Abstract The Chortis Block of Central America is a cratonic-type peri-Gondwanan terrane and is commonly included in Neoproterozoic–Paleozoic palaeogeographical reconstructions. At present, most research has focused on the Mesozoic evolution of the Chortis Block, however, its earlier history remains poorly constrained. As a result, there is considerable debate surrounding the internal complexities of the Chortis Block and its tectonothermal evolution has not been well established by geochronological and geochemical data. New field investigations from the Nueva Segovia Schist (Northern Nicaragua), considered one of the oldest exposed parts of the Chortis Block, reveal it is composed primarily of deformed sequences of greenschist facies marine clastic and chemical sediments in conformable contact with felsic volcanics. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) detrital zircon data from two samples taken from the Nueva Segovia Schist reveal a youngest age peak of c. 250 Ma with other significant peaks at c. 500 Ma, c. 1.0 Ga and c. 1.2 Ga. Taken together with field observations, these data suggest the Nueva Segovia Schist was likely deposited between c. 250 and 110 Ma proximal to Amazonia during the Late Paleozoic, and they support a Precambrian age for the basement of the East Chortis Terrane. Taken together the data support a Pangaean position of the Chortis Block, adjacent to Amazonia inboard of Oaxaquia.

ABSTRACT We generated low-temperature thermochronological data on crystalline rocks from the Chiapas Massif in southern Mexico to constrain the complex relationship among tectonics, exhumation, and sedimentation in the region. Our data show that the first recorded cooling event occurred at ca. 40–25 Ma due to denudation of the sedimentary cover of the Chiapas Massif at slow rates of ~0.1 km/m.y. This was followed by a period of tectonic quiescence from ca. 25 to 14 Ma. Between ca. 14 and 7 Ma, cooling implying exhumation of the massif at rates of up to ~0.7 km/m.y. was renewed, and this was associated with, and possibly responsible for, the Miocene “Chiapanecan” deformational event observed in the Chiapas fold-and-thrust belt to the northeast of the massif. This younger uplift was also accompanied by the onset of arc-related magmatism beneath the massif, between ca. 13 and 9 Ma, along the Tonalá shear zone at the Pacific coast. Since ca. 7 Ma, additional but slower cooling and exhumation are indicated along the length of the Chiapas Massif, and arc magmatism has jumped north by ~125 km from the Tonalá shear zone into the Chiapas fold-and-thrust belt. Concurrently, subsidence and sedimentation have persisted along the offshore Tehuantepec Shelf to the south, suggesting that the Tonalá shear zone has been recently active (despite no magnitude 4 or larger earthquakes), with up-to-the-north vertical displacement. We interpret the exhumation at ca. 40–25 Ma to pertain to displacement of the Chortis block along the paleo–Motagua fault zone, either as a northward propagation of a basement thrust beneath the massif within a regional transpressional setting, or as a deep, ductile crustal thickening and attendant isostatic uplift of the southern flank of the massif during the transpressional passage of the Chortis block. The ensuing quiescence (25–14 Ma) coincided, we believe, with the passage of the “western tail” of Chortis, which is internally deformed and perhaps transferred compressive stress less effectively than had the central, continental core of the Chortis block earlier. Renewed uplift and exhumation of the region began by ca. 14–10 Ma. An onset at ca. 10 Ma is probably the best estimate for the beginning of exhumation of the northwestern and central portions of the Chiapas Massif, whereas the present-day southeastern tip of the massif (potentially an allochthonous sliver belonging to the Chortis block) started to exhume earlier, at ca. 14 Ma. By ca. 13 Ma, arc magmatism had moved into the western Tehuantepec area, marking the onset of subduction of the Cocos plate beneath the Chiapas Massif. Hence, we interpret the main period of uplift of the Chiapas Massif and primary shortening of the Chiapas fold-and-thrust belt (ca. 14–7 Ma) as being driven by the establishment of Cocos subduction beneath the area.

ABSTRACT New low-temperature thermochronological data analyses (apatite fission track and apatite and zircon [U-Th]/He) on rocks from the southern (Pacific) margin of Mexico between Acapulco and the western Gulf of Tehuantepec, where pre–middle Eocene arc and forearc complexes are expected but missing, show that this continental margin was subjected to an important Tertiary exhumational event. Exhumation is constrained to ca. 32–20 Ma in the west (Acapulco) and to ca. 19–11 Ma in the east (Puerto Angel) and was thus eastwardly diachronous. The diachroneity is interpreted as relating to the migration of the Chortis block, representing the western end of the Caribbean plate. The amount of exhumation along the trend is constrained to roughly 4–5 km (~0.3–0.6 km/m.y.). These magnitudes and rates are much less than previous estimates of 2.5–4 km/m.y. during the Oligocene, which are likely overestimated. These faster rates have been employed in a competing model for arc removal by orthogonal subduction erosion (i.e., Chortis block not involved), which is accordingly questioned. The exhumation was not due to shearing or fault-related uplift as the Chortis block migrated, but rather to the inception of subduction along Mexico in the wake of Chortis block migration. A four-part history applies to southern Mexico that is eastwardly diachronous: (1) inception of arc magmatism as the Chortis block first moved over the ~150 km depth contour of the Farallon/Cocos Benioff zone; (2) uplift and exhumation of basement as southern Mexico encountered and overrode the site of the Farallon/Cocos Benioff zone; (3) northward migration of arc magmatism as the Chortis block left the cross section and North America continued to advance further onto the Cocos plate, producing flat slab subduction geometry; and (4) resumption of forearc subsidence once the Mexican margin had acquired a subduction zone hanging-wall geometry. The missing arc terrane along southern Mexico is the Chortis block.

Abstract Northern Honduras and its offshore area include an active transtensional margin separating the Caribbean and North American plates. We use deep-penetration seismic-reflection lines combined with gravity and magnetic data to describe two distinct structural domains in the Honduran offshore area: (1) an approximately 120 km-wide Honduran Borderlands (HB) adjacent to the Cayman Trough characterized by narrow rift basins controlled by basement-involving normal faults subparallel to the margin; and (2) the Nicaraguan Rise (NR), characterized by small-displacement normal faulting and sag-type basins influenced by Paleocene–Eocene shelf sedimentation beneath an Oligocene–Recent, approximately 1–2 km-thick carbonate platform. Thinning of continental crust from 25–30 km beneath the NR to 6–8 km beneath the oceanic Cayman Trough is attributed to an Oligocene–Recent phase of transtension. Five tectonostratigraphic phases established in the HB and NR include: (1) a Late Cretaceous uplift in the north and south-dipping thrusting related to the collision in the south, between the Chortis continental block and arc and oceanic plateau rocks of the Caribbean; (2) Eocene sag basins in the NR and minor extension in the HB; two phases (3) and (4) of accelerated extension (transtension) across the subsidence mainly of the HB; and (5) Pliocene–Recent minor fault activity in the HB and a stable carbonate platform in the NR.

Early Mesozoic arc magmatism of the southern Sierra Nevada region records the onset of plate convergence–driven magmatism resulting from subduction initiation near the end of Permian time along a prior transform margin. We provisionally adopt the term California-Coahuila transform for this complex boundary transform system, which bounded the southwest margin of the Cordilleran passive margin, its offshore marginal basin, and fringing island arc. In Pennsylvanian–Early Permian time, this transform cut into the arc-marginal basin and adjacent shelf system, calved off a series of strike-slip ribbons, and transported them differentially southward through ∼500–1000-km-scale sinistral displacements. These strike-slip ribbons constitute the principal Neoproterozoic–Paleozoic metamorphic framework terranes for the superposed Mesozoic batholithic belt in the Sierra Nevada and Mojave plateau regions. The southern Sierra Nevada batholith intruded along the transform truncation zone where marginal basin ribbons were juxtaposed against the truncated shelf. Strike-slip ribbons, or blocks, liberated from the truncated shelf occur today as the Caborca block in northwest Mexico, and possibly parts of the Chortis block, farther south. The oldest arc plutons in the Sierra region were emplaced between 256 and 248 Ma, which matches well with ca. 255 Ma high-pressure metamorphism recorded in the western Sierra Foothills ophiolite belt, interpreted to approximate the time of subduction initiation. The initial phases of arc plutonism were accompanied by regional transpressive fold-and-thrust deformation, kinematically marking the transition from transform to oblique convergent plate motion. Early arc volcanism is sparsely recorded owing to fold-and-thrust–driven exhumation having accompanied the early phases of arc activity. By Late Triassic time, the volcanic record became quite prolific, owing to regional subsidence of the arc into marine conditions, and the ponding of volcanics in a regional arc graben system. The arc graben system is but one mark of regional suprasubduction-zone extension that affected the early SW Cordilleran convergent margin from Late Triassic to early Middle Jurassic time. We interpret this extension to have been a dynamic consequence of the subduction of exceptionally aged Panthalassa abyssal lithosphere, which is well represented in the Foothills ophiolite belt and other ophiolitic remnants of the SW Cordillera. Middle and Late Jurassic time was characterized by important tangential displacements along the SW Cordil-leran convergent margin. In Middle Jurassic time, dextral impingement of the Insular superterrane intra-oceanic arc drove a migrating welt of transpressional deformation through the SW Cordillera while the superterrane was en route to its Pacific Northwest accretionary site. Dextral transtensional spreading in the wake of the obliquely colliding and translating arc opened the Coast Range and Josephine ophiolite basins. In Late Jurassic time, a northwestward acceleration in the absolute motion of the North American plate resulted in an ∼15 m.y. period of profound sinistral shear along the Cordilleran convergent margin. This shear is recorded in the southern Cordillera by the Mojave-Sonora megashear system. Late Jurassic intrusive units of the southern Sierra region record sinistral shear during their magmatic emplacement, but we have not observed evidence for major Late Jurassic sinistral displacements having run through the Sierran framework. Possible displacements related to the megashear in the California to Washington regions are likely to have: (1) followed preexisting transforms in the Coast Range ophiolite basin and (2) been accommodated by oblique closure of the Josephine ophiolite basin, and the northern reaches of the Coast Range ophiolite basin, proximal to the southern Insular superterrane, which in Late Jurassic–earliest Cretaceous time was obliquely accreting to the inner Cordillera terranes of the Pacific Northwest.

An aeromagnetic survey of Honduras and its northeastern Caribbean coastal area covering a continuous area of 137,400 km 2 was acquired by the Honduran government in 1985 and provided to the University of Texas at Austin for research purposes in 2002. We correlate regional and continuous aeromagnetic features with a compilation of geologic data to reveal the extent, structural grain, and inferred boundaries of tectonic terranes that compose the remote and understudied, Precambrian-Paleozoic continental Chortis block of Honduras. A regional geologic map and a compilation of isotopic age dates and lead isotope data are used in conjunction with and geo-referenced to the aero-magnetic map. These combined data provide a basis for subdividing the 531,370 km 2 Chortis block into three tectonic terranes with distinctive aeromagnetic expression, lithologies, structural styles, metamorphic grade, isotopically and paleontologically determined ages, and lead isotope values: (1) The Central Chortis terrane occupies an area of 110,600 km 2 , exhibits a belt of roughly east-west–trending high magnetic values, and exposes small, discontinuous outcrops of Grenville to Paleozoic continental metamorphic rocks including greenschist to amphibolite grade phyllite, schist, gneiss, and orthogneiss that have been previously dated in the range of 1 Ga to 222 Ma; the northern 59,990 km 2 margin of the Central Chortis terrane along the northern Caribbean coast of Honduras exhibits an irregular pattern of east-west–trending magnetic highs and lows that correlates with an east-west–trending belt of early Paleozoic to Tertiary age metamorphic rocks intruded by Late Cretaceous and early Cenozoic plutons in the range of 93.3–28.9 Ma. (2) The Eastern Chortis terrane occupies an area of 185,560 km 2 , exhibits belts of roughly northeast-trending high magnetic values, and correlates with outcrops of folded and thrusted Jurassic metasedimentary phyllites and schists forming a greenschist-grade basement; we propose that the Eastern and Central terranes are distinct terranes based on the strong differences in their structural style and aeromagnetic grain, sedimentary thickness, metamorphic grade, and lead isotope values. (3) The Southern Chortis terrane occupies an area of 120,100 km 2 , contains one known basement outcrop of metaigneous rock, exhibits a uniformly low magnetic intensity that contrasts with the rest of the Chortis block, and is associated with an extensive area of Miocene pyroclastic strata deposited adjacent to the late Cenozoic Central American volcanic arc. The outlines of the terranes as constrained by the aeromagnetic, lithologic, age, and lead isotope data are restored to their pre–early Eocene position along the southwestern coast of Mexico by a 40° clockwise rotation and 1100 km of documented post–early Eocene (ca. 43 Ma) left-lateral offset along the strike-slip faults of the northern Caribbean strike-slip plate boundary. The inner continental and outboard oceanic terranes of Chortis and the 120,100 km 2 Siuna terrane to the south trend roughly north-south and align with terranes of similar magnetic trend, lithology, age, and crustal character in southwestern Mexico. Additional progress in mapping and isotopic dating is needed for the proposed Chortis terranes in Honduras in order to constrain this proposed position against much better mapped and dated rocks in southwestern Mexico.

This study describes the geology of a well-exposed but previously unmapped section of Paleozoic–early Cenomanian metamorphic, sedimentary, and igneous rocks in the Frey Pedro study area of the Agalta Range of east-central Honduras. The objective of the study is to use these new structural, stratigraphic, biostratigraphic, and geochemical data to better constrain the geologic and tectonic history of this part of the Chortis block during the period of time from Aptian to early Cenomanian. The study revealed that the topographic Agalta Range exposes a thick stratigraphic section (3.5 km) deposited in an Albian-Aptian intra-arc rift and on the rift shoulders. This rift feature, named here the Agua Blanca rift, presently trends northwest and is parallel to three other belts of deformed Cretaceous rocks in Honduras (the Comayagua, Minas de Oro, and Montaña de la Flor belts) that also may correspond to Cretaceous intra-arc rifts produced during the same phase of intra-arc extension. These other three deformed belts are west of the Agalta Range and also form topographically elevated mountain ranges. Albian-Aptian calc-alkaline volcanic rocks and pyroclastic flows of the Manto Formation record arc affinity, while the volcaniclastic wedges of the Tayaco Formation record syn-rift deposition. These rift and arc-related units occupy the stratigraphic position between two major, extensive, shallow-water carbonate units, the lower and upper Atima Formations, also of middle Cretaceous age. Thickening of volcaniclastic rocks of the Tayaco Formation strata in the Agua Blanca rift accompanied erosion of the adjacent rift shoulders and eruption of Manto calc-alkaline volcanic rocks both within and adjacent to the Agua Blanca rift. The Agua Blanca intra-arc rift was inverted by a regional shortening event presumably in the Late Cretaceous. Rocks within the rift were intensely shortened, while rocks on the rift shoulders were shortened less because they are underlain by more competent metamorphic basement rocks. In order to better understand the Aptian–early Cenomanian tectonic setting for intra-arc rifting and subsequent rift inversion on the Chortis block, we reconstructed the Chortis block relative to the terranes studied by previous workers in southern Mexico. Five geologic and tectonic features were selected for realigning the two now widely separated areas: (1) areas of Precambrian basement outcrops, (2) areas of similar Mesozoic stratigraphy, (3) aligned trends of Mesozoic volcanic arc rocks exhibiting a similar arc geochemistry, (4) aligned trends of Late Cretaceous folds and thrusts, and (5) alignment of magnetic boundary.

We document a previously unrecognized, thin-skinned arc-continental collisional zone, termed here the Colon fold-thrust belt, which trends northeastward for 350 km near the Honduras-Nicaragua border region. The Colon belt occurs in three collinear segments: (1) a 200-km-long belt of remote but well-exposed Jurassic–Late Cretaceous rock outcrops described from original geologic mapping presented in this study; (2) a 75-km-long subsurface belt of Jurassic–Late Cretaceous rocks known from onland seismic reflection studies and exploration drilling for oil; and (3) an offshore 75-km-long subsurface belt of Late Cretaceous to Eocene rocks known from exploration studies. These three segments share a continuity of the deformation front and associated folds, as well as a similar timing of fold-thrust deformation (segment one: post-Campanian; segment two: post–Late Cretaceous; segment three: post-Cretaceous and possible to Eocene); and all segments display southeastward-dipping thrusts and related northeastward-verging folds that structurally elevate Cretaceous rocks. The structural position of the Siuna belt of oceanic island arc affinity to the south of the Colon fold-thrust belt, its association with calc-alkaline volcanic rocks of the Caribbean arc, and its Campanian (75 Ma) emplacement age, suggest that the Siuna belt was overthrust to the north and northwest onto the hanging wall of the Colon fold-thrust belt. The northwestward-transported Colon fold-thrust belt and adjacent Siuna belt document a Late Cretaceous collisional event between a south-facing continental margin of the Chortis block of northern Central America and an eastward and north-eastward-moving, Early to Late Cretaceous Caribbean arc system.

Early Tertiary volcanic clasts were collected from the streambed of the Rio Morti near the village of Morti, eastern Panama, as part of a reconnaissance study. The samples range from basalts to rhyolites. K-Ar dates cluster around 58 Ma. The phenocryst mineralogy of the samples is typical of that found in arc-related volcanics: plagioclase (the dominant phase), clinopyroxene, titanomagnetite, and minor orthopyroxene. The clinopyroxenes are augites that plot in the field of orogenic lavas. The geochemistry of the rocks—high ratios of large-ion lithophile elements to high-field strength elements, negative Nb and Ta anomalies, positive Ba anomalies, and relatively low Th to U values—confirms that they are arc related (specifically the calc-alkaline series) and strongly suggests that the samples are not cogenetic. Volcanic rocks with similar ages are exposed in several other localities throughout Panama and Costa Rica (e.g., the Azuero and Sona Peninsulas in Panama and the Nicoya Peninsula in Costa Rica as well as other areas in the Darien of eastern Panama). We suggest that there may have been a more or less continuous arc from South America to the Chortis block of Nicaragua during the Paleocene-Early Eocene. The presence of the arc would imply that the breakup of the Farallon plate and its subduction below the new Caribbean plate in present-day southern Central America probably started some time close to the Cretaceous-Tertiary boundary.