Multiple models (steady state, episodic, and early growth followed by crustal reworking) have been postulated to explain the evolution of Earth’s continental crust. An independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using U-Pb and Hf isotopes. A compilation of ∼200,000 published zircon dates reveals a global age distribution (U-Pb) with six prominent, statistically significant peaks: 3.2–3.0, 2.7–2.5, 2.0–1.7, 1.2–1.0, 0.7–0.5, and 0.3–0.1 Ga. In most cases, these peaks are recorded on all seven continents. Most of the peaks are detectable also in modern sediments. A comparison of grains with both U-Pb ages and Hf isotope model ages, available for ∼5100 detrital zircons, suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth. The zircon age distributions and associated data provide consistent ev...

https://www.jstor.org/stable/10.1086/658295

Quantifying the Timing and Rate of Crustal Evolution: Global Compilation of Radiometrically Dated Detrital Zircon Grains

Summary. This paper reports a detailed re-examination of the spatial distribution of shallow- and intermediate-depth earthquakes located beneath Peru between latitudes 3° and 18°S. The events are classified into two groups according to the quality of the locations. The results of this paper confirm and considerably strengthen the findings of Barazangi & Isacks, which have taken on significance beyond Peru because of their implications regarding the tectonics of western North America during the Laramide orogeny. The more accurately located events that occurred beneath central and northern Peru clearly define a shallow-dipping (about 10°) inclined seismic zone that extends inland for a distance of about 700 km from the Peru trench. Although the intermediate-depth events are not uniformly and continuously distributed along the strike of the arc, the available data are best explained by a single Benioff zone that defines the descending Nazca plate beneath central and northern Peru. In southern Peru the Benioff zone is well determined and has a dip of about 30°. The transition from the relatively flat Benioff zone in central Peru and the steeper Benioff zone in southern Peru is abrupt and is interpreted as a tear in the descending Nazca plate. Considerable seismic activity occurs within the upper 50 km of the overriding continental South America plate. This activity is well separated from the shallow-dipping Benioff zone in central and northern Peru, and probably occurs in the crustal part of the South American lithosphere. All events that occur outside the spatial pattern of events as described above are based on few and generally insufficient data. Azimuthal coverage at teleseismic distances is poor, and no reliable pP readings are available to control hypocentral depths. A search for ScS-to-P converted phases at the upper boundary of the descending Nacza plate beneath central Peru has failed to provide any clear and consistent data. This is in contrast to the well-documented observations from Japan.

The flat geometry of the descending Nazca plate beneath central and northern Peru requires that the thickness of the continental South American lithosphere does not exceed about 130 km in this region. A remarkable correlation exists between the flat geometry of the descending Nazca plate and the absence of Quaternary volcanism on the overriding South America plate. This is in marked contrast to the presence of abundant volcanism above the steeper segment of the descending Nazca plate in southern Peru, and suggests that generation of arc-type volcanism requires a mantle wedge of asthenospheric material between the subducted and overriding plates. The flat geometry of the descending plate may also be the cause for the broad deformation of the Andean Cordilleras. Many factors may be responsible for the observed flat geometry, such as the fact that the Nazca plate is relatively young (< 50 Myr old), and hence is thinner and more buoyant than the older western Pacific plate. Also, it may be significant that the upper South American lithosphere is thick, in comparison to the upper western Pacific plates, and is possibly actively overriding the descending Nazca plate. Finally, the width of the flat Peru segment along the strike of the arc is probably controlled by the existence of the aseismic submarine structural features near the southern limit (the Nazca ridge) and the northern limit (the Carnegie ridge) of the flat segment. These features are probably zones of weakness along which the Nazca plate is being transversely torn.

Subduction of the Nazca plate beneath Peru: evidence from spatial distribution of earthquakes

Numerous models have been presented to explain the late Cretaceous/early Cenozoic Laramide orogeny, which affected the foreland region of the western cordillera within the US1. The most attractive models invoke low-angle subduction2–4, which can develop for various reasons5,6. Evidence from South America indicates that one such reason may be the subduction of buoyant ocean floor. We here extend the low-angle subduction models by attributing the shallowing of the subduction angle during the Laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust. Pacific plate history indicates that a twin to the Hess Rise may represent the oceanic plateau that triggered Laramide events.

Was the Laramide orogeny related to subduction of an oceanic plateau

Stable continental regions, free from tectonic activity, are generally found only within ancient cratons—the centres of continents which formed in the Archaean era, 4.0–2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium–osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.

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Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.

Anomalous subsidence of continental crust occurred during the Late Cretaceous in a region centred about Colorado, southern Wyoming and eastern Utah. The amount of subsidence is greater than can be explained by eustatic sea level rise and supracrustal loading by water and sediment. Analysis of thickness and distribution of Upper Cretaceous strata of the Western Interior Seaway, USA, shows two distinct basinal trends. The subsidence of the earliest (100–80 Myr BP) is attributed partly to isostatic flexure in front of the rising orogenic welt. The subsidence of the later (80-70 Myr BP) is attributed to subcrustal loading induced by a shallowly subducted plate.

B. C. Burchfiel

Papers

Grundprinzipien der alpinen Deckentektonik; eine Systemanalyse am Beispiel der noerdlichen Kalkalpen