Nadine McQuarrie

Greater India Basin hypothesis and a two-stage Cenozoic collision between India and Asia

Cenozoic convergence between the Indian and Asian plates produced the archetypical continental collision zone comprising the Himalaya mountain belt and the Tibetan Plateau. How and where India–Asia convergence was accommodated after collision at or before 52 Ma remains a long-standing controversy. Since 52 Ma, the two plates have converged up to 3,600 ± 35 km, yet the upper crustal shortening documented from the geological record of Asia and the Himalaya is up to approximately 2,350-km less. Here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic Indian lithosphere within the Himalaya between approximately 50 and 25 Ma. Paleomagnetic data show that this extended continental and oceanic “Greater India” promontory resulted from 2,675 ± 700 km of North–South extension between 120 and 70 Ma, accommodated between the Tibetan Himalaya and cratonic India. We suggest that the approximately 50 Ma “India”–Asia collision was a collision of a Tibetan-Himalayan microcontinent with Asia, followed by subduction of the largely oceanic Greater India Basin along a subduction zone at the location of the Greater Himalaya. The “hard” India–Asia collision with thicker and contiguous Indian continental lithosphere occurred around 25–20 Ma. This hard collision is coincident with far-field deformation in central Asia and rapid exhumation of Greater Himalaya crystalline rocks, and may be linked to intensification of the Asian monsoon system. This two-stage collision between India and Asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography.

Detrital zircon geochronology of pre-Tertiary strata in the Tibetan-Himalayan orogen

Detrital zircon data have recently become available from many different portions of the Tibetan-Himalayan orogen. This study uses 13,441 new or existing U-Pb ages of zircon crystals from strata in the Lesser Himalayan, Greater Himalayan, and Tethyan sequences in the Himalaya, the Lhasa, Qiangtang, and Nan Shan-Qilian Shan-Altun Shan terranes in Tibet, and platformal strata of the Tarim craton to constrain changes in provenance through time. These constraints provide information about the paleogeographic and tectonic evolution of the Tibet-Himalaya region during Neoproterozoic to Mesozoic time. First-order conclusions are as follows: (1) Most ages from these crustal fragments are <1.4 Ga, which suggests formation in accretionary orogens involving little pre-mid-Proterozoic cratonal material; (2) all fragments south of the Jinsa suture evolved along the northern margin of India as part of a circum-Gondwana convergent margin system; (3) these Gondwana-margin assemblages were blanketed by glaciogenic sediment during Carboniferous-Permian time; (4) terranes north of the Jinsa suture formed along the southern margin of the Tarim-North China craton; (5) the northern (Tarim-North China) terranes and Gondwana-margin assemblages may have been juxtaposed during mid-Paleozoic time, followed by rifting that formed the Paleo-Tethys and Meso-Tethys ocean basins; (6) the abundance of Permian-Triassic arc-derived detritus in the Lhasa and Qiangtang terranes is interpreted to record their northward migration across the Paleo- and Meso-Tethys ocean basins; and (7) the arrival of India juxtaposed the Tethyan assemblage on its northern margin against the Lhasa terrane, and is the latest in a long history of collisional tectonism. Copyright 2011 by the American Geophysical Union.

The kinematic history of the central Andean fold-thrust belt, Bolivia: Implications for building a high plateau

This paper presents a model for the kinematic evolution of the central Andean plateau based on balanced cross sections across the Bolivian Andes. The proposed model links the formation of the Andean plateau to the development of the Andean fold-thrust belt through the creation and propagation of two large basement mega thrusts. Support for large, basement- involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the Andean fold-thrust belt. The structurally highest basement thrust raised folds and faults in predominantly lower Paleozoic rocks of the Eastern Cordillera with respect to Tertiary rocks in the broad, internally drained basin of the Altiplano to the west, and east- verging folds and faults in upper Paleozoic rocks of the Interandean zone to the east. The Interandean zone was in turn raised (both structurally and topographically) with respect to the frontal folds and faults of the fold-thrust belt (the Subandean zone) by a second, structurally lower basement thrust sheet. Thus, these two megathrusts divide the Andean fold-thrust belt into four areas of markedly different structural elevations. The Eastern Cordillera can be further subdivided into two zones of west- and east-vergent folds and thrusts. Shortening accommodated by the fold-thrust belt can be divided among these tectono-structural zones and linked to shortening accommodated by the inferred basement megathrusts. The proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed ∼105 km of slip into the Eastern Cordillera along east-vergent and west-vergent faults. This structure also fed ∼90 km of eastward slip into the Interandean zone. The initiation and eastward propagation of a lower basement thrust structurally elevated the Interandean zone with respect to the foreland while feeding ∼65 km of slip into the Subandean zone. Out-of-sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated ∼40 km of shortening within the Altiplano. Total cumulative shortening within the cover rocks of the Andean fold-thrust belt (300–330 km) can be balanced by an equivalent amount of shortening along two basement megathrusts. To the first order, the eastern margin of the central Andean plateau (defined by the 3 km topographic contour) is contiguous with the leading edge of the upper basement megathrust. This relationship between the basement highs and the physiographic boundaries of the Andean plateau suggests that extensive megathrust sheets (involving strong rocks such as crystalline basement or quartzite) play an important role in the formation of the central Andean plateau, and a similar link between megathrust sheets and plateaus may be found in other orogens.

An animated tectonic reconstruction of southwestern North America since 36 Ma

We present tectonic reconstructions and an accompanying animation of deformation across the North America–Pacific plate boundary since 36 Ma. Intraplate deformation of southwestern North America was obtained through synthesis of kinematic data (amount, timing, and direction of displacement) along three main transects through the northern (40°N), central (36°N– 37°N), and southern (34°N) portions of the Basin and Range province. We combined these transects with first-order plate boundary constraints from the San Andreas fault and other areas west of the Basin and Range. Extension and strike-slip deformation in all areas were sequentially restored over 2 m.y. (0–18 Ma) to 6 m.y. (18–36 Ma) time intervals using a script written for the ArcGIS program. Regions where the kinematics are known constrain adjacent areas where the kinematics are not well defined. The process of sequential restoration highlighted misalignments, overlaps, or large gaps in each incremental step, particularly in the areas between data transects, which remain problematic. Hence, the value of the reconstructions lies primarily in highlighting questions that might not otherwise be recognized, and thus they should be viewed more as a tool for investigation than as a final product. The new sequential reconstructions show that compatible slip along the entire northsouth extent of the inland right-lateral shear zone from the Gulf of California to the northern Walker Lane is supported by available data and that the east limit of active shear has migrated westward with respect to North America since ca. 10 Ma. The reconstructions also highlight new problems regarding strain-compatible extension east and west of the Sierra Nevada– Great Valley block and strain-compatible deformation between southern Arizona and the Mexican Basin and Range. Our results show ~235 km of extension oriented ~N78°W in both the northern (50% extension) and central (200% extension) parts of the Basin and Range. Following the initiation of east-west to southwest-northeast extension at 15–25 Ma (depending on longitude), a significant portion of right-lateral shear associated with the growing Pacific– North America transform jumped into the continent at 10–12 Ma, totaling ~100 km oriented N25°W, for an average of ~1 cm/yr since that time.

Retrodeforming the Arabia-Eurasia collision zone: Age of collision versus magnitude of continental subduction

The Arabia-Eurasia collision has been linked to global cooling, the slowing of Africa, Mediterranean extension, the rifting of the Red Sea, an increase in exhumation and sedimentation on the Eurasian plate, and the slowing and deformation of the Arabian plate. Collision age estimates range from the Late Cretaceous to Pliocene, with most estimates between 35 and 20 Ma. We assess the consequences of these collision ages on the magnitude and location of continental consumption by compiling all documented shortening within the region, and integrating this with plate kinematic reconstructions. Shortening estimates across the orogen allow for ~350 km of Neogene upper crustal contraction, necessitating collision by 20 Ma. A 35 Ma collision requires additional subduction of ~400‐600 km of Arabian continental crust. Using the Oman ophiolite as an analogue, ophiolitic fragments preserved along the Zagros suture zone permit ~180 km of subduction of the Arabian continental margin plus overlying ophiolites. Wholesale subduction of this more dense continental margin plus ophiolites would reconstruct ~400‐500 km of postcollisional Arabia-Eurasia convergence, consistent with a ca. 27 Ma initial collision age. This younger Arabia-Eurasia collision suggests a noncollisional mechanism for the slowing of Africa, and associated extension.

Geometry and crustal shortening of the Himalayan fold-thrust belt, eastern and central Bhutan

We present a new geologic map of eastern and central Bhutan and four balanced cross sections through the Himalayan fold-thrust belt. Major structural features, from south to north, include: (1) a single thrust sheet of Subhimalayan rocks above the Main Frontal thrust; (2) the upper Lesser Himalayan duplex system, which repeats horses of the Neoproterozoic–Cambrian(?) Baxa Group below a roof thrust (Shumar thrust) carrying the Paleoproterozoic Daling-Shumar Group; (3) the lower Lesser Himalayan duplex system, which repeats horses of the Daling-Shumar Group and Neoproterozoic–Ordovician(?) Jaishidanda Formation, with the Main Central thrust (MCT) acting as the roof thrust; (4) the structurally lower Greater Himalayan section above the MCT with overlying Tethyan Himalayan rock in stratigraphic contact in central Bhutan and structural contact above the South Tibetan detachment in eastern Bhutan; and (5) the structurally higher Greater Himalayan section above the Kakhtang thrust. Cross sections show 164–267 km shortening in Subhimalayan and Lesser Himalayan rocks, 97–156 km structural overlap across the MCT, and 31–53 km structural overlap across the Kakhtang thrust, indicating a total of 344–405 km of minimum crustal shortening (70%–75%). Our data show an eastward continuation of Lesser Himalayan duplexing identified in northwest India, Nepal, and Sikkim, which passively folded the overlying Greater Himalayan and Tethyan Himalayan sections. Shortening and percent shortening estimates across the orogen, although minima, do not show an overall eastward increase, which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates.

Raising the Colorado Plateau

Shallow-marine rocks exposed on the 2-km-high, 45-km-thick Colorado Plateau in the western United States indicate that it was near sea level during much of the Phanerozoic. Isostatic calculations, however, illuminate the difficulty in maintaining a 45-km-thick crust at or near sea level. We propose that an isostatically balanced, 30-km-thick, proto‐Colorado Plateau crust was thickened during the Late Cretaceous to early Tertiary by intracrustal flow out of an overthickened Sevier orogenic hinterland. This plateau would have been supported by a thick (>70 km) crustal root, which is proposed to have been the source region for hot and weak mid-crustal material that flowed eastward from the plateau toward the low-elevation proto‐Colorado Plateau.

Geometry and structural evolution of the central Andean backthrust belt, Bolivia

The central Andean backthrust belt is a large-scale west vergent thrust system along the western side of the Eastern Cordillera in the generally east vergent Andean fold-thrust belt of Bolivia. Although west vergent structures in the central Andes have been recognized previously, we describe the backthrust belt at a regional scale, emphasizing its implications for the kinematic development of the Andes and the subsequent influence of these kinematics on amounts of tectonic shortening. We use techniques such as line length balancing, restorability, and the viability of the progressive development of the structures to construct balanced cross sections across the backthrust belt and Altiplano. The cross sections are taken to a regional depth of detachment (basement) to examine the relationship between mapped surface structures and inferred subsurface structures. The relationship of the backthrust belt to the Altiplano suggests that the Altiplano basin is a crustal-scale piggyback basin created as a basement megathrust propagated up and over a half-crustal scale ramp located just west of the physiographic boundary of the Eastern Cordillera. This basement megathrust was the means by which a narrow Paleocene fold-thrust belt located to the west of the Altiplano propagated eastward and emerged in the present Eastern Cordillera. The relationship between the basement thrusts and the physiographic boundaries of the Central Andean plateau (as defined by Isacks [1988]) suggests that extensive megathrust sheets (involving crystalline basement or quartzite) may play an important role in the formation of orogenic plateaus. The kinematic development of the Andean fold-thrust belt indicates that the backthrust belt developed as a taper-building mechanism after the basement megathrust overextended the system eastward. The mechanism proposed in this study for the development of the central Andean backthrust belt requires a minimum of 200 km of shortening within the Altiplano/Eastern Cordillera alone. This increases minimum shortening estimates across the fold-thrust belt in Bolivia to as much as 300–340 km.

Initial plate geometry, shortening variations, and evolution of the Bolivian orocline

Comparisons of newly published cross sections across the Bolivian Andes with existing cross sections through Argentina emphasize significant along-strike changes in crustal shortening. A sharp decrease in the magnitude of crustal shortening from ∼530 km to ∼150 km (north to south) occurs at ∼23°S. A 20–40 m.y. difference in the ages at which deformation was initiated accompanies the abrupt decrease in the magnitude of shortening. Extending the western margin of South America to account for 530 km of shortening in the Bolivian Andes and 150 km of shortening in Argentina produces a central Andean salient that is perpendicular to the Nazca plate shortening direction from 60 to 26 Ma. During this same time interval, the Chilean coast south of 23°S was in an orientation sufficiently oblique to oceanic convergence to allow for predominantly strike-slip offset and backarc extension. Deformation within the Andean mountain chain may be a function of plate convergence where the oblique nature of convergence south of ∼23°S inhibited mountain building, whereas north of ∼23°S, normal convergence to a central Andean salient facilitated contractional deformation. The magnitude of deformation north of ∼23°S is a consequence of both plate-convergence direction, providing a longer period of contractional deformation (from ca. 70 Ma to the present), and a thick Phanerozoic sedimentary package that permitted large magnitudes of thin-skinned deformation. Significant along-strike changes in the shape of the South American margin—allowing for convergence to change from compression to extension along the strike of the orogen—may help explain the dramatic differences in timing, amount, and style of deformation in the Andes.

Tectonostratigraphy of the Lesser Himalaya of Bhutan: Implications for the along-strike stratigraphic continuity of the northern Indian margin

New mapping in eastern Bhutan, in conjunction with U-Pb detrital zircon and δ 13 C data, defi nes Lesser Himalayan tectonostratigraphy. The Daling-Shumar Group, 2–6 km of quartzite (Shumar Formation) overlain by 3 km of schist (Daling Formation), contains ~1.8–1.9 Ga intrusive orthogneiss bodies and youngest detrital zircon peaks, indicating a Paleoproterozoic deposition age. The Jaishidanda Formation, 0.5– 1.7 km of garnet-biotite schist and quartzite, stratigraphically overlies the Daling Formation beneath the Main Central thrust, and yields youngest detrital zircon peaks ranging from ~0.8–1.0 Ga to ca. 475 Ma, indicating a Neoproterozoic–Ordovician(?) deposition age range. The Baxa Group, 2–3 km of quartzite, phyllite, and dolomite, overlies the DalingShumar Group in the foreland, and yields ca. 0.9 Ga to ca. 520 Ma youngest detrital zircon peaks, indicating a Neoproterozoic– Cambrian(?) deposition age range. Baxa dolo mite overlying quartzite containing ca. 525 Ma detrital zircons yielded δ 13 C values between +3‰ and +6‰, suggesting deposition during an Early Cambrian positive δ 13 C excursion. Above the Baxa Group, the 2–3 km thick Diuri Formation diamictite yielded a ca. 390 Ma youngest detrital zircon peak, suggesting correlation with the late Paleo zoic Gondwana supercontinent glaciation. Finally, the Permian Gondwana succession consists of sandstone, siltstone, shale, and coal. Our deposition age data from Bhutan: (1) reinforce suggestions that Paleoproterozoic (~1.8–1.9 Ga) Lesser Himalayan deposition was continuous along the entire northern Indian margin; (2) show a likely east ward continuation of a Permian over Cambrian unconformity in the Lesser Himalayan section identifi ed in Nepal and northwest India; and (3) indicate temporal overlap between Neoproterozoic–Paleozoic Lesser Himalayan (proximal) and Greater Himalayan–Tethyan Himalayan (distal) deposition.