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Lithium resources. When lithium emerged as a critical element in the transition from gas-powered to electric cars, it became clear that lithium itself is a finite resource and that lithium deposits have never been a high research priority in global metallogeny. It became one for me starting in 2009. I’m working on ore-genesis models for the two deposit types that account for virtually all of today's lithium production: brines and pegmatites.
Pegmatites account for about one third of the world’s lithium production, most of the tantalum, and all of the cesium. Pegmatites enriched in these elements (LCT pegmatites) are widely interpreted as extreme fractionation products of orogenic granitic melts, although it is not always possible to tie a particular pegmatite to a known granite of the same age. A key question that bears on both metallogenesis and exploration strategies is: Why are some orogenic belts well endowed with LCT pegmatites, while other, seemingly similar orogens are barren? Andrew McCauley and I have found that the global age distribution of LCT pegmatites is similar to the age distributions of common pegmatites, of orogenic granites, and of detrital zircons. LCT pegmatite maxima at ca. 2650, 1800, 525, 350, and 100 Ma correspond to times of collisional orogeny, and except for the comparatively minor peak at 100 Ma, to times of supercontinent assembly. Between these pulses are long intervals of few or no LCT pegmatites. The lows overlap with supercontinent tenures at ca. 2450–2225, 1625–1000, 875–725, and 300-200 Ma (see heading below on Secular Trends). We are currently dating a number of LCT pegmatites in order to assess connections with plate-tectonic events. The main targets are the Phanerozoic pegmatites of the Pangean collisional orogens, especially the Appalachians. This work is a collaboration with Sam Bowring, Robert Buchwaldt, and Erin Shea at MIT's CA-TIMS facility. Our 2012 abstract at the Annual Meeting of the Geological Society of America can be found at https://gsa.confex.com/gsa/2012AM/webprogram/Paper208315.html. We have an LCT pegmatite ore-deposit model in review; a preliminary, shortened version of it came out in early 2013 (click here - pdf file 858 kb).
Brines are already the most important lithium source and have the added near-term advantage of being easiest to look for and exploit. These brines occur as shallow groundwater beneath certain salars (salt flats) in the Basin & Range and elsewhere. The obvious favorable factors include (1) closed basin; (2) arid climate; (3) lithium-rich, high-silica glassy volcanics in the catchment; (4) geothermal activity; (5) tectonically driven basin subsidence; (6) active faulting; and (7) time. The broader question is: Why are brines enriched in lithium in only a small subset of salars that meet these criteria? I'm studying the tectonic controls of Li-brine genesis in Clayton Valley, Nevada in collaboration with LeeAnn Munk from University of Alaska and Scott Hynek from University of Utah. Click here (Word doc 668 kb) to download a 2011 SGA abstract by Lee Ann Munk and others on our lithium brine research. Click here (pdf file 1.1 mb) for our most recent publication: a preliminary ore-deposit model for lithium brines.
Secular trends in the geologic record, ore deposits, and the supercontinent cycle. Plate reconstructions are a time-honored way to synthesize earth history, but this approach has two disadvantages: it is futile in the deep past and it doesn’t reveal much about evolution of the Earth system. Much of my research in global tectonics over the past decade has focused on an alternative: geologic secular trends. Countless geologic variables can be tracked through time—even times when plate reconstructions are out of the question—making it possible to sidestep the unknown specifics of plate paleogeography and instead focus on evolution of the Earth system as a whole. My 2011 paper in Earth-Science Reviews (link pdf file 4.6 mb) is a state-of-the-art global synthesis of >100 secular trends including age distributions of rocks and minerals, geochemical trends, censuses of tectonic settings, numerical model results, and more. Many secular trends show episodic fluctuations on a roughly half-billion-year time scale that reflect the supercontinent cycle, whereas other trends paint the changing backdrop against which the succession of supercontinents came and went. I argue that the supercontinent-related fluctuations are driven by global plate reorganizations that inevitably happen when continents either break up or come together. Here is the abstract:
Geologic secular trends are used to refine the timetable of supercontinent assembly, tenure, and breakup. The analysis rests on what is meant by the term supercontinent, which here is defined broadly as a grouping of formerly dispersed continents. To avoid the artificial pitfall of an all-or-nothing definition, quantitative measures of “supercontinentality” are presented: the number of continents, and the area of the largest continent, which both can be gleaned from global paleogeographic maps for the Phanerozoic. For the secular trends approach to be viable in the deep past when the very existence of supercontinents is debatable and reconstructions are fraught with problems, it must first be calibrated in the Phanerozoic against the wellconstrained Pangea supercontinent cycle. The most informative geologic variables covering both the Phanerozoic and Precambrian are the abundances of passive margins and of detrital zircons. Both fluctuated with size of the largest continent during the Pangea supercontinent cycle and can be quantified back to the Neoarchean. The tenure of Pangea was a time represented in the rock record by few zircons and few passive margins. Thus, previously documented minima in the abundance of detrital zircons (and orogenic granites) during the Precambrian (Condie et al., 2009a, Gondwana Research 15, 228–242) now can be more confidently interpreted as marking the tenures of supercontinents. The occurrences of carbonatites, granulites, eclogites, and greenstone-belt deformation events also appear to bear the imprint of Precambrian supercontinent cyclicity. Together, these secular records are consistent with the following scenario. The Neoarchean continental assemblies of Superia and Sclavia broke up at ca. 2300 and ca. 2090 Ma, respectively. Some of their fragments collided to form Nuna by about 1750 Ma; Nuna then grew by lateral accretion of juvenile arcs during the Mesoproterozoic, and was involved in a series of collisions at ca. 1000 Ma to form Rodinia. Rodinia broke up in stages from ca. 1000 to ca. 520 Ma. Before Rodinia had completely come apart, some of its pieces had already been reassembled in a new configuration, Gondwana, which was completed by 530 Ma. Gondwana later collided with Laurentia, Baltica, and Siberia to form Pangea by about 300 Ma. Breakup of Pangea began at about 180 Ma (Early Jurassic) and continues today. In the suggested scenario, no supercontinent cycle in Earth history corresponded to the ideal, in which all the continents were gathered together, then broke apart, then reassembled in a new configuration. Nuna and Gondwana ended their tenures not by breakup but by collision and name change; Rodinia's assembly overlapped in time with its disassembly; and Pangea spalled Tethyan microcontinents throughout much of its tenure. Many other secular trends show a weak or uneven imprint of the supercontinent cycle, no imprint at all. Instead, these secular trends together reveal aspects of the shifting background against which the supercontinents came and went, making each cycle unique. Global heat production declined; plate tectonics sped up through the Proterozoic and slowed down through the Phanerozoic; the atmosphere and oceans became oxidized; life emerged as a major geochemical agent; some rock types went extinct or nearly so (BIF, massif-type anorthosite, komatiite); and other rock types came into existence or became common (blueschists, bioclastic limestone, coal).
The most tantalizing new line of inquiry to come out of this research is what I call the “zircon plate speedometer”—the idea that most zircons are the products of plate convergence, and hence that global zircon abundance through time is a function of global subduction flux. Subject to testing of the hypothesis, calibration, correction for a number of artifacts, and analysis of a carefully selected global sample suite (ca. 600 samples; 60,000 individual zircons), the detrital zircon record has the untapped potential to quantify global (but not individual) plate motions back to at least 3 Ga. A semi-quantitative appraisal suggests that during four intervals of supercontinent tenure (ca. 2300, 1500, 850, and 300 Ma), global subduction flux dwindled to less than half of its value before or after these “dead times”.
Passive margins through Earth history. Secular decline in global heat production through Earth history might be expected to have been matched by a secular global spreading rates, and hence, in the tempo of Wilson Cycles. This is testable, but as it turned out, time consuming: I spent parts of six years on a global compilation of ancient passive margins. The main findings are that the abundance of passive margins has fluctuated significantly over time, marking the repeated assembly and breakup of continental groupings, and that the tempo of Wilson Cycles appears to have been slower—not faster—in the Precambrian. My 2008 paper in Earth-Science Reviews, which includes Appendices detailing the histories of 80+ passive margins, with 250+ references, can be downloaded at this link (link - pdf file 5.41mb).
Here are the abstract and two figures:
Passive margins have existed somewhere on Earth almost continually since 2740 Ma. They were abundant at 1900-1890, 610-520, and 150-0 Ma, scarce at ca. 2445-2300, 1600-1000, and 300-275 Ma, and absent before ca. 3000 Ma and at 1740-1600 (Fig. 1). The fluctuations in abundance of passive margins track the first-order fluctuations of the independently derived seawater 87Sr/86Sr secular curve, and the compilation thus appears to be robust (Fig. 1). The 76 ancient passive margins for which lifespans could be measured have a mean lifespan of 181 m.y. The world-record holder, with a lifespan of 590 m.y., is the Mesoproterozoic eastern margin of the Siberian craton. Subdivided into natural age groups, mean lifespans are 186 m.y. for the Archean to Paleoproterozoic, 394 m.y. for the Mesoproterozoic, 180 m.y. for the Neoproterozoic, 137 m.y. for the Cambrian to Carboniferous, and 130 m.y. for the Permian to Neogene. The present-day passive margins, which are not yet finished with their lifespans, have a mean age of 104 m.y. and a maximum age of 180 m.y. On average, Precambrian margins thus had longer, not shorter, lifespans than Phanerozoic ones—and this remains the case even discounting all post-300 Ma margins, most of which have time left. Longer lifespans deeper in the past is at odds with the widely held notion that the tempo of Wilson Cycles was faster in the Precambrian than at present. It is entirely consistent, however, with recent modeling by Korenaga (2006. Archean geodynamics and thermal evolution of Earth. AGU Geophysical Monograph Series 164, 7-32), which showed that plate tectonics was more sluggish in the Precambrian. The abundance of passive margins clearly tracks the assembly, tenure, and breakup of Pangea. Earlier parts of the hypothesized supercontinent cycle, however, are only partly consistent with the documented abundance of passive margins. The passive-margin record is not obviously consistent with the proposed breakup of Nuna (Columbia), the assembly of Rodinia, or the assembly or breakup of the putative Pannotia. An alternative model is put forth involving (a) formation of two or more supercratons during the late Paleoproterozoic, (b) a Mesoproterozoic interval dominated by lateral accretion of arcs rather than by continental breakup and dispersal, (c) wholesale collision to form Rodinia by the end of the Mesoproterozoic, and (d) staged breakup of Rodinia through much of the Neoproterozoic.
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|Figure 1. (a) Distribution of passive margins through time. The ancient margins are shown in color, with the colors of bins reflecting the quality of the age control (red is tightest). The modern margins, with still incomplete lifespans, are in gray. (b) Continental contribution to seawater Sr, from Shields (2007). The general correspondence between the two totally independent datasets suggest that the passive margin distribution is robust, and not an artifact of a flawed census.|
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|Figure 2. Lifespans of passive margins, subdivided into age groupings implied by Figure 1. Precambrian margins lasted longer, and Mesoproterozoic ones had the longest lifespans of all. This unexpected finding is at odds with conventional wisdom but supports recent modeling by Korenaga on the thermal evolution of Earth, which predicted that plate tectonics was more sluggish in the Precambrian.|
Detrital zircons of the Alaska, the Lower 48, and the world. The rapidly growing field of detrital zircon geochronology has revitalized regional and global tectonic analysis. Detrital zircon data yield information about provenance, depositional age, plate interactions (at a surprisingly vast scale), Earth history, and geodynamics. The "problem" that looms is a glut of underutilized but potentially valuable data: worldwide, at least 2000 individual detrital zircons from at least 30 sandstones are being dated every day; dated zircon grains must already number in the millions. At present, new results can only be assimilated and pondered in an ad hoc manner, diminishing the potential impact of each detrital zircon study. To better take advantage of this new technology, my colleague Heather Bleick and I have begun a systematic program to capture detrital zircon data, and to develop a set of GIS tools for data analysis and display. Data from Alaska are being compiled first (Fig. 3), to be followed by data from the Lower 48, then from elsewhere in North America, then from places such as Siberia and West Africa that were once adjacent to parts of the United States, and finally other parts of the world. The prototype database will be made public in late 2008 or early 2009. We invite collaboration with others having similar aims.Most of my detrital zircon research has focused on the older rocks of Alaska: the Farewell, Ruby, and Yukon-Tanana terranes. Bradley et al. (2007) (link - pdf file 934k), which covers results obtained through 2004. A more recent summary (link - pdf file 667k) updates the 2007 paper and includes new findings from the oldest known supracrustal rocks in Alaska: a Mesoproterozoic? carbonate-dominated metasedimentary assemblage in the Kilbuck terrane.
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|Figure 3. Locations of Alaskan detrital zircon samples (some already published, others yet to be published) that had been captured in the statewide database as of October 2008. Another hundred or so DZ samples still need to be compiled. Map courtesy of Heather Bleick, USGS, Anchorage.|
Tectonics and regional geology of southwest Alaska. Southwestern Alaska is one of the few remaining geologic frontiers in North America. Some parts have simply never been geologically mapped even at the reconnaissance scale of 1:250,000, despite the impression given by state-scale compilation maps. In 2004, under the lead of Marti Miller, the USGS began a new mapping effort focused on key parts of the Taylor Mountains 1:250,000-scale quadrangle (link), where the Cretaceous Kuskokwim flysch basin buries the suture zone between the Togiak-Goodnews arc/subduction complex and the Farewell microcontinent. The main resource potential lies in ~60 Ma tin granites and ~70-Ma epithermal mercury and pluton-related gold. Previous work in southwest Alaska, with project chief Marti Miller and Tom Bundtzen, resulted in a new geologic map of the Sleetmute 1:250,000-scale quadrangle (not yet published), a detailed study of the genesis of the Donlin Creek gold deposits (link - pdf file 2.16mb), and a synthesis of the history of two regional-scale strike-slip faults, the Denali and Iditarod-Nixon Fork (link - pdf file 2.33mb).
Mauritania. The USGS had a two-year contract in 2006-2008 with the Mauritanian government to assess the nation's mineral resource potential. My main roles were to compile a national-scale geologic map of this west African nation, and geochronology. In October-November 2007, during a 5000+ kilometer Saharan expedition, we collected a huge suite of igneous rocks (U-Pb zircon), metamorphic rocks (40/39 white mica and biotite), and metasandstones and sandstones (detrital zircon U-Pb). The detrital zircon samples include key supracrustal formations of the Archean and Paleoproterozoic of the Reguibat Shield, the Neoproterozoic to Paleozoic Mauritanide orogen, and the Mesoproterozoic to Paleozoic Taudeni Basin. This sample suite has immense potential for clarifying the regional tectonic and metallogenic history, including the place (if any) of the West Africa in Rodinia. All work was mothballed for geopolitical reasons relating to a coup d'etat in August 2008.
The Mauritania work is now back on track. In 2012 we'll complete the geologic map of Mauritania at 1,000,000 scale (thanks in large part to Holly Motts) and will have some awesome new geochronology results including a tight U-Pb TIMS age (by Sam Bowring and Jahan Ramezani at MIT) from three meters above a Neoproterozoic cap carbonate.
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|Figure 4. Map of the west African nation of Mauritania showing main tectonic elements and the route (in red) of the 2007 joint U.S-Mauritanian geology expedition. Mauritania is about the size of Texas but has only 3 million people.|
Ridge subduction and slab window tectonics (Paleocene-Eocene, Alaska). During the early Tertiary, subduction of a spreading center left a clear trail along Alaska's Pacific margin, in the form of a 2200-km-long belt of near-trench plutons. This project is addressing the hypothesis that an enormous area of interior Alaska was likewise affected at this time by a "slab window"-a gap between subducted but still diverging plates. Various project members (Alison Till, Peter Haeussler, Sue Karl, Marti Miller, Rich Goldfarb, Robert Ayuso, David Farris and others) are studying aspects of deformation, metamorphism, basin evolution, mineralization, and magmatism in Alaska, with a particular emphasis on geochronology (with Rich Friedman, Jim Mortensen, and Paul Layer), which will ultimately link all these things into a coherent picture. Evidence now in hand suggests that seven types of ore deposits formed above the postulated slab window (or above the thin, young edge of the pre-window slab) and that a slab window may have played a significant role in the evolution of petroleum systems of Cook Inlet forearc basin and the North Slope. Publications include:
Origin of Mississippi Valley-type (MVT) lead-zinc deposits. Only twenty years ago, few workers could see a meaningful connection between this important deposit type and plate tectonics. Thanks in large part to the career contributions of my colleague Dave Leach, we now know that most MVT deposits are hosted in pre-orogenic carbonate sequences in orogenic forelands and that they form from huge, continental-scale hydrothermal systems (link - pdf file 4.18mb). Taking advantage of new ages of MVT mineralization, we have been able to draw specific connections between mineralization and foreland tectonic features including forebulge unconformities and flexurally induced normal faults (link - pdf file 2.56mb).
Chugach accretionary complex, south-central Alaska. During the late 1980s and early 1990s, Tim Kusky, Peter Haeussler, Sue Karl, and I completed the first geologic map of the Seldovia 1:250,000-scale quadrangle, which features superb coastal exposures of the Chugach accretionary complex-Alaska's equivalent of the Franciscan. Although the coast of the Seldovia quadrangle had been mapped during the early 1900's, the inaccessible interior was essentially unmapped. Our work on ridge subduction started here. We also focused on the accretionary history: the Chugach terrane is about all that remains of a vast tract of paleo-Pacific seafloor that has been subducted since at least the Jurassic, and it is therefore key to plate reconstructions. In addition to those having to do with ridge subduction (see above), publications include:
Farewell terrane, a "Siberian" fragment in interior Alaska. The Farewell terrane is a microcontinental fragment, about the size of Switzerland, surrounded by other accreted terranes in the Alaskan interior. It includes Neoproterozoic (908-850 Ma) basement, a carbonate platform (Nixon Fork subterrane), and a deep-water basin (Dillinger subterrane). The terrane was long regarded as a displaced fragment of the continental margin of western Canada, but now turns out to have Siberian faunal affinities at least from Cambrian to Devonian time. This finding has calls into question the entire assembly history of Alaska, which have traditionally rested on a North American origin for the Farewell. This project involved collaboration with Julie Dumoulin, Anita Harris, Dave Sunderlin, Bill McClelland, Paul Layer, Sarah Roeske, and others. Publications include:
Acadian Tectonics, Northern Appalachians. Since the early 1970s, it has been recognized thathere the Acadian Orogeny in the Northern Appalachians was diachronous, younger toward the craton. My earliest work on the Acadian involved developing the first tectonic model to relate orogenic loading to foreland basin subsidence and migration (Bradley, 1983). In the late 1990s, with collaborators Bob Tucker, Anita Harris, Dan Lux, and Colin McGregor, I revisited the Acadian to try to quantify rates of plate motion during collision. We precisely dated a number of pre-, syn-, and post-Acadian rock units across a transect through Maine and Quebec. These included carbonate-platform and foreland-basin deposits, which were dated by conodonts and palynomorphs, and plutons, which were dated by U/Pb and 40Ar/ 39Ar. Using the new time scale (see next heading), we demonstrated that collision was underway between 423 and 384 Ma, that the plate convergence rate was at least 1 cm/yr over this time, and that at least 400 km of plate convergence occurred (link). A separate paper was devoted to the early Emsian configuration of the orogen, when an enigmatic magmatic province straddled the deformation front (link - pdf file 694k).
Devonian time scale. As of the mid-1990s, existing time scales were based almost entirely on old Rb-Sr and conventional K-Ar dates with large error bars, poor paleontological age control, and we now realize, very poor accuracy. This work was a collaboration with Bob Tucker, Anita Harris, Chuck ver Straaten, and others. Using isotope dilution thermal ionization mass spectrometry (TIMS), we obtained precise, concordant U/Pb ages for four ash beds with tight fossil control. Our dating has moved most of the Devonian stage boundaries back by 10-15 million years (link - pdf file 1.21mb). The subsequent 2004 Geologic Time Scale by Gradstein and Ogg incorporated all of our age constraints, plus a few new ones. In general, their age picks for the Devonian stage boundaries are younger than ours by one or two million years.
Paleoslope and paleocurrent analysis in orogenic belts. Much of my earlier work in the Acadian orogen of Maine involved sedimentology of metasedimentary rocks, in strata that push the useful limits of paleocurrent and paleoslope analysis. Two papers resulted; both studies bear directly on the Devonian paleogeography of the Acadian orogen, but the methods are applicable worldwide.
Flexural extension of the upper continental crust in collisional foredeeps. A 1991 Bradley & Kidd paper in GSA Bulletin (click here) was the first published synthesis of the regional-scale structural response of continental lithosphere to extreme flexure. We coined the term "flexural extension" for this process. Flexural extension differs fundamentally from whole-lithosphere extension resulting from plate divergence or gravitational spreading, because in flexural extension, the lower lithosphere is shortened while the upper crust is extended. Normal faulting on a regional scale does not always equal rifting; this has provided a reasonable explanation for otherwise inexplicable extension events in a variety of collisional forelands including the Taconic, the Ouachita, the Carpathian of Poland, the Alps, the Argentine Precordillera, the Antler of Canada, and the Archean Barberton belt of Africa. More recently, Schoonmaker et al. (2005) presented evidence for slab failure in the Acadian orogenic foreland (link - pdf file 376k). The new problem is how to distinguish between the geologic effects of extensional slab failure and flexural extension in foredeeps like the Taconics.
Quantifying plate motions from foreland-basin stratigraphy: The Taconic Orogeny. Quantitative data on ancient plate motions are essential to reconstructing global geologic history. Because the marine magnetic-anomaly record is so fragmentary before about 80 Ma, quantitative reconstructions of past motions of continents and continental fragments has relied very heavily on paleomagnetism. Valuable as it is, paleomagnetism has three deficiencies: it cannot detect east-west (longitudinal) motions, the timing of magnetization is rarely known, and uncertainties in paleolatitude are typically in the range of 500-1000 km. With this shortcoming in mind, I spent the latter 1980's working on an alternative way of determining rates of plate motion during collisional orogeny, an approach based on stratigraphic diachronism in foredeep sequences. The method uses the migration of the foreland basin - which is amenable to biostratigraphic analysis - as a proxy for the migration of the convergent plate boundary itself. The rate so determined for plate convergence during the Ordovician Taconic collision in the Appalachians was about 20 km per million years.
Turnagain Arm and Resurrection Bay. This is certainly one of the most scenic transects through an accretionary complex in the world: some 50 km of nearly continuous roadcuts and beach exposures through the Chugach terrane. Download this link (pdf file 3.41mb) the Bradley & Miller (2006) guidebook published by the Alaska Geological Society. If you want to do just a couple of the best outcrops near Anchorage in half a day, I suggest Stop 2 (McHugh melange), Stop 3 (McHugh Conglomerate), and Stop 4 (Valdez Group turbidites).
Kachemak Bay, between Homer and Seldovia. This trip features even better exposures of the McHugh Complex, and also some relatively accessible (by Alaskan standards) coastal exposures of the ~190-Ma Seldovia blueschists. The best way to see everything is by small boat with 25 h.p. outboard, such as a zodiak or aluminum skiff. The tidal range is huge, typically more than 20 feet. Kachemak Bay is relatively sheltered from the North Pacific but the seas can be dangerous, even in the summer months, so use caution and common sense. Very early morning is often the calmest time of day to cross the Bay from Homer. Alternatively, the Seldovia exposures can be reached by taking a small plane or water taxi to Seldovia village, then walking a mile or so to coastal exposures at and near Outside Beach. Click on this link (pdf file 7.54mb) to download the 2000 guidebook published by the Alaska Geological Society.
Bleick, H.A., Till, A.B., Bradley, D.C., O'Sullivan, P., Wooden, J.L., Bradley, D.B., Taylor, T.A., Friedman, S.B., and Hults, C.P., 2012, Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska: U.S. Geological Survey Open-File Report 2012-1232, 1 plate.
Bradley, D.C., 2011, Secular trends in the geologic record and the supercontinent cycle: Earth-Science Reviews, v. 108, p. 16-33, and Supplementary Data, in electronic format, 17 p. doi: 10.1016/j.earscirev.2011.05.003. (link - pdf file 4.4 mb)Karl, S.M., Bradley, D.C., Combellick, R.A., and Miller, M.L., 2011, Field Guide to south-central Alaska's accretionary complex and neotectonics, Anchorage to Seward: Alaska Geological Society, 44 p.
Leach, D.L., Bradley, D.C., Huston, D., Pisarevky, S.A., Taylor, R.D., and Gardoll, S.J., 2010, Sediment-hosted lead-zinc deposits in Earth history: Economic Geology, v. 105, p. 593-625. (link - pdf file 1.81mb)
Goldfarb, R.J., Bradley, D.C., and Leach, D.L., 2010. Secular variation in economic geology. Economic Geology 105, 459-465. (link - pdf file 307kb)
Bradley, Dwight C., Haeussler, Peter, O’Sullivan, Paul, Friedman, Rich, Till, Alison, Bradley, Dan, and Trop, Jeff, 2009, Detrital zircon geochronology of Cretaceous and Paleogene strata across the south-central Alaskan convergent margin, in Haeussler, P.J., and Galloway, J.P., Studies by the U.S. Geological Survey in Alaska, 2007: U.S. Geological Survey Professional Paper 1760-F, 36 p. (link - pdf file 11.47mb)
Taylor, R.D., Leach, D.L., Bradley, D.C., and Pisarevsky, S.A., 2009, Compilation of mineral resource data for Mississippi Valley-type and clastic-dominated sediment-hosted lead-zinc deposits: U.S. Geological Survey Open-File Report 2009-1297, 42 p. (link - pdf file 5.77mb)
Bradley, Dwight C., 2008, Passive margins through Earth history: Earth-Science Reviews, v. 91, p. 1-26. doi:10.1016/j.earscirev.2008.08.001. (link - pdf file 6.02mb)
Ayuso, Robert A., Haeussler, Peter J., Bradley, Dwight C., Farris, David W., Foley, Nora K., Wandless, Gregory A., 2008, The role of ridge subduction in determining the geochemistry and Nd-Sr-Pb isotopic evolution of the Kodiak batholith in Southern Alaska, Tectonophysics, v. 64, p. 137-163. doi: 10.1016/j.tecto.2008.09.029. (link - pdf file 4.78mb)
Bradley, D., O’Sullivan, P., Friedman, R., Miller, M., Till, A., Dumoulin, J., and Blodgett, R., 2008, Detrital zircon geochronology of Proterozoic to Devonian rocks in Interior Alaska: Newsletter of the Alaska Geological Society, v. 38, 5, p. 1-5. (link - pdf file 667k)
Till, A.B., Roeske, S.M., Bradley, D.C., Layer, P., Friedman, R., and Layer, P.W., 2007, Early Tertiary transtension-related deformation and magmatism along the Tintina fault system, Alaska, in Till, A.B., Roeske, S.M., Sample, J.C., and Foster, D.A., editors, Exhumation Associated with Continental Strike-Slip Fault Systems: Geological Society of America Special Paper 434, p. 233-264. (link - pdf file 2.77mb)
Bradley, D.C., McClelland, W., Wooden, J., Till, A.B., Roeske, S., Miller, M.L., Karl, S., and Abbott, G., 2007, Detrital zircon geochronology of some Neoproterozoic to Triassic rocks in interior Alaska in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O'Neill, J.M., editors, Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 155-180. (link - pdf file 934k)
Miller, M.L., Bradley, D.C., Bundtzen, T.K., Pessagno, E.A., Jr., Blodgett, R.B., Tucker, R., and Wooden, J., The restricted Gemuk Group—A Triassic to Early Cretaceous succession in southwest Alaska, in press, 2007, in Ridgway, K.D., Trop, J.M., Glen, J.M.G., and O'Neill, J.M., editors, Tectonic growth of a collisional continental margin: Crustal evolution of southern Alaska: Geological Society of America Special Paper 431, p. 273-305. (link - pdf file 11.54mb)
Bradley, D., and Miller, M., 2006, Field guide to south-central Alaska's accretionary complex, Anchorage to Seward: Alaska Geological Society Field Guide Series, Anchorage, Alaska, 32 p. (link - pdf file 3.41mb)
Schoonmaker, A., Kidd, W.S.F., and Bradley, D.C., 2005, Foreland/forearc collisional mafic and granitoid magmatism caused by lower-plate lithospheric slab-breakoff: the Acadian of Maine, and other orogens: Geology, v. 33, p. 961-964. (link - pdf file 376k)
Richard J. Goldfarb, Robert Ayuso, Marti L. Miller, Shane W. Ebert, Erin E. Marsh, Scott A Petsel, Lance D. Miller, Dwight Bradley, Craig Johnson, and William McClelland, 2004, The Late Cretaceous Donlin Creek gold deposit, southwestern Alaska: Controls on epizonal ore formation: Economic Geology, v. 99, p. 643-672. (link - pdf file 2.16mb)
Bradley, D.C., and Leach, D.L., 2003, Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands: Mineralium Deposita, v. 38, p. 652-667. (link - pdf file 2.56mb)
Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection Plate: Evidence for an additional plate in the northeastern Pacific in Paleocene-Eocene time: Geological Society of America Bulletin, v. 115, p. 867-880. (link - pdf file 2.26mb)
Bradley, Dwight C., Julie Dumoulin, Paul Layer, David Sunderlin, Sarah Roeske, Bill McClelland, Anita G. Harris, Grant Abbott, Tom Bundtzen, and Timothy Kusky, 2003, Late Paleozoic Orogeny in Alaska's Farewell Terrane: Tectonophysics, 372, p. 23-40. (link - pdf file 1.54mb)
Kusky, Timothy M., Bradley, Dwight C., Donley, D. Thomas, Rowley, D., and Haeussler, Peter J., 2003, Controls on intrusion of near-trench magmas of the Sanak-Baranof Belt, Alaska, during Paleogene ridge subduction, and consequences for forearc evolution: Geological Society of America Special Paper 371, p. 269-292. (link - pdf file 3.72mb)
Garven, G., Raffensperger, J.P., Dumoulin, J.A., Bradley, D.A., Young. L.E., Kelley, K.D., and Leach, D.L., 2003, Coupled heat and fluid flow modeling of the Carboniferous Kuna Basin, Alaska: implications for the genesis of the Red Dog Pb-Zn-Ag-Ba ore district: Journal of Geochemical Exploration, v. 78-79, p. 215-219. (link - pdf file 673k)
Haeussler, P.J., Bradley, D. C., and Goldfarb, R.J., 2003, Along-strike variations in response to ridge subduction in southern Alaska from structural characteristics of dikes and gold-bearing quartz veins: Geological Society of America Special Paper 371, p. 119-140. (link - pdf file 3.11mb)
Bradley, D., Kusky, T., Haeussler, P., Goldfarb, R., Miller, M., Dumoulin, J., Nelson, S., and Karl, S., 2003, Geologic signature of early Tertiary ridge subduction in Alaska in V.B. Sisson, S. Roeske, and T.L. Pavlis, eds., Geology of a transpressional orogen developed during ridge-trench interaction along the north Pacific margin: Geological Society of America Special Paper 371, p. 19-49. (link - pdf file 9.29mb)
Miller, M.L., Bradley, D.C., Bundtzen, T.K., and McClelland, W., 2002, Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska: Journal of Geology, v. 110, p. 247-270. (link - pdf file 2.33mb)
Bradley, Dwight C., and Hanson, Lindley S., 2002, Paleocurrent analysis of synorogenic clastic rocks within the Acadian orogen of Maine: Sedimentary Geology, v. 148, p. 425-447. (link - pdf file 2.85mb)
Dumoulin, J.A., Harris, A.G., Gagiev, M., Bradley, D.C., and Repetski, J.E., 2002, Lithostratigraphic, conodont, and other faunal links between lower Paleozoic strata in northern and central Alaska and northeastern Russia, in E. Miller et al., eds., Tectonic evolution of the Bering Shelf-Chuckchi Sea-Arctic margin and adjacent landmasses: Geological Society of America Special Paper 360, p. 2911-312. (link - pdf file 360k)
Bradley, Dwight C., and Tucker, R.D., 2002, Emsian synorogenic paleogeography of the Maine Appalachians: Journal of Geology, v. 110, p. 483-492. (link - pdf file 694k)
Leach, D.L., Bradley, D.C., Lewchuck, M., Symons, D.T.A., Brannon, J., and de Marsily, G., 2001, Mississippi Valley-type lead-zinc deposits through geological time: Implications from recent age-dating research: Mineralium Deposita, v. 36, p. 711-740. (link - pdf file 4.18mb)
Bradley, D.C., and Wilson, F.H., 2000, Reconnaissance bedrock geology of the southeastern Kenai quadrangle, Alaska: U.S. Geological Survey Professional Paper 1615, p. 59-63. (link - pdf file 339k)
Dumoulin, J., Bradley, D.C., and Harris, A.G., 2000, Lower Paleozoic deep-water facies of the Dyckman Mtn. area, northeastern Medfra quadrangle, Alaska: U.S. Geological Survey Professional Paper 1615, p. 43-57. (link - pdf file 3.7mb)
Bradley, D.C., Parrish, R., Clendenen, W., Lux, D., Layer, P., Heizler, M., and Donley, D.T., 2000, New geochronological evidence for the timing of early Tertiary ridge subduction in southern Alaska: U.S. Geological Survey Professional Paper 1615, p. 5-21. (link - pdf file 1.01mb)
Bradley, Dwight C., Kusky, Tim, Karl, S., Till, A., and Haeussler, P., 2000, Field guide to the Mesozoic accretionary complex in Kachemak Bay and Seldovia, south-central Alaska: Anchorage, Alaska Geological Society, 19 p. (link - pdf file 7.54mb)
Dumoulin, J.A., Harris, A.G., Bradley, D.C., and deFreitas, T.A., 2000, Facies patterns and conodont biogeography in Arctic Alaska and the Canadian Arctic Islands: Evidence against juxtaposition of these areas during early Paleozoic time: Polarforschung, v. 68, p. 257-266. (link - pdf file 1.19mb)
Bradley, D.C., Tucker, R.D., Lux, Dan, Harris, A.G., and McGregor, D.C., 2000, Migration of the Acadian orogen and foreland basin across the Northern Appalachians: U.S. Geological Survey Professional Paper 1615, 49 p. (link)
Kusky, T.M., and Bradley, D.C., 1999, Kinematic analysis of melange fabrics: Examples and applications from the McHugh Complex, Kenai Peninsula, Alaska: Journal of Structural Geology, v. 21, p. 1773-1796. (link - pdf file 1.44mb)
Bradley, D.C., Kusky, T., Haeussler, P., Karl, S., and Donley, D.T., 1999, Geologic map of the Seldovia quadrangle, Alaska: USGS Open-File Report OF 99-18, scale 1:250,000, with text. (link)
Dumoulin, J.A., Bradley, D.C., Harris, A.G., and Repetski, J.E., 1999, Lower Paleozoic deep-water facies of the Medfra area, central Alaska, in Kelley, K.D., ed., Geologic Studies in Alaska by the U.S. Geological Survey, 1997: U.S. Geological Survey Professional Paper 1614, p. 73-103. (link - pdf file 7.0mb)
Bradley, D.C., and Hanson, L., 1998, Paleoslope analysis of slump folds in the Devonian flysch of Maine: Journal of Geology, v. 106, p. 305-318. (link - pdf file 1.54mb)
Dumoulin, J.A., Bradley, D.C., and Harris, A.G., 1998, Sedimentology, conodonts, structure, and correlation of Silurian and Devonian metasedimentary rocks in Denali National Park, Alaska: U.S. Geological Survey Professional Paper 1595, p. 71-98. (link - pdf file 7.69mb)
Tucker, R.D., Bradley, D.C., ver Straeten, C., Harris, A.J., Ebert, J.R., and McCutcheon, S.R., 1998, New U-Pb zircon ages and the duration and division of Devonian time: Earth and Planetary Science Letters, v. 158, p. 175-186. (link - pdf file 1.21mb)
Robinson, P., Tucker, R.D., Bradley, D.C., Berry, H.N., IV, and Osberg, P.H., 1998, Paleozoic orogens in New England, U.S.A.: GFF (journal of the Geological Society of Sweden), vol. 120, p. 119-148. (link - pdf file 28.9mb)
Wilson, F.H., Dover, J.H., Bradley, D.C., Weber, F.R., Bundtzen, T.K., and Haeussler, P.J., 1998, Geologic map of central (interior) Alaska: U.S. Geological Survey Open-File Report 98-133, 64 p., 3 plates, scale 1:500,000 (also released as a CD-ROM). (link)
Stevens, C.H., Davydov, V.I., and Bradley, D.C., 1997, Permian Tethyan fusilinids from the Kenai Peninsula, Alaska: Journal of Paleontology, v. 71, p. 985-994. (link - pdf file 1.62mb)
Kusky, T., Bradley, D., and Haeussler, P., 1997, Progressive deformation of the Chugach accretionary complex, Alaska, during a Paleogene ridge-trench encounter: Journal of Structural Geology, v. 19, p. 139-157. (link - pdf file 2.99mb)
Kusky, T., Bradley, D., and Haeussler, P., and Karl, S.J., 1997, Controls on accretion of flysch and melange belts at accretionary margins: Evidence from the Chugach Bay thrust and Iceworm melange, Chugach accretionary wedge, Alaska: Tectonics, v. 16, p. 855-878. (link - pdf file 5.16mb)
Bradley, Dwight C., Kusky, Timothy M., Karl, Susan M., and Haeussler, Peter J., 1997, Field guide to the Mesozoic accretionary complex along Turnagain Arm and Kachemak Bay, south-central Alaska: in S.M. Karl, N.R. Vaughan, and T.J. Ryherd, eds., 1997 Guide to the Geology of the Kenai Peninsula, Alaska: Alaska Geological Society, Anchorage, Alaska, p. 2-12. Also see road log, p. 83-128. (link - pdf file 3.47mb)
Karl, S., Reger, R., Pinney, D., Bradley, D., Swenson, R., Combellick, R., Kurtak, J., Haeussler, P., and Brimberry, D., 1997, Road Log for the 1997 Guide to the Geology of the Kenai Peninsula, Alaska: in S.M. Karl, N.R. Vaughan, and T.J. Ryherd, eds., 1997 Guide to the Geology of the Kenai Peninsula, Alaska: Alaska Geological Society, Anchorage, Alaska, p. 83-128.
Bradley, D.C., 1997, The Northern Appalachians, p. 445-450 in van der Pluijm, B. and Marshak, S., editors, Earth structure- An introduction to structure and tectonics: WCB/McGraw-Hill, New York, 495 pp. (link - pdf file 4.74mb)
Haeussler, Peter, Bradley, D., Goldfarb, Rich, and Snee, Lawrence, 1995, A link between ridge subduction and gold mineralization in southern Alaska: Geology, v. 23, p. 995-998. (link - pdf file 852k)
Goldfarb, R.J., Christie, T., Skinner, D., Haeussler, P., and Bradley, D., 1995, Gold deposits of Westland, New Zealand and southern Alaska --Products of the same tectonic processes?, in Mauk, J., ed., PACRIM '95: Symposium Volume, p. 239-244.
Kusky, T., Bradley, D., Winsky, P., Caldwell, D., and Hanson, L., 1994, Paleozoic stratigraphy and tectonics of Ripogenus Gorge and nearby areas, Maine: 86th annual New England Intercollegiate Geological Conference, Guidebook to Field Trips, Salem State College, Salem, Mass, p. 183-192. (link - pdf file 4.92mb)
Bradley, D.C., and Bradley, L.M., 1994, Geometry of an outcrop-scale duplex in the Devonian flysch of Maine: Journal of Structural Geology, v. 16, p. 371-380. (link - pdf file 2.04mb)
Hanson, L. S., Bradley, D. C., and Caldwell, D. W., 1993, Geology and geomorphology of the Acadian Orogen, central Maine: Field Trip Guidebook for the northeastern United States: Contrib. No. 67, Dept. of Geology and Geography, Univ. of Massachusetts, Amherst, Mass., p. CC1-27
Bradley, D.C., 1993, Role of lithosphere flexure and plate convergence in the genesis of some Appalachian zinc deposits: U.S. Geological Survey Bulletin 2039, p. 35-43.
Bradley, D.C., Haeussler, P., and Kusky, T.M., 1993, Timing of Early Tertiary ridge subduction in southern Alaska: U.S. Geological Survey Bulletin 2068, p. 163-177. (link - pdf file 1.27mb)
Bradley, D.C., and Kusky, T.M., 1992, Deformation history of the McHugh Complex, Seldovia quadrangle, south-central Alaska: U.S. Geological Survey Bulletin 1999, p. 17-32. (link - pdf file 1.34mb)
Bradley, D.C., and Ford, A., editors, 1992, Geologic Studies in Alaska by the U.S. Geological Survey, 1990: U.S. Geological Survey Bulletin 1999, 242 p.
Cieutat, B.A., Goldfarb, R.J., Bradley, D.C., and Roushey, B.H., 1992, Placer gold of the Kenai Lowland: U.S. Geological Survey Bulletin 2041, p. 23-29. (link - pdf file 770k)
Bradley, D.C., and Dusel-Bacon, C., editors, 1992, Geologic Studies in Alaska by the U.S. Geological Survey, 1990: U.S. Geological Survey Bulletin 2041, 256 p.
Bradley, D. C., and Kidd, W. S. F., 1991, Flexural extension of the upper continental crust in collisional foredeeps: Geological Society of America Bulletin, v. 103, p. 1416-1438. (link - pdf file 6.3mb)
Bradley, D. C., and Kusky, T. M., 1990, Kinematics of late faults along Turnagain Arm, Mesozoic accretionary complex, south-central Alaska: U.S. Geological Survey Bulletin 1946, p. 3-10. (link - pdf file 580k)
De Paor, D. G., Bradley, D. C., Eisenstadt, G., and Phillips, S. M., 1989, The Arctic Eurekan Orogen: A most unusual fold-and-thrust belt: Geological Society of America Bulletin, v. 101, p. 952-967.
Bradley, D. C., 1989, Description and analysis of early faults based on geometry of fault-bed intersections: Journal of Structural Geology, v. 11, p. 1011-1019. (link - pdf file 959k)
Bradley, D. C., 1989, Taconic plate kinematics as revealed by foredeep stratigraphy: Tectonics, v. 8, p. 1037-1049. (link - pdf file 1.87mb)
Hanson, L. S., and Bradley, D. C., 1989, Sedimentary facies and tectonic interpretation of the Carrabassett Formation, north-central Maine, in R. Marvinney and R. Tucker, eds., Studies in Maine Geology, V. 2, Maine Geological Survey, Augusta, Me., p. 101-125. (link - pdf file 19.28mb)
Bradley, D. C., and Hanson, L. S., 1989, Turbidites and mélanges of the Madrid Formation, central Maine, in Berry, A.W., Jr., ed., Guidebook for Field Trips in southern and west-central Maine, 81st New England Intercollegiate Geological Conference, Univ. of Maine, Farmington, Me., p. 183-199. (link - pdf file 9.62mb)
De Paor, D. G., and D. C. Bradley, 1988, Balanced sections in thrust belts, Part 2: Computerized line and area balancing: Geobyte, May 1988, p. 33-37.
Bradley, D. C., 1987, Paleocurrent directions from apparent dip of cross laminae in the Devonian flysch of Maine: Journal of Geology, v. 95, p. 271-279.
Kusky, T., Kidd, W., and Bradley, D. C., 1987, Oblique closure of the Acadian Ocean: evidence from the Northern Arm Fault in Newfoundland: Journal of Geodynamics, v. 7, p. 105-133.
Bradley, D. C., and Bradley, L. M., 1986, Tectonic significance of the Carboniferous Big Pond Basin, Cape Breton Island, Nova Scotia: Canadian Journal of Earth Science, v. 23, p. 2000-2011. (link - pdf file 3.13mb)
Bradley, D. C., and Kusky, T., 1986, Geologic evidence for the rate of plate convergence during the Taconic arc-continent collision: Journal of Geology, v. 94, p. 667-681. (link - pdf file 2.09mb)
Bradley, D. C., 1984, Late Paleozoic strike slip tectonics of the Northern Appalachians: Ph.D. Dissertation, State University of New York at Albany, 286 pp., 3 plates, scales 1:1,000,000, 1:12,300, and 1:10,000.
Bradley, D. C., 1983, Tectonics of the Acadian Orogeny in New England and adjacent Canada: Journal of Geology, v. 91, p. 381-400. (link - pdf file 29.13mb)
Mann, W. P., Hempton, M., Bradley, D. C., and Burke, K., 1983, Development of pull-apart basins: Journal of Geology, v. 91, p. 529-554.
Bradley, D. C., 1982, Subsidence in late Paleozoic basins in the Northern Appalachians: Tectonics, v. 1, p. 107-123. (link - pdf file 1.01mb)
Bradley, D. C., 1981, Late Wisconsinan mountain glaciation in the northern Presidential Range, New Hampshire: Arctic and Alpine Research, v. 13, p. 319-327.