The Nome Complex is a large metamorphic unit that sits along the southern boundary of the Arctic Alaska–Chukotka terrane, the largest of several microcontinental fragments of uncertain origin located between the Siberian and Laurentian cratons. The Arctic Alaska–Chukotka terrane moved into its present position during the Mesozoic; its Mesozoic and older movements are central to reconstruction of Arctic tectonic history. Accurate representation of the Arctic Alaska–Chukotka terrane in reconstructions of Late Proterozoic and early Paleozoic paleogeography is hampered by the paucity of information available. Most of the Late Proterozoic to Paleozoic rocks in the Alaska–Chukotka terrane were penetratively deformed and recrystallized during the Mesozoic deformational events; primary features and relationships have been obliterated, and age control is sparse. We use a variety of geochemical, geochronologic, paleontologic, and geologic tools to read through penetrative deformation and reconstruct the protolith sequence of part of the Arctic Alaska–Chukotka terrane, the Nome Complex. We confirm that the protoliths of the Nome Complex were part of the same Late Proterozoic to Devonian continental margin as weakly deformed rocks in the southern and central part of the terrane, the Brooks Range. We show that the protoliths of the Nome Complex represent a carbonate platform (and related rocks) that underwent incipient rifting, probably during the Ordovician, and that the carbonate platform was overrun by an influx of siliciclastic detritus during the Devonian. During early phases of the transition to siliciclastic deposition, restricted basins formed that were the site of sedimentary exhalative base-metal sulfide deposition. Finally, we propose that most of the basement on which the largely Paleozoic sedimentary protolith was deposited was subducted during the Mesozoic.

The Arctic Alaska–Chukotka terrane is a microcontinent with an origin exotic to Laurentia. We used a sensitive high-resolution ion microprobe (SHRIMP) to date nine samples of Neoproterozoic rock and five samples of Devonian rock from the Brooks Range and Seward Peninsula of Alaska and from the Chukotka Peninsula of northeastern Russia. Felsic magmatism occurred at 968 Ma and 742 Ma in the Brooks Range and at 865 Ma and 670–666 Ma on Seward Peninsula. Felsic igneous rocks in Chukotka were dated at 656 Ma and 574 Ma. Devonian igneous rocks are found throughout the Arctic Alaska–Chukotka terrane, and we dated samples with ages of 391 Ma, 390 Ma, 385 Ma, 371 Ma, and 363 Ma. The felsic character of the Neoproterozoic rocks suggests formation at least in part through crustal melting. The age of the crustal source rocks that melted to form the Neoproterozoic rocks is inferred to be Mesoproterozoic based on Nd model ages ranging from 1.6 to 1.4 Ga. Rocks of this age range have been reported from the basement of Baltica but are rare in Laurentia. The 565 Ma orthogneisses on Seward Peninsula have ca. 1.1 Ga Nd model ages. Devonian igneous rocks have a wide range of model ages ranging from 1.6 to 0.8 Ga. The tectonic setting of the 968 Ma, 865 Ma, and 742 Ma rocks is unknown. The ca. 670 Ma magmatism on Seward Peninsula is interpreted to have occurred in an arc setting based on geochemistry and similarities in their ages to the Avalonian–Cadomian arc system peripheral to Gondwana. Latest Neoproterozoic magmatism is inferred to have occurred in a rift setting based on composition and the Paleozoic passive margin sequence that was deposited across the Arctic Alaska–Chukokta terrane. Devonian magmatism likely occurred in an arc and/or backarc rift setting. Significant uncertainties remain concerning the age of the Arctic Alaska–Chukotka terrane basement, particularly the age of the host rocks for Neoproterozoic intrusions.

Paleozoic carbonate strata deposited in shallow platform to off-platform settings occur across the Seward Peninsula and range from unmetamorphosed Ordovician–Devonian(?) rocks of the York succession in the west to highly deformed and metamorphosed Cambrian–Devonian units of the Nome Complex in the east. Faunal and lithologic correlations indicate that early Paleozoic strata in the two areas formed as part of a single carbonate platform. The York succession makes up part of the York terrane and consists of Ordovician, lesser Silurian, and limited, possibly Devonian rocks. Shallow-water facies predominate, but subordinate graptolitic shale and calcareous turbidites accumulated in deeper water, intraplatform basin environments, chiefly during the Middle Ordovician. Lower Ordovician strata are mainly lime mudstone and peloid-intraclast grainstone deposited in a deepening upward regime; noncarbonate detritus is abundant in lower parts of the section. Upper Ordovician and Silurian rocks include carbonate mudstone, skeletal wackestone, and coral-stromatoporoid biostromes that are commonly dolomitic and accumulated in warm, shallow to very shallow settings with locally restricted circulation. The rest of the York terrane is mainly Ordovician and older, variously deformed and metamorphosed carbonate and siliciclastic rocks intruded by early Cambrian (and younger?) metagabbros. Older (Neoproterozoic–Cambrian) parts of these units are chiefly turbidites and may have been basement for the carbonate platform facies of the York succession; younger, shallow- and deep-water strata likely represent previously unrecognized parts of the York succession and its offshore equivalents. Intensely deformed and altered Mississippian carbonate strata crop out in a small area at the western edge of the terrane. Metacarbonate rocks form all or part of several units within the blueschistand greenschist-facies Nome Complex. The Layered sequence includes mafic metaigneous rocks and associated calcareous metaturbidites of Ordovician age as well as shallow-water Silurian dolostones. Scattered metacarbonate rocks are chiefly Cambrian, Ordovician, Silurian, and Devonian dolostones that formed in shallow, warmwater settings with locally restricted circulation and marbles of less constrained Paleozoic age. Carbonate metaturbidites occur on the northeast and southeast coasts and yield mainly Silurian and lesser Ordovician and Devonian conodonts; the northern succession also includes debris flows with meter-scale clasts and an argillite interval with Late Ordovician graptolites and lenses of radiolarian chert. Mafic igneous rocks at least partly of Early Devonian age are common in the southern succession. Carbonate rocks on Seward Peninsula experienced a range of deformational and thermal histories equivalent to those documented in the Brooks Range. Conodont color alteration indices (CAIs) from Seward Peninsula, like those from the Brooks Range, define distinct thermal provinces that likely reflect structural burial. Penetratively deformed high-pressure metamorphic rocks of the Nome Complex (CAIs ≥5) correspond to rocks of the Schist belt in the southern Brooks Range; both record subduction during early stages of the Jurassic–Cretaceous Brooks Range orogeny. Weakly metamorphosed to unmetamorphosed strata of the York terrane (CAIs mainly 2–5), like Brooks Range rocks in the Central belt and structural allochthons to the north, experienced moderate to shallow burial during the main phase of the Brooks Range orogeny. The nature of the contact between the York terrane and the Nome Complex is uncertain; it may be a thrust fault, an extensional surface, or a thrust fault later reactivated as an extensional fault. Lithofacies and biofacies data indicate that, in spite of their divergent Mesozoic histories, rocks of the York terrane and protoliths of the Nome Complex formed as part of the same lower Paleozoic carbonate platform. Stratigraphies in both areas feature Lower Ordovician and mid-Silurian shallow-water deposits with some deeper water facies of late Early to Middle Ordovician age. Most significantly, Ordovician conodont faunas in both successions contain a characteristic, distinctive mixture of Laurentian and Siberian-Alaskan endemic forms. Lithologic and faunal resemblances also link Seward Peninsula platform strata with coeval successions in the Brooks Range and in interior Alaska (Farewell and White Mountains terranes) and imply that all of these rocks were once part of a single carbonate platform situated between Laurentia, Siberia, and Baltica. Little is known about the basement on which Alaskan platform strata formed, and correlations between Cambrian and older rocks in these areas remain tentative. Similarities between strata and fossils in northern and interior Alaska are strongest during the Ordovician, and diminish by Middle Devonian; correlations between Seward Peninsula and Brooks Range rocks, however, extend into the Carboniferous. Ordovician mafic volcanism in the Nome Complex and the White Mountains terrane could reflect a rifting episode that began to separate platform rocks of the interior from those of Arctic Alaska. Lower Paleozoic off-platform successions on Seward Peninsula also correlate well with equivalent sections in northern and interior Alaska, and have some similarities with strata in southeast Alaska (Alexander terrane). Silurian (mainly Wenlock–Ludlow) mass flow deposits derived at least in part from a carbonate source overlie condensed graptolitic shales in most of these successions; this coeval influx of calcareous detritus suggests a common tectonic cause.

Detrital zircons from the Nome Complex, a metamorphic terrane in northern Alaska, reveal important constraints on the early Paleozoic history of the Arctic Alaska–Chukotka terrane, a microcontinental block with an origin exotic to Laurentia. Twenty-two samples (17 in this study, five previously published) produce three detrital zircon population patterns (called themes), indicating that at least three distinguishable source areas contributed to the metamorphic protolith. Detrital zircon populations from metamorphosed rift-related mafic volcaniclastic rocks, a lithologic subunit of the Nome Complex, contain a dominant population of 740–550 Ma zircons. Samples from three other lithologic units yielded populations dominated by early Paleozoic zircons and characterized by a large population of 450–420 Ma zircons. A few samples, taken from two different lithologic units, yielded populations dominated by Mesoproterozoic zircons (most around 1.25–0.9 Ga) and lacked zircons younger than 900 Ma. None of the 22 samples contained more than a few Archean zircons. The ages of the youngest detrital zircon populations indicate that little of the protolith for the Nome Complex can be as old as Proterozoic, as previously thought. Further, a significant part of the protolith sequence is Devonian or younger; these rocks are likely correlative with Devonian or Mississippian units in the Brooks Range, specifically marine parts of the Endicott or Lisburne Groups. Based on detrital zircon data, limiting factors can be placed on the paleogeographic history of the Nome Complex and associated parts of the Arctic Alaska–Chukotka terrane: (1) 740–550 Ma zircons were deposited in a rift-related basin formed on a continental margin in the early Paleozoic; at least some of those zircons may have been sourced from local basement; (2) a transition to new sediment sources is reflected in Devonian or younger protoliths with the appearance of 450–420 Ma and 1.25–0.9 Ga detrital zircons; and (3) 450–420 Ma and 1.25–0.9 Ga zircons may have been supplied from sources outside the Arctic Alaska–Chukotka terrane.

The chemical character of mafic rocks from the Arctic Alaska–Chukotka terrane records rifting of continental crust during the early Paleozoic, possibly during the Ordovician. The mafic rocks are part of a metamorphosed Neoproterozoic to Devonian continental margin sequence preserved in a Mesozoic metamorphic terrane, the Nome Complex, of Seward Peninsula, Alaska. Protoliths of the mafic rocks include basalt and mafic clastic rocks, which were interlayered with calcareous, pelitic, and feldspathic sediments, and gabbro and diabase, likely feeder dikes and sills to the basalt. Major-element, trace-element, and rare-earth element (REE) analyses of these mafic rocks, together with analyses of Nd, Pb, and Sr isotopes, form two compositional groups. The two groups differ in Nb/Y (one plots as basalt, the other as alkali to subalkali basalt), TiO 2 , P 2 O 5 , and Nb (and other elements). The high-Ti group is characterized by enrichment of light REE; the low-Ti group lacks such enrichment. The trace-element and isotopic characteristics of the two groups resemble typical non-arc magmas derived from the mantle: the low-Ti group has compositions between normal mid-ocean ridge basalt (N-MORB) and enriched mid-ocean ridge basalt (E-MORB), while those of the high-Ti group are between E-MORB and ocean-island basalt (OIB). The two groups have overlapping positive values of ε Nd (+0.34 to +7.40). TiO 2 /Yb ratios suggest the high-Ti group formed from melts generated under normal thickness of continental crust, while the low-Ti group formed from melts generated at shallower conditions, presumably after rift-related crustal thinning had progressed. Geologic, paleontologic, and geochronologic characteristics of the Nome Complex support an origin along the NE margin of Baltica. The rift-related magmatism in the Nome Complex likely occurred during the opening of the Uralian ocean along that margin; by implication, related parts of the Arctic Alaska–Chukotka terrane may have experienced a similar origin.

Stratabound base-metal sulfide deposits and occurrences are present in metasedimentary rocks of the Neoproterozoic and Paleozoic Nome Complex on south-central Seward Peninsula, Alaska. Stratabound and locally stratiform deposits including Aurora Creek (Zn-Au-Ba-F), Wheeler North (Pb-Zn-Ag-Au-F), and Nelson (Zn-Pb- Cu-Ag), consist of lenses typically 0.5–2.0 m thick containing disseminated to semimassive sulfides. Host strata of the Aurora Creek and Wheeler North deposits are variably calcareous and graphitic siliciclastic metasedimentary rocks of Middle Devonian or younger age based on detrital zircon geochronology; the Nelson deposit is within Ordovician–Devonian marble (Till et al., this volume, Chapter 4). Deformed veins such as Quarry (Zn-Pb-Ag-Ba-F) and Galena (Pb-Zn-Ag-F) occur in a unit composed mainly of marble and schist; fossil and detrital zircon data indicate that this unit contains rocks of Ordovician, Silurian, and Devonian age. None of these Zn- and Pbrich deposits or occurrences has spatially associated metavolcanic or intrusive rocks. All were deformed and metamorphosed to blueschist facies and then retrograded to greenschist facies during the Jurassic and Early Cretaceous Brookian orogeny. Disseminated Cu-rich deposits including Copper King (Cu-Bi-Sb-Pb-Ag-Au) and Wheeler South (Cu-Ag-Au) occur in silicified carbonate rocks and have textures that indicate a pre- to syn-metamorphic origin. The Zn- and Pb-rich sulfide deposits and occurrences consist mainly of pyrite, sphalerite, and/or galena in a gangue of quartz and carbonate. Minor minerals include arsenopyrite, chalcopyrite, magnetite, pyrrhotite, tetrahedrite, barite, fluorite, and chlorite; gold and electrum are trace to minor constituents locally. Sphalerite is uniformly unzoned and commonly aligned in the dominant foliation. These textures, together with the presence of folded layers of barite at Aurora Creek and folded sulfi de layers at Wheeler North, indicate that mineralization in the stratabound deposits predated deformation and metamorphism. Electron microprobe (EMP) analyses of the carbonate gangue show three major compositions comprising siderite, ankerite, and lesser dolomite. The Cu-rich deposits differ in containing chalcopyrite and bornite in a quartzose matrix. Altered wall rocks surrounding the Zn- and Pb-rich deposits and occurrences have aluminous assemblages composed of muscovite + chloritoid + siderite + chlorite + quartz ± tourmaline ± ilmenite ± apatite ± monazite. Muscovite within these assemblages and in sulfide-rich samples is phengitic and locally enriched in barium; chloritoid at Aurora Creek is enriched in zinc. Minor minerals including pyrite, sphalerite, galena, chalcopyrite, barite, and hyalophane occur as fine-grained disseminations. These altered rocks vary from small lenses a few meters thick to large zones tens of meters in thickness that extend along strike, discontinuously, for 4 km or more. Whole-rock geochemical analyses of the altered rocks from deposit-proximal and deposit-distal settings reveal generally lower SiO 2 /Al 2 O 3 ratios and higher Fe 2 O 3 T /MgO ratios compared to those of unaltered clastic metasedimentary rocks of the Nome Complex and of average shale or graywacke. The deposit-proximal samples are also characterized by anomalously high Zn, Pb, Hg, and Sb, relative to the unaltered metasediments. These data, together with mass change calculations, suggest that the aluminous rocks formed as replacements of permeable graywacke in semi-conformable alteration zones, beneath the seafloor contemporaneously with Zn-and/or Pb-rich sulfide mineralization. Exposures of all three stratabound Zn-Pb deposits show evidence of deformation and recrystallization that occurred in a largely brittle deformational regime. This evidence includes small faults and veins that cut foliation and localized zones of breccia. Sulfide minerals, fluorite, quartz, chlorite, and carbonate minerals crystallized within these structures, which probably formed during Cretaceous deformation of the Nome Complex. Previous studies of the Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences have invoked models of epigenetic veins, volcanogenic massive sulfides (VMS), or carbonate- replacement deposits (CRD). In contrast, our field and laboratory data (including sulfur isotopes; Shanks et al., this volume) suggest that these Zn- and/or Pb-rich deposits represent different levels of sediment-hosted, seafloor-hydrothermal systems, with stratabound and locally stratiform deposits such as Aurora Creek and Wheeler North having formed on the seafloor and/or in the shallow subsurface like many sedimentary-exhalative (SEDEX) deposits worldwide. The deformed veins such as Quarry and Galena are interpreted to have formed deep in the subsurface, possibly as feeders to overlying SEDEX deposits such as Aurora Creek. Formation of all of the Zn- and Pb-rich deposits and occurrences took place during episodic rifting of the continental margin between the Ordovician and Mississippian(?). Regional relationships are consistent with at least some of the deposits having formed in Late Devonian–Mississippian(?) time.

A detailed study of the Pb isotope geochemistry of Zn-Pb(-Ag-Au-Ba-F) stratabound sulfide deposits within metasedimentary rocks of the Neoproterozoic to Mississippian(?) Nome Complex provides key information for understanding deposit genesis and crustal evolution. A total of 106 new analyses of galena (and other sulfi des) and metasedimentary rocks hosting the deposits shows that (1) Pb isotope signatures of the deposits are heterogeneous when considered as a group; (2) the stratabound Nelson deposit, and deformed veins at Quarry and Galena, are isotopically similar; (3) stratabound and locally stratiform lenses such as Wheeler North and Aurora Creek had different isotopic evolutions; and (4) the occurrence at Bluff and the postmetamorphic, undeformed Pb-Zn-Ag veins and replacements at Hannum, Independence, Foster, and Omilak show the highest values of 206 Pb/ 204 Pb in the region. Pb isotope data for the stratabound Zn-Pb deposits and occurrences do not lie along similar secondary or anomalous lead evolution lines, and there is no shared, two-stage lead line that would provide intersections with a primary or single-stage lead isotope growth curve. Lead isotopic characteristics of the Nelson stratabound deposit and the deformed veins at Quarry and Galena indicate that they largely shared metal and fluid sources. Quarry and Galena also display sufficient Pb isotopic contrast compared to Aurora Creek and Wheeler North to eliminate such veins as subsurface “feeders” for these stratabound deposits, if the deformed veins and deposits formed as closed isotopic systems (without a contribution from externally derived lead). The Pb isotope composition of galena from Aurora Creek formed by a multistage process. It is thus possible that the Aurora Creek deposit originally contained Pb isotope compositions that resembled those from Quarry and Galena. That early-formed Pb was probably remobilized and mixed with radiogenic lead contributed by Mesozoic hydrothermal fluids similar to those associated with the gold-quartz veins in the region. Values of 207 Pb/ 204 Pb and 206 Pb/ 204 Pb from each of the deposits and occurrences plot within the Pb isotope fields of the host metasedimentary rocks and Mesoproterozoic basement rocks of Seward Peninsula; Pb isotope compositions in the deposits thus reflect a local source control. The processes that generated the premetamorphic Zn-Pb(-Ag-Au-Ba-F) sulfide deposits in the Nome Complex differed from those that generated Zn-Pb-Ag deposits in the western Brooks Range, such as the giant Red Dog ore body. Taken as a group, the stratabound lenses and deformed veins in the Nome Complex did not form in a single, widespread, homogeneous hydrothermal system. The Brooks Range deposits, which consist of a range of host rock types and styles of mineralization distributed over a large area, have a high degree of regional Pb isotope homogeneity. The Wheeler North deposit is isotopically similar to Red Dog and related deposits and may have formed in a related hydrothermal system. A preliminary comparison of the Pb isotope compositions of sedimentary-exhalative (SEDEX)–type deposits within the Arctic Alaska–Chukotka terrane and deposits in crustal blocks of Laurussia shows: (1) noteworthy Pb isotopic overlap exists between some of Zn-Pb-Ag deposits in Ireland and the deposits in Arctic Alaska ; but (2) no exact isotopic match exists between any of the deposits in Arctic Alaska and any deposit in crustal blocks involved in the Paleozoic evolution of Laurussia.

Results of sulfur and oxygen isotopic studies of sedimentary-exhalative (SEDEX) Zn-Pb(-Ag-Au-Ba-F) deposits hosted in metamorphosed Paleozoic clastic and carbonate rocks of the Nome Complex, Seward Peninsula, Alaska, are consistent with data for similar deposits worldwide. Stable isotopic studies of the Nome Complex are challenging because the rocks have undergone Mesozoic blueschist- and greenschistfacies metamorphism and deformation at temperatures estimated from 390 to 490 °C. Studies of sulfur and oxygen isotopes in other areas suggest that, in the absence of chemical and mineralogical evidence for metasomatism, the principal effect of metamorphism is re-equilibration between individual minerals at the temperature of metamorphism, which commonly leads to a narrowing of the overall range of isotope values for a suite of rocks but generally does not significantly modify the average whole-rock value for that suite. Sulfur isotopic studies of the stratabound and locally stratiform sulfide lenses at the Aurora Creek–Christophosen deposit, which is of possible Late Devonian–Early Carboniferous age, show a large range of δ 34 S sulfide values from –9.7‰ to 39.4‰, suggesting multiple sulfur sources and possibly complex processes of sulfide formation that may include bacterial sulfate reduction, thermochemical sulfate reduction in a restricted sulfide basin, and Rayleigh distillation. Low δ 34 S values probably represent bacterial sulfide minerals remobilized from the host metasedimentary rocks either during the original seafloor mineralization or are related to a Cretaceous mineralizing event that produced Au-vein and base-metal replacement deposits; the latter process is supported by Pb isotope data. The Wheeler North deposit is similar to Aurora Creek–Christophosen but does not have negative δ 34 S values. It probably formed in an euxinic subbasin. The stratabound Nelson deposit, and the deformed veins at the Galena and Quarry deposits, may be older than the Aurora Creek–Christophosen and Wheeler North deposits. The Nelson deposit has a lower and narrower range of δ 34 S values (1.9‰–10.4‰), averaging ~8‰. The Galena and Quarry vein deposits display δ 34 S values that are similar to those of the stratabound Nelson deposit. Barite samples from the Aurora Creek–Christophosen, Wheeler North, and Quarry deposits have 34 S-enriched δ 34 S values between 25‰ and 30‰ that are consistent with derivation of the sulfur from coeval (Paleozoic) seawater sulfate. Given their δ 34 S values, it is likely that the Aurora Creek–Christophosen and Wheeler North deposits formed in closed subbasins with euxinic conditions that led to extreme Rayleigh distillation to produce the very large range and very high δ 34 S values. The Nelson deposit probably formed within an anoxic but not euxinic subbasin. At Nelson, sulfide was likely derived by a subsurface thermochemical sulfate reduction (TSR) reaction, similar to reactions that are inferred to have produced the sulfides in the Galena and Quarry deposits, which are interpreted as feeder veins for the stratabound deposits. Calculations of oxygen isotope temperatures are based on the assumption that evolved seawater with δ 18 O of 3‰ was the mineralizing and altering fluid related to the formation of the sulfide deposits. Temperatures of aluminous alteration and sulfide mineralization were between 109 and 209 °C, determined on the basis of oxygen isotope fractionations between the mineralizing fluid and proportionate amounts of quartz and muscovite in the rocks. These temperature estimates agree well with known temperatures of SEDEX mineralization worldwide. Sulfur isotope values also are generally consistent with the known ranges in SEDEX deposits worldwide (δ 34 S ≈ −5‰ to 25‰).

The tectonic evolution of the South Island of New Zealand records the consequences of a transition from nearly translational to transpressional plate motions since the Late Miocene. Although it is clear how that transition was accommodated in the upper crust—primarily through the development of the Southern Alps orogen—how the lithospheric system responds to this change in plate kinematics is unclear. Coupling kinematic and deformational modeling with an analysis of the existing seismic data that images the deformational fabric in the lithospheric mantle leads me to propose that a substantial amount of the plate boundary strain is accommodated by a reorientation of the plate boundary structure and maintenance of simple shear deformation as plate motions change. This leads to a developing geographical mismatch between the location of upper crustal strain and that within the lower crust and lithospheric mantle. One possible result of this offset in the locus of plate boundary strain can be the development of a detachment surface within the lower crust that effectively decouples the upper crust from its underlying foundation. Consequences of this include different styles of deformation of the lower crust with position along the orogen and significantly different strain histories for crustal rocks involved in the Southern Alps orogen as a function of whether they are part of the thin-skinned or thick-skinned regime.

The majority of the San Andreas fault zone is convergently oblique to relative plate motion. The commonness of transpression makes it significant for understanding deformation of the continental lithosphere. We have quantified the distribution of transpressional deformation along the San Andreas fault zone with respect to variations in boundary conditions along its length and distance from the fault zone itself. Rock uplift was used as a proxy for transpressional deformation. The pattern of exhumation along the fault was synthesized based on previously determined apatite fission-track and (U-Th)/He ages from 210 locations within 40 km of the fault trace. Patterns of mean elevation and slope in swaths along the fault were used as rough proxies of surface uplift and erosion. Relatively higher exhumation rates and mean elevations occur most commonly along the most oblique sections of the fault, such as in the Transverse Ranges. The highest rates of exhumation (>0.5 mm/yr) and highest and steepest topography also occur almost exclusively in the near field (i.e., within ∼10 km) of the fault trace. These trends are consistent with the strain-partitioning model of transpression, in which distributed deformation is concentrated in the fault zone and the degree of partitioning between simple and pure shear is a function of obliquity. However, the pattern of rock uplift also exhibits considerable variability. Neither the degree of obliquity nor the distance to the fault trace is enough to predict where high exhumation or mean elevation will occur. This suggests that heterogeneity in boundary conditions, including mechanical weaknesses and variations in erodibility, is equally important for controlling the pattern of transpressional deformation.

The Niğde Massif, south-central Turkey, experienced two complete cycles of burial and exhumation during orogenesis and is, therefore, an excellent example of yo-yo tectonics. We propose that burial and exhumation of the metamorphic basement and, in the second cycle, the basement and its sedimentary cover rocks, were driven largely by transpression and transtension in an intracontinental strike-slip zone. The eastern margin of the massif, where it is adjacent and subparallel to the sinistral Central Anatolian fault zone, is comprised of Upper Cretaceous basement that was the source of, and is unconformably overlain by, early Tertiary sedimentary rocks. The contact between the Tertiary rocks and basement is an unconformity that is locally sheared and characterized by a low-angle oblique-normal shear system with cataclasite in the basement and brittle-ductile shear zones in the sedimentary rocks. These relationships, documented by geo/thermochronology to encompass 80 million years, define the timing and magnitude of the yo-yo process: burial and heating of Mesozoic sedimentary rocks during Late Cretaceous transpression to form the high-grade metamorphic basement (peak metamorphism at 85–91 Ma); Late Cretaceous (ca. 80–60 Ma) unroofing by transtension and erosion, with early Tertiary deposition of massif-derived clastic material at the edge of a marine basin along the Central Anatolian fault zone; reburial of basement and cover rocks involving folding, shearing, and greenschist facies metamorphism of the sedimentary cover in late Eocene through early Miocene time (ca. 50–20 Ma); and final exhumation in the middle Miocene (17–9 Ma) along strike-slip and normal faults.

The Walker Lane, a zone of northwest-striking dextral faults east of the Sierra Nevada, accommodates 15%–25% of Pacific–North American plate motion. A distinctive feature of the Walker Lane is the coexistence of parallel dextral and normal faults, which either developed sequentially or are related by strain partitioning. In the northern Walker Lane, three en echelon dextral faults strike northwest parallel to relative motion between the Sierra Nevada and Great Basin. Each fault cuts major basins but is parallel to and 1–5 km basinward of range-front normal faults. Basins in this area have anomalous northwest trends, whereas basins and major normal faults in the adjoining Basin and Range province trend north to north-northeast. Major normal fault systems straddling the northern Walker Lane dip toward one another, thereby producing a structural low. Significant exhumation related to strike-slip faulting is restricted to one, more westerly striking, probably transpressional segment of one dextral fault. Geologic data suggest that northwest-striking range-front faults were active during a ca. 3 Ma episode of extension but have been inactive in the Quaternary. Strike-slip faulting probably started immediately after 3 Ma, either cutting or reactivating deeper parts of the northwest-striking range-front normal faults as dextral faults. North-striking normal faults are active and kinematically compatible with the northwest-striking dextral faults. The unusual northwest strike of normal faults in the northern Walker Lane may reflect reactivation of a major northwest-striking basement structure indicated by gravity data. Strain partitioning between parallel dextral and normal faults is unnecessary because the dextral faults parallel Sierra Nevada–Great Basin motion and can take up all required displacement. In contrast, Sierra Nevada–Great Basin motion in Owens Valley in the southern Walker Lane is strongly oblique to faults, so strain partitioning between parallel dextral and normal faults may be necessary.