Contrasting lithospheric strength between terranes often results in the concentration of strain and deformation within the weaker material. Dramatic alternating asymmetric topography of the central and eastern Alaska Range along the active Denali fault is due to contrasting lithospheric strength between terranes and a suture zone, controlled by fault location with respect to the irregular boundary of a relatively stronger terrane backstop. Highest topography and greatest Neogene exhumation in the central Alaska Range occur on the concave side of the arcuate Denali fault, yet to the north and on the convex side of the fault in the eastern Alaska Range. The Denali fault largely lies along a Mesozoic suture zone between two large composite terranes (Yukon and Wrangellia composite terranes: YCT and WCT), but the McKinley strand of the fault cuts across an embayment of weaker suture-zone rocks (Alaska Range suture-zone, ARSZ) within the irregular southern boundary of the YCT (Hines Creek fault). Deformation (and uplift of the Alaska Range) is driven by slip and partitioning of strain along the Denali fault, occurring preferentially in weaker rocks of the ARSZ against the stronger YCT. Where the YCT lies well north of the McKinley strand, deformation is primarily to the north of the fault (eastern Alaska Range). Where the YCT is close to the fault, deformation is primarily to the south (central Alaska Range). While the trace of the McKinley strand approximates a small circle, two restraining bends (McKinley and Hayes) pinned equidistant from the ends of this strand localize uplift and exhumation.

Ar-40/Ar-39, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from similar to 33 Ma to similar to 18 Ma for Ar-40/Ar-39 biotite, similar to 18 Ma to similar to 6 Ma for K-feldspar minimum closure ages, and similar to 15 Ma to similar to 1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from similar to 4 Ma to similar to 1 Ma. There has been at least similar to 11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite Ar-40/Ar-39 thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least similar to 22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (similar to past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

Two hypotheses have been offered to account for the transport and accretion history of the Yakutat terrane in southern Alaska. To investigate these two options, we deconvolved fission-track (FT) and U/Pb ages of detrital zircons from stratigraphically coordinated samples collected in the northern Robinson Mountains into component populations. The strata of the Yakutat terrane include the Middle Eocene Kulthieth Formation, the Lower Oligocene to Lower Miocene Poul Creek Formation, and the Miocene-Pleistocene Yakataga Formation. The Kulthieth and Poul Creek formations record erosion of a simple, uniform, long-lived, nonvolcanic source terrain that crystallized from ∼50 to 220 Ma and cooled from ∼40 to 110 Ma. Miocene cooling episodes recorded in the source to the Kulthieth and Poul Creek formations are likely associated with plutons in the northern Coast Plutonic Complex and the Kuiu-Etoilin belt. The Upper Miocene to Pleistocene Yakataga Formation records erosion of rocks that crystallized from ∼50 to 53 Ma and cooled below the zircon FT closure at ∼70–20 Ma. Upper Miocene strata are likely derived from erosion of the Chugach–Prince William terranes and the superimposed Sanak-Baranof plutonic belt. The uniform provenance of the Kulthieth and Poul Creek formations, the overall FT grain age distribution, and the distinct lack of volcanic zircons favor a northern position of the Yakutat terrane since the Eocene. However, a far-traveled southern option for the basement rocks cannot be ruled out, but it is unlikely that the Eocene and younger cover strata were deposited far to the south.

The north central Peruvian Andes (c . 9-11 0S) are divided into the Cordillera Oriental and the Cordillera Occidental. Uplift of this part of the Andes has occurred s ince ~ 10-20 Ma , and since that time deep canyons have been incised (3-4 km of relief) and rugged ranges have been emerged along the crest of the uplift (peak eJevations 5-7 km and relief of 3-4 km) . High-relief ranges are distinct in the Cordillera Oriental, which has the highest peaks in Peru: these include Huscaran (6768 m) in the Cordillera Blanca, and Yerupaja (6634 m) in the Cordillera Huayhuash. These ranges represent the largest glaciated region in equatorial South America, and therefore glacial erosion rate s are presumably higher than adjacent areas. Deep canyon incision has been profound in Pacific-draining rivers that have their source in this glaciated terrain. These deeply inc ised rivers include the Rio Santa, which drains the Cordillera Blanca, and the Rio Pativilica, which drains the Cordillera Huayhuash. The timing of uplift and local intense exhumation varies along strike, but the majority of regional uplift is generally viewed as being Miocene to Plio-Pleistocene. We have focused our thermochronological studies on three areas in this region. 1) The Cordillera Blanca, along the Andean crest and which is bound on its western edge by the Active Cordillera Blanca Normal Fault; 2) the Cordillera Huayhuash, also along the crest of Andes, south of the Blanca, and 3) The Rio Pativilica, whose headwaters drain the Cordillera Huayhuash.

The Cordillera Huayhuash is a north-south-oriented range along the drainage divide of the northern Peruvian Andes. The range has high topography with peaks in excess of 5500 m and the second-highest peak in Peru, Nevados Yerupaja ( 6617 m). Bedrock is dominated by folded Mesozoic miogeoclinal rocks unconformably overlain by mid-Tertiary volcanics intruded by Late Tertiary granitic rocks and silicic dikes. Zircon fission track ( ZFT) and ( U-Th)/He ( ZHe) dating of zircons along a west-east transect elucidates the thermal evolution of exhumed and uplifted rocks. The stability of fission tracks in zircons is a function of single-grain radiation damage. In samples with grain-to-grain variability in radiation damage, resetting results in variable resetting and multiple age populations. Low retentive zircons ( LRZs), which have a partly disordered crystalline structure, have significant radiation damage and a low temperature of annealing ( ca. 180degrees- 200degreesC). High retentive zircons ( HRZs), which are nearly crystalline, fully anneal at temperatures in excess of ca. 280degrees- 300degreesC. Partly reset samples are those where LRZs are reset and HRZs are not reset, and therefore the cooling age is not concordant, but the young population of grain ages records the youngest thermal event. Full resetting of both LRZs and HRZs results in cooling ages that are concordant or nearly so. Lower Cretaceous quartzites show ZFT ages with a wide range of cooling ages, but most have LRZ reset ages at ca. 27 and 63 Ma. The ZFT ages from three quartzites and two granites from the core of the range yielded a single mean reset age of Ma. The ZHe ages from four samples in these rocks ranged from 10 to 7 Ma, with older ages away 11.4 +/- 1 from the high topography. Together, the ZFT and ZHe cooling ages near the core of the range indicate moderate to rapid postintrusive cooling in the Miocene and a highMiocene geothermal gradient ( ca. 40degrees- 50degreesC/km). This widespread cooling age represents a falling geotherm, not a period of significant exhumation. Estimations of the thickness of preexhumation cover rock suggest that nearly 5 km of unroofing has occurred since the eruption of the Puscanturpa Formation ( Huayllay Formation) at ca. 6.2 Ma. Exhumation was driven by valley incision initiated by uplift of this part of the Andes between 5 and 6 Ma. The high topography may have been formed by isostatic response to canyon incision. Therefore, the thermochronologic record of uplift and canyon incision is not yet apparent in the low-temperature thermochronology ( for zircons) of these rocks.