Daroca thrust (Iberian Chain, Spain) and the Azuara impact structure -the controversy continues.

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ERNSTSON CLAUDIN IMPACT STRUCTURES –
METEORITE CRATERS
Research on impact geology, geophysics, petrology,
and impact cratering
Daroca thrust (Iberian Chain, Spain)
and the Azuara impact structure – the
controversy continues
Comment on
Sanchez, M.A. ; Gil, A. y Simón, J.L. (2017): Las rocas de falla del cabalgamiento de
Daroca (sector central de la Cordillera Ibérica): Interpretación reológica y
cinemática. Geogaceta, 61: 75-78.
(http://www.sociedadgeologica.es/archivos/geogacetas/geo61/geo61_19p75_78.pd
f)
Casas-Sainz, A.M., Gil-Imaz, A., Simón, J.L., Izquierdo Llavall, Aldega, E.L.,
Román-Berdiel, T., Osácar, M.C., Pueyo-Anchuela, O., Ansón, M., García-Lasanta,
C., Corrado, S., Invernizzi, C., Caricchi, C. (2018): Strain indicators and magnetic
fabric in intraplate fault zones: Case study of Daroca thrust, Iberian Chain, Spain.
Tectonophysics, 730: 29-47 (10.1016/j.tecto.2018.02.013)
(https://zaguan.unizar.es/record/78325/les/texto_completo.pdf
Gutierrez, F, Carbonela, D., Sevil, J., Moreno, D., Linares, R, Comas, X., Zarroca,
M., Roqué,C., McCalpin, J.P. (2020): Neotectonics and late Holocene paleoseismic

evidence in the Plio-Quaternary Daroca Half-graben, Iberian Chain, NE Spain.
Implications for fault sorce characterization. Journal of Structural Geology, 131: 1-17
(https://doi.org/10.1016/j.jsg.2019.103933)
by Ferran Claudin & Kord Ernstson (March 2020)
The town of Daroca in the Spanish Province of Zaragoza hides a peculiar geologic
scenario – an enigma for geologists from time out of mind. Being enthroned above
the town the geologic stratigraphy shows with a very sharp cut Cambrian dolomite
(Ribota dolomite) over Tertiary young sediments (Fig. A). Older layers over younger
ones are not uncommon in geology, and overthrust and thrust faulting are related
processes. But Daroca is dierent. The Cambrian plate is kilometer-sized and
fragmented into larger blocks, and a Tertiary 180° overtrust can reasonably be
excluded. Early geologists confronted with the situation in sheer desperation
thought of a preexisting Cambrian autochthonous plate and a vast undercutting by
the Tertiary. Today this explanation is left out of consideration and simple thrust
faulting is being favored. But the case is all but simple. There is no root zone and
not any relief from where the giant plate could have started to override the Tertiary
around Daroca. Nevertheless, the thrust kinematics are developed further by
geologists (e.g., Capote et al. 2002), and tens of kilometers long faults are drawn
within models of syn-tectonic sedimentation (Casas et al. 2000; Fig. 3).
Fig. A. The prominent Daroca exposure.
In 2012 we published an extended article (Claudin and Ernstson 2012) under the
title “Azuara impact structure: The Daroca thrust geologic enigma – solved? A Ries
impact structure analog“.. which proposes a new and in our opinion reasonable

model of formation and a physically plausible solution of the enigma. To cut the
story short, the Daroca thrust originated in the meanwhile generally established
Azuara impact event, when according to the spall plate model of Melosh (1989) the
Daroca Ribota dolomite plate started from the developing excavation crater and the
near-surface interference zone with extreme velocity (Fig. B), supported by
enormous volumes of rock melt, water and gases (water vapor and carbon dioxide
from the shocked target) as a kind of hovercraft. This is no speculation but has
much earlier been discussed for the so-called role-and-glide mode in the
excavation process of the Ries impact crater (Germany). In our paper on the Daroca
thrust we write about the anity of both events and point to Ries giant megablocks
having been excavated and transported over enormous distances.
In the case of the Daroca thrust this impressive way of transport can so nicely and
conclusively be observed on the geological general map 1 : 200 000, sheet Daroca,
which we show in simplied manner below (Fig. B) and in a copy of the geological
map in Fig. 11 of the Spanish complete version).

Fig. B. The impact cratering model for the Daroca thrust – no intraplate fault zone
(see text).
We do not know with certainty whether the authors of the three papers have read
our paper and whether they have understood the herein presented simple
explanation, but strangely enough since the publication of our paper on the Daroca
thrust with the close relation to the Azuara impact structure, practically
“overnight” a series of publications has appeared to demonstrate that the Daroca
thrust has a normal tectonic fault origin, after not any geologist from Spain or
elsewhere had paid attention to the enigma for decades (apart from the Casas et al.
(2000) and Capote et al. (2002) papers, only seeing a tectonic fault despite all non-
tectonic eld evidence).
Of course, science thrives on controversy on certain topics and especially on new
discoveries and models, but one principle is that the dierent views should be on a
scientic level and that both views should be carefully discussed and balanced.
In all three papers we miss the observance of this basic scientic constitution. Not a
single word is used to mention the Daroca article and the Azuara impact structure
in general, and not a single one of the abundant publications on one of the most
spectacular geological scenarios in Spain is found in their works. Today the Internet
is a common medium and broadly used to get information about serious scientic
publications (ResearchGate e.g.), and on preparing their papers a few clicks by the
authors of the three papers had of course opened a host of literature about the
Azuara impact event and the Daroca thrust.
In principle our Comment article could therefore end here. But we do not want to
make the same omission ignoring the papers under discussion here. In the main
part of our Comment paper following below, F. Claudin has compiled an exhaustive
analysis of the three Daroca “tectonic” papers confronting their in many respects
rather questionable claims with standard geologic literature and with the
impressive meteorite impact-related features, once more described here point by
point with a host of gures that do not exist for the authors.
Exemplarily, one point of importance is addressed already here, the Casas-Sainz et.
al. paper about magnetic fabric from AMS (anisotropy of magnetic susceptibility)
and strain indicators. From the text we learn that magnetic AMS analyses were
performed at 6 sites, but only a single one (no.16) is located in Daroca at the thrust

exposure (their Fig. 2A). Since the site is navigated at fractures of a second, we have
to proceed from a spot analysis (their Table. 1), and the rock for 12 specimen
measurements is described as a fault microbreccia (their Table 3). The rest of the 5
AMS sites is located roughly 1 km southeast of Daroca (their Fig. 2A).
We imagine: For the Daroca thrust as described in very detail in our paper (Claudin
& Ernstson 2012), a spot few meters sized at best served for an AMS analysis of a
microbreccia (we assume of Ribota dolomite) for which it is obviously not known
when it acquired the brecciation and a resulting AMS texture.
Considering now the Daroca thrust impact model of an enormously dislocated spall
plate, which Casas-Sainz et al. completely ignore, what will a point AMS tell us
about old in situ tectonics and intraplate fault zones? Nothing. The Daroca plate
may have transported magnetic textures from its original place more than 10 km to
the east, intense brecciation and other deformation (which can be seen in Daroca
outcrops) in the excavation, ejection, transport and emplacement processes should
have produced a completely new texture, not to forget possible strong temperature
overprint in the impact cratering process.
The same holds true for the ve other sites of AMS analyses. Since the aim and
outcome of the paper is basically the AMS of the thrust zone, more than a little
methodological insight into the authors’ working cannot be recognized. The paper
of Gutierrez et al. (2020) does not dier in any way in this respect. Their ground
penetrating radar (GPR) and resistivity measurements at Daroca are good to look
at, but for the topic under discussion they are absolutely meaningless. The idea
arises that the visual impression of scientic evidence for the so-called Daroca
Half-graben is to be created by the pure application of a few geophysical
measurements on a very small area.
The complete article (in Spanish) may be clicked here.
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