Jefferson Yarce
The main hypothesis to test in this point is that during the Middle Cretaceous, there was a magmatic flare-up composed mostly of tonalite, granodiorite and granite. A good modern analog for this volume/flux of magmatism is the one occurring in the Altiplano-Puna region in the Central Andes of South America. Authors usually show this magmatic volume/flux values with different calculation methods. The most common is the apparent magmatic flux (AMF), with units of volume per unit year (or million year [Ma]) normalized with the strike length continuity of the magma. The magma production rate (MPR) is essentially the same quantity as AMF without being normalized to strike length [Bogolub, 2016]. The above statement is tested with the comparison of magmatic flux per unit time, per unit length for the Cretaceous magmatic Sierra Nevada and Coast Mountains and the same flux values in the Central Andes.
For the Coast Mountains batholiths, that are part of the “Cordilleran batholithic phase”, Hildebrand (2009, 2013) mentioned that the main voluminous period occurred in the interval of time: ~120 to 80 Ma, while the magmatic fluxes show an increasing flux for three different periods 160–140 Ma (~35 km3/Ma per km strike length), 120–78 Ma (35–45 km3/Ma per km strike length), and 55–48 Ma (~50 km3/Ma per km strike length) [Gehrels et al., 2009]. Also in the Coast Mountains, [Miller et al., 2009] found AFMs at two different pulses (between 96-89 Ma and between 78-71 Ma) with the first pulse with flux 1.2x10-5 km3/yr/km (equivalent to 12 km3/Ma/km) strike, the second pulse shows a decreasing of magmatic flux with 2.6x10-6 km3/yr/km (equivalent to 2.6 km3/Ma/km). These values differ in almost an order of magnitude to the first values presented. This probably has to due with the estimated plutonic thickness. For the Sierra Nevada magmatism, MPRs are calculated for the John Muir Intrusive suite with magmatic fluxes on the order of 0.001–0.002 km3/yr [Davis et al., 2012].
The central portion of the Andean Cordillera (~13°S to 28°S), represents a good moderna analog for the magmatism in the western US. It is composed for Late Cenozoic volcanoes of andesitic composition, ignimbrites related cover a great extension of the Altiplano-Puna region [Baker and Francis, 1978].
In the Altiplano-Puna Volcanic complex (APVC) it is possible to find good and large examples of exposed large ignimbrites with an age distribution from the Late Tertiary to Recent. This volcanic complex is located between >21°S and 24°S, the APVC covers some 70,000 km2 and represents > 30,000 km3 of intermediate composition ignimbrites, that are in particular well exposed/preserved due to the arid weather condition present in this region of the Andes [Lindsay et al., 2001].
There are estimated MPRs for this region of the Andes with estimated rates of emplacement ranging between the long-term rates of 10-2 km3/yr to short-terms rates in a range from 1 to 0.1 km3/yr [Paterson and Tobisch, 1992]. According to de Silva and Gosnold (2007), the MPR in the APVC lies around 0.004 to 0.012 km3/yr, which is similar or slightly higher than the John Muir Suite. Compared to the MPRs suggested for the western US, the Altiplano volcanic complex has had similar to up to around one to two orders of magnitude greater production rates than the Sierra Nevada.
Between latitudes 19°30'S and 22°30'S, a total of 12x103 km3 of volcanic rocks has been erupted over the last 20 Ma. This corresponds to about 1.6 km3/Ma/km of the cordillera. If the rate of intrusion is five times that of extrusion, the AFM of all kinds in the Central Andes must be of the order of 8 km3/Ma/km [Baker and Francis, 1978]. In this case, this value is an order of magnitude less than in the United States and would confirm the anomalous volume accumulation in the Middle Cretaceous for North America.
In conclusion, the anomalously voluminous accumulation in the Mesozoic arc in the cordilleran batholithic arc, are not quite anomalous to what Hildebrand (2009, 2013) suggested for the western United States. The apparent/average magma flux and magma production rates are fluctuating around the values published for the Central Andes volcanism.
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