to facilitate the use of U-Pb dating of detrital zircons to better understand basins that formed under changing geodynamic plate boundary conditions.